IB Biology SL Interaction and Interdependence Concepts

Interaction and Interdependence explains how cells, organisms and ecosystems exchange information, energy and matter through regulated relationships and environmental feedback.

Exam analysis

Published Concept evidence in Interaction and Interdependence clusters around regulated processes, signalling, defence and ecological relationships. The recurring pattern is explaining how interactions control biological outcomes, from enzyme activity and energy transfer to population change and ecosystem cycling.

Most tested topics

C1.1 Enzymes and metabolism

Enzymes and metabolism explain how catalysts, active sites, reaction conditions and pathway regulation control cellular chemical reactions in living systems.

An enzyme is a biological catalyst. Most are proteins, although some RNA molecules also catalyse reactions. A small amount can act repeatedly because the enzyme emerges unchanged after products are released.

The enzyme provides an alternative pathway with lower activation energy, so more collisions can reach the transition state at the same temperature. It does not change the reactants, products or overall free-energy difference, and it does not remove the barrier completely.

Metabolism is the complete set of enzyme-controlled reactions in a cell or organism. Because each step needs a particular enzyme, cells regulate which routes operate by controlling enzyme amount, activity and location.

Route	Change in matter	Typical chemistry	Energy relationship
anabolism	smaller units → larger, more complex molecules	condensation and reduction	requires an energy input
catabolism	complex molecules → smaller products	hydrolysis and oxidation	releases usable energy

The two routes are coupled: catabolic reactions release energy and provide small molecules; anabolic reactions use both to build cell components. Respiration is mainly catabolic, while protein synthesis is anabolic.

A typical enzyme is a globular protein. Folding brings particular amino-acid side chains together to form a small active-site pocket; most of the protein maintains the three-dimensional environment that makes this pocket work.

Three-stage diagram showing substrate entering an enzyme active site, an enzyme-substrate complex, and products leaving the unchanged enzyme.

Substrate collides with and binds to the active site.

A short-lived enzyme–substrate complex forms and catalysis occurs.

Products no longer fit in the same way, so they leave; the enzyme can bind another substrate.

The active site is flexible, not a rigid lock. Initial contact with the correct substrate changes the conformation of both enzyme and substrate: this is induced fit.

Sequence showing substrate approach, induced-fit closure and substrate strain toward the transition state.

Induced fit can:

align catalytic groups with the bonds that must change
strain or polarize bonds in the substrate
stabilize the transition state

These effects lower the activation energy; complementary shape alone does not explain catalysis.

Three graphs showing enzyme rate against temperature, pH and substrate concentration, with optima and a saturation plateau labelled.

Curve feature	Molecular explanation
temperature rises toward an optimum	faster molecular motion gives more frequent energetic substrate–active-site collisions
temperature falls beyond the optimum	heat increasingly disrupts the enzyme's tertiary structure and active sites are lost
pH has an optimum range	changing H⁺ concentration alters charges and bonding that maintain active-site shape
substrate curve reaches a plateau	nearly all active sites are occupied; enzyme concentration now limits turnover

An optimum is the condition giving the highest measured rate for that enzyme under that experiment. It is not a universal temperature or pH shared by all enzymes.

An enzyme is specific because the active site's three-dimensional arrangement of shape, charge and chemical groups permits productive binding of only particular substrate molecules.

High temperature or extreme pH can disrupt hydrogen bonds, ionic interactions and other weak forces that maintain tertiary structure. The active site changes shape or charge, so the substrate no longer binds productively: the enzyme is denatured.

Denaturation does not normally hydrolyse the peptide bonds of the primary structure. Moderate pH effects may be reversible when the original bonding pattern reforms; severe heating commonly causes irreversible aggregation or misfolding.

Substrate concentration	Active-site occupancy	Effect of adding substrate
low	many active sites are free	collision frequency and rate rise almost proportionally
intermediate	active sites are occupied more often	rate still rises, but by smaller increments
high	nearly every active site is continually occupied	rate approaches a maximum and extra substrate has little effect

The plateau is not caused by substrate molecules stopping. It appears because a fixed number of enzyme molecules can process only a limited number of substrates per unit time.

At saturating substrate concentration, increasing enzyme concentration adds active sites and can raise the maximum rate, provided substrate remains in excess.

Reaction rate is measured as substrate disappearance or product formation per unit time. In a catalase assay, oxygen production is convenient; in an amylase assay, disappearance of starch can be timed with iodine.

Catalase assay apparatus in a water bath, with yeast added to hydrogen peroxide and oxygen bubbles carried through a delivery tube for measurement.

$\text{initial rate}=\frac{\Delta \text{product}}{\Delta t}=-\frac{\Delta \text{substrate}}{\Delta t}$

Use the gradient of a tangent at the start of a product–time curve. Later rates are lower because substrate is depleted, products accumulate or conditions change, so comparing arbitrary final amounts can be misleading.

Design decision	Example for testing temperature on amylase	Why it matters
independent variable	water-bath temperature	defines the factor being tested
dependent variable	time for starch to disappear; convert to relative rate, 1/t	produces a comparable rate measure
controlled variables	pH, enzyme and starch concentrations and volumes	prevents alternative causes of a rate change
negative control	starch plus water without enzyme	checks that loss of starch requires enzyme activity

Pre-incubate enzyme and substrate separately to the chosen temperature, mix to start the reaction, sample at fixed intervals, and use the same endpoint rule each time.

Repeat each condition, calculate a mean and inspect variation. An anomalous result should be investigated, not silently removed; uncertainty in timing, temperature and volume limits the precision of the conclusion.

globular fold → active-site chemistry → induced fit → lower activation energy → faster turnover → controlled metabolic pathways

When explaining a rate pattern, identify what changed, then connect it to:

molecular motion and successful collisions
active-site occupancy
active-site shape and charge
depletion or changing conditions during measurement

A strong investigation measures initial rate, changes one independent variable, controls alternatives, includes a relevant control and uses repeats to reveal variation.

[ 3 ]

Explain how enzymes catalyse chemical reactions.

[ 4 ]

Explain the role of enzymes in metabolic pathways.

[ 2 ]

Identify the following processes as either anabolism or catabolism by placing a tick ( ✓ ) in the correct box.

Process	Anabolism	Catabolism
Light-independent reactions of photosynthesis	$\square$	$\square$
Glycolysis	$\square$	$\square$

[ 1 ]

State a role of the active site of an enzyme.

[ 6 ]

Describe the lock and key model of enzyme activity and how the induced fit model extends it.

[ 1 ]

Which statement applies to enzymes?

[ 6 ]

Some proteins in membranes act as enzymes. Outline enzyme-substrate specificity.

[ 8 ]

Explain the effect of changes of pH , substrate concentration and temperature on enzyme activity.

[ 3 ]

Suggest how the percentage of decolorization could be obtained experimentally.

[ 1 ]

The graph shows energy changes during a reaction both with and without an enzyme present.

Which statement correctly identifies two of the regions labelled X, Y and Z in the graph?

C1.2 Cell respiration

Cell respiration transfers energy from organic compounds into ATP through glycolysis, anaerobic pathways and aerobic mitochondrial stages, supporting usable cellular work and measurable biological activity.

ATP is a small, soluble nucleotide found in all living cells. It transfers manageable amounts of energy between energy-yielding reactions and energy-requiring processes, and can move between nearby reaction sites in the aqueous cell.

$ATP + H_2O \rightarrow ADP + P_i$

$ADP + P_i \rightarrow ATP$

Cells recycle ATP rapidly rather than storing large reserves of it. Glucose, glycogen and lipids are more stable energy stores; respiration transfers some of their energy into ATP when and where work is needed.

Work powered by ATP	Example	How coupling helps
transport	membrane pump moves ions against a gradient	phosphorylation changes pump conformation
chemical synthesis	amino acids joined into a protein	coupled reactions make an otherwise unfavourable step proceed
mechanical movement	motor proteins move chromosomes or contractile filaments	ATP-driven conformational cycles generate force

ATP hydrolysis is useful because it is coupled directly to another process, often by transferring a phosphate or changing a protein's conformation. Free energy is not released as a detached substance that a cell later collects.

One ATP transfer supplies a small task; continuous ATP regeneration allows many molecular events to combine into cell movement, active transport and growth.

Cell respiration is the enzyme-controlled release of energy from organic compounds to produce ATP. Stepwise oxidation transfers energy in small amounts instead of releasing it suddenly as heat and light.

Glucose is converted to pyruvate, which enters a high-ATP aerobic route with oxygen or low-ATP anaerobic routes producing lactate or ethanol and carbon dioxide.

In humans	Aerobic route	Anaerobic route
oxygen	required for continued electron transport	not required
location	glycolysis in cytoplasm, then mitochondria	cytoplasm only
glucose oxidation	complete	incomplete
products	CO₂ and H₂O	lactate
ATP yield	high	net 2 ATP per glucose

Variable	Why rate may change
ATP demand	active muscle or transport increases ADP supply and pathway flux
organism size	smaller endotherms lose heat faster per unit mass and often need higher mass-specific respiration
oxygen	shortage limits aerobic electron transport and shifts ATP production toward low-yield anaerobic pathways
respiratory substrate	availability and entry route limit carbon and electron supply
temperature	enzyme activity rises to an optimum, then denaturation lowers rate
pH	altered charges and protein structure change respiratory enzyme activity

A measured correlation does not prove a single cause. For example, exercise changes ATP demand, oxygen delivery, temperature and substrate use together; a controlled experiment isolates one variable.

The organism consumes O₂ and releases CO₂. Soda lime absorbs the CO₂, so total gas volume falls by the volume of O₂ taken up; reduced pressure moves the capillary fluid toward the organisms.

A sealed respirometer contains a respiring organism and carbon dioxide absorbent; oxygen uptake lowers pressure and draws capillary fluid toward the chamber.

$\text{oxygen uptake rate}=\frac{\pi r^2 d}{t}$

Use a control tube with an equal volume of inert beads, keep temperature constant in a water bath, allow equilibration before sealing, repeat measurements, and prevent direct contact between organisms and soda lime.

organic substrate → stepwise oxidation in respiration → ATP regeneration → coupled transport, synthesis or movement → ADP + Pᵢ recycled

With adequate oxygen, aerobic respiration completes oxidation and yields much more ATP. Without enough oxygen, human cells can regenerate NAD through lactate formation so glycolysis continues, but only its small ATP yield remains.

Respiration rate can be estimated from O₂ uptake only when CO₂ is absorbed and temperature, pressure, organism amount and measurement time are controlled.

[ 3 ]

Explain the properties of ATP that make it useful for distributing energy within cells.

[ 5 ]

Outline, with examples, the wide range of uses of adenosine triphosphate (ATP) in cells.

[ 1 ]

Which reaction does not cause a net release of energy?

[ 8 ]

Explain the need for energy in cells and how energy is released through cell respiration.

[ 8 ]

Carbon dioxide is released during cell respiration. Explain anaerobic and aerobic respiration.

[ 3 ]

Describe how the apparatus measures the oxygen consumption of the mouse.

C1.3 Photosynthesis

Photosynthesis converts light energy into chemical energy by using pigments, thylakoid reactions and Calvin-cycle carbon fixation to build organic compounds from carbon dioxide.

Photosynthesis converts light energy into chemical energy stored in organic carbon compounds. It supplies the carbon compounds and most of the chemical energy that enter ecosystems.

Photoautotroph	Where chlorophyll captures light
plant or alga	thylakoid membranes inside chloroplasts
cyanobacterium	internal photosynthetic membranes; no chloroplast

Chlorophyll-containing thylakoids in a plant chloroplast and photosynthetic membranes in a cyanobacterium absorb light.

light absorbed by pigments → excited electrons → ATP and reduced NADP → CO₂ reduced into carbon compounds

Overall matter equation

6CO₂ + 6H₂O + light energy →
C₆H₁₂O₆ + 6O₂

Light-dependent reactions use water and release oxygen, while ATP and reduced NADP support carbon dioxide fixation in the stroma.

Atom or product	Source and fate
carbon	CO₂ is fixed and reduced into carbohydrate
hydrogen	water supplies hydrogen carried by reduced NADP
released O₂	produced by photolysis of water, not removed from CO₂

Plants, algae and cyanobacteria carry out oxygenic photosynthesis. Glucose is a useful summary product, but fixed carbon also feeds the synthesis of many other compounds.

Pigments separate because each partitions differently between a moving solvent and the stationary phase. A pigment that is more soluble in the solvent and less strongly attracted to the stationary phase moves farther.

A pigment chromatogram labels the origin, pigment distance and solvent-front distance used to calculate Rf.

$R_f = \frac{d_{pigment}}{d_{solvent\ front}}$

Extract pigments in an organic solvent and place a concentrated spot on the origin.
Keep the origin above the solvent; let the solvent rise.
Mark the solvent front immediately, then measure from the origin to each spot's centre.
Compare colour and Rf only with standards run under the same solvent, temperature and stationary phase.

Absorption spectra show carotenoids, chlorophyll b and chlorophyll a absorbing different parts of the visible spectrum, especially blue and red wavelengths.

A photon is absorbed only when its energy matches an allowed change in a pigment's electrons. Absorption raises an electron to an excited state, beginning the energy transfers of the light-dependent reactions.

Chlorophylls absorb strongly in blue and red regions but weakly in much of the green region, so green light is reflected or transmitted. Accessory pigments absorb additional wavelengths and transfer excitation energy toward chlorophyll a.

Spectrum	What is varied	What the y-axis measures	What it tests
absorption	wavelength	light absorbed by a pigment or extract	which wavelengths pigments capture
action	wavelength	rate of photosynthesis, such as O₂ production or CO₂ uptake	which wavelengths drive the whole process

Both normally show strong effects in blue and red regions and weaker effects in much of the green region. The curves are related, not identical: an action spectrum integrates all pigments, energy transfer and every later rate-limiting step.

To compare wavelengths fairly, keep incident photon supply, temperature, CO₂ availability and plant material constant; a coloured filter that also changes intensity confounds the conclusion.

A limiting factor is the condition in shortest effective supply relative to the requirements of photosynthesis. Increasing it raises the rate only until another factor becomes limiting.

Photosynthesis rate rises with light intensity and then plateaus; high carbon dioxide permits a higher plateau than low carbon dioxide.

Region of curve	Interpretation
steep rise as light increases	light is limiting
plateau at low CO₂	added light cannot overcome carbon limitation
higher plateau at high CO₂	relieving CO₂ limitation reveals a higher maximum under those conditions

Estimate photosynthesis from oxygen volume produced per unit time or carbon dioxide removed per unit time. Bubble number is less valid because bubble size varies.

An illuminated Elodea shoot supplies oxygen to a capillary, where gas-bubble displacement is measured with a microburette and stopwatch.

Experimental role	Example
independent variable	light intensity, CO₂ concentration or temperature
dependent variable	initial O₂-production or CO₂-uptake rate
controls	plant species and amount, measurement time, wavelength, the other limiting factors
reliability	repeat each treatment and compare means with variation

Allow the apparatus to equilibrate, change one independent variable across a suitable range, measure an initial rate, reset the gas bubble or sensor, and repeat. Use a heat shield or water bath when changing lamp distance so temperature does not change with light.

Photosynthesis rate rises with temperature to an optimum and then falls steeply above the optimum.

Temperature region	Why rate changes
below optimum	greater kinetic energy increases successful enzyme–substrate collisions
optimum	combined photosynthetic processes reach their highest rate under these conditions
above optimum	carbon-fixation performance falls and other heat stresses increase; do not assume immediate wholesale enzyme denaturation

Light and CO₂ usually give rising curves that level off as limitation changes. Temperature affects enzyme-controlled reactions, so its curve normally rises to an optimum and then declines.

Extra CO₂ can raise photosynthesis or growth only while light, temperature, water, mineral nutrients and biochemical capacity remain sufficient. An enrichment result therefore supports a conditional prediction, not unlimited future growth.

Towers around a forest plot release carbon dioxide into open air for a FACE enrichment experiment.

Design	Strength	Main limitation
greenhouse or chamber	close control of CO₂ and other conditions	enclosure changes light, temperature, airflow and scale
FACE field plot	exposes intact crops or ecosystems under realistic open-air conditions	rainfall, light and other variables cannot be controlled and must be monitored

Compare enriched and ambient-CO₂ replicates, measure photosynthesis and biomass over time, and report variation. Differences among species, years or water treatments show interaction with other limiting factors.

light energy → excited pigment electrons → ATP and reduced NADP → CO₂ reduced using hydrogen from water → organic carbon compounds; photolysis of water releases O₂

A chromatogram separates pigments; Rf compares their movement under matched conditions.
An absorption spectrum measures captured wavelengths; an action spectrum measures whole-process rate.
A limiting-factor experiment changes one condition, controls the others and measures O₂ production or CO₂ uptake per unit time.

When a rate curve plateaus, identify which factor was relieved and which may now be limiting. When evaluating CO₂ enrichment, separate strong control from ecological realism and do not extrapolate beyond the tested conditions.

[ 8 ]

Explain the processes by which light energy is converted into chemical energy.

[ 4 ]

Outline how photosynthesis produces glucose.

[ 1 ]

Which group(s) produce(s) oxygen as a by-product of photosynthesis?

I. Algae
II. Cyanobacteria
III. Fungi

[ 3 ]

Describe the process used to obtain this chromatogram.

[ 4 ]

Outline how plants make use of the different wavelengths of light.

[ 3 ]

Draw a fully labelled graph of the action spectrum for photosynthesis.

[ 9 ]

Explain methods by which the rate of photosynthesis can be measured, including conditions that affect the rate.

[ 2 ]

Using the data in the graph, discuss whether rising carbon dioxide levels in the atmosphere will increase growth rates in maize.

C2.2 Neural signalling

Neural signalling transmits information through neuron structure, ion gradients, action potentials, synapses and chemical modulation of postsynaptic responses, linking cellular mechanisms to rapid communication in nervous systems.

Neuron	Information route	Decisive structural pattern
sensory	receptor → CNS	long dendron toward an off-centre cell body; axon continues into CNS
relay	within CNS	many short dendrites and a short axon connect neurons locally
motor	CNS → muscle or gland	many dendrites around the cell body; one long axon reaches an effector

Sensory, relay and motor neurons compared by cell-body position, fibre arrangement and direction of impulse flow.

Dendrites and dendrons carry impulses toward the cell body; an axon carries impulses away from it. A nerve is a bundle of many nerve fibres, not one neuron.

At rest, an axon's inside is about −70 mV relative to the outside. The membrane is polarized because ions and large negative solutes are distributed unequally across it.

The sodium–potassium pump hydrolyses ATP to move 3 Na⁺ out for every 2 K⁺ in, against both concentration gradients. It establishes and maintains high Na⁺ outside and high K⁺ inside.

At rest the membrane is much more permeable to K⁺ than to Na⁺. K⁺ leakage leaves behind impermeant negative charge, so the inside becomes negative. The pump maintains the gradients that this leakage continually uses.

A resting axon is shown with a negative interior relative to the positively charged exterior.

An action potential is a brief reversal of membrane potential, typically from about −70 mV to about +40 mV. A nerve impulse is this action potential repeatedly regenerated along a nerve fibre.

Below threshold, the membrane returns to rest without firing.
At or above threshold, a full action potential occurs.
Stronger stimuli do not make larger action potentials; they produce a higher frequency of standard-sized impulses.

Traces show no action potential below threshold, one standard-sized action potential just above threshold, and higher impulse frequency with stronger sustained stimuli.

Ion flow through open channels is passive, down electrochemical gradients. ATP is used by pumps to maintain and restore those gradients, not to push each ion through an open channel.

Structural change	Immediate physical effect	Consequence for speed
larger axon diameter	lower resistance to local current inside the axon	adjacent membrane reaches threshold sooner
myelin sheath	raises membrane resistance and limits ion exchange to nodes of Ranvier	local current travels farther before another action potential is generated

Within both myelinated and unmyelinated groups, conduction velocity increases with diameter. At the same diameter, a myelinated axon is much faster; giant unmyelinated axons partly compensate through size.

Diameter and myelination are independent adaptations. Neither changes the amplitude of each action potential; they change how quickly the next region reaches threshold.

The presynaptic terminal stores neurotransmitter in vesicles and releases it.
The synaptic cleft is a narrow extracellular gap, typically about 20 nm.
The postsynaptic membrane carries receptors that convert the chemical signal into a membrane response.

Transmission is normally one-way because release machinery is presynaptic while matching receptors are postsynaptic. Synapses can connect neuron to neuron, neuron to muscle, or neuron to a gland cell.

A presynaptic terminal contains neurotransmitter vesicles above a narrow synaptic cleft and a postsynaptic membrane bearing receptor proteins.

An action potential depolarizes the presynaptic terminal.
Voltage-gated Ca²⁺ channels open.
Ca²⁺ diffuses into the terminal down its electrochemical gradient.
Ca²⁺ triggers neurotransmitter vesicles to dock and fuse with the presynaptic membrane.
Exocytosis releases neurotransmitter, creating a high concentration beside the cleft.

This is an electrical → chemical conversion. Ca²⁺ acts as an intracellular signal that couples membrane depolarization to vesicle fusion.

Presynaptic structure	Contribution
mitochondria	supply ATP for vesicle cycling and ion-gradient maintenance
Golgi apparatus and vesicles	process, package and store transmitter components
presynaptic membrane	provides voltage-gated Ca²⁺ channels and the fusion surface

neurotransmitter diffuses across the cleft → binds a complementary postsynaptic receptor → ligand-gated cation channels open → Na⁺ entry depolarizes the membrane → an excitatory postsynaptic potential (EPSP) forms

An EPSP is graded: more transmitter can produce a larger local depolarization. It makes an action potential more likely, but only a summed depolarization reaching threshold triggers the all-or-none spike.

Acetylcholine binding opens a postsynaptic nicotinic receptor channel and allows sodium ions to enter.

The signal must be removed. At cholinergic synapses, acetylcholinesterase hydrolyses acetylcholine; receptors close, and choline can be taken up for reuse. Other transmitters may be removed by reuptake or diffusion.

Within a neuron: ion gradients create a resting potential; threshold opens a regenerative membrane event; axon diameter and myelin change how quickly it travels.

Across a synapse: arriving depolarization opens Ca²⁺ channels → Ca²⁺ triggers exocytosis → transmitter diffuses → postsynaptic receptors open channels → an EPSP moves the next cell toward threshold.

If transmission fails here…	Inspect…
resting voltage cannot be maintained	ATP supply, Na⁺/K⁺ pump and membrane permeability
presynaptic impulse arrives but no transmitter appears	voltage-gated Ca²⁺ channels and vesicle fusion
transmitter reaches the cleft but no EPSP forms	receptor match, postsynaptic channel and transmitter removal

[ 5 ]

Draw a labelled diagram of a motor neuron.

[ 4 ]

Outline how neurons generate a resting potential.

[ 3 ]

Outline how nerve impulses are transmitted along a nerve fibre.

[ 1 ]

The diagrams represent sections through different axons. Which axon has the slowest speed of impulse?

[ 2 ]

Distinguish between the presynaptic and postsynaptic membranes by giving two differences.

[ 3 ]

Describe how neurotransmitters are released from a presynaptic neuron membrane.

[ 3 ]

Describe how an excitatory postsynaptic potential is generated.

Up to one additional mark is available for the construction of your answer.

C3.1 Integration of body systems

Integrated body systems coordinate neural, hormonal and plant responses through receptors, control centres and effectors, maintaining function across organisms and changing environmental conditions.

Integration is the coordination of interacting parts so the whole biological system performs an overall function. An emergent property appears through those interactions and is not a property of any isolated part.

Genes and molecular, cellular and organ integration contribute to an organism that also interacts with its ecosystem.

If the claim describes…	Classify it as…
what one component can do by itself	a component property
what coordinated components achieve together	an emergent property
information or material passing between components	evidence of integration

A smooth-muscle cell can contract, but one cell cannot propel a meal through an intestine. Peristalsis emerges when many cells, tissues and neural signals coordinate waves of contraction.

cells → tissues → organs → organ systems → organism

Each level contains interacting subsystems from the level below; moving upward adds organisation, not merely more material.

Epithelial and smooth-muscle cells form tissues, the tissues form the small intestine, organs form the digestive system and organ systems form an organism.

Level	Contribution to the intestine
epithelial tissue	forms a selective surface for secretion and absorption
smooth-muscle tissue	contracts and relaxes repeatedly
nervous and connective tissues	coordinate movement and hold working tissues together
whole organ	combines movement, digestion, absorption and transport

The organ's function depends on different tissue types interacting. Calling an organ ‘a group of similar cells’ misses the transition from tissue to organ.

Integrating route	What travels	Characteristic job
nervous signalling	action potentials along neurons; neurotransmitter across synapses	rapid, precise control of particular effectors
hormonal signalling	hormones in blood	coordinated effects in target cells with matching receptors
blood transport	gases, nutrients, substrates, hormones and wastes	physically links organ inputs and outputs

During exercise, motor neurons stimulate skeletal muscle, epinephrine changes several target organs, and blood delivers O₂ and glucose while carrying CO₂ away. No route replaces the others.

Blood reaches many tissues, but a hormone changes only cells with the appropriate receptor. Nervous output is targeted mainly by the physical wiring of the pathway.

Material	Important route	Why the route matters
glucose and amino acids	small intestine → hepatic portal vein → liver → body tissues	liver processes and buffers nutrient supply before wider distribution
O₂	lungs → red blood cells → respiring tissues	supports aerobic ATP production
CO₂	respiring tissues → blood → lungs	removes a respiratory waste and helps restore pH
urea	liver → blood → kidneys	transfers nitrogenous waste to the organ that excretes it

Long-chain lipids are rebuilt into triglycerides and packaged with protein before entering lymph, then the blood near the heart. Their hydrophobicity changes the transport route.

Transport integrates organs because one organ's output becomes another organ's substrate, signal or waste load. The circulation is the connecting route; it does not itself perform digestion, respiration or excretion.

Region	Main integration role
cerebral hemispheres	conscious perception, memory, planning and voluntary motor commands
cerebellum	compares intended movement with sensory feedback to refine timing, force, posture and balance
hypothalamus	links nervous information and blood conditions to autonomic and endocrine control
medulla	adjusts involuntary functions including heart rate and ventilation

External and sectional brain views identify the cerebral hemispheres, cerebral cortex, hypothalamus, pituitary gland and cerebellum.

The brain is an integration organ because it receives, compares, stores and redistributes information. A region's role is not an isolated behaviour; it contributes to coordinated networks.

Spinal-cord role	Information route	Outcome
relay	sensory input ascends to the brain; motor commands descend to peripheral neurons	links brain and body
local integration	sensory neuron synapses through relay neuron(s) to a motor neuron	rapid unconscious response can begin without waiting for the brain

A spinal reflex is integrated in the spinal cord, but sensory impulses can still travel to the brain. Rapid withdrawal and later conscious pain are therefore compatible, not alternative explanations.

Spinal circuits also contribute to involuntary processes such as urination, defecation and some sweating. ‘Unconscious’ means not under direct voluntary control; it does not mean unregulated.

receptor transduces a stimulus → sensory neuron carries impulses into the CNS → relay neurons in brain networks support conscious perception → an upper motor neuron carries a command downward → a lower motor neuron reaches skeletal muscle → neurotransmitter at the neuromuscular junction triggers muscle action potentials and contraction

Sensory input from a receptor travels to the spinal cord and cerebral cortex, while upper and lower motor neurons carry output to a skeletal muscle.

Fibre	Direction relative to CNS	Function
sensory (afferent)	receptor → CNS	delivers information for integration
motor (efferent)	CNS → skeletal muscle	delivers the coordinated response

The cerebral hemispheres coordinate voluntary action, but they do not contract the muscle directly. The final output is carried by motor neurons to the neuromuscular junction.

Structure	What it encloses
endoneurium	one nerve fibre
perineurium	one fascicle, or bundle of fibres
epineurium	several fascicles plus blood vessels, forming the whole nerve

A peripheral nerve cutaway shows many axons grouped into fascicles, surrounded by endoneurium, perineurium and epineurium.

A neuron is one cell. A nerve fibre is a neuron's long conducting process, usually an axon. A nerve is a connective-tissue package containing many fibres.

A mixed nerve can carry sensory impulses toward the CNS and motor impulses away from it at the same time because those signals travel in different fibres.

A damaging stimulus activates a pain receptor in the skin.
A sensory neuron carries impulses into the spinal cord.
A relay neuron transfers the signal to a motor neuron.
The motor neuron stimulates a flexor skeletal muscle.
The muscle contracts and withdraws the limb.

A pain receptor in skin sends an impulse through a sensory neuron to a spinal relay neuron and motor neuron, contracting a flexor muscle to withdraw the hand while information also travels to the brain.

Route	Earliest outcome
spinal reflex arc	rapid involuntary withdrawal
ascending pathways to brain	conscious pain, awareness and memory
descending input from brain	may modify the response after integration

The reflex trades deliberation for speed: a predictable protective response starts through a short spinal pathway while the brain receives and interprets the same event.

intended movement from cerebral cortex + sensory feedback about position and balance → cerebellum compares intention with performance → timing and force of motor output are adjusted → movement becomes smooth and posture remains stable

The cerebral cortex supplies an intended motor command, sensory receptors return information about the movement, and the cerebellum adjusts output so the movement becomes smooth.

Cerebellar contribution	Example
timing	activates muscles in the correct sequence
force	prevents contraction from overshooting or undershooting
balance and posture	uses receptor input to correct body position
learned precision	refines writing, speech and practiced hand movements

The cerebellum coordinates and corrects skeletal-muscle contraction. Voluntary movement is initiated elsewhere, particularly in the cerebral cortex.

A circadian rhythm is an approximately 24-hour pattern in physiology or behaviour. The pineal gland secretes melatonin as a hormonal timing signal within this clock system.

Environmental state	Melatonin secretion	Likely effect
darkness, including longer nights	rises	supports sleepiness and night-time timing
light	falls	supports preparation for wakefulness
abruptly shifted light–dark cycle	internal rhythm and local time are misaligned	jet lag persists while the clock resets

Melatonin modulates the timing of sleep; it is not a sedative switch that alone causes every transition into sleep. Light resets a continuing biological rhythm rather than producing an immediate, complete change.

During danger or stress, the adrenal medulla releases epinephrine into the blood. One hormone then coordinates target organs for vigorous activity.

Target response	Immediate change	Contribution to intense muscle contraction
heart	heart rate and cardiac output increase	delivers O₂ and glucose more rapidly
ventilation system	breathing rate and depth increase	increases gas exchange
liver	glycogen is broken down and glucose enters blood	raises available respiratory substrate
skeletal muscle and other responsive tissues	metabolic pathways are shifted toward rapid ATP demand	supports forceful activity

The important outcome is matched supply and demand: circulation, ventilation and fuel mobilisation rise together, allowing a higher rate of aerobic respiration and ATP production.

Epinephrine is an amino-acid-derived catecholamine, not a peptide hormone. It acts only in cells with appropriate adrenergic receptors.

Pituitary part	Hypothalamic route	What the pituitary does
anterior	releasing or inhibiting hormones travel through portal blood vessels	endocrine cells synthesize and release pituitary hormones
posterior	axons of hypothalamic neurosecretory cells end in the gland	stores and releases ADH and oxytocin made in the hypothalamus

Hypothalamic neurosecretory cells regulate the anterior pituitary through a portal capillary system and send axons directly to the posterior pituitary.

Anterior-pituitary hormones regulate processes including growth, reproduction and homeostasis and can control endocrine glands such as the thyroid, gonads and adrenal cortex.

Calling the pituitary the ‘master gland’ is incomplete: the hypothalamus largely controls its activity, linking neural and endocrine information.

Receptor	Variable detected	Important locations	What a change implies
baroreceptor	stretch caused by blood pressure	carotid sinus, aortic arch, right atrium	pressure/venous return has changed
chemoreceptor	mainly CO₂-linked pH; also O₂	carotid bodies, aortic bodies and brain/medulla	gas exchange or metabolic demand has changed

High arterial pressure increases baroreceptor signalling and calls for a slower heart rate.
Low pressure calls for a faster, stronger response.
High CO₂ or low pH calls for increased blood flow so CO₂ reaches the lungs faster.

Sensory impulses converge on the cardiovascular centre in the medulla. The medulla integrates the evidence; receptors detect change but do not directly set the SA node.

Integrated condition	Medulla increases output through…	Transmitter at SA node	SA-node effect
low pressure, high CO₂ or low pH	sympathetic cardiac nerves	norepinephrine	firing rate rises; heart rate increases
high pressure or low CO₂ demand	parasympathetic vagus nerve	acetylcholine	firing rate falls; heart rate decreases

disturbance → receptor firing changes → medulla integrates input → sympathetic or parasympathetic output changes → SA-node rate changes → blood pressure and gas transport move toward the required range

The SA node initiates each heartbeat within cardiac muscle. Autonomic nerves adjust its firing rate; they do not supply every impulse that causes the heart to contract.

Circulating epinephrine can also increase SA-node rate, so nervous and endocrine signals may reinforce one another during stress.

Cellular respiration raises blood CO₂.
CO₂ forms carbonic acid, increasing H⁺ and lowering pH.
Central and peripheral chemoreceptors increase signalling to respiratory centres in the medulla.
Motor output to the diaphragm and external intercostal muscles increases rate and depth of contraction.
Ventilation removes CO₂ faster, so H⁺ falls and pH moves back toward its set range.

Chemoreceptors in the medulla, aortic bodies and carotid bodies signal the respiratory centre, which controls intercostal muscles and the diaphragm.

Observation	Best interpretation
CO₂ rises and pH falls	ventilation should increase
exercise stops and CO₂ falls	ventilation can decrease gradually
O₂ falls strongly	peripheral chemoreceptors add a weaker but important drive

The principal routine stimulus is CO₂-linked H⁺, not a direct measurement of how hard the muscles are working. Voluntary control can temporarily override the automatic rhythm but does not replace it.

Part of the route	Main control	Status
start of swallowing	CNS and somatic motor pathways	voluntary initiation
most movement through gut	enteric nervous system, modulated by autonomic/CNS input	involuntary
defecation boundary	spinal/CNS reflexes plus voluntary control of external sphincter	mixed control

Peristalsis is a travelling wave of contraction and relaxation in circular and longitudinal smooth muscle that propels gut contents forward.

The enteric nervous system contains sensory neurons, relay neurons and motor neurons in the gut wall. Local stretch and chemical information can therefore coordinate adjacent regions without a separate conscious command for every contraction.

Enteric is not a synonym for all autonomic control. The enteric nervous system specifically serves the gastrointestinal tract and is one specialised part of involuntary regulation.

System architecture: specialised cells form tissues and organs; nervous signals, hormones and circulating materials cross the boundaries between them; coordinated interaction creates emergent functions.

Control route: disturbance or stimulus → receptor evidence → integration centre → targeted motor or endocrine output → effector response → changed internal condition.

Observation	Most useful place to inspect
touch is detected but no voluntary movement follows	CNS integration, descending motor route or neuromuscular junction
pressure rises but heart rate does not fall	baroreceptors, medulla, vagus nerve or SA node
CO₂ rises but breathing remains unchanged	chemoreceptors, respiratory centre, phrenic/intercostal output
gut smooth muscle is intact but waves are uncoordinated	enteric sensory, relay and motor circuits

Stabilising loop	Coordinated response without a fixed set point
heart-rate and ventilation feedback oppose a disturbance	epinephrine temporarily raises several outputs together for anticipated demand

[ 3 ]

Using the diagram, explain the concept of emergent properties of biofilms.

[ 1 ]

Why do multicellular organisms have emergent properties?

[ 3 ]

Pancreatic secretions contain sodium hydrogen carbonate, making them basic.
Deduce the significance of the response by the pancreas to secretin.

[ 6 ]

Explain two methods that scientists have used to determine the different functions of the brain.

[ 1 ]

The spinal cord is involved in the coordination of involuntary actions such as the pain reflex.

Where are synapses between sensory neurons and interneurons found?

[ 1 ]

Between which structures do sensory neurons carry nerve impulses?

[ 1 ]

What is the main role of nerves in human movement?

[ 1 ]

Numerous health benefits are associated with diets that include omega-6 fatty acids and omega-3 fatty acids in a ratio between 1:1 and 4:1. When consumed in excess, omega-6 inhibits uptake of omega-3. Many people in developed countries eat large amounts of processed foods and oils, so they consume omega-6 fatty acids and omega-3 fatty acids at a ratio of between 10:1 and 25:1. Such high ratios are associated with many chronic diseases.

Oils	Ratio of omega-6 to omega-3
Flaxseed oil	0.24: 1
Canola oil	2: 1
Walnut oil	5: 1
Olive oil	13: 1
Sunflower oil	19: 1
Corn oil	46: 1
Sesame oil	138: 1
Grapeseed oil	696: 1

State the role of the vagus nerve.

[ 4 ]

Draw a labelled diagram of a reflex arc for a pain withdrawal reflex.

[ 2 ]

Describe the use of fMRI to identify the role of the cerebellum.

[ 2 ]

Outline the role of melatonin in humans.

[ 2 ]

Describe two ways in which epinephrine in the body facilitates intense muscle contraction. [2]

1.

2.

[ 7 ]

Explain the roles of named hormones in the development and function of the sexual reproductive systems in males and females.

[ 7 ]

Explain the control mechanism of the heart rate.

[ 8 ]

Explain the causes of a decreased blood pH and its effects on the ventilation rate in humans.

[ 3 ]

Outline how food is moved from the stomach to the large intestine.

C3.2 Defence against disease

Defence against disease combines physical barriers, clotting, phagocytes, lymphocyte specificity, vaccination, antibiotics and evolutionary responses to pathogens across human populations.

A pathogen is a disease-causing biological agent. Infection means that it has entered and established itself in a host; infectious disease is the harmful condition that can result.

Comparison of viruses, bacteria, fungi, protists, parasitic animals and archaea as possible or known pathogen groups.

Group	Disease-causing examples?
viruses, bacteria, fungi, protists	yes
parasitic animals, including helminths	yes
archaea	no human infectious disease is currently known

Viruses are pathogens but are not cells. Exposure also does not guarantee disease: entry, replication, host defence and tissue damage all affect the outcome.

Keratinized cells form a tough, dry physical barrier.
Continuous shedding removes attached microbes.
Skin secretions create chemical conditions that inhibit many microbes.

Body map showing skin and the mucous membranes of the respiratory and digestive systems, with shedding, lysozyme, mucus and cilia.

A cut, bite or puncture bypasses this barrier. The defence is strongest while the surface remains intact.

Respiratory and digestive surfaces must exchange materials, so they cannot be sealed by thick dry skin. They use a trap-and-remove system instead.

Mechanism	Protective result
mucus	traps microbes and particles
cilia	sweep trapped material toward the throat
swallowing or coughing	removes material from the airway
lysozyme in tears, saliva and mucus	hydrolyses bacterial cell-wall peptidoglycan

Mucus alone is not the complete defence: protection depends on capture + movement + removal or chemical damage.

Clotting limits blood loss and closes an entry route for pathogens.

Damaged tissue and activated platelets start a cascade of clotting factors.
The cascade produces thrombin from its inactive precursor.
Thrombin converts soluble fibrinogen into insoluble fibrin.
Fibrin fibres trap platelets and blood cells, forming a clot that dries into a scab.

The clot is not simply a pile of platelets: the fibrin network gives the seal its reinforcing mesh.

Feature	Innate immunity	Adaptive immunity
onset	rapid	slower on first exposure
recognition	broad pathogen patterns or damage	one antigen specificity per selected clone
main cells here	phagocytes	B and T lymphocytes
memory	no antigen-specific memory	memory cells produce an improved secondary response

The systems cooperate. Innate cells contain infection and present antigens; adaptive cells add precise targeting and long-term memory.

chemical signals → amoeboid movement → receptor attachment → engulfment into a phagosome → lysosome fusion → enzymatic digestion

The pathogen is destroyed inside a membrane-bound vesicle. Lysosomal enzymes are contained, reducing damage to the phagocyte's own cytoplasm.

Sequence of phagocytosis from chemotaxis and receptor attachment through engulfment, lysosome fusion and digestion.

All lymphocytes arise from stem cells in bone marrow. B cells mature in bone marrow; T cells mature in the thymus. Mature cells circulate through blood and lymph and concentrate in lymph nodes.

Cell	Core job after activation
helper T cell	coordinates responses and activates matching B cells
killer T cell	destroys infected body cells displaying its antigen
B cell	forms plasma cells and memory B cells
plasma cell	secretes one specific antibody

A resting B cell does not yet release large amounts of antibody; high-output secretion follows activation and differentiation into a plasma cell.

An antigen is a molecular structure recognized specifically by an antibody or lymphocyte receptor and capable of contributing to an immune response. Many are proteins or glycoproteins.

Recognition depends on complementary three-dimensional shape and chemical interactions. One selected lymphocyte clone therefore responds to a particular antigenic feature, not to every foreign molecule.

A pathogen antigen binding to a complementary antibody variable region and a matching B-cell receptor.

A B-cell receptor binds its complementary antigen.
The B cell internalizes and digests the antigen.
Antigen fragments are displayed on MHC II proteins.
An activated helper T cell recognizing the same antigen binds and delivers co-stimulation and cytokines.
The B cell is fully activated.

The two linked recognition events act as a specificity check: the B cell captures the target, and a matching helper T cell confirms that response.

An antigen does not instruct a B cell to invent a receptor. It selects the rare pre-existing B cell whose receptor already binds it.

After activation	Result
repeated mitosis	a clone with the same antigen specificity
differentiation into plasma cells	rapid secretion of large amounts of matching antibody
differentiation into memory B cells	long-lived capacity for a faster later response

Plasma cells contain extensive rough endoplasmic reticulum and Golgi apparatus because antibodies are proteins made and exported in large quantities.

Antibody binding can…	Consequence
cover a toxin or viral attachment site	neutralize its interaction with host cells
cross-link many targets	agglutinate them into easier-to-clear clusters
coat a pathogen	make recognition and engulfment by phagocytes easier

The antibody supplies specific binding. Clearance is then completed by phagocytes, complement and other immune mechanisms.

Response	Starting state	Typical outcome
primary	only rare matching naive cells are present	slower clonal expansion; symptoms may develop
secondary	many long-lived matching memory cells are present	faster, larger response; pathogen may be controlled before disease

Memory cells do not provide a permanent high antibody concentration. They preserve ready-to-expand antigen-specific clones, so re-exposure starts farther along the response pathway.

keep out with skin and mucosa → seal breaches with fibrin → contain broadly with innate phagocytes → recognize specifically with lymphocytes → amplify selected clones → clear with antibodies and effector cells → remember with long-lived B- and T-cell clones

If this step fails…	The immediate consequence is…
surface barrier	easier pathogen entry
phagocytosis	weaker early containment
helper-T coordination	weaker B-cell and killer-T responses
clonal expansion	too few specific effector cells
memory formation	no improved response on re-exposure

Protection is not one reaction. It is a connected sequence of barriers, broad responses, specific selection and retained memory.

Transmission becomes possible when enough HIV in blood, semen, vaginal or rectal fluid, or breast milk reaches susceptible tissue or the bloodstream.

Route	Why transmission can occur
unprotected sexual contact	infected fluid contacts vulnerable mucosa or damaged tissue
shared needles or contaminated blood	virus enters blood directly
pregnancy, birth or breastfeeding	maternal virus can reach the child

HIV is not transmitted by intact-skin contact, sharing food, toilets, coughing, or insect bites. Risk depends on fluid + access + viral load, not on ordinary social contact.

attachment to CD4 and co-receptors → entry and uncoating → reverse transcriptase copies viral RNA into DNA → viral DNA integrates into host DNA → host machinery produces new virus

Integration lets viral information persist in the cell. Replication and immune attack progressively reduce functional helper T cells.

HIV attaches to a CD4-bearing helper T cell, releases RNA, uses reverse transcriptase to make viral DNA and integrates that DNA before immune coordination declines.

HIV is the virus. AIDS is an advanced syndrome diagnosed when HIV has caused severe immune damage, indicated by very low CD4 helper-T-cell counts and/or characteristic opportunistic disease.

fewer helper T cells → weaker activation of matching B cells and killer T cells → reduced antibody and cell-mediated responses → normally controlled pathogens and cancers can cause severe disease

A person can transmit HIV without having AIDS. Antiretroviral therapy suppresses viral replication and can prevent or greatly delay progression.

Selective toxicity means disrupting a bacterial structure or process more than the patient's cells.

Bacterial target	Why it can be selective
peptidoglycan wall synthesis	human cells have no cell wall
70S ribosome function	bacterial ribosomes differ from cytoplasmic human 80S ribosomes
bacterial DNA or metabolic enzymes	key molecules can differ enough for selective inhibition

Viruses have no peptidoglycan wall, ribosomes or independent metabolism. Antibiotics therefore do not treat viral infections; antivirals target stages of viral entry or replication instead.

Mutation or acquired DNA creates heritable variation before treatment.
The antibiotic kills or inhibits susceptible bacteria.
Resistant bacteria survive more often.
Survivors reproduce, so resistance alleles become more frequent.

An individual bacterium does not evolve because it ‘tries’ to survive. The population evolves as allele frequencies change across generations.

A bacterial population with susceptible and resistant variants is exposed to antibiotic; resistant survivors reproduce and can transfer a resistance plasmid.

Process	Contribution
mutation	creates a new resistance allele
plasmid transfer	moves resistance genes between bacteria, sometimes across species
reproduction	passes genes to descendants
antibiotic exposure	repeatedly favours carriers of useful resistance genes

Unnecessary prescribing, incorrect doses, poor infection control and routine agricultural use can increase selection or spread. They do not deliberately cause mutation; they make resistant lineages more likely to survive and transmit.

A single bacterium can accumulate several resistance genes, producing a strain for which multiple treatments fail.

Route	Example
direct contact or bite	rabies
food, droplets or close animal exposure	bovine tuberculosis
arthropod vector	Japanese encephalitis via mosquitoes
spillover followed by sustained human transmission	COVID-19, with probable animal origin

A reservoir maintains the pathogen in nature. A vector carries it between hosts. They may be different organisms and should not be used as synonyms.

Animal reservoirs pass zoonotic pathogens to humans directly or through a vector, with examples including rabies, bovine tuberculosis, Japanese encephalitis and COVID-19.

Platform	What enters the body	How antigen appears
attenuated or inactivated pathogen	whole modified pathogen	its antigens are processed and presented
subunit or toxoid	selected antigenic material	antigen is taken up by antigen-presenting cells
mRNA or DNA vaccine	genetic instructions	host cells make the antigen, then display or release it

All routes aim to activate matching helper T cells, B cells and often killer T cells without the uncontrolled disease caused by the pathogen. Clonal expansion then forms effector and memory cells.

Genetic vaccines do not alter the purpose of the response: they change how antigen is supplied, not the antigen-specific logic of adaptive immunity.

Immunity	Source	Memory formed?
active natural	infection exposes the person to antigen	usually
active artificial	vaccination exposes the person to antigen safely	usually
passive natural	maternal antibodies cross placenta or enter milk	no
passive artificial	prepared antibodies are injected	no

Active means the person's own lymphocytes respond and form memory. Passive means ready-made antibodies arrive from elsewhere, giving immediate but temporary protection.

Vaccination is called artificial because the exposure is deliberately provided—not because the antibodies or memory cells are artificial.

When many people are immune, an infected person is less likely to contact a susceptible person. Transmission chains end more often, indirectly reducing exposure for people who are not immune.

There is no universal threshold. It depends on contagiousness, route of transmission, vaccine effectiveness, duration of immunity, behaviour and how evenly immunity is distributed.

Population protection lowers risk; it does not guarantee that every susceptible individual is safe.

Transmission networks showing many immune people interrupting spread, compared with continued spread when few people are immune.

$\%\text{ change}=\frac{\text{new}-\text{original}}{\text{original}}\times100$

$\%\text{ difference}=\frac{|A-B|}{(A+B)/2}\times100$

$I=\frac{N}{P}$

$VE=\frac{I_u-I_v}{I_u}\times100$

Question	Measure
How far did one value move from its baseline?	percentage change
How different are two values with no baseline?	percentage difference
How quickly are new cases arising?	incidence
How much lower is incidence in vaccinated people?	vaccine efficacy

Use incidence or risk, not raw case counts, when group sizes or observation time differ. Here, Iᵤ and Iᵥ are incidences in unvaccinated and vaccinated groups.

Before comparing groups, check:

the same case definition and testing access
population size or person-time at risk
the same observation period
age, prior infection and health differences
exposure patterns, circulating variants and time since vaccination
uncertainty, missing data and source reliability

Trial arm	Cases	Participants	Risk
unvaccinated	185	15,210	1.216%
vaccinated	11	15,210	0.072%

$VE=(1-0.059)\times100=94.1\%$

A correlation or group difference supports a causal claim only when design and confounding have been addressed. Report what the data measure—not more than they can show.

Stage in the disease system	Control logic
reservoir or transmission route	reduce contact, block vectors, use barriers and infection control
pathogen replication	choose a treatment with a real pathogen-specific target
population evolution	use antibiotics only when appropriate and limit spread of resistant strains
susceptible host	vaccinate to build antigen-specific memory before exposure
population network	raise and distribute immunity so transmission chains break
evidence and policy	compare rates fairly, quantify effects and account for confounding

Ask three questions in order: Where is the causal bottleneck? Which intervention acts there? What evidence would show that it worked?

Effective disease control joins cellular mechanisms, evolutionary consequences and population data. A correct intervention aimed at the wrong stage—or supported by an unfair comparison—can still fail.

[ 6 ]

Describe the cause, transmission and effects of malaria.

[ 3 ]

Describe how disease-causing bacteria are prevented from entering the body.

[ 7 ]

Explain how blood clotting occurs and the consequence for a person who has hemophilia.

[ 1 ]

Distinguish between innate and adaptive immune response.

The graph shows the magnitude of the immune response after infection or after vaccination over time.

[ 4 ]

Describe how phagocytic leucocytes may act as a defence against disease.

[ 7 ]

Explain how cells in the bloodstream cause a specific immune response.

[ 1 ]

What is a characteristic of antigens?

[ 7 ]

Explain the production of antibodies when a patient is infected with the TB bacterium.

[ 1 ]

What is a role of activated B cells?

[ 3 ]

Suggest possible reasons for the observed pattern of presence of antibodies in vaccinated mice.

The summer after vaccination, the prevalence of B. burgdorferi infection in tick nymphs collected on mice from the two sites was measured.

	State of infection of tick nymphs with B. burgdorferi
	Site 1		Site 2
Host mice	Infected	Not infected	Infected	Not infected
Control mice	90	315	57	89
Vaccinated mice	87	288	49	121

[ 1 ]

How has the transmission of HIV been reduced?

[ 4 ]

Discuss the consequences of infection with HIV.

[ 3 ]

(i) Estimate the diameter of the zone of inhibition of chloramphenicol.
(ii) Distinguish between the action of tetracycline and penicillin on B. subtilis.
(iii) Suggest a reason for the result with disc X .

[ 9 ]

Explain how natural selection can lead to evolution using antibiotic resistance in bacteria as an example.

[ 1 ]

The table shows the number of amino acids that are different in the sequences of the protein FOXP2 from a human compared with four other primate species.

Primate species	Number of amino acid differences compared to human
Pan paniscus	2
Pan troglodytes	2
Gorilla gorilla	2
Pongo pygmaeus	3

State the term used for an infectious disease that can transfer from other species to humans.

[ 8 ]

Some prokaryotes cause infectious disease in humans. Explain the principles of vaccination.

C4.1 Populations and communities

Populations and communities describe abundance, sampling, growth limits, species interactions and trophic controls that shape ecological patterns across habitats and ecosystems.

A population is all interacting individuals of the same species living in a defined area at the same time. In a sexually reproducing species, members have opportunities to interbreed.

Two groups can belong to the same species but form different populations when geography, behaviour or timing prevents regular gene flow between them.

Two groups of the same rabbit species interbreed within each group but are separated from one another by a geographic barrier.

A sample must represent the study area. Random placement reduces selection bias, but the correct design also depends on spatial structure.

Site or question	Sampling design
one relatively uniform habitat	random points or coordinates
two or more habitat types	stratified: sample each in proportion to its area
change along an environmental gradient	systematic points at fixed intervals on a transect

More repeated samples reduce random sampling error. They do not repair a systematic bias such as choosing only species-rich patches.

Mark the site's boundaries and coordinate axes.
Generate random coordinate pairs.
Place the same-sized quadrat with its centre at each coordinate.
Identify the target species and record the chosen response.
Repeat enough times to estimate a mean and variation.

Record	Best when…
density	individuals can be distinguished and counted
frequency	presence or absence across quadrats matters
percentage cover	individuals overlap or cannot be separated easily
abundance score	a defined ordinal scale is appropriate

A coordinate grid marks random quadrat centres across a habitat and shows one quadrat placed at a selected coordinate.

$\hat N=\bar n\times\frac{A}{a}$

The estimated population size equals the mean number of individuals per quadrat multiplied by total habitat area ÷ quadrat area.

Evidence	Value
mean count per 0.5 m² quadrat	6.4 plants
habitat area	250 m²
estimate	6.4 × (250 ÷ 0.5) = 3,200 plants

This is an estimate, not a full count. Report sampling method, number of quadrats and variation so the uncertainty and representativeness can be judged.

After marked animals mix back into the population, the marked fraction in the second sample should approximate the marked fraction in the whole population.

$\frac{R}{N}\approx\frac{M}{\hat N}\qquad\hat N\approx\frac{M\times N}{R}$

If M = 50, N = 40 and R = 10, then the estimated population size = (50 × 40) ÷ 10 = 200 animals. Fewer marked recaptures imply a larger estimated population.

Capture-mark-release-recapture sequence showing the first marked sample M, second capture N, marked recaptures R and the Lincoln estimate.

Assumption	If violated…	Likely effect
marked animals mix completely	second sample contains too many nearby marked animals	R too high → underestimate
marks persist and are recognized	marked animals appear unmarked	R too low → overestimate
marking does not alter survival or capture	marked animals are lost or avoided	R too low → overestimate
population is closed	births, deaths or movement change the fractions	estimate no longer describes one stable population
equal catchability	some individuals dominate both samples	biased in either direction

Use a harmless, inconspicuous, durable mark; allow time for mixing; recapture in the same defined area; and keep the interval short enough that demographic change is limited.

Carrying capacity, K, is the largest population that the current environment can sustain over time without exhausting the resources and conditions on which that population depends.

Plants may be limited by light, water, mineral nutrients, carbon dioxide, temperature or space.
Animals may be limited by food, water, mates, territory, nesting sites, disease or predation.

K is not permanently fixed. Seasonal change, disturbance, habitat management or evolution can alter the limiting environment and therefore alter carrying capacity.

A sigmoid population curve rises and then fluctuates around carrying capacity as competition, food and disease limit growth.

population density rises → competition, pathogen transmission, waste effects or predation intensify → birth rate falls and/or death rate rises → net growth falls → density moves back toward K

Factor	Density-dependent?	Regulatory pattern
competition, disease, predation	yes	effect becomes stronger as density rises
drought, frost, fire or storm	usually no	can change population size regardless of density

‘Negative’ describes opposition to change, not harm. The result is often fluctuation around K rather than a perfectly constant population.

A rabbit and lynx cycle represents higher prey density supporting more predators, which then lowers prey density before predator numbers decline.

$\Delta N=(B+I)-(D+E)$

Population increases when births B plus immigration I exceed deaths D plus emigration E. The graph's slope shows the net rate of change.

Pattern	What happens to slope?	Biological conditions
exponential growth	becomes progressively steeper	abundant resources; weak limiting factors
sigmoid growth	steepens, then becomes less steep near K	density-dependent resistance increasingly offsets growth

A rising curve can still be slowing: population size increases while its slope decreases during the transition toward carrying capacity.

Phase	Population pattern	Mechanism
lag or establishment	small rise	few reproductive individuals; acclimation
exponential	increasingly steep rise	resources abundant; births greatly exceed deaths
transition	rise continues but slows	competition, disease or predation strengthen
plateau	fluctuates around K	average gains and losses are approximately balanced

The smooth sigmoid is an idealized model. Real populations show irregular departures caused by weather, disturbance, migration and changing resource supply.

Start equal cultures with a small known number of duckweed thalli or a measured yeast inoculum.
Use at least five replicates per condition.
Hold temperature, light, volume and nutrients constant unless one is the independent variable.
Count or measure population size at regular intervals.
Plot the mean against time and show variation.

Observation	Interpretation
progressively steeper early rise	net growth rate is increasing
curve bends toward a plateau	limiting factors are strengthening
different plateau under changed nutrients	the treatment changed carrying capacity
large scatter among replicates	estimate is less precise; investigate uncontrolled variation

Counting duckweed area or yeast absorbance may be a proxy rather than a direct count. State what was actually measured and do not treat a smooth fitted curve as exact reality.

define the population → choose a representative design → sample sessile or motile organisms appropriately → calculate an estimate → test assumptions and uncertainty → follow size through time → explain slope and fluctuations using limiting factors and feedback

Evidence pattern	First question to ask
quadrat estimate differs between repeats	were placement, sample number and habitat structure representative?
few marked animals are recaptured	is the population large, or were marks lost / mixing poor?
growth slows near a plateau	which density-dependent factor is strengthening?
population drops far below K after a storm	is a density-independent disturbance responsible?

A population number becomes biologically useful only when the sampling inference and the mechanism of change are both explicit.

Intraspecific interaction	Immediate effect	Possible fitness consequence
competition for food, light, territory or mates	each competitor gains less access to a limiting resource	reduced survival or reproduction
dominance hierarchy	some individuals gain priority; repeated fighting is reduced	lower injury cost for the group, unequal access remains
cooperative hunting	individuals share pursuit and capture	prey too large or fast for one hunter becomes available
parental or colony care	time and energy support related young	more shared genes may survive into the next generation

Members of one species overlap strongly in resource needs, so competition can be intense. Cooperation is favoured when its direct or inclusive-fitness benefit exceeds its cost in that context.

Ecological level	Includes
population	one species in one area
community	all interacting populations in that area
ecosystem	the community plus abiotic conditions and their interactions

A change in one population can alter food, habitat, disease, pollination or competition for several others. Community structure is therefore a network outcome, not a list of species.

Plant, herbivore, predator, parasite and mutualist populations are connected in a simple community interaction network.

Relationship	Species A	Species B	Defining transfer or effect
mutualism	+	+	each gains a resource or service
competition	−	−	both lose access or spend resources contesting
predation	+	−	predator kills and consumes prey
herbivory	+	−	animal consumes plant or algal tissue
parasitism	+	−	parasite gains resources while living on or in host
pathogenicity	+	−	pathogen reproduces while damaging host

The signs describe effects relative to no interaction. They do not mean every individual receives the same benefit or cost, and the outcome can change with conditions.

Examples classify mutualism as positive-positive, competition as negative-negative, and predation, herbivory, parasitism and pathogenicity as positive-negative interactions.

In mutualism, each species receives a net benefit from the interaction under the stated conditions. The exchanged benefits may be nutrients, energy-rich compounds, protection, habitat or transport.

Partners	One partner gives	The other gives
legume ↔ Rhizobium	carbohydrates and a root-nodule habitat	fixed nitrogen compounds
orchid ↔ mycorrhizal fungus	photosynthetic carbohydrates after establishment	water and mineral nutrients; crucial seedling nutrition
coral ↔ zooxanthellae	CO₂, mineral waste and protected habitat	glucose and O₂ from photosynthesis

Mutualism is not unlimited generosity: both partners incur costs, and the relationship persists when the received benefit exceeds those costs. Dependence may be facultative or obligatory.

Term	Meaning
native	occurs in an area without human introduction
endemic	native and restricted to a particular geographic area
alien / introduced	moved outside its native range by human activity
invasive	introduced, spreads rapidly and causes ecological harm

In Britain, introduced grey squirrels can digest acorns earlier and use a wider diet than red squirrels. Their niches overlap, so more grey squirrels compete for hazelnuts, pine cones and habitat while also carrying squirrelpox that strongly affects red squirrels.

Escape from predators or pathogens, rapid reproduction and broad resource use can expand an introduced species' realized niche. The native competitor may then lose resources and occupy a smaller realized niche.

Approach	What it can show	Main limitation
field observation / random sampling	species distributions are associated	shared abiotic conditions may cause the pattern
laboratory comparison	performance changes when species are grown alone versus together	simplified conditions may not transfer to the field
field removal or addition	one species changes when the other is manipulated	difficult controls and other field changes remain

Strong evidence predicts a response, manipulates the proposed competitor, measures the response, uses controls and replication, and rules out alternative explanations such as desiccation or substrate.

One species doing better where another is absent suggests competition; it does not by itself prove that competition caused the distribution.

Observation or manipulation	Inference
Semibalanus absent high on shore even when competitors are removed	desiccation restricts its fundamental niche
Chthamalus normally occupies upper shore	its observed realized niche is narrow
removing Semibalanus lets Chthamalus survive lower down	Semibalanus competition excludes Chthamalus from part of its fundamental niche

The fundamental niche is the range tolerated without biotic exclusion. The realized niche is the range actually occupied after competition and other biotic interactions.

Natural barnacle zonation is compared with Semibalanus removal, after which Chthamalus expands downward into the middle shore, revealing a broader fundamental niche.

For two species scored present or absent in random quadrats, the null hypothesis H₀ states that their distributions are independent: there is no association between presence of one and presence of the other.

200 quadrats	Bell present	Bell absent	Row total
ling present	89	45	134
ling absent	31	35	66
column total	120	80	200

$E=\frac{\text{row total}\times\text{column total}}{\text{grand total}}$

Expected both present = (134 × 120) ÷ 200 = 80.4. Keep expected values unrounded during the calculation so rounding does not distort χ².

$\chi^2=\sum\frac{(O-E)^2}{E}$

Cell	O	E	(O−E)²/E
both present	89	80.4	0.920
ling present, bell absent	45	53.6	1.380
ling absent, bell present	31	39.6	1.868
both absent	35	26.4	2.802
sum			6.97

For a 2 × 2 table, df = (2−1)(2−1) = 1. At p = 0.05 the critical value is 3.84. Because 6.97 > 3.84, reject H₀: the species show a statistically significant association.

Association is not proof of competition or causation. The species may respond to the same abiotic factor or interact in another way; biological evidence must identify the mechanism.

more hare → more food raises lynx survival and reproduction after a delay → more lynx increases hare mortality → fewer hare reduce food for lynx after a delay → lynx decline → reduced predation allows hare recovery

The predator curve lags behind the prey curve because births, maturation and deaths take time. Predation is density-dependent, but weather, vegetation and other predators can also affect the real cycle.

Snowshoe hare population peaks precede lynx peaks; more hare supports more lynx, and more lynx then lowers hare numbers.

Control	Initial change	Example cascade
top-down	predator abundance or predation	fewer predators → more herbivores → fewer producers
bottom-up	nutrients, habitat or primary production	fewer nutrients → fewer producers → fewer herbivores → fewer predators

Identify where the disturbance begins, then follow its direct resource or predation effects one trophic level at a time. Many communities contain both controls, but one may dominate under particular conditions.

A trophic chain shows top-down effects beginning with predators and bottom-up effects beginning with nutrients and producers.

Allelopathy is chemical inhibition of another organism. An allelopathic plant releases an allelochemical that reduces a competitor's germination, photosynthesis or growth.

Release route	How a neighbour is exposed
root exudate	chemical diffuses through soil and is absorbed by roots
volatile release from leaves	gas reaches nearby shoots
decomposing litter	compounds enter soil as leaves break down

Broccoli residues can inhibit later brassica seedlings, so crop rotation reduces carry-over effects. Dense bracken stands release compounds that help exclude other plants and can reduce local diversity.

Poor growth beside a plant does not prove allelopathy; shading, water use and mineral competition must be controlled or measured.

Microbes also compete chemically. Penicillium releases penicillin, which inhibits susceptible bacteria nearby and reduces their competition for space and nutrients.

On an agar plate, a clear zone of inhibition around the fungal colony shows that a diffusible substance prevents bacterial growth. Zone size also depends on diffusion and dose, so it is not a direct measure of ecological success by itself.

Humans use antibiotics as medicines, but their original biological role is an interaction among organisms in a community.

Broccoli roots release allelochemicals that inhibit a nearby seedling, while Penicillium produces a clear inhibition zone among Staphylococcus colonies.

Question	What a strong explanation identifies
Who interacts?	populations and whether each gains or loses
What is transferred or limited?	food, habitat, nutrients, protection, disease or a chemical
Where does change begin?	individual pair, predator level, producer/resource level or introduced species
How does it propagate?	competition, feedback, trophic cascade or niche restriction
What supports causation?	manipulation, control, replication and a predicted response
What supports association?	representative sampling, expected counts and a statistical decision

Use observations to find a pattern, statistics to judge whether it is unlikely under a null model, and experiments or field manipulations to test the proposed mechanism.

Community structure emerges from many population interactions. Naming a relationship is only the start; explanation requires its mechanism, direction of effect, feedback through the network and the limits of the evidence.

[ 1 ]

State the biological term for a group of organisms of the same species living in an area.

[ 6 ]

Discuss how wild fish populations are assessed to ensure sustainable fishing practices.

[ 3 ]

Describe a method that can be used to measure the diversity of herbaceous plants in one of the green areas at different distances from the main road.

[ 6 ]

Explain the technique used to estimate the population size of a named species of organism that is able to move.

[ 7 ]

Explain why populations that have grown exponentially reach a maximum size, rather than continue to grow.

[ 2 ]

Explain the change in numbers of the cactus moth throughout the study period.

[ 6 ]

Discuss the factors affecting population growth that can result in an exponential growth curve.

[ 2 ]

A group of nine semi-wild Asian elephants (Elephas maximus) from an elephant camp in Myanmar were trained to pull two ends of a single rope to get access to food in food trays located on a table behind a barrier. When both elephants pulled the rope, the table moved towards them making the food trays accessible. The task required two elephants to work together in cooperation for mutual benefit to obtain food.

The graph shows the rate of cooperation between pairs of elephants in pulling the rope when there were two food trays, one at each end of the table (as shown in the diagram), or when there was one food tray placed in the centre of the table. The cooperation rate was the number of successful cooperative attempts divided by the number of all attempts in one session. The experiment was repeated for many sessions on successive days.

Compare and contrast the frequency of monopoly and fighting when there is a change from two trays to one tray of food.

[ 3 ]

Torre Guaceto is a marine protected area which covers over $22 \mathrm{~km}^{2}$ of the Adriatic Sea in south-eastern Italy. The diagram shows a food chain within this area.

\begin{gathered} \text { Seagrass } \\ (\text { Cymodocea nodosa }) \end{gathered} \rightarrow \quad \begin{gathered} \text { Sea urchin } \\ (\text { Paracentrotus lividus }) \end{gathered} \rightarrow \quad \begin{gathered} \text { Sea bream } \\ \text { (Sparus aurata) } \end{gathered}

The data refers to the numbers of these three species inside the marine protected area, where fishing is limited, and outside the marine protected area, where fishing is allowed.

	Within the fully protected area	Outside the fully protected area
Sea bream individuals per $100 \mathrm{~m}^{2}$	30	3
Sea urchin individuals per $100 \mathrm{~m}^{2}$	70	690
% algae cover	47	15

Compare and contrast the community structure within and outside the marine protected area.

[ 3 ]

Evaluate whether crop yield may be improved through enhancement of pollinator richness.

[ 3 ]

Analyse the relationship between plants and their pollinators.

[ 6 ]

Discuss, giving an example, the possible effects of the introduction of an alien species into an ecosystem.

[ 3 ]

Explain the results shown in this experiment.

[ 3 ]

Outline how chi-squared can be used to test for an association between the distributions of the two species.

[ 3 ]

Describe the effect of removing trout on frog density in Upper and Lower LeConte Lakes.

[ 3 ]

Explain how nutrients can have a positive or negative bottom-up effect on seagrass.

[ 3 ]

Explain how a named plant can reduce competition by allelopathy.

C4.2 Transfers of energy and matter

Transfers of energy and matter connect sunlight, feeding, trophic levels, decomposers, heat loss and carbon cycling to ecosystem structure, function and sustainability.

An ecosystem is a community interacting with its abiotic environment. It is an open system because both energy and matter can cross its chosen boundary.

Crossing the boundary	Example
energy enters	sunlight reaches producers
energy leaves	respiratory heat dissipates
matter enters	rain, mineral ions, migrating organisms
matter leaves	runoff, gases, harvested biomass

A closed system exchanges energy but not matter. Natural ecosystems are not closed, even when much of their matter is recycled internally.

An open system receives and releases both energy and matter across its boundary.

sunlight → photosynthetic pigments capture light → chemical energy is stored in carbon compounds → producer biomass supports consumers

Sunlight is the principal energy source for most ecosystems because photoautotrophs form the base of most food webs.

Low-light ecosystem	Initial energy source	Producer route
deep-ocean hydrothermal vent	oxidation of inorganic chemicals	chemoautotrophic bacteria or archaea
permanently dark cave	organic matter imported from outside, or local chemosynthesis	detrital food web or chemoautotrophs

The word most matters: sunlight is the dominant global input, but a food web can begin with chemical energy or with organic matter imported from another ecosystem.

Feature	Energy	Matter
common input	sunlight or chemical energy	CO₂, water and mineral ions
movement through biota	chemical energy in biomass and food	atoms in organic molecules
major exit / return	heat dissipates to surroundings	decomposition returns inorganic forms
recycled by ecosystem?	no	yes, although matter can also enter or leave

external energy → producer carbon compounds → feeding → ATP and biomass → heat

inorganic matter → producer biomass → feeding and waste → detritus → decomposers → inorganic matter

Energy is conserved, but after it disperses as low-temperature heat it is no longer available to organisms as the chemical-energy input for another trophic transfer.

A food-chain arrow shows the direction in which chemical energy and biomass are transferred: from the organism being eaten to the consumer.

grass → grasshopper → robin → hawk means grasshopper feeds on grass, robin feeds on grasshopper, and hawk feeds on robin.

Read each arrow as ‘provides energy to’, not ‘eats’. The arrow therefore points toward the feeder.

A food chain and food web use arrows from each food organism toward the organism that consumes it.

Model	What it shows	What it hides
food chain	one possible transfer sequence	alternative foods and indirect effects
food web	interconnected chains in a community	quantities and interaction strength unless data are added

To predict a change: locate the altered population → follow arrows to its consumers → follow arrows backward to its foods → check whether consumers have alternative pathways.

If one prey declines, its specialist predator loses an energy source. A generalist predator may switch prey, transferring stronger predation pressure to another population.

A food web is qualitative unless arrow widths or accompanying data quantify biomass, energy flow or feeding frequency.

Detritus includes dead organisms, shed tissue, fallen leaves and faeces. Its carbon compounds still contain chemical energy.

Organism	Action	Immediate result
detritivore	ingests and fragments detritus	smaller particles and faeces
saprotrophic bacterium or fungus	secretes enzymes, digests externally and absorbs products	carbon for respiration and biomass
decomposer respiration	oxidizes absorbed carbon compounds	ATP, CO₂, water and heat

Decomposition releases inorganic molecules and ions into soil, water or air. Producers absorb them, so matter returns even though the detrital chemical energy ultimately leaves as heat.

Dead material and waste pass to decomposers, which release inorganic nutrients that producers can absorb again.

An autotroph uses an external energy source to synthesize carbon compounds from simple inorganic substances. Carbon fixation brings inorganic carbon into organic molecules.

Input type	Example	Job
carbon source	CO₂	supplies carbon atoms
energy source	light or oxidation of inorganic substances	powers carbon fixation and synthesis
other matter	water and mineral ions	supplies atoms needed for biomass

‘Self-feeding’ does not mean creating matter or energy. Autotrophs transform external energy and inorganic matter into their own organic biomass.

Carbon dioxide, water, mineral ions and an external energy source enter an autotroph, which produces carbon compounds and biomass.

Producer	External energy source	Example	Shared outcome
photoautotroph	light captured by pigments	plant, alga, cyanobacterium	inorganic carbon fixed into organic molecules
chemoautotroph	energy released by oxidation of inorganic substances	nitrifying or iron-oxidizing bacterium	inorganic carbon fixed into organic molecules

An iron-oxidizing bacterium can obtain energy when Fe²⁺ is oxidized to Fe³⁺. The iron supplies energy through electron transfer; it is not the organism's organic-carbon source.

Both routes are autotrophic because the organism builds carbon compounds from inorganic carbon; only the external energy source differs.

A photoautotroph uses light, while an iron-oxidizing chemoautotroph uses energy from converting iron two-plus ions to iron three-plus ions; both build organic molecules.

A heterotroph obtains carbon compounds from other organisms or organic matter. It does not fix all the carbon it needs from inorganic carbon.

organic matter → digestion inside a gut or outside the body → small molecules absorbed into cells → assimilation into organism-specific proteins, lipids, polysaccharides and nucleic acids

Heterotroph	How organic matter is obtained
consumer	ingests living or recently killed organisms
detritivore	ingests dead material and waste
saprotroph	digests externally and absorbs soluble products

A heterotroph receives organic matter and directs it toward respiration and heat or toward synthesis of its own biomass.

Autotrophs and heterotrophs both oxidize carbon compounds in cell respiration. Energy is transferred to ATP for cellular work, while some becomes heat.

Organism	How it first gets carbon compounds	What respiration does
autotroph	synthesizes them from inorganic carbon	releases stored chemical energy
heterotroph	obtains them from other organisms	releases stored chemical energy

Plants respire as well as photosynthesize. Photosynthesis stores energy in carbon compounds; respiration releases usable energy from those compounds.

A producer, herbivore and carnivore each oxidize carbon compounds in cell respiration, transferring energy to ATP and heat.

Follow…	Start	Through organisms	End or return
energy	sunlight or inorganic oxidation	producer biomass → feeding → respiration	heat dissipates
carbon	CO₂ or organic matter crossing boundary	fixation → feeding → waste/death	CO₂ from respiration or decay; some enters longer-term stores
mineral element	inorganic ion	producer uptake → feeding	waste/death → decay → inorganic ion

For any arrow, ask: what is moving? In what chemical form? Which process moves it? Does it cross the ecosystem boundary, enter biomass, or return to an inorganic store?

Feeding can transfer energy and matter together, but their later fates differ: usable energy disperses as heat, while atoms can be rearranged and reused.

Trophic level	Feeding position	Example in one chain
1	producer	grass
2	primary consumer	grasshopper eating grass
3	secondary consumer	robin eating grasshopper
4	tertiary consumer	hawk eating robin

A trophic level is a feeding position in a particular food chain. It describes the transfer being traced, not an unchanging property of a species.

A human eating beans is a primary consumer; the same human eating a herbivorous cow is a secondary consumer. An omnivore can therefore occupy different trophic levels in different chains.

Decomposers receive material from organisms at every trophic level, so forcing them into one fixed level hides the detrital network.

A pyramid of energy shows the energy incorporated into biomass at each trophic level per unit area per unit time, for example kJ m⁻² yr⁻¹.

Use equal-height horizontal bars → centre every bar on the same vertical axis → make width proportional to the measured energy-flow value → place producers at the base.

Pyramid	Quantity represented	Can be inverted?
energy	rate of biomass-energy production	no; transfer losses make each higher rate smaller
biomass	standing biomass at one time	yes
numbers	count of organisms at one time	yes

A generalized energy pyramid has a wide producer base and progressively narrower primary-, secondary- and tertiary-consumer bars.

Energy route at one level	Available to next grazing level?	Destination
biomass never eaten	no	detritus and decomposers
eaten but not digested	no	faeces and decomposers
absorbed then excreted	no	waste pathway
oxidized in respiration	no	ATP transfer and heat
retained as new biomass	yes, if eaten	growth and reproduction

energy in existing biomass → fraction ingested → fraction assimilated → fraction left after respiration → new consumer biomass

Transfer efficiency varies with digestibility, metabolic rate and how much biomass is eaten. Carnivore diets are often more digestible than lignified plant tissue; ectotherms often spend less energy maintaining body temperature than endotherms.

‘Lost to the next consumer’ does not always mean lost from the ecosystem immediately. Uneaten biomass and faeces can still supply detritivores and decomposers before respiration releases heat.

$\eta=\frac{E_{higher}}{E_{lower}}\times100\%$

Trophic level	Energy flow
producers	18,000 kJ m⁻² yr⁻¹
primary consumers	2,160 kJ m⁻² yr⁻¹
efficiency	(2,160 ÷ 18,000) × 100 = 12%

Here η is transfer efficiency, E higher is the energy-flow value at the next trophic level and E lower is the value at the level supplying it. Both measurements must cover the same area and time interval.

About 10% is a useful rough model, not a fixed biological law. Use the supplied values whenever data are available, and draw bar widths to those values rather than forcing every transfer to exactly 10%.

carbon-compound oxidation → energy transferred to ATP plus heat → ATP powers cellular work and more heat is released → heat spreads into the surroundings

Energy is not destroyed. It becomes dispersed thermal energy that organisms cannot concentrate back into the chemical bonds required to restart the food chain.

Because usable energy is continually degraded to heat, ecosystems need a continuing external input—usually sunlight—even though atoms can be recycled.

Light energy enters a producer and chemical energy passes through consumers and detritivores, while respiration releases heat from every trophic level.

producer energy budget → incomplete transfer → smaller primary-consumer budget → another incomplete transfer → still smaller upper-level budget → an additional stable trophic level eventually becomes impossible

Toward the top of a food chain:

Energy flow decreases with every incomplete transfer.
Less total biomass can usually be supported.
Fewer large consumers can usually be supported.
A top predator often requires a larger feeding area.

Higher levels do not always contain fewer individual organisms. Body size matters; many small parasites can live on one host. The reliable restriction is the smaller energy budget.

Energy pyramids for tundra, temperate forest, temperate grassland and tropical rainforest show progressively narrower upper trophic levels.

$NPP=GPP-R$

Quantity	Meaning
GPP	total rate at which producers fix carbon / store energy by photosynthesis
R	producer respiratory loss
NPP	rate of new producer biomass accumulation available for growth, reproduction and consumers

If GPP = 2,400 g C m⁻² yr⁻¹ and producer respiration = 1,350 g C m⁻² yr⁻¹, then NPP = 1,050 g C m⁻² yr⁻¹.

Gross primary production is divided between producer respiration and new producer biomass called net primary production.

NPP falls when a resource or condition restricts new biomass production:

Low light: less energy is captured by photosynthesis.
Water shortage: stomata close, limiting CO₂ uptake, and growth slows.
Low temperature: enzyme-controlled reactions and nutrient cycling slow.
Excessive temperature: water loss rises and enzymes may lose function.
Short growing season: there are fewer days of positive carbon gain.

Biome	Typical NPP explanation
tropical rainforest	warm, wet and illuminated through most of the year
temperate forest or grassland	seasonal light and temperature shorten production
tundra	cold, short growing season and slow nutrient cycling

Higher NPP builds a larger producer energy budget, which can support more biomass and sometimes more trophic levels. Low NPP constrains the whole food web from its base.

NPP is a rate of new biomass production, not the amount of standing biomass visible on one date.

$GSP=I-F$

$NSP=GSP-R$

Symbol	Meaning
I	food energy or biomass ingested
F	faecal loss; not assimilated
GSP	energy or biomass absorbed by the consumer
R	consumer respiratory loss
NSP	new consumer biomass available to growth, reproduction and the next trophic level

A caterpillar ingests 900 kJ, loses 360 kJ in faeces and uses 420 kJ in respiration: GSP = 540 kJ and NSP = 120 kJ.

Food energy enters a consumer; some is not assimilated and leaves in faeces, while assimilated energy can support respiration or new biomass.

Stage	Calculation or decision	What becomes available next?
producer	NPP = GPP − producer R	new producer biomass
consumer assimilation	GSP = ingestion − faecal loss	absorbed energy
consumer production	NSP = GSP − consumer R	new consumer biomass
trophic transfer	higher-level energy ÷ lower-level energy × 100	transfer efficiency

Observation	First mechanism to test
low NPP	light, water, temperature or short season
much food but low GSP	low digestibility / large faecal loss
high GSP but low NSP	high respiration
short food chain	compounded transfer losses leave too little upper-level energy

An energy pyramid narrows because each level keeps only the fraction converted into new biomass. Every other route either feeds the detrital pathway or ultimately dissipates energy as heat.

Diagram feature	Meaning	Example
box	carbon store or reservoir	atmospheric CO₂; plant biomass
labelled arrow	carbon flux between stores	photosynthesis; feeding; respiration
arrowhead	direction of carbon transfer	atmosphere → producers
number on arrow	rate over a stated interval	Gt C yr⁻¹

Name the stores first → add biological fluxes → add long-term geological/fossil stores if required → check every arrow has a direction and process label → include atmospheric CO₂.

Draw stores as boxes and processes as arrows. A plant, animal or factory picture does not by itself state the carbon store, transfer direction or process.

A simplified carbon cycle links atmospheric carbon dioxide, green plants, animals, decomposers, soil organic carbon and fossil fuels with labelled process arrows.

Flux	From	To	Carbon form change
photosynthesis	atmospheric or dissolved CO₂	producer biomass	inorganic → organic
feeding	one organism's biomass	consumer biomass	organic → organic
respiration	organic carbon in cells	atmospheric or dissolved CO₂	organic → inorganic
death and waste	living biomass	detritus	organic → organic
decomposition	detritus	decomposer biomass and CO₂ / inorganic nutrients	organic redistributed and mineralized

CO₂ → photosynthesis → plant carbohydrate → feeding → animal biomass → respiration or death → CO₂ through respiration and decomposition

Producers, consumers and decomposers all respire. The carbon cycle therefore has several arrows returning CO₂, not one ‘animal-only’ respiration arrow.

A cycle diagram shows routes, not necessarily equal rates. Add values or arrow widths only when evidence supplies them.

Net balance over a stated interval	Classification	Store change
carbon in > carbon out	sink	increases
carbon out > carbon in	source	decreases
carbon in = carbon out	neither net sink nor net source	unchanged

$\Delta C=C_{in}-C_{out}$

Reservoir	Can act as a sink when…	Can act as a source when…
forest	photosynthesis and biomass growth exceed losses	respiration, fire, decay or harvest exceed uptake
soil / peat	organic inputs exceed decomposition	drainage and warming accelerate oxidation
ocean	uptake exceeds release	warming or circulation shifts favour release

A forest, soil or ocean is not permanently a sink. Classification depends on all relevant fluxes and on the time interval chosen.

carbon-rich fuel + O₂ → CO₂ + H₂O + released energy

Fuel	Approximate storage timescale before burning	Why combustion matters
fresh biomass	years to decades	rapidly returns recently fixed carbon; regrowth determines later uptake
peat	centuries to millennia	drainage exposes stored organic carbon to oxidation and fire
coal, oil, natural gas	millions of years	transfers geologically stored carbon into the active atmosphere-ocean cycle

Fire converts organic carbon in biomass or fuel to atmospheric CO₂ quickly. It can also weaken a sink by killing vegetation that would otherwise remove CO₂ through photosynthesis.

Combustion occurs naturally after lightning, but human fossil-fuel use and deliberate biomass burning have greatly increased the flux to the atmosphere.

The Keeling Curve is the long-running atmospheric CO₂ record begun at Mauna Loa in the late 1950s. Its rising baseline shows a long-term net gain of CO₂ in the atmosphere.

fossil-fuel combustion + land-use emissions + other sources > uptake by oceans and terrestrial photosynthesis → atmospheric CO₂ accumulates

Graph feature	Timescale	Main interpretation
rising baseline	decades	persistent net source to atmosphere
small repeating teeth	one year	seasonal biological fluxes

The curve records atmospheric concentration. It supports a net-balance conclusion; identifying individual sources requires additional measurements and carbon-cycle evidence.

Atmospheric carbon dioxide rises from about 315 parts per million in 1958 to above 400 parts per million, with a repeated seasonal oscillation around the rising baseline.

Northern Hemisphere season	Dominant change in terrestrial biota	Atmospheric CO₂ tendency
spring and summer	leaf area and photosynthesis increase; uptake exceeds biological release	falls
autumn and winter	photosynthesis falls while respiration and decomposition continue	rises

Northern Hemisphere land vegetation dominates the global seasonal signal because most of Earth's land and terrestrial plant biomass lie north of the equator.

Within each year CO₂ can fall for several months, while each year's average remains above the previous one. Seasonal uptake therefore coexists with the long-term rise.

Do not explain the annual fall by reduced combustion alone. The repeating biological drawdown is driven mainly by photosynthesis during Northern Hemisphere growth.

Process	Main gas input	Main gas output	Energy role
photosynthesis	CO₂	O₂	stores captured energy in carbon compounds
aerobic respiration	O₂	CO₂	releases energy from carbon compounds to ATP and heat

photosynthetic organisms replenish atmospheric O₂ used by aerobic respiration ↔ respiration by producers, consumers and decomposers returns CO₂ that photoautotrophs can fix

The processes are linked but are not simple reversals inside one organelle. They use different pathways, enzymes and energy conversions, and both occur in photosynthetic organisms.

Large opposing fluxes can occur at the same time. Atmospheric concentration changes only by the difference between total inputs and outputs.

inorganic molecule or ion → producer uptake and assimilation → organic molecules in biomass → feeding → waste or death → detritus and decomposers → inorganic form available again

Carbon is one example. Hydrogen, oxygen, nitrogen, phosphorus, sulfur, calcium, potassium and other required elements are also recycled between organisms and the atmosphere, hydrosphere or lithosphere.

Without decomposers, essential elements would remain locked in dead organic matter and waste. Recycling restores availability; it does not create a new supply of atoms.

Inorganic molecules or ions are absorbed by producers, move to consumers by feeding and digestion, enter decomposers after death, and return to the inorganic environment through decomposition.

Question	Energy answer	Matter / carbon answer
What enters?	usually sunlight	CO₂, water, ions, organisms and detritus can cross boundaries
What moves through food webs?	chemical energy in biomass	atoms in organic molecules
What returns?	no usable-energy cycle; heat dissipates	decomposition and respiration return inorganic forms
What can accumulate?	biomass energy temporarily	carbon in biomass, soil, peat, ocean or fossil stores
What reveals imbalance?	shrinking energy budgets up trophic levels	changing store size or atmospheric concentration

Use the correct evidence scale: energy-pyramid data for trophic transfer; GPP/NPP and GSP/NSP budgets for production; labelled store-and-flux diagrams for cycling; the Keeling trend for multi-decade atmospheric accumulation; seasonal teeth for annual biological fluxes.

Matter can be rearranged and reused because atoms remain available in biotic and abiotic stores. Usable energy must be replaced because each biological transfer disperses part of it as heat.

[ 6 ]

Describe the movement of energy and nutrients in an ecosystem.

[ 7 ]

Explain how the energy supply in an ecosystem is dependent on sunlight.

[ 7 ]

Describe how populations in communities rely on each other for supplies of energy.

[ 6 ]

Describe what is meant by a food chain and a food web.

[ 2 ]

Detritus accumulates on coral reefs damaged by ocean acidification.
Suggest two possible impacts of an increase in detritus on the organisms in this food web.

[ 1 ]

Define the term autotroph.

[ 1 ]

Which organisms use oxidation of simple inorganic substances as an energy source?

[ 5 ]

Compare and contrast how different types of heterotrophs obtain the energy that they need to produce ATP.

[ 1 ]

State one process that results in the loss of carbon dioxide from a marine organism such as a crustacean or a jellyfish.

[ 4 ]

Explain, using an example of a food chain, how trophic levels can be deduced.

[ 6 ]

Ecologists sometimes display data from an ecosystem using a diagram called a pyramid of energy. Describe what is shown in pyramids of energy.

[ 3 ]

The efficiency of energy transfer along the various food chains in a food web varies. Suggest reasons for the differences.

[ 1 ]

Plankton are major producers in marine ecosystems. Only a small proportion of the energy harvested by plankton is passed to the primary consumers. Which process in phytoplankton results in the largest loss of energy that could otherwise be utilized by consumers?

[ 1 ]

Outline why the number of trophic levels is limited in a food chain.

[ 3 ]

In each forest, there are two or three trial plots per CO2 treatment. The bar chart shows the allocation of carbon from net primary production to different parts of the trees in these trial plots.

Evaluate the evidence from the bar chart that increases in carbon dioxide cause increases in carbon storage in young, developing forests.

[ 1 ]

Calculate how much food would be required to produce 20 kg of boar meat.

kg

[ 9 ]

Living organisms at every trophic level are part of the carbon cycle. Draw a labelled diagram of the carbon cycle to show the processes involved.

[ 2 ]

The graph shows long-term fluctuations in atmospheric $\mathrm{CO}_{2}$ concentration of about 80 ppm . The decreases in $\mathrm{CO}_{2}$ concentration are probably due to oceans acting as a sink by holding large quantities of dissolved $\mathrm{CO}_{2}$ .

Identify two other examples of natural sinks that can remove carbon from the carbon cycle and reduce atmospheric $\mathrm{CO}_{2}$ concentration.

[ 1 ]

The graph shows how the worldwide use of fossil fuels has increased from 1800 to 2019.

How has the increased combustion of fossil fuels contributed significantly to global warming?

[ 1 ]

The graph shows data collected at Mauna Loa, USA, for monthly mean carbon dioxide concentration.

What causes the decreases in monthly mean carbon dioxide concentration each year?

[ 7 ]

Explain how carbon is recycled in a terrestrial ecosystem.

Exam analysis

Most tested topics

Enzymes as catalysts

Role in metabolism

Anabolic and catabolic reactions

Enzymes as globular proteins

Induced-fit binding

Molecular motion

Relationships between structure and function

Effects on enzyme activity

Measuring enzyme reactions

Effect of enzymes on activation energy

ATP distributes energy

Life processes using ATP

ATP ↔ ADP interconversions

Cell respiration system

Anaerobic vs. aerobic respiration in humans

Variables affecting rate

Light energy → chemical energy

CO₂ → glucose

Oxygen as by-product

Photosynthetic pigment separation

Absorption of specific wavelengths

Absorption vs. action spectra

Limiting factors investigation

CO₂ enrichment experiments

Neurons exam focus

Resting potential generation

Nerve impulses as action potentials

Variation in impulse speed

Synapses as junctions

Neurotransmitter release

Excitatory postsynaptic potential

System integration

Hierarchy in multicellular organisms

Integration of organs

Brain as information integration organ

Spinal cord

Input through sensory neurons

Output through motor neurons

Nerves as bundles

Pain reflex arcs

Cerebellum role

Melatonin and sleep

Epinephrine (adrenaline)

Hypothalamus and pituitary control

Heart rate feedback control

Ventilation rate feedback control

Peristalsis control

Pathogens cause infectious diseases

Skin and mucous membranes

Blood clotting

Innate vs. adaptive immune system

Phagocytes exam focus

Lymphocytes exam focus

Antigens exam focus

B-lymphocyte activation

Clones of plasma cells

Immunity from memory cells

HIV transmission

HIV infection and AIDS

Antibiotics exam focus

Antibiotic resistance evolution

Zoonoses exam focus

Vaccines and immunization