2 Structure and functions in living organisms

Exam analysis

Most tested topics

(a) Level of organisation

Organisation in a multicellular organism forms a hierarchy: specialised parts combine into progressively larger units that coordinate a shared function.

Level	Meaning	Example in one animal hierarchy
organelle	a structure within a cell that performs a particular function	a mitochondrion in a muscle cell
cell	the living unit that contains organelles	a cardiac muscle cell
tissue	a group of similar cells working together for a function	cardiac muscle tissue
organ	different tissues working together for a particular function	the heart
organ system	several organs working together for a major body function	the circulatory system

The direction is: organelle $ightarrow$ cell $ightarrow$ tissue $ightarrow$ organ $ightarrow$ organ system. At each step, the smaller level becomes a component of the next; the heart, for example, contains muscle tissue but also other tissues that let the organ work as a whole.

A tissue is not simply any collection of cells: its similar cells cooperate in a function. An organ contains more than one tissue type, whereas an organ system contains multiple organs. Do not include the whole organism as one of the five levels required by this objective.

(b) Cell structure

A cell contains distinct structures that can be described by their position, boundary and appearance before their functions are considered.

Structure	Description
nucleus	a compartment in the cytoplasm that contains chromosomes
cytoplasm	the material filling the inside of the cell
cell membrane	a thin boundary enclosing the cytoplasm
cell wall	a rigid layer outside the cell membrane in plant cells
mitochondria	small structures distributed through the cytoplasm
chloroplasts	chlorophyll-containing structures in photosynthetic plant cells
ribosomes	very small structures in the cytoplasm
vacuole	a fluid-filled space; mature plant cells usually have a large central vacuole containing cell sap

The cell wall and cell membrane are separate layers: the wall lies outside the membrane. Ribosomes are much smaller than nuclei, chloroplasts and mitochondria, so a simple light-microscope drawing may not show every structure.

Do not infer that every plant cell contains chloroplasts: only photosynthetic plant cells require them. A vacuole is a space within the cell, whereas cytoplasm surrounds the cell structures.

Each cell structure has a specific job, and the abundance of a structure reflects how strongly the cell depends on that job.

Structure	Function
nucleus	contains genetic material and controls cell activities
cytoplasm	site of many metabolic reactions
cell membrane	controls movement of substances into and out of the cell
cell wall	supports the cell and helps maintain its shape
mitochondria	site of aerobic respiration, which releases energy for cell activities
chloroplasts	absorb light using chlorophyll and carry out photosynthesis
ribosomes	site of protein synthesis
vacuole	contains cell sap and helps support a plant cell when full

A cell with a high energy demand often contains many mitochondria. A palisade cell exposed to light contains many chloroplasts, while a root cell underground usually contains none because it does not photosynthesise.

Mitochondria release energy by respiration; they do not create energy. Ribosomes are the site of protein synthesis, while the nucleus supplies genetic instructions rather than assembling proteins itself.

Plant and animal cells share the core structures needed for cellular control and metabolism, while plant cells have additional structures for support and photosynthesis.

Structure	Plant cell	Animal cell
nucleus	present	present
cytoplasm	present	present
cell membrane	present	present
mitochondria	present	present
ribosomes	present	present
cellulose cell wall	present	absent
chloroplasts	present in photosynthetic cells	absent
large permanent vacuole containing cell sap	usually present	absent

The rigid wall and large vacuole often give a plant cell a more regular outline. An animal cell has only a membrane at its outer boundary, so its outline can be less regular.

Cell shape alone is not enough for identification. Use the combination of wall, chloroplasts and large permanent vacuole, and remember that a non-photosynthetic plant cell may lack chloroplasts but still has a cellulose wall.

Differentiation is the process by which an unspecialised cell develops the structures and abilities needed for a particular function.

An unspecialised cell divides, then daughter cells can differentiate into specialised types. Their structures become suited to different jobs; groups of specialised cells can therefore form tissues and organs with a division of labour.

Without differentiation	With differentiation
cells remain similar and cannot perform the full range of body functions efficiently	different cell types perform particular jobs efficiently
complex tissues and organs cannot develop normally	specialised cells combine into working tissues and organs

A stem cell is unspecialised and can continue to divide. If it differentiates into a red blood cell, it develops a specialised role in oxygen transport; if it differentiates into a muscle cell, it develops the ability to contract.

Differentiation is not ordinary growth: a cell changes what it is specialised to do, rather than merely becoming larger. Most specialised animal cells have a more limited range of possible future cell types than stem cells.

Stem cells can divide repeatedly and differentiate, so medicine can use them to replace damaged or missing specialised cells.

Potential advantage	Corresponding disadvantage or condition
can produce specialised cells to repair damaged tissue	differentiation must be controlled so the required cell type forms
repeated division can supply many replacement cells	uncontrolled division may form a tumour
a patient's own cells are genetically matched, reducing immune rejection	donor cells may be rejected and may require immunosuppressant treatment
donor or embryonic cells may provide options when a patient's cells are unsuitable	transplanted material can carry infection; using embryonic cells raises ethical objections for some people

For a damaged heart, stem cells may divide and differentiate into replacement muscle cells. Bone-marrow stem cells can form blood-cell types and may therefore help treat some blood conditions.

A justified medical decision weighs likely benefit against tumour, rejection, infection and ethical risks for the particular cell source. A possible treatment is not automatically safe or effective simply because stem cells can differentiate.

(c) Biological molecules

Carbohydrates, proteins and lipids are carbon-based biological molecules, but their usual elemental compositions are not identical.

Molecule group	Elements present	Symbols
carbohydrate	carbon, hydrogen and oxygen	C, H, O
lipid (fat or oil)	carbon, hydrogen and oxygen	C, H, O
protein	carbon, hydrogen, oxygen and nitrogen	C, H, O, N

Carbon, hydrogen and oxygen occur in all three groups. Nitrogen is the distinguishing element in the syllabus comparison because proteins are built from amino acids.

Do not infer the molecule group from carbon alone. Carbohydrates and lipids contain the same three named elements, so their structure—not a different element list—distinguishes them.

Large biological molecules are assembled from smaller basic units; the type and arrangement of those units determine the molecule group.

Large molecule	Smaller basic unit or units
starch	many simple sugar units
glycogen	many simple sugar units
protein	amino acids
lipid	fatty acids and glycerol

Starch and glycogen are both large carbohydrate molecules made from simple sugars, but they are different storage molecules. A protein uses amino acids rather than sugars; a lipid combines glycerol with fatty acids.

A lipid is not described here as a chain of only one repeating unit: it requires both glycerol and fatty acids. Do not confuse a molecule's building units with the chemical elements inside those units.

A food test is valid only when the reagent, treatment and positive colour change are matched to the molecule being detected.

Molecule	Method	Positive result
glucose	add Benedict's solution and heat in a hot-water bath	blue changes through green, yellow or orange to brick-red
starch	add iodine solution	orange-brown changes to blue-black
protein	add Biuret reagent	blue changes to lilac or purple
lipid (fat)	shake with ethanol, then add water	a cloudy white emulsion forms

Crush a solid sample and mix it with water when an extract is needed. Use comparable sample and reagent volumes when comparing foods, and include known positive and negative samples if the test must be checked.

Wear eye protection. Heat Benedict's tubes in a water bath rather than directly in a flame; ethanol is flammable, so keep it away from ignition sources. A negative result means the test did not detect the molecule, not that the food contains nothing else.

An enzyme is a biological catalyst: it increases the rate of a metabolic reaction without being used up by that reaction.

A substrate with a complementary shape binds to the enzyme's active site, forming an enzyme–substrate complex. The reaction occurs, products leave, and the unchanged enzyme can catalyse another reaction.

ext{enzyme}+ ext{substrate}
ightarrow ext{enzyme–substrate complex}
ightarrow ext{enzyme}+ ext{products}

Metabolism means the chemical reactions occurring in cells or organisms. Enzymes make these reactions fast enough under cellular conditions, and active-site shape gives each enzyme specificity for particular substrate shapes.

An enzyme changes reaction rate, not the final products, and it is not consumed as a reactant. A substrate must fit the active site; merely colliding with any enzyme is insufficient.

Enzyme activity usually rises with temperature to an optimum, then falls sharply when higher temperature changes the active site.

Temperature region	Molecular explanation	Effect on rate
below optimum	enzyme and substrate have less kinetic energy, so successful collisions and complexes form less often	rate is lower but rises as temperature increases
at optimum	active sites retain their shape and successful complex formation is most frequent	maximum rate
above optimum	bonds maintaining enzyme shape are disrupted; the active site changes shape	substrate fits less well or not at all, so rate falls

When the active site changes permanently enough that the substrate is no longer complementary, the enzyme is denatured. The reaction may stop even though substrate remains.

Low temperature does not usually denature an enzyme; it slows molecular movement and is often reversible on warming. Do not say the enzyme itself is killed—enzymes are molecules, not living organisms.

To test temperature fairly, change only temperature and measure enzyme activity with a repeatable endpoint or rate.

Step	Action
1	prepare water baths across a suitable temperature range
2	keep enzyme and substrate volumes and concentrations, pH and total volume constant
3	equilibrate enzyme and substrate separately at each temperature, then mix and start timing
4	for amylase and starch, sample at fixed intervals onto iodine until iodine remains orange-brown
5	repeat each temperature, calculate a mean time and compare rate using $1/ ext{time}$

Use smaller temperature intervals around the fastest result to estimate the optimum more accurately. A shorter endpoint time means a faster reaction, so time itself is inversely related to rate.

Use water baths and eye protection. Equilibrating before mixing prevents the reaction starting at an unintended temperature; a single reading cannot reveal random variation, so repeats are required.

Each enzyme has an optimum pH at which its active site has the most effective shape for binding its substrate.

Changing pH changes the chemical conditions around the enzyme. Away from the optimum, bonds maintaining its three-dimensional shape can be disrupted, altering the active site so fewer enzyme–substrate complexes form.

Condition	Active site and rate
optimum pH	substrate is complementary; complex formation and rate are greatest
moderately away from optimum	fewer successful complexes form; rate decreases
sufficiently extreme pH	active site may change so much that the enzyme is denatured

Optimum pH is enzyme-specific; it is not always neutral. pH changes enzyme activity by altering the active site, not by changing the amount of substrate present.

A pH investigation uses buffer solutions to set different pH values while every other factor affecting enzyme rate is controlled.

Step	Action
1	prepare labelled tubes containing buffers across a suitable pH range
2	add equal volumes and concentrations of amylase and starch; keep temperature constant in a water bath
3	mix one pH treatment, start timing and sample at fixed intervals onto iodine
4	record the time when iodine stays orange-brown, showing no starch is detected
5	repeat each pH, calculate mean time and compare rate using $1/ ext{time}$

Independent variable: buffer pH. Dependent variable: time to the starch endpoint or calculated rate. Controls include temperature, enzyme and substrate concentration and volume, total volume, sampling interval and iodine volume.

Use a buffer rather than an unmeasured amount of acid or alkali so pH is known and stable. Test smaller pH intervals around the fastest result before claiming an optimum, and wear eye protection when handling reagents.

(d) Movement of substances into and out of cells

Cells exchange substances with their surroundings by diffusion, osmosis and active transport. The three processes differ in what moves, its direction and whether cellular energy is required.

Process	What moves	Direction	Membrane and energy
diffusion	particles of a substance	net movement from higher to lower concentration	no energy from respiration; a membrane is not essential
osmosis	water molecules only	from higher water potential (more dilute) to lower water potential (more concentrated)	through a partially permeable membrane; no energy from respiration
active transport	dissolved ions or other solutes	from lower to higher concentration, against the concentration gradient	across a living cell membrane using carrier proteins and energy from respiration

Mineral ions can enter root hair cells by active transport when their concentration is lower in the soil than in the cell. Water can then move across partially permeable membranes by osmosis, while gases such as oxygen and carbon dioxide commonly move down concentration gradients by diffusion.

Osmosis is the movement of water, not solute. Active transport can move a substance against its gradient because it uses energy; diffusion and osmosis are passive. At equilibrium particles still move randomly, but there is no net movement in either direction.

The rate of movement into or out of cells depends on how much exchange surface is available and how quickly particles can cross it.

Change	Effect on rate	Explanation
larger surface-area-to-volume ratio	faster	more membrane area is available for exchange for each unit of cell volume
shorter diffusion distance or thinner exchange surface	faster	particles have a shorter path to travel
higher temperature	faster, within a suitable range	particles have more kinetic energy and move more rapidly
steeper concentration gradient	faster net movement	the difference in concentration across the surface is greater

Small cells have a larger surface-area-to-volume ratio than equally shaped large cells. Exchange surfaces can therefore be adapted by being folded to increase area and thin to reduce distance, while circulation or ventilation can maintain a steep concentration gradient.

These factors change rate, not the direction set by the gradient. Surface area alone is not the same as surface-area-to-volume ratio, and active transport additionally depends on carrier proteins and a supply of energy from respiration.

Diffusion and osmosis can be investigated in living tissue and in a non-living model. A valid investigation changes one factor, measures movement quantitatively and controls the other rate factors.

System	Example method	Quantitative evidence
living	equal potato cylinders placed in a range of sucrose or salt concentrations	change in mass or percentage change in mass
non-living	partially permeable Visking or dialysis tubing containing solution, immersed in another solution	change in mass, volume or liquid-column height

Step	Action
1	cut equal potato cylinders, blot them in the same way and record each initial mass
2	place them in equal volumes of a suitable range of solution concentrations
3	keep time, temperature, cylinder dimensions and potato source constant
4	remove after the same time, blot consistently and record final mass
5	calculate percentage mass change; repeat each concentration and calculate a mean

% ext{ change in mass}=rac{ ext{final mass}- ext{initial mass}}{ ext{initial mass}} imes100

A positive value means net water entry; a negative value means net water loss. The concentration giving approximately zero change has a similar water potential to the tissue. In the tubing model, a mass or height increase shows net water movement into the partially permeable bag without requiring living cells.

Blotting removes surface liquid that would otherwise distort mass, while percentage change allows samples with different starting masses to be compared. Use repeats to reveal variation, handle cutting tools safely, and use a water bath rather than direct heating when temperature is investigated.

(e) Nutrition

Photosynthesis transfers light energy into chemical energy stored in glucose. It occurs mainly in chloroplasts, where chlorophyll absorbs light.

The absorbed energy drives the conversion of carbon dioxide and water into glucose; oxygen is released. The glucose can be respired, stored as starch, transported as sucrose, or used to make substances such as cellulose that support growth.

This conversion supplies plant biomass and stores energy in chemical bonds. That chemical energy can pass to consumers through food chains and can be released during respiration.

Photosynthesis does not create energy: it converts and stores it. Chlorophyll absorbs light energy; glucose is the energy-containing chemical product.

Photosynthesis uses carbon dioxide and water to make glucose and oxygen. Light supplies energy and chlorophyll absorbs it; neither is a reactant consumed in the equation.

ext{carbon dioxide}+ ext{water}\longrightarrow ext{glucose}+ ext{oxygen}

6\mathrm{CO}{2}+6\mathrm{H}{2}\mathrm{O}\longrightarrow\mathrm{C}{6}\mathrm{H}{12}\mathrm{O}{6}+6\mathrm{O}{2}

The coefficients balance six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms on each side. Changing a subscript would change the substance, so balance an equation only by changing coefficients.

The rate of photosynthesis is controlled by the factor in shortest effective supply: the limiting factor. Increasing that factor raises the rate until a different factor becomes limiting.

Factor increased	Why rate can rise	Why the rise stops or reverses
light intensity	more light energy is absorbed by chlorophyll	another factor, such as carbon dioxide or temperature, becomes limiting
carbon dioxide concentration	more reactant is available for carbon fixation	light or temperature becomes limiting
temperature	enzyme-controlled reactions proceed faster as particles have more kinetic energy and successful collisions increase	above the optimum, enzymes denature and the rate falls

A plateau does not mean photosynthesis has stopped; it means changing the plotted factor no longer changes the rate under those conditions. Two curves can plateau at different rates because their other conditions differ.

State which factor is limiting from the evidence. More light or carbon dioxide cannot keep increasing the rate indefinitely, and high temperature can reduce rather than increase the rate.

A leaf is broad and thin, giving a large surface for light absorption and a short diffusion distance for gases.

Structure	Adaptation for photosynthesis
transparent cuticle and upper epidermis	allow light to reach mesophyll cells while the waxy cuticle limits water loss
palisade mesophyll near the upper surface	tightly packed cells contain many chloroplasts to absorb light
spongy mesophyll	large air spaces create internal surfaces and allow carbon dioxide and oxygen to diffuse
stomata and guard cells	pores allow gas exchange; guard cells control opening
xylem in veins	supplies water and mineral ions
phloem in veins	carries products such as sucrose away from the leaf

Palisade cells contain more chloroplasts than spongy cells, not all the leaf's chloroplasts. Air spaces aid gas movement; they are not empty because the leaf is poorly packed.

Plants need mineral ions because carbohydrates made by photosynthesis do not supply every element required to build new cells.

Mineral ion	Required to make	Likely effect of deficiency
nitrate	amino acids, then proteins needed for growth and enzymes	stunted growth
magnesium	chlorophyll needed to absorb light for photosynthesis	yellow leaves (chlorosis) and reduced photosynthesis

A valid mineral-deficiency comparison uses a complete mineral solution as the control and a solution lacking only one ion as the treatment. Any growth difference can then be linked to that missing ion if other conditions are controlled.

Mineral ions do not provide energy. Nitrate supplies nitrogen for amino acids, while magnesium is a component needed for chlorophyll; their functions are not interchangeable.

Photosynthesis can be demonstrated by measuring oxygen production and by showing that starch forms only where light, carbon dioxide and chlorophyll are available.

Claim tested	Method and comparison	Positive evidence
oxygen evolves from a water plant	illuminate equal lengths of pondweed; collect gas by displacement or measure its volume over time	gas volume rises; collected oxygen relights a glowing splint
light is required	destarch a plant, cover part of a leaf, then expose it to light	only the uncovered region tests blue-black for starch
carbon dioxide is required	enclose a destarched leaf with potassium hydroxide or soda lime; compare with a control containing water	control leaf forms starch; carbon-dioxide-free leaf does not
chlorophyll is required	expose a destarched variegated leaf to light	green regions form starch; non-green regions do not

For a starch test, boil the leaf in water, heat it in ethanol using a water bath to remove chlorophyll, rinse it, then add iodine solution. Blue-black shows starch; orange-brown shows no starch.

Destarching in darkness makes any later starch attributable to the experiment. Keep temperature, exposure time and plant material comparable, use repeats, and never heat flammable ethanol directly over a flame.

A balanced diet supplies carbohydrate, protein, lipid, vitamins, minerals, water and dietary fibre in proportions that meet the body's needs without persistent excess or deficiency.

Component group	Main contribution to balance
carbohydrate and lipid	energy supply; lipid also provides longer-term energy storage
protein	material for growth and repair
vitamins and mineral ions	required in small amounts for specific body functions
water	medium for transport and chemical reactions
dietary fibre	supports movement of material through the gut

Appropriate proportions are not identical for everyone. Energy and nutrient needs vary with factors such as activity, age and pregnancy, while the overall diet must still include all required component groups.

Balanced does not mean equal amounts of every component or a single 'healthy' food. A food rich in one useful nutrient cannot by itself supply the full diet.

Each dietary component has characteristic sources and functions, so a varied diet reduces the risk that one essential function is left unsupported.

Component	Example sources	Main functions
carbohydrate	bread, rice, pasta, potatoes	energy released in respiration
protein	meat, fish, eggs, beans, pulses	growth and tissue repair; making enzymes
lipid	oils, butter, nuts, seeds	energy store, insulation and organ protection
vitamin A	liver, eggs, dairy, orange or dark-green vegetables	normal vision
vitamin C	citrus fruits and vegetables	healthy connective tissue; prevents scurvy
vitamin D	oily fish, eggs, fortified foods; made in skin in sunlight	helps calcium absorption and healthy bones
calcium ions	milk and dairy foods, leafy vegetables	bones and teeth
iron ions	red meat, liver, beans, leafy vegetables	haemoglobin in red blood cells
water	drinks and water-rich foods	solvent, transport medium and reaction medium
dietary fibre	wholegrains, fruit and vegetables	adds bulk and supports peristalsis, reducing constipation

Fibre is not digested for energy, but it still has an important gut function. Vitamins and mineral ions have specific roles and cannot replace energy-providing carbohydrate or lipid.

Energy requirements change when the amount of energy used for movement, growth or reproduction changes.

Factor	Effect on requirement	Why
greater activity level	increases	contracting muscles need more ATP, so respiration releases more energy
age	varies	children and adolescents require energy for growth; requirements often fall when growth ends or activity decreases
pregnancy	increases	energy supports growth of the fetus and changes in maternal tissues

If energy intake persistently exceeds energy use, surplus energy is stored, mainly as fat, and body mass can increase. If intake is lower than use, stored fat and glycogen are respired and body mass can decrease.

These factors change requirements, not a fixed amount for every person. Age alone does not determine need: body size, activity and physiological state also matter.

The alimentary canal is a continuous tube that ingests food, digests large molecules, absorbs useful products and egests undigested material.

Part	Main function
mouth	ingestion, chewing, mixing with saliva and starting starch digestion
oesophagus	carries the bolus to the stomach by peristalsis
stomach	churns food; acid kills many pathogens and provides conditions for protease
duodenum	receives pancreatic enzymes and bile; much chemical digestion occurs
ileum	completes digestion and absorbs soluble products through villi
colon	absorbs water and compacts undigested material
rectum	stores faeces before egestion
pancreas	produces digestive enzymes released into the duodenum

The pancreas is an associated digestive organ, not part of the tube food passes through. Digestion breaks molecules down; absorption moves soluble products into blood or lymph.

Peristalsis is a coordinated wave of muscle contraction that pushes food along the gut.

Circular muscle contracts behind a bolus and relaxes ahead of it, narrowing the gut behind the food. Coordinated contractions of the gut wall then move the bolus forward through the oesophagus and intestines.

Because the movement is produced by muscle, food can travel through the digestive system even when gravity is not acting in the direction of movement. Dietary fibre adds bulk that helps the gut contents stimulate this process.

Peristalsis moves food; it does not chemically digest it. The wave is coordinated contraction and relaxation, not the food sliding down only because of gravity.

Digestive enzymes hydrolyse large, insoluble food molecules into smaller soluble molecules that can be absorbed.

Substrate	Enzyme	Product
starch	amylase	maltose
maltose	maltase	glucose
protein	proteases	amino acids
lipid	lipases	fatty acids and glycerol

Carbohydrate digestion therefore uses two named steps: amylase changes starch to maltose, then maltase changes maltose to glucose. Each enzyme is specific to its substrate because its active site has a complementary shape.

Amylase does not convert starch directly to glucose in the named syllabus pathway, and bile is not an enzyme. Mechanical breakdown increases surface area but does not replace chemical digestion.

Bile is produced continuously by the liver and stored in the gall bladder until it is needed.

After food enters the duodenum, bile is released from the gall bladder through the bile duct into the small intestine. Storage allows a supply to be released when lipid-containing food arrives.

The gall bladder stores and releases bile; it does not make it. The pancreas produces digestive enzymes, not bile.

Bile helps digestion in the duodenum by neutralising acid and emulsifying lipids.

Role	Mechanism and benefit
neutralisation	bile neutralises acidic chyme from the stomach, providing a more suitable pH for enzymes in the small intestine
emulsification	bile separates large lipid globules into many small droplets, increasing surface area for lipase action

Emulsification is physical subdivision, not chemical digestion. Bile contains no digestive enzyme and does not convert lipid into fatty acids and glycerol; lipase does that.

The ileum absorbs digested nutrients rapidly because its inner surface is folded into villi, and each villus has structures that shorten transport paths and maintain gradients.

Feature	How it improves absorption
many villi and microvilli	provide a very large surface area
epithelium one cell thick	gives a short diffusion distance
dense capillary network	quickly carries away glucose and amino acids, maintaining concentration gradients
lacteal	absorbs and transports products of lipid digestion
many mitochondria in epithelial cells	supply ATP for active transport when needed

Small soluble products cross the epithelium by diffusion or active transport. Blood capillaries carry water-soluble nutrients away, while lipid products enter lacteals and are transported in lymph.

Villi absorb products after digestion; they do not produce the digestive enzymes listed in the previous step. A large surface area is effective only because it is paired with a thin barrier and rapid transport away.

Food energy can be estimated by burning a measured mass of food and measuring the temperature rise of a known mass of water.

Step	Action
1	record the mass of food and the mass or volume of water
2	record the initial water temperature, ignite the food and heat the water
3	stir the water and record its highest temperature
4	reweigh the remaining food to find the mass actually burned
5	repeat, calculate a mean and compare energy transferred per gram

E=mc\Delta T

For water, use $c=4.2\,\mathrm{J\,g^{-1}\,^{\circ}C^{-1}}$ . Divide $E$ by the mass of food burned to obtain joules per gram. Keep water mass, flame-to-container distance and apparatus the same when comparing foods.

The estimate is usually lower than the food's true energy value because combustion may be incomplete and heat is lost to the air and apparatus. Insulation, a lid, stirring and an oxygen supply improve accuracy; wear eye protection and secure hair around flames.

(f) Respiration

Respiration is a series of enzyme-controlled reactions that transfers chemical energy from glucose into ATP in living cells.

Breaking down glucose releases energy in controlled steps. Cells capture part of this energy by making ATP rather than releasing it all at once as heat. Aerobic respiration uses oxygen and transfers more energy to ATP than anaerobic respiration.

A cell's respiration rate rises when its ATP demand rises. For example, active muscle cells need more ATP, so they consume glucose and—during aerobic respiration—oxygen more rapidly.

Respiration is not the same as breathing: breathing moves air, whereas respiration is a chemical process in cells. ATP is the immediate energy carrier made using energy transferred from glucose.

ATP provides cells with small, immediately usable transfers of energy for energy-requiring processes.

Cellular process	Why ATP is needed
active transport	moves substances across membranes against a concentration gradient
synthesis of large molecules	joins smaller units to support growth and repair
muscle contraction	powers movement of contractile proteins
cell division and movement	powers chromosome movement and other cellular work

When ATP releases energy it is converted to ADP and phosphate. Energy transferred during respiration is then used to regenerate ATP, linking glucose breakdown to the cell's changing energy demands.

ATP is not a long-term energy store like glycogen or lipid. It is continually used and regenerated, so reduced respiration can quickly limit active transport, synthesis and movement.

Aerobic and anaerobic respiration both transfer energy from glucose to ATP, but they differ in oxygen use, completeness of breakdown, energy yield and products.

Feature	Aerobic respiration	Anaerobic respiration
oxygen	required	not required
glucose breakdown	complete	incomplete
ATP yield per glucose	greater	smaller
products in animals	carbon dioxide and water	lactic acid
products in plants and yeast	carbon dioxide and water	ethanol and carbon dioxide

During vigorous exercise, oxygen delivery may not meet muscle demand, so anaerobic respiration supplies some ATP and lactic acid accumulates. Afterwards, breathing remains elevated while the body deals with the lactic acid and repays the oxygen debt.

Anaerobic does not mean no energy is transferred; it means less ATP is obtained from each glucose molecule because breakdown is incomplete.

Aerobic respiration uses glucose and oxygen and produces carbon dioxide and water while transferring energy to ATP.

ext{glucose}+ ext{oxygen}\longrightarrow ext{carbon dioxide}+ ext{water}

\mathrm{C}{6}\mathrm{H}{12}\mathrm{O}{6}+6\mathrm{O}{2}\longrightarrow6\mathrm{CO}{2}+6\mathrm{H}{2}\mathrm{O}

The balanced equation has six carbon, twelve hydrogen and eighteen oxygen atoms on each side. Energy transferred to ATP is a consequence of the reaction, not a chemical atom that is balanced into the symbol equation.

Without sufficient oxygen, cells can transfer some energy from glucose by anaerobic respiration, but animals and plants form different products.

Organisms	Word equation
animals	glucose $\longrightarrow$ lactic acid
plants and yeast	glucose $\longrightarrow$ ethanol + carbon dioxide

The products still contain chemical energy because glucose is only partly broken down. This explains why anaerobic respiration transfers less energy to ATP than aerobic respiration.

Animals do not produce ethanol and carbon dioxide in this pathway, and plant/yeast anaerobic respiration does not produce lactic acid in the syllabus equations. Oxygen is absent from both reactant sides.

Respiring seeds provide measurable evidence that respiration releases carbon dioxide and transfers some energy as heat.

Product	Experimental comparison	Evidence
carbon dioxide	place equal masses of germinating seeds and boiled or dry control seeds in separate sealed apparatus; keep temperature and time equal	a carbon-dioxide sensor rises, or gas from germinating seeds turns limewater cloudy
heat	place equal masses of germinating and boiled control seeds in insulated flasks with thermometers; use the same starting temperature	a larger temperature rise in the germinating-seed flask shows heat release

Use the same seed species, age or stage, mass, moisture, oxygen supply and measurement time. Repeat each condition and compare mean changes. A control separates respiration from temperature drift or other environmental change.

Disinfect seeds to reduce microbial respiration, but do not kill the experimental seeds. Carbon dioxide production alone does not distinguish aerobic from anaerobic respiration, so ensure oxygen is available when claiming aerobic respiration.

(g) Gas exchange

Gas exchange occurs by diffusion: the net movement of gas particles from a region of higher concentration to a region of lower concentration.

Oxygen diffuses across an exchange surface when its concentration is higher on one side, while carbon dioxide diffuses in the opposite direction when its gradient is reversed. Thin, permeable surfaces shorten the diffusion path; a large area allows more particles to cross at once.

Ventilation replaces air and circulation carries gases away or supplies them, maintaining steep concentration gradients. A steeper gradient produces faster net diffusion.

Diffusion is passive and does not pump gases against a gradient. Gas particles move in both directions, but net movement is toward the lower concentration.

Leaf cells respire continuously, while photosynthesis occurs only when light is available; both processes affect oxygen and carbon dioxide exchange.

Process	Gas used	Gas produced	When it occurs
respiration	oxygen	carbon dioxide	day and night
photosynthesis	carbon dioxide	oxygen	when sufficient light is available

The gases measured outside a leaf show the net result of both processes. In strong light, photosynthesis can exceed respiration, so carbon dioxide enters and oxygen leaves overall. In darkness, only respiration affects net exchange, so oxygen enters and carbon dioxide leaves.

Plants do not stop respiring in daylight. Oxygen released by photosynthesis and carbon dioxide released by respiration can be reused inside the leaf, so net exchange is the difference between the two rates.

A leaf is adapted to move carbon dioxide and oxygen rapidly between the atmosphere and photosynthesising or respiring cells.

Feature	Gas-exchange advantage
broad, thin blade	large external area and short diffusion distance
many stomata, mainly in the lower epidermis	pores connect outside air to internal spaces while limiting exposure to direct sunlight
loosely packed spongy mesophyll	large air spaces let gases spread through the leaf
moist mesophyll cell surfaces	gases dissolve before diffusing across cell membranes
thin mesophyll cell walls	short path between air spaces and cells

Internal air spaces are not themselves photosynthetic cells; they create routes and surfaces for diffusion. Floating leaves may place stomata on the upper surface because the lower surface is under water.

A stoma is a pore in the epidermis controlled by two guard cells; changing the pore width regulates gas exchange.

Stoma state	Gas and water movement
open	carbon dioxide diffuses in; oxygen and water vapour can diffuse out
partly closed or closed	carbon dioxide entry and oxygen exit fall, but water loss by transpiration is reduced

Opening supports photosynthesis by maintaining carbon dioxide supply. Closing conserves water during dry conditions but can limit photosynthesis, so stomatal control is a compromise between gas exchange and water loss.

Stomata do not actively pull gases into the leaf; they change the resistance of the diffusion pathway. Guard cells control the pore, while gradients determine net gas direction.

Respiration continues at every light intensity, but increasing light raises photosynthesis until another factor limits it; net gas exchange depends on which rate is greater.

Light condition	Relative rates	Net exchange
darkness or very low light	respiration exceeds photosynthesis	oxygen enters; carbon dioxide leaves
compensation point	photosynthesis equals respiration	no net oxygen or carbon dioxide exchange
bright light	photosynthesis exceeds respiration	carbon dioxide enters; oxygen leaves

No net exchange at the compensation point does not mean both processes have stopped. Their opposite gas movements are equal, so they cancel at the whole-leaf boundary.

Hydrogen-carbonate indicator reveals changes in carbon dioxide concentration around a leaf: yellow means carbon dioxide increased, orange-red means little or no change, and dark red or purple means it decreased.

Step	Action
1	place equal leaf areas in sealed tubes containing equal volumes of indicator; include a tube with indicator but no leaf
2	expose tubes to a measured range of light intensities using lamp distance, neutral filters or darkness
3	keep leaf species, temperature and exposure time constant
4	record final colour or use a colorimeter; repeat each intensity and compare means

Yellow shows net respiration and carbon dioxide release. Purple or dark red shows net photosynthesis and carbon dioxide uptake. Orange-red suggests the compensation point or no biological change in the empty control.

Keep tubes sealed and prevent lamp heating from becoming a second variable. The indicator measures net carbon dioxide change, not photosynthesis alone.

The thorax contains the lungs and the structures that conduct air, exchange gases and change chest volume during ventilation.

Structure	Position or role
trachea	main airway supported by cartilage; divides into two bronchi
bronchi	one enters each lung and branches into bronchioles
bronchioles	narrow branching airways ending at alveoli
alveoli	microscopic air sacs where gases diffuse between air and blood
ribs	protect the lungs and move to change thoracic volume
intercostal muscles	lie between ribs and move the rib cage
diaphragm	muscular sheet below the lungs that changes thoracic volume
pleural membranes	surround the lungs with a thin fluid layer that reduces friction during movement

During inhalation, air follows trachea $ightarrow$ bronchus $ightarrow$ bronchioles $ightarrow$ alveoli. The oesophagus is part of the digestive system, not this airway.

Ventilation moves air because the diaphragm and intercostal muscles change thoracic volume, which changes pressure inside the lungs.

Event	Inhalation	Exhalation at rest
diaphragm	contracts and flattens	relaxes and becomes dome-shaped
external intercostal muscles	contract	relax
ribs	move up and out	move down and in
thoracic volume	increases	decreases
pressure in lungs	falls below atmospheric pressure	rises above atmospheric pressure
air movement	into lungs	out of lungs

The lungs do not contain muscles that pull themselves open. Air flows down a pressure gradient created by changing thoracic volume; inhalation does not occur because the lungs actively suck air in.

Millions of alveoli form an exchange surface that lets oxygen diffuse into blood and carbon dioxide diffuse from blood into air rapidly.

Adaptation	Effect on diffusion
many tiny alveoli	very large total surface area
alveolar and capillary walls one cell thick	very short diffusion distance
moist lining	oxygen and carbon dioxide dissolve before crossing membranes
dense capillary network and continuous blood flow	carries oxygen away and brings carbon dioxide, maintaining gradients
ventilation	renews alveolar air, maintaining steep oxygen and carbon dioxide gradients

Oxygen concentration is higher in alveolar air than in deoxygenated blood, so oxygen diffuses into capillaries. Carbon dioxide concentration is higher in the blood, so it diffuses into alveoli.

A large surface area alone is insufficient: thin walls and maintained concentration gradients are also required. Diffusion crosses both the alveolar wall and capillary wall.

Cigarette smoke damages both the lungs and circulatory system, reducing oxygen delivery and increasing disease risk.

Smoke component or damage	Biological consequence
tar damages cilia	mucus and pathogens accumulate, increasing bronchitis and infection risk
carcinogens in tar	mutations increase lung-cancer risk
alveolar walls break down in emphysema	surface area and elasticity fall, causing breathlessness and reduced diffusion
carbon monoxide binds strongly to haemoglobin	less oxygen is transported to tissues
nicotine and damage to artery linings	heart workload and risk of narrowed or blocked coronary arteries increase

Coronary heart disease restricts blood flow through coronary arteries, reducing oxygen supply to heart muscle. Combined lung and blood-vessel damage therefore limits aerobic respiration in tissues.

Smoking raises risk; it does not guarantee one outcome. Explaining a consequence requires the causal link—for example, damaged alveoli reduce surface area, which reduces oxygen diffusion.

Breathing investigations can show that exhaled air contains more carbon dioxide and that exercise increases ventilation to meet greater respiratory demand.

Question	Method and evidence
is more carbon dioxide exhaled?	bubble equal volumes of inhaled and exhaled air through separate limewater samples using one-way tubing; exhaled air turns limewater cloudy faster
how does exercise affect breathing?	count breaths per minute at rest, immediately after a fixed exercise, and at regular recovery intervals; increased rate and slower return indicate greater ventilatory demand
how does exercise affect carbon dioxide release?	compare the time for equal volumes of exhaled air before and after exercise to change equal indicator or limewater samples

Standardise exercise type, duration and intensity; use the same participant for paired readings where possible; keep solution volume and temperature constant. Repeat trials or participants and calculate means.

Breathing rate counts breaths, while ventilation rate also depends on volume per breath. Exercise can increase both, so breathing rate alone is not a complete measure of ventilation.

(h) Transport

A unicellular organism exchanges substances directly with its surroundings by diffusion because its single cell is always next to the environment.

Its small size gives a large surface-area-to-volume ratio and a short diffusion distance. Oxygen and dissolved food can enter, while carbon dioxide and other wastes can leave, fast enough for the cell's relatively low demand.

A unicellular organism does not need a specialised transport system. Diffusion is sufficient because distances and demand are small—not because diffusion becomes faster inside a single cell.

Large multicellular organisms need specialised transport systems because diffusion across the outer surface alone cannot supply every cell fast enough.

Feature of a large organism	Consequence
small surface-area-to-volume ratio	too little exchange surface for the volume of living tissue
many internal cells	some cells are far from the external environment
long diffusion distances	substances take too long to reach cells or leave them
high metabolic demand	oxygen and nutrients are used, and wastes produced, faster

A transport system moves materials rapidly between exchange surfaces and cells. Continuous delivery and removal also maintain steep concentration gradients for diffusion at both ends.

Transport does not replace diffusion: bulk flow covers long distances, while diffusion still moves substances across exchange surfaces and cell membranes.

Phloem translocates dissolved sucrose and amino acids between leaves and other parts of a plant.

Source or sink	Typical role
photosynthesising leaf	source: loads sucrose made from photosynthetic products
growing root, bud, flower or fruit	sink: uses sucrose and amino acids for respiration, growth and synthesis
storage organ	can be a sink while storing and a source when reserves are mobilised

Translocation can occur up or down the plant because the source–sink relationship changes. Phloem is living tissue; movement is not restricted to the root-to-leaf direction.

Phloem transports sucrose and amino acids, not starch molecules. Do not confuse it with xylem, which carries water and mineral ions mainly upward.

Xylem carries water and dissolved mineral ions from roots through stems to leaves and the rest of the plant.

Feature	Transport advantage
dead cells joined end to end	forms a continuous hollow pathway
no end walls or cytoplasm	reduces resistance to water flow
lignified walls	resist collapse under tension and support the plant

Water loss from leaves creates transpiration pull, drawing a continuous column of water upward through xylem. Mineral ions dissolved in that water travel with the transpiration stream.

Xylem flow is mainly one-way from roots upward. It is not the tissue that translocates sucrose and amino acids between sources and sinks.

Water enters root hair cells by osmosis, from soil with higher water potential to cell contents with lower water potential through a partially permeable cell membrane.

Adaptation	Effect
long hair-like extension	large contact area with water between soil particles
thin wall and membrane	short pathway into the cell
concentrated cell sap	helps maintain a lower water potential than the soil solution

Water then moves across the root into xylem. Mineral ions may be absorbed by active transport and can help lower cell water potential, but the water itself enters by osmosis.

Osmosis describes water movement only. Do not say water is actively transported into the root hair cell or that mineral ions enter by osmosis.

Transpiration is the loss of water vapour from a plant's surface, mainly through stomata in the leaves.

Stage	Event
1	water evaporates from moist mesophyll cell surfaces into leaf air spaces
2	water vapour diffuses through open stomata down its concentration gradient
3	water leaving mesophyll is replaced from leaf xylem
4	tension pulls a continuous water column upward through xylem from the roots

The transpiration stream supplies water for photosynthesis and turgor, carries mineral ions and can cool leaves by evaporation.

Evaporation occurs inside the leaf before water vapour diffuses out. Transpiration is the overall water loss, not a pump located in the stomata.

Environmental conditions alter transpiration by changing evaporation, the water-vapour gradient or stomatal opening.

Change	Effect on rate	Mechanism
lower humidity	increases	steepens the water-vapour gradient from leaf to air
greater wind speed	increases	removes the moist boundary layer around the leaf
higher temperature	increases	gives water molecules more kinetic energy, increasing evaporation and diffusion
brighter light	usually increases	opens stomata for carbon dioxide entry, reducing resistance to water loss

The converse changes usually reduce the rate. Once stomata are fully open or another factor becomes limiting, further change may have little additional effect.

State the mechanism, not only the direction. Light acts mainly through stomata; wind does not heat the leaf in the standard explanation, and high humidity slows loss by reducing the gradient.

A bubble potometer measures water uptake by a cut leafy shoot. Water uptake is used as a proxy for transpiration because most absorbed water is lost from leaves.

Step	Action
1	cut the shoot under water and connect it to water-filled apparatus
2	seal every joint, dry the leaves and check that the apparatus is airtight
3	introduce one air bubble and record its starting position
4	expose the shoot to one measured condition for a fixed time
5	measure bubble distance; calculate volume as capillary cross-sectional area × distance, then divide by time
6	reset the bubble, repeat and calculate a mean

Change one factor such as wind speed, light intensity, humidity or temperature. Control the other environmental factors, shoot species, leaf area and time; allow the shoot to acclimatise before readings.

A potometer does not measure transpiration directly: some absorbed water is used in photosynthesis, growth or turgor. Avoid air entering the xylem by cutting under water, and keep water away from electrical equipment.

Blood is a tissue made of red blood cells, white blood cells and platelets suspended in liquid plasma.

Component	Main function
red blood cells	transport oxygen using haemoglobin
phagocytes and lymphocytes (white blood cells)	defend against pathogens
platelets	trigger clotting at damaged vessels
plasma	transports cells and dissolved substances, and distributes heat

Platelets are cell fragments, not white blood cells. Plasma is the liquid carrier; it is not the same as the whole blood or the clear fluid inside every cell.

Plasma is the liquid part of blood that carries dissolved substances between the organs that produce, absorb, use or remove them.

Cargo	Main route
carbon dioxide	respiring tissues → lungs
digested food such as glucose and amino acids	small intestine → liver and body cells
urea	liver → kidneys
hormones	endocrine glands → target organs
heat	redistributed from active organs such as muscles and liver to the rest of the body

Oxygen is transported mainly bound to haemoglobin inside red blood cells, whereas these listed substances travel in plasma. Always name both the substance and a biologically correct source or destination.

A red blood cell is specialised to load oxygen in the lungs and release it to respiring tissues.

Adaptation	Oxygen-transport advantage
biconcave disc	large surface-area-to-volume ratio and short diffusion path
no nucleus when mature	leaves more internal space for haemoglobin
packed with haemoglobin	haemoglobin binds oxygen reversibly
small and flexible	squeezes through narrow capillaries close to cells

Red blood cells contain cytoplasm and haemoglobin but no nucleus. Iron is part of haemoglobin; the cell does not carry oxygen because it is hollow.

White blood cells defend the body in two distinct ways: phagocytes ingest pathogens, while lymphocytes make specific antibodies.

Cell	Recognition and action
phagocyte	surrounds and engulfs a pathogen, then digests it with enzymes
lymphocyte	recognises a particular antigen and produces antibodies with complementary binding sites

Antibodies bind specifically to their matching antigens and can clump pathogens or mark them for destruction. Some activated lymphocytes form memory cells.

Phagocytes do not produce antibodies, and lymphocytes do not normally engulf pathogens. Antibody specificity depends on complementary antigen-binding shape.

Vaccination exposes the immune system to harmless pathogen antigens so that protection develops without the full disease.

Stage	Immune event
1	dead, weakened, inactive or antigen-containing material is introduced
2	specific lymphocytes recognise the antigen and produce antibodies
3	some lymphocytes remain as memory cells
4	later exposure to the same antigen triggers a secondary response
5	antibodies are produced sooner, faster and in greater quantity, often removing the pathogen before symptoms develop

A vaccine does not kill every future pathogen directly and does not cause antibiotics to be made. Memory is antigen-specific, so protection against one pathogen may not protect against another.

Blood clotting rapidly seals a damaged vessel, limiting blood loss and blocking microorganisms from entering the body.

Stage	Event
1	a vessel is damaged and platelets collect at the site
2	clotting reactions produce strands of fibrin
3	the fibrin mesh traps blood cells and forms a clot
4	the clot dries into a scab while repair occurs underneath

Platelets help start clotting but are not antibodies or phagocytes. A clot prevents both excessive bleeding and pathogen entry; these are separate benefits.

The heart is a muscular double pump: its right side sends deoxygenated blood to the lungs, and its left side sends oxygenated blood around the body.

Flow step	Structure
1	vena cava → right atrium
2	right atrium → valve → right ventricle
3	right ventricle → pulmonary artery → lungs
4	lungs → pulmonary vein → left atrium
5	left atrium → valve → left ventricle
6	left ventricle → aorta → body

Feature	Function
valves	prevent backflow
septum	prevents oxygenated and deoxygenated blood mixing
thick left-ventricle wall	generates high pressure for the whole body
thinner right-ventricle wall	pumps only to nearby lungs
coronary arteries	supply heart muscle with oxygen and glucose

Arteries carry blood away from the heart and veins return it; oxygen content does not define the vessel. The pulmonary artery is deoxygenated and the pulmonary vein oxygenated.

Heart rate rises when muscles need faster delivery of oxygen and glucose and faster removal of carbon dioxide during exercise.

More muscular contraction increases respiration and ATP demand. Increasing heart rate raises blood flow, supporting aerobic respiration and helping transport heat and respiratory products away.

Adrenal glands release the hormone adrenaline into the blood during stress or excitement. Adrenaline acts on the heart's pacemaker to increase heart rate as part of the fight-or-flight response.

Adrenaline is a hormone carried in plasma; it is not produced by the heart. A higher heart rate supports increased respiration—it is not itself cellular respiration.

Coronary heart disease occurs when coronary arteries become narrowed or blocked, reducing blood and oxygen supply to heart muscle.

Risk factor	How risk can increase
smoking	damages artery lining, raises clot risk and blood pressure
high saturated-fat or cholesterol diet	promotes fatty deposits that narrow coronary arteries
inactivity and obesity	increase strain on the circulatory system and often raise blood pressure
persistent high blood pressure or stress	damages vessels and increases heart workload
diabetes, inherited alleles and increasing age	can increase susceptibility independently or with lifestyle factors

A narrowed lumen limits oxygen and glucose delivery. Complete blockage can stop aerobic respiration in part of the heart muscle, causing tissue death and a heart attack.

A risk factor changes probability; it does not prove that one individual will develop CHD. Correlation in population data also does not isolate cause unless other variables are controlled.

Arteries, veins and capillaries have structures matched to the pressure, direction and exchange role of the blood they carry.

Vessel	Structure	Structure–function link
artery	thick muscular, elastic wall; relatively narrow lumen; no valves along most of its length	withstands high pressure; elastic recoil smooths flow away from heart
vein	thinner wall with less muscle and elastic tissue; wide lumen; valves	low-resistance return at low pressure; valves prevent backflow
capillary	wall one cell thick; extremely narrow lumen; branching network	short diffusion distance, slow close flow and large total exchange area

Vessel type is defined by direction relative to the heart, not oxygen content. Capillaries are not simply tiny veins: their one-cell-thick walls are specialised for exchange.

Humans have double circulation: blood passes through the heart once in the pulmonary circuit and again in the systemic circuit during one complete journey.

Organ route	Named vessels and direction
lungs	heart → pulmonary artery → lungs → pulmonary vein → heart
body tissues	heart → aorta → organ arteries → capillaries → organ veins → vena cava → heart
liver	hepatic artery brings oxygenated blood; hepatic portal vein brings nutrient-rich blood from the gut; hepatic vein drains to vena cava
kidneys	renal arteries bring blood from aorta; renal veins return blood to vena cava

The pulmonary circuit oxygenates blood. The systemic circuit distributes oxygen and nutrients and returns carbon dioxide and other wastes. Separate pumps allow high systemic pressure without exposing lung capillaries to the same pressure.

The hepatic portal vein is unusual because it connects gut capillaries to liver capillaries before blood returns to the heart. The renal vein carries blood away from the kidney, even though all veins carry blood toward the heart overall.

(i) Excretion

Leaf metabolism can produce carbon dioxide and oxygen faster than the plant uses them; the excess gases diffuse out through stomata.

Metabolic process	Gas produced	When it becomes a waste product
respiration	carbon dioxide	production exceeds its use in photosynthesis, especially in darkness
photosynthesis	oxygen	production exceeds its use in respiration, commonly in sufficient light

Each gas moves from a higher concentration in the leaf air spaces to a lower concentration outside. Stomata provide the pore through which this diffusion occurs.

Plants respire in both light and darkness. Calling oxygen or carbon dioxide a waste product depends on net production at that time; neither gas is always waste to the plant.

Excretion removes waste products of metabolism and substances present in excess; the lungs, kidneys and skin remove different mixtures.

Organ	Excretory products	Route out
lungs	carbon dioxide and water vapour	exhaled air
kidneys	urea, excess water and excess mineral ions	urine
skin	water, mineral ions and a small amount of urea	sweat

Excretion is not egestion: faeces contain mainly unabsorbed food leaving the gut, whereas excretory products were made by metabolism or are excess internal substances.

Kidneys clean the blood by excreting urea and regulate blood water and ion content by adjusting what the nephrons return to the blood.

Nephron process	Contribution
ultrafiltration	forces water and small dissolved substances out of glomerular blood
selective reabsorption	returns all useful glucose and required ions from filtrate to blood
variable water reabsorption	returns more or less water according to the body's water balance
urine formation	leaves urea plus excess water and ions for removal

Osmoregulation maintains a suitable blood water concentration. Dehydration causes more water reabsorption and a smaller volume of concentrated urine; excess body water causes less reabsorption and a larger volume of dilute urine.

Excretion removes metabolic waste such as urea; osmoregulation controls water and ion balance. They occur in the same organ but are not identical functions.

The urinary system makes urine in two kidneys, carries it through two ureters, stores it in the bladder and releases it through the urethra.

Structure	Direction and function
renal artery	brings unfiltered blood to each kidney
kidney	filters blood and forms urine in nephrons
renal vein	returns adjusted blood from each kidney
ureter	carries urine from one kidney to the bladder
bladder	stores urine temporarily
urethra	carries urine from the bladder out of the body

A ureter connects a kidney to the bladder; the urethra leaves the bladder. Blood enters by the renal artery, but urine never travels through a blood vessel.

A nephron is the kidney's microscopic processing unit; its connected regions filter blood, recover useful substances and form urine.

Route through nephron	Main structural relationship
1. glomerulus inside Bowman's capsule	capillary knot where filtrate is forced into the capsule
2. proximal convoluted tubule	first coiled region leaving the capsule
3. loop of Henle	long U-shaped section extending into the kidney medulla
4. distal convoluted tubule	second coiled region returning toward the cortex
5. collecting duct	receives fluid from nephrons and carries it toward the renal pelvis and ureter

The glomerulus is a blood-capillary network; Bowman's capsule surrounds it and receives filtrate. The collecting duct is downstream of the convoluted tubules and loop, not part of the blood supply.

Ultrafiltration occurs when high pressure in the glomerulus forces water and small dissolved substances through the filtration barrier into Bowman's capsule.

Substance in blood	Enters glomerular filtrate?	Reason
water, glucose, urea and mineral ions	yes	small enough to pass the filtration barrier
blood cells and large plasma proteins	no	too large to cross and remain in the blood

The glomerulus has high hydrostatic pressure, helped by blood entering through a wider arteriole than the one leaving. The filtrate collected in Bowman's capsule then enters the proximal convoluted tubule.

Ultrafiltration is non-selective among small molecules: useful glucose enters the filtrate alongside urea. Selective recovery happens later, so filtrate and urine do not have the same composition.

Water moves from fluid in the collecting duct into the surrounding kidney tissue and then the blood by osmosis.

The medulla around the collecting duct has a lower water potential because it contains a high concentration of dissolved ions. When the duct wall is permeable, water moves down this water-potential gradient and is carried away by capillaries.

Water reabsorbed	Urine outcome
more	smaller volume and higher concentration
less	larger volume and lower concentration

Water is reabsorbed by osmosis, not active transport. The amount depends on collecting-duct permeability, which ADH regulates in the next control step.

Glucose enters glomerular filtrate because it is small, but a healthy kidney selectively reabsorbs it from the proximal convoluted tubule into the blood.

Glucose is a useful respiratory substrate, so losing it in urine would waste chemical energy. Proximal-tubule cells use active transport, requiring energy from ATP, to move glucose from filtrate even against its concentration gradient; it then returns to nearby capillaries.

Selective reabsorption is different from ultrafiltration: filtration lets small molecules leave blood without deciding whether they are useful, while reabsorption retrieves specific useful substances. Normally, no glucose remains in urine.

ADH regulates blood water content by changing the permeability of collecting ducts and therefore how much water is reabsorbed.

Blood state	Control response	Kidney effect	Urine
too little water; blood too concentrated	hypothalamic osmoreceptors stimulate more ADH release from the pituitary	collecting ducts become more permeable; more water returns to blood	small volume, concentrated
too much water; blood too dilute	less ADH is released	collecting ducts become less permeable; less water returns	large volume, dilute

As blood concentration returns toward normal, the original stimulus falls and ADH release adjusts. This negative feedback prevents uncontrolled correction.

ADH is released from the pituitary and acts on collecting ducts; it does not add water to urine or filter urea. More ADH produces less urine, not more.

Urine contains water, urea and mineral ions that remain after filtration and selective reabsorption.

Component	Why it remains
water	the amount not required to maintain blood water balance
urea	nitrogen-containing metabolic waste made in the liver and excreted by kidneys
mineral ions	quantities present in excess of the body's needs

Urine volume and concentration vary with water balance: reabsorbing more water concentrates the urea and ions left in the tubule.

Healthy urine normally lacks blood cells, large proteins and glucose. Urine is not simply filtered blood: useful substances and much water have been reabsorbed before it reaches the bladder.

(j) Co-ordination and response

An organism responds when it detects a change in its internal or external environment and produces an action that can improve survival.

Stage	Role	Example
stimulus	detectable environmental change	increasing light, heat, sound or touch
detection and coordination	receptors detect the change and information is processed	sensory cells and a coordination system
response	an effector changes activity	muscle contracts, gland secretes or a plant changes growth direction

Responses can move an organism toward useful conditions or away from harm. Repeated harmless stimulation can sometimes reduce a response, preventing unnecessary energy use.

A stimulus is the change, not the action it causes. A response need not involve conscious choice: plants, microorganisms and reflex pathways all respond without deliberate decision-making.

Homeostasis is the maintenance of a constant internal environment within narrow limits despite internal or external change.

Regulated condition	Why control matters	Corrective examples
body temperature	enzyme-controlled reactions work best in a suitable range	sweating and vasodilation when hot; reduced sweating and vasoconstriction when cold
body water content	cells need a suitable water balance	ADH changes kidney water reabsorption and urine concentration

A deviation is detected, a coordination system activates effectors, and the response opposes the deviation. As the condition returns toward its set range, the corrective response is reduced: this is negative feedback.

Homeostasis does not keep every value perfectly fixed; it holds conditions within a tolerable range. It concerns the internal environment, not simply keeping the external environment unchanged.

A coordinated response links a stimulus to a receptor and then to an effector whose action produces the response.

Component	What it does
stimulus	changes a condition, such as light intensity or temperature
receptor	detects the stimulus and converts it into information for the coordination system
coordinator	processes information and sends instructions
effector	carries out the response; usually a muscle or gland in animals

Touching a hot object is the stimulus; temperature or pain receptors in skin detect it; the nervous system coordinates; arm muscles contract as effectors, withdrawing the hand.

A receptor detects; an effector acts. The organ that senses a change should not be named as the effector unless it also performs the response.

Plants detect environmental stimuli and alter growth or cell activity even though they have no nervous system or muscles.

Stimulus	Plant response and value
directional light	shoots change growth direction, improving light capture
gravity	roots and shoots grow in opposite directions, placing organs in useful positions
water shortage	stomata close, reducing water loss
day length	flowering can occur in a season favourable for reproduction

Plant responses are often slower than animal movements because many depend on unequal growth, but speed does not determine whether something is a response. A stimulus can also change reversible cell activity, such as stomatal opening.

A tropism is a directional growth response: positive growth is toward a stimulus and negative growth is away from it.

Organ	Light response	Gravity response	Benefit
shoot	positive phototropism: toward light	negative geotropism: away from gravity	exposes leaves to light for photosynthesis
root	usually negative phototropism: away from light	positive geotropism: toward gravity	anchors the plant and grows into soil for water and ions

Positive and negative describe direction relative to the stimulus, not whether the response is beneficial. A shoot growing upward is negatively geotropic because it grows opposite to gravity.

When light reaches a shoot from one side, auxin causes unequal cell elongation so the shoot bends toward the light.

Stage	Event
1	auxin is produced near the shoot tip
2	one-sided light causes more auxin to accumulate on the shaded side
3	in shoots, auxin stimulates greater cell elongation on the shaded side
4	the shaded side grows faster, curving the tip toward the light

Bending exposes developing leaves to stronger light, supporting a higher photosynthetic rate.

Auxin does not pull the shoot toward light and light does not make the illuminated side grow faster. In this shoot response, greater elongation occurs on the shaded side.

Nervous and hormonal systems both coordinate responses, but they carry different signals through different routes and produce different response patterns.

Feature	Nervous communication	Hormonal communication
signal	electrical impulse along neurones; neurotransmitter at synapses	chemical hormone
route	specific nerves	blood plasma throughout the body
target	precise connected effector	only cells with the correct receptor respond
speed	usually rapid	usually slower
duration	often short-lived	often longer-lasting

Hormones travel throughout the body but do not affect every cell; target cells need matching receptors. Nervous impulses travel along neurones rather than being carried in blood.

The central nervous system consists of the brain and spinal cord; nerves connect it to receptors in sense organs and to effectors.

Direction	Route and purpose
into CNS	receptors in sense organs detect stimuli; sensory neurones carry impulses toward brain or spinal cord
within CNS	relay neurones connect pathways and allow information to be processed
out of CNS	motor neurones carry impulses to muscles or glands

The nerves outside the brain and spinal cord belong to the peripheral nervous system, not the CNS. A sense organ contains receptors but is not itself part of the CNS.

Stimulation of receptors in a sense organ generates electrical impulses that travel along neurones into and out of the CNS to produce a rapid response.

Stage	Information flow
1	a receptor detects a stimulus
2	a sensory neurone carries electrical impulses into the CNS
3	the CNS coordinates the response
4	a motor neurone carries impulses out to an effector
5	a muscle contracts or a gland secretes

The dedicated pathway and electrical transmission along neurones allow responses to occur rapidly. Wider axons can conduct impulses faster because they offer less internal resistance to current flow.

The signal is electrical along a neurone but chemical across most synapses. Do not describe an impulse as blood-borne or say receptors themselves perform the final response.

At a synapse, a neurotransmitter carries a signal across the tiny gap from one neurone to the next cell.

Stage	Event
1	an electrical impulse reaches the end of the presynaptic neurone
2	neurotransmitter is released into the synaptic cleft
3	molecules diffuse across the gap
4	neurotransmitter binds to complementary receptors on the postsynaptic membrane
5	a new electrical impulse is triggered if stimulation is sufficient

Neurotransmitter is released on one side and receptors are concentrated on the other, making transmission one-way. Chemical diffusion also creates a small synaptic delay.

The neurotransmitter does not travel along the whole axon; the electrical impulse does. Neurotransmitter crosses only the synaptic cleft before being removed or broken down.

A withdrawal reflex is a rapid, automatic response that reduces tissue damage before conscious processing is required.

Stage	Structure and event
1	heat or pain stimulates a receptor in the finger
2	a sensory neurone carries impulses to the spinal cord
3	neurotransmitters cross synapses to a relay neurone and then a motor neurone
4	the motor neurone carries impulses to an arm muscle
5	the muscle contracts as the effector and withdraws the hand

Impulses can also travel to the brain so the person becomes aware of pain, but the protective withdrawal can begin through the spinal cord first.

A reflex is involuntary and stereotyped, not necessarily unconscious forever. The relay neurone lies inside the CNS; sensory and motor neurones connect the receptor and effector to it.

The eye is a sense organ whose structures refract light, control how much enters and convert focused light into nerve impulses.

Structure	Function
cornea	transparent curved surface that provides most refraction
iris and pupil	iris muscles change pupil diameter to regulate light entry
lens	changes curvature to fine-focus light on the retina
ciliary muscles and suspensory ligaments	alter tension and therefore lens shape
retina	contains light-sensitive receptor cells
fovea	retinal region with high visual acuity
optic nerve	carries electrical impulses from retina to brain
sclera	tough outer layer that protects and supports the eye

The pupil is a hole, not a muscle or receptor. The iris controls pupil size, while photoreceptors in the retina detect light after the cornea and lens have focused it.

The eye focuses by changing lens curvature and responds to light intensity by changing pupil diameter; these are separate control mechanisms.

Viewing distance	Ciliary muscles	Suspensory ligaments	Lens	Refraction
near object	contract	loosen	thicker and more curved	bends light more
distant object	relax	tighten	thinner and less curved	bends light less

Light condition	Iris response	Pupil outcome
bright	circular muscles contract; radial muscles relax	constricts, reducing light entry and retinal damage
dim	radial muscles contract; circular muscles relax	dilates, increasing light entry

The lens does not move forward and backward to focus. Accommodation changes lens shape; the pupil reflex changes light quantity but does not focus the image.

The skin helps regulate body temperature by changing sweat production and blood flow near the surface.

Condition	Sweating	Skin blood vessels	Effect on heat transfer
body too hot	increases; evaporation removes thermal energy	arterioles vasodilate, increasing blood flow through surface capillaries	more heat lost by radiation and convection
body too cold	decreases	arterioles vasoconstrict, reducing surface-capillary blood flow	less heat transferred to the environment

Temperature receptors and the brain coordinate these opposing responses. When temperature returns toward its normal range, the responses reduce through negative feedback.

Capillaries do not constrict or dilate because they lack muscular walls; arterioles controlling blood supply to them do. Sweat cools only when it evaporates, not merely when it is secreted.

Hormones are chemical messengers carried in blood; each is released by a source gland and acts on target tissues with matching receptors.

Hormone	Main source	Required role or effect
adrenaline	adrenal glands	prepares for fight or flight: increases heart rate and blood supply to muscles and raises blood glucose availability
insulin	pancreas	lowers high blood glucose by promoting glucose uptake and conversion of glucose to glycogen
testosterone	testes	stimulates male secondary sexual characteristics and supports sperm production
oestrogen	ovaries	stimulates female secondary sexual characteristics and rebuilds the uterus lining
progesterone	ovary, especially corpus luteum	maintains the uterus lining

A hormone's source is not necessarily its target. Insulin is made by the pancreas rather than the liver; adrenaline by adrenal glands; testosterone by testes; oestrogen and progesterone mainly by ovaries.

ADH, FSH and LH are released from the pituitary into blood, but they act on different target organs and regulate different processes.

Hormone	Trigger or target	Main role and effect
ADH	released more when blood water content is low; targets kidney collecting ducts	increases duct permeability and water reabsorption, producing a smaller volume of concentrated urine
FSH	targets ovaries	stimulates maturation of an ovarian follicle and secretion of oestrogen
LH	targets ovaries	its surge triggers ovulation and supports formation of the corpus luteum

More ADH produces a smaller volume of more concentrated urine. FSH acts earlier in the ovarian cycle to mature a follicle; LH triggers release of the egg.

At this syllabus level, the pituitary is the required release source for all three. FSH and LH are not produced by the ovaries, and ADH does not directly add water to the blood—it changes kidney permeability.