IB Biology HL Interaction and Interdependence Concepts

Enzymes are biological catalysts that increase reaction rate without being consumed.

They provide an alternative pathway with lower activation energy, so more substrate molecules can react at a given temperature. The enzyme is regenerated after products leave its active site.

For a catalyst claim, check:

• reaction rate increases
• activation energy decreases
• enzyme is not used up
• equilibrium position is unchanged

Adding catalase to hydrogen peroxide makes oxygen bubbles appear faster, but the catalase remains available for further reactions.

An enzyme changes kinetics, not the overall energy difference or equilibrium constant.

Metabolism is the complex network of interdependent and interacting chemical reactions occurring in a living organism; each step is catalysed by a specific enzyme.

Enzyme specificity requires many different enzymes, because each active site catalyses only a limited reaction. Linking enzyme-controlled steps lets the products of one reaction become substrates for another.

Cells control metabolic flux by changing enzyme synthesis, activity, location or access to substrate. Regulation at one key step can alter the output of an entire pathway while other pathways continue independently.

In a pathway A → B → C, one enzyme catalyses A → B and a different enzyme catalyses B → C; inhibiting the first step reduces both B formation and downstream C production.

Metabolism is not one reaction or only energy release: it includes all interacting anabolic and catabolic reactions and the controls acting through their enzymes.

Anabolic reactions build larger molecules from smaller units and require energy; catabolic reactions break down or oxidize molecules and often release usable energy.

Anabolism commonly joins monomers by condensation, forming covalent bonds and releasing water. Catabolism includes hydrolysis of macromolecules in digestion and oxidation of respiratory substrates.

Catabolic respiration can supply ATP and reducing power that drive anabolic protein or glycogen synthesis, linking the two parts of metabolism.

Energy input or release is a typical consequence, not the sole classification rule: identify whether the cellular pathway constructs or breaks/oxidizes material.

Most enzymes are globular proteins whose overall three-dimensional fold creates a small active-site pocket for substrate binding and catalysis.

Only a few amino-acid residues directly form the active site, but interactions among many residues elsewhere—hydrogen bonds, ionic attractions, hydrophobic effects and sometimes disulfide bonds—position those catalytic residues correctly.

The active site binds substrate to form an enzyme–substrate complex and presents chemical groups with the charge, polarity and geometry needed to stabilize the transition state and promote reaction.

Changing a residue far from the pocket can disrupt the protein fold and reposition an active-site residue, reducing catalysis even though that altered residue never contacts substrate.

The active site is only a small part of the enzyme, but it depends on the entire globular conformation; a matching outline alone is not enough for catalytic chemistry.

In induced-fit binding, initial substrate contact changes the conformation of both the enzyme active site and the substrate, producing a catalytically effective fit.

Enzyme side chains move to align catalytic groups, while the substrate can be bent, strained or have bonds polarized. These changes make the transition state easier to reach.

Sequence: initial recognition → enzyme–substrate complex → reciprocal conformational change → transition-state stabilization → products form and leave → enzyme returns to a reusable state.

Hexokinase closes around glucose and ATP, aligning them and excluding water; after phosphate transfer, the differently shaped products have weaker interactions and are released.

Induced fit is not a rigid lock-and-key event and does not mean the enzyme permanently changes. Specificity remains because only suitable substrates trigger productive interactions.

Enzyme catalysis requires random molecular motion to bring a substrate into a productive collision with an active site.

Higher kinetic energy increases motion and collision frequency up to the point where enzyme structure becomes unstable. A collision must also have suitable orientation and enough energy for binding and reaction.

Sometimes a large substrate is effectively immobilized, so enzyme molecules diffuse to exposed sites. In other systems the enzyme is immobilized in a membrane, and moving substrate molecules collide with its fixed active sites.

A membrane-embedded enzyme remains in one location while dissolved substrate diffuses through the membrane environment and collides with the active site.

Immobilized does not mean inactive, and faster motion alone cannot guarantee catalysis: molecular complementarity, orientation and enzyme conformation still matter.

Substrate specificity depends on the three-dimensional arrangement and chemistry of amino acids in the active site; denaturation disrupts that arrangement and lowers activity.

A suitable substrate forms complementary shape, charge, polarity and hydrogen-bond interactions. Temperature extremes or unsuitable pH can disrupt bonds maintaining tertiary structure, changing the active site's geometry.

Trace the relationship: amino-acid interactions maintain fold → fold positions active-site residues → substrate binds specifically → catalysis occurs. Denaturation breaks this chain without normally hydrolysing peptide bonds.

If heating moves a charged catalytic residue away from the substrate-binding position, fewer enzyme–substrate complexes form and the reaction rate falls even after the solution is cooled.

Denaturation is a structural loss, not simply temporary active-site occupancy. The protein may remain present and its peptide sequence intact while its function is lost.

Enzyme activity depends on temperature, pH, substrate concentration and enzyme concentration within an appropriate range.

Warming usually increases collisions until bonds in the protein are disrupted; pH changes alter ionization and active-site interactions. More substrate or enzyme increases rate only while another factor is not limiting.

Interpret a rate curve by locating:

• optimum region
• limiting factor
• denaturation or inhibition
• plateau from saturated active sites

Increasing substrate raises rate until every active site is occupied; further substrate then produces little additional increase.

The optimum is not a universal constant: it depends on the enzyme’s structure and cellular environment.

An enzyme reaction rate is measured from the change in substrate or product concentration per unit time.

Choose a signal proportional to concentration, keep conditions controlled and use the initial linear section before substrate depletion or product inhibition changes the rate.

A sound measurement includes:

• dependent signal and calibration
• controlled temperature and pH
• initial-rate interval
• repeats and uncertainty

A colorimeter can track product colour every ten seconds; the slope of absorbance against time estimates the initial rate.

A final product amount alone cannot distinguish a fast reaction from a slow reaction allowed to run longer.

Enzymes lower activation energy by providing an alternative reaction pathway that stabilizes the transition state; reactant and product energy levels remain unchanged.

Energy must be supplied to distort or break bonds in substrate before new product bonds can form. Bond formation then releases energy; the balance between starting and ending states determines the overall energy change.

On an energy profile, catalysed and uncatalysed curves begin and end at the same levels, but the catalysed curve has a lower peak. More molecules can reach that lower transition-state barrier at the same temperature.

Read the vertical gap from reactants to the peak as activation energy: the enzyme reduces this gap but does not alter the vertical difference between reactants and products.

An enzyme does not remove the barrier, add net energy or make an endergonic reaction exergonic. It changes rate, not the equilibrium position.

Intracellular enzyme reactions occur inside cells, whereas extracellular enzymes are secreted and catalyse reactions outside the cell that produced them.

Glycolysis in the cytoplasm and the Krebs cycle in the mitochondrial matrix are intracellular pathways. Digestive enzymes secreted into the gut lumen catalyse extracellular chemical digestion.

Location matches access to substrate and conditions. Extracellular products must often be small enough for absorption, while intracellular products can feed directly into other compartmentalized pathways.

Pancreatic amylase is secreted into the small intestine and hydrolyses starch outside cells; absorbed sugars can then enter intracellular metabolic reactions.

Extracellular means outside the producing cell, not outside the organism. An enzyme enclosed in a cytoplasmic organelle is still intracellular.

Metabolic reactions transfer energy, and some of the released energy appears as heat rather than being captured in ATP or products.

Cells use coupled reactions and ATP to conserve part of the energy, but inefficiency and thermodynamic constraints produce heat. Heat generation can help endotherms maintain body temperature but must be regulated.

Trace energy allocation:

• chemical energy in substrate
• captured work or ATP
• unavoidable heat release
• heat loss or regulation

During respiration, not all glucose energy becomes ATP; some warms tissues and must be dissipated or used for thermoregulation.

Heat is an energy transfer, not a separate metabolic substance or proof that ATP production stopped.

A linear metabolic pathway converts an initial substrate through a sequence to a terminal product; a cyclical pathway regenerates an acceptor needed for the next turn.

Glycolysis is linear: glucose passes through intermediates and ends as pyruvate. The Krebs cycle regenerates oxaloacetate, and the Calvin cycle regenerates ribulose bisphosphate while processing new carbon input.

Identify pathway form by tracing carbon or the acceptor: if a starting acceptor is re-formed, the pathway is cyclical; if intermediates lead to a different terminal product without regeneration, it is linear.

Each Krebs-cycle turn combines an acetyl group with oxaloacetate and later regenerates oxaloacetate, allowing another acetyl group to enter when substrates and cofactors are available.

A cycle is not self-sustaining perpetual motion: it requires new inputs and energy transfers. A linear pathway can also contain reversible steps without becoming a cycle.

An allosteric site is a specific regulatory binding site separate from the active site; reversible inhibitor binding there can produce non-competitive inhibition.

Only molecules with suitable interactions bind the allosteric site. Binding changes interactions within the enzyme and causes a conformational change that alters the active site enough to prevent or reduce catalysis.

Because the inhibitor does not compete for the active site, adding more substrate does not fully restore activity. When the inhibitor dissociates, the enzyme can return to its active conformation.

If maximum rate remains lower at high substrate concentration in the presence of a reversible allosteric inhibitor, the pattern is consistent with non-competitive inhibition.

Non-competitive does not mean irreversible or nonspecific. The inhibitor binds a specific separate site and affects activity through conformation.

A competitive inhibitor binds reversibly to the enzyme's active site and prevents substrate binding while it occupies that site.

The inhibitor often resembles part of the substrate. Increasing substrate concentration raises the chance that substrate rather than inhibitor occupies the active site, so the inhibition can be reduced.

Statins competitively inhibit an active site in the cholesterol-synthesis pathway, reducing pathway flux. In the ideal kinetic model, more substrate raises the apparent substrate requirement while the original maximum rate remains reachable.

If a reaction recovers its uninhibited maximum rate at sufficiently high substrate concentration, reversible active-site competition is a plausible mechanism.

Competitive inhibition does not permanently destroy enzyme. Contrast it with reversible allosteric inhibition, whose effect is not overcome simply by adding substrate.

Feedback inhibition regulates a metabolic pathway when its end product reversibly inhibits an enzyme near the beginning of that pathway.

As end product accumulates, binding to a regulatory site lowers early-pathway activity and prevents unnecessary substrate and energy use. When product concentration falls, inhibition decreases and flux resumes.

In isoleucine synthesis, accumulated isoleucine inhibits an early enzyme in its own pathway. This negative-feedback loop matches production to demand without reversing the intervening reactions.

High isoleucine → early enzyme inhibited → fewer downstream intermediates → isoleucine production slows; use by the cell lowers isoleucine → inhibition eases → production increases.

The end product is a regulator, not a substrate for the inhibited step, and feedback inhibition controls pathway flux rather than proving that the pathway runs backwards.

A mechanism-based inhibitor undergoes enzyme-catalysed chemistry and then binds irreversibly or causes an irreversible active-site change, disabling that enzyme molecule.

Penicillin acts on bacterial transpeptidase involved in peptidoglycan cross-linking. Interaction at the active site forms a stable covalent complex, preventing further wall cross-link formation.

Without effective cross-linking, growing bacterial walls are weakened. Resistance can arise when a transpeptidase changes so penicillin binds or reacts less effectively while the enzyme still performs its normal function.

Unlike a reversible competitive inhibitor, increasing the normal substrate cannot reactivate a transpeptidase molecule already irreversibly modified by penicillin; new functional enzyme is required.

Mechanism-based inhibition is not merely temporary active-site competition. Penicillin targets bacterial wall synthesis; it does not directly inhibit an equivalent human cell-wall pathway.

ATP—adenosine triphosphate—is a small soluble nucleotide that distributes energy between energy-releasing reactions and energy-requiring cell work.

ATP can move within the cell, release a manageable amount of energy rapidly by hydrolysis and transfer its terminal phosphate to coupled reactions. It is continuously regenerated rather than stored in large quantities.

Its adenine, ribose and three-phosphate structure supports reversible cycling with ADP. Respiration supplies energy to form ATP; ATP hydrolysis then drives transport, synthesis or movement near the point of use.

A membrane pump couples ATP hydrolysis to a conformational change that moves ions against an electrochemical gradient, while ADP and phosphate are returned to ATP-producing pathways.

ATP is an energy-transfer currency, not the original source of energy and not long-term energy storage. Energy comes from reactions such as respiration and is partly transferred through ATP.

Hydrolysis of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and inorganic phosphate releases energy that can be coupled to cell work.

Energy from respiration is required to phosphorylate ADP + phosphate back to ATP. Rapid ATP ↔ ADP cycling links energy-yielding reactions to energy-requiring processes without requiring a large ATP store.

ATP supplies active transport across membranes, anabolic synthesis of macromolecules, movement of whole cells and movement of components such as chromosomes or motor proteins.

ATP hydrolysis powers a membrane pump; respiration then provides energy for ADP + Pi → ATP, allowing the same carrier system to support another round of transport.

ATP hydrolysis releases sufficient energy for many cell tasks, but a numerical kilojoule value is not required. ATP is recycled rather than used once.

Cell respiration is an enzyme-controlled system that transfers energy released from carbon compounds into ATP; glucose and fatty acids are major substrates, although other organic compounds can also be used.

Word equations: glucose + oxygen → carbon dioxide + water; glucose → lactate. In both cases energy released is transferred to ATP, but mitochondrial aerobic stages produce far more.

Cell respiration is chemical energy transfer inside cells; gas exchange is movement of oxygen and carbon dioxide across a surface. Mitochondria are required for aerobic but not human anaerobic respiration.

Cell-respiration rate can be measured as substrate use or product formation per unit time, with results normalized when organisms differ in mass or number.

In a respirometer, soda lime absorbs carbon dioxide, so a pressure or gas-volume decrease reflects oxygen uptake. Keep temperature constant in a water bath and use a control without respiring material to correct for non-biological pressure changes.

Use the calibrated capillary displacement to find oxygen-volume change, subtract the control change, divide by elapsed time, then divide by sample mass if samples are being compared.

A respirometer measures oxygen uptake only when carbon dioxide is appropriately absorbed and the system is sealed. Temperature, pressure, mass, activity and acclimation time must be controlled.

NAD is a hydrogen and electron carrier: it is reduced when it accepts hydrogen during dehydrogenation and later oxidized when it donates electrons.

Oxidation is electron loss. Removing hydrogen together with its electron oxidizes a respiratory substrate, while NAD gains that reducing power; oxidation and reduction therefore occur together in a redox reaction.

Carrier cycle: substrate loses hydrogen/electrons → NAD is reduced → reduced NAD transfers a pair of electrons to the mitochondrial electron transport chain → NAD is regenerated for further dehydrogenation.

Reduced NAD made during glycolysis, the link reaction or the Krebs cycle is reoxidized when its electrons enter the chain, allowing those pathways to keep accepting hydrogen.

NAD is not the terminal electron acceptor. In aerobic respiration oxygen ultimately accepts the electrons, while NAD cycles between oxidized and reduced forms.

Glycolysis is a linear cytoplasmic pathway in which one glucose is converted stepwise into two pyruvate, with a net yield of ATP and reduced NAD.

Phosphorylation uses ATP to activate the six-carbon molecule; lysis splits it into two three-carbon compounds; oxidation transfers hydrogen to NAD; ATP formation then occurs by substrate-level phosphorylation.

Each step is catalysed by a different enzyme. Track the required net outputs rather than intermediate names: two pyruvate, two ATP net and reduced NAD per glucose.

The ATP investment precedes later ATP-producing steps, so gross production is larger than the net gain. NAD must be regenerated for the oxidation step to continue.

Glycolysis does not require oxygen or mitochondria and does not completely oxidize glucose. Do not count invested ATP as part of the net yield.

When oxygen is unavailable, human and yeast cells regenerate NAD using pyruvate-derived reactions so glycolysis can continue with a net yield of two ATP per glucose.

In humans, pyruvate accepts hydrogen/electrons from reduced NAD and becomes lactate, regenerating NAD. In yeast, pyruvate loses carbon dioxide and is reduced to ethanol, also regenerating NAD.

The pathways are the same through glycolysis; they differ in the reaction that regenerates NAD and therefore in final products: human cells form lactate, whereas yeast forms ethanol + carbon dioxide.

Carbon dioxide released by yeast expands bread dough, while ethanol production is used in brewing. Neither fermentation route adds ATP beyond the two net ATP made in glycolysis.

The purpose of reducing pyruvate-derived molecules is NAD regeneration, not a high ATP yield. Human lactate formation does not release carbon dioxide.

In the mitochondrial matrix, the link reaction oxidizes and decarboxylates pyruvate and transfers the remaining two-carbon acetyl group to coenzyme A.

One carbon is released as carbon dioxide, hydrogen/electrons reduce NAD, and the 2C acetyl group joins coenzyme A to form acetyl-CoA for entry into the Krebs cycle.

Per pyruvate: one CO₂, one reduced NAD and one acetyl-CoA; no ATP is formed directly. Per glucose, these outputs double because glycolysis produces two pyruvate.

Carbohydrate-derived pyruvate and fatty-acid breakdown can both supply 2C acetyl groups; coenzyme A carries those groups into the Krebs cycle.

The link reaction is not a cycle and does not directly synthesize ATP. It links glycolysis or lipid breakdown to acetyl-group oxidation.

In the mitochondrial matrix, each 2C acetyl group from acetyl-CoA joins 4C oxaloacetate to form 6C citrate; subsequent reactions regenerate oxaloacetate.

The cycle includes two decarboxylations that release the two incoming carbons as carbon dioxide and four oxidations/dehydrogenations that transfer hydrogen to NAD or FAD.

Each turn yields carbon dioxide, reduced NAD, reduced FAD and one ATP or equivalent while regenerating oxaloacetate. One glucose supplies two acetyl groups, so the cycle turns twice.

Citrate is the named 6C intermediate and oxaloacetate the regenerated 4C acceptor; intermediate names between them are not required for this objective.

The Krebs cycle produces only a small amount of ATP directly. Most captured energy leaves in reduced carriers for later oxidative phosphorylation.

Reduced NAD from glycolysis, the link reaction and the Krebs cycle transfers a pair of electrons to the first carrier of the electron transport chain in the inner mitochondrial membrane.

As electrons pass between carriers, reduced NAD is converted back to NAD and released energy drives proton pumping from the matrix into the intermembrane space. Reduced FAD can also contribute electrons downstream.

The inner membrane's low proton permeability maintains both a concentration difference and electrical potential: an electrochemical proton gradient. Protein-complex names are not required.

Electron donation reoxidizes NAD so earlier respiration stages can continue, while proton pumping stores transferred energy in a gradient ready for ATP synthase.

The electron transport chain builds the gradient; it does not phosphorylate ADP directly. Pumping is matrix → intermembrane space, and chemiosmosis is the return flow.

Chemiosmosis converts the proton gradient into ATP. The inner mitochondrial membrane restricts proton movement except through ATP synthase, so H⁺ diffuses from the intermembrane space back into the matrix through this enzyme. The flow drives phosphorylation of ADP to ATP; this is oxidative phosphorylation. At the end of the electron transport chain, oxygen accepts electrons and H⁺ to form water. Without oxygen, electron flow stops, the gradient collapses and oxidative ATP production cannot continue.

• H⁺ flows down its electrochemical gradient through ATP synthase.
• ATP synthase uses this energy to phosphorylate ADP.
• Oxygen is the terminal electron acceptor and forms water.
• Oxygen removal stops the electron transport chain and oxidative phosphorylation.

Lipids release more energy per gram than carbohydrates because they contain less oxygen and more oxidizable carbon and hydrogen; they also yield more metabolic water.

Carbohydrates are hydrolysed to sugars that enter glycolysis quickly. Fatty acids are broken into 2C acetyl groups that enter aerobic respiration as acetyl-CoA and generate many reduced electron carriers.

Glycolysis and anaerobic respiration occur only when carbohydrate is the substrate. Lipid oxidation depends on mitochondrial aerobic pathways, whereas glycerol from triglycerides can enter carbohydrate metabolism.

During prolonged aerobic activity, repeated fatty-acid breakdown supplies many acetyl-CoA molecules and reduced carriers, giving a high ATP yield; rapid anaerobic ATP demand instead depends on carbohydrate.

Higher energy per gram does not mean lipid is always the fastest or only fuel. Oxygen availability, mobilization, enzyme capacity and activity intensity determine substrate use.

Photosynthesis captures light energy and stores it as chemical energy in organic molecules.

Pigments absorb photons and excite electrons; electron transfer creates ATP and reduced NADP, which then drive carbon fixation. The process depends on light, pigments, membranes, water, carbon dioxide and suitable conditions.

Trace the energy conversion:

• photon absorbed by pigment
• electron transfer and proton gradient
• ATP and reduced NADP formed
• carbon compounds synthesized

A leaf exposed to light can use ATP and reduced NADP from the thylakoid reactions to build carbohydrate in the stroma.

Light provides energy but is not itself converted directly into glucose molecules.

The overall photosynthesis equation shows carbon dioxide and water converted to carbohydrate using light energy: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon dioxide supplies the carbon skeleton and is reduced using hydrogen derived from water. Oxygen is released when water is split by photolysis in the light-dependent reactions, not directly from CO₂. Glucose represents a carbohydrate product; the products can be used to make starch, sucrose and other organic compounds.

• Word equation: carbon dioxide + water → glucose + oxygen, using light energy.
• Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• O₂ comes from photolysis of H₂O.
• Plants, algae and cyanobacteria perform oxygenic photosynthesis.

Photosynthetic pigments can be separated and identified because they differ in solubility in the mobile solvent and attraction to the stationary phase.

Extract pigments, place a small concentrated spot on paper or a thin-layer plate, keep the spot above the solvent, allow the solvent front to rise, then mark the solvent front immediately and observe the separated bands.

If a pigment travels 4.2 cm while the solvent front travels 6.0 cm, Rf = 4.2 ÷ 6.0 = 0.70. Identify a pigment using both its colour and an Rf reference obtained with the same solvent and stationary phase.

Rf has no unit and normally lies between 0 and 1. It depends on the solvent and stationary phase, so an Rf from different conditions is not a secure identification; band distance alone is not pigment abundance.

An absorption spectrum shows the proportion of each wavelength absorbed by a pigment; an action spectrum shows the measured rate or effectiveness of photosynthesis at each wavelength.

Only photons with suitable energies are absorbed and excite electrons in pigment molecules. Chlorophylls and accessory pigments absorb different wavelength ranges, so their combined absorption helps explain the action spectrum.

Plot wavelength in nanometres, with the corresponding light colours, on the horizontal axis. For an action spectrum, calculate photosynthesis rate from oxygen production or carbon-dioxide consumption at each wavelength and plot rate on the vertical axis.

If oxygen production is greatest in blue and red light and lowest in green light, the action spectrum has blue and red peaks that broadly match pigment absorption; accessory pigments can make the curves differ.

Absorption and photosynthesis rate are different dependent variables. Do not label an action-spectrum y-axis as absorbance, and compare rates only when intensity and other limiting factors are controlled.

At any moment, the limiting factor is the condition in shortest effective supply relative to photosynthetic demand; increasing it raises rate until another factor becomes limiting.

State a testable hypothesis, vary one independent variable—carbon-dioxide concentration, light intensity or temperature—and measure photosynthesis rate as the dependent variable. Control the other two, plant material, time and measurement conditions; repeat measurements.

Vary light with lamp distance or a light meter, CO₂ with known hydrogencarbonate concentrations or gas control, and temperature with a thermostatically controlled water bath. Measure an initial oxygen-production or CO₂-consumption rate.

A rate rising with light intensity and then reaching a plateau supports the hypothesis that light was initially limiting; at the plateau, CO₂ concentration, temperature or biochemical capacity may limit instead.

A hypothesis is provisional and needs repeated testing. A plateau does not mean photosynthesis has stopped, and moving a lamp can also change temperature unless heat is controlled.

Carbon-dioxide enrichment experiments test how higher atmospheric CO₂ may alter photosynthesis and plant growth, but responses depend on other limiting factors.

Compare enriched and ambient-CO₂ treatments with replication. Record photosynthetic rate and longer-term growth or biomass while monitoring controlled variables such as light, temperature, water, nutrients and plant age.

A crop may show greater CO₂ uptake under enrichment when light and nutrients are sufficient, but little additional biomass under shade or nutrient limitation. This conditional response improves predictions of future growth.

Higher CO₂ does not guarantee a proportional or permanent rise in photosynthesis or yield. Greenhouse results cannot be transferred to natural ecosystems without considering enclosure effects and field interactions.

A photosystem is a membrane-bound molecular array of chlorophyll and accessory pigments surrounding a special reaction-centre chlorophyll.

Different antenna pigments absorb different wavelengths and transfer excitation energy toward the reaction centre. The reaction-centre chlorophyll emits an excited electron to an electron acceptor, initiating electron flow.

Photosystems occur in chloroplast thylakoid membranes and photosynthetic membranes of cyanobacteria. The structured array broadens light capture and funnels energy efficiently to one reaction centre.

A wavelength weakly absorbed by the reaction-centre chlorophyll can still drive electron emission when an accessory pigment absorbs it and transfers the excitation energy through the array.

A single isolated chlorophyll or accessory-pigment molecule cannot perform any part of photosynthesis by itself; the organized pigment–protein membrane system is essential.

In photosystem II, light-driven photolysis splits water; later, photosystem I supplies excited electrons that reduce NADP on the stromal side of the thylakoid.

Photolysis replaces electrons lost by photosystem II. Its protons contribute to the thylakoid proton gradient and its oxygen is released as waste; oxygen generation profoundly changed Earth's living and geological systems.

At photosystem I, NADP accepts two excited electrons originating from photosystem I and one H⁺ from the stroma to form reduced NADP (NADPH), which carries reducing power to the Calvin cycle.

Use one consistent pair of names: NADP/reduced NADP or NADP⁺/NADPH. The released O₂ comes from water, not carbon dioxide, and photolysis does not directly make glucose.

The thylakoid is an integrated membrane system that positions photosystems, electron carriers and ATP synthase for the light-dependent reactions.

Electron transfer releases energy that pumps H⁺ from the stroma into the thylakoid lumen. H⁺ then moves down its electrochemical gradient through ATP synthase, coupling that flow to ADP + Pi → ATP in the stroma.

Photolysis occurs at photosystem II, ATP synthesis across the membrane, and NADP reduction at photosystem I on the stromal side. Non-cyclic electron flow begins with electrons from photosystem II and makes ATP plus reduced NADP; cyclic flow returns electrons from photosystem I and supports ATP production without reducing NADP.

The membrane separates the H⁺-rich lumen from the stroma, so electron-carrier proton pumping stores energy as a gradient and ATP synthase converts that gradient energy into chemical energy in ATP.

Electron carriers build the gradient; ATP synthase uses it. Chemiosmosis is proton movement through ATP synthase, not electron passage through the enzyme.

Rubisco catalyses carbon fixation in the chloroplast stroma by adding CO₂ to the five-carbon acceptor RuBP.

The unstable six-carbon product immediately divides into two molecules of glycerate 3-phosphate (GP). This is carbon's entry into the Calvin cycle.

Substrates: CO₂ and RuBP; enzyme: Rubisco; product: two GP molecules. Rubisco is the most abundant enzyme on Earth, and plants maintain high stromal concentrations because it works relatively slowly and is ineffective at low CO₂ concentrations.

Fixing three CO₂ molecules to three RuBP molecules produces six GP molecules before their reduction to triose phosphate.

Rubisco does not directly produce glucose or triose phosphate. It catalyses the fixation step that produces GP, which is modified in later Calvin-cycle reactions.

Glycerate 3-phosphate (GP) is converted to triose phosphate (TP) using ATP and reduced NADP; most TP is then recycled to regenerate RuBP using ATP.

ATP supplies energy and reduced NADP supplies hydrogen/electrons for GP reduction. Regenerating the five-carbon CO₂ acceptor RuBP allows carbon fixation to continue.

For every six TP produced, five TP molecules are rearranged using ATP to form three RuBP molecules. Thus five-sixths of TP is recycled when glucose is treated as the photosynthesis product; one-sixth is available for product synthesis.

Three fixed CO₂ yield six TP after reduction: five TP regenerate three RuBP, while the carbon from one TP is the net gain. Two such net TP gains can supply the six carbons needed for one glucose.

The individual regeneration reactions are not required. Do not imply that each turn releases glucose or that all TP leaves the cycle.

The Calvin cycle supplies carbon intermediates for carbohydrates, amino acids and other carbon compounds, while depending on ATP and reduced NADP from the light-dependent reactions.

All carbon in compounds made by a photosynthesizing organism is first fixed through the Calvin cycle. TP and other intermediates enter metabolic pathways; mineral nutrients supply elements such as nitrogen needed for amino acids and other products.

The light-dependent reactions supply ATP and reduced NADP; the Calvin cycle returns ADP, phosphate and NADP. Without light, ATP and reduced NADP production stops. Without CO₂, carbon fixation stops and the coupled recycling of electron acceptors is disrupted, eventually preventing photosystem II from functioning.

Some TP contributes to sucrose or starch, while carbon skeletons combined with mineral nitrogen can form amino acids. Most TP remains in the cycle to regenerate RuBP.

‘Light-independent’ means light is not absorbed directly by these reactions; it does not mean they can continue indefinitely without light-reaction products. Glucose is not formed in one Calvin-cycle step.

A receptor is a protein with a binding site specific to a signalling chemical, called a ligand.

Complementary shape and chemical interactions allow a ligand to bind selectively. Binding changes the receptor's state or conformation and initiates a sequence of responses inside the target cell.

Identify four linked parts: ligand → specific receptor binding site → receptor activation → intracellular response. A cell is a target only if it has the appropriate receptor.

Insulin can circulate past many cells, but only cells expressing an insulin receptor bind it and initiate the corresponding signal-transduction pathway.

Specific does not mean that a receptor completes the response by itself. It detects its ligand and starts a pathway; downstream cell machinery produces the response.

Quorum sensing allows bacteria to coordinate gene expression by releasing and detecting signalling molecules whose concentration indicates local population density.

Each cell releases an autoinducer. At low density it diffuses away, but as population density rises its local concentration increases; once a threshold is reached, receptor activation switches on coordinated genes.

Sequence: autoinducer release → concentration rises with cell density → threshold detection → coordinated gene expression across the bacterial population.

The marine bacterium Vibrio fischeri produces bioluminescence only when a dense population in its host's light organ accumulates enough autoinducer to cross the quorum threshold.

Quorum sensing estimates local density through chemical concentration; it is not nervous communication. Dilution or flow can prevent threshold activation even when bacteria are present.

Animal chemical signals differ in source, distance, speed and target-cell effect. Hormones are released by endocrine cells into blood and act at distant targets, usually more slowly. Neurotransmitters are released by neurons across synapses and act rapidly on nearby target cells. Cytokines are released by immune and other cells and coordinate local or systemic responses. Ca²⁺ ions act as short-lived intracellular signals that regulate target proteins.

• Hormone: endocrine source; bloodstream; distant target; relatively slow and longer-lasting.
• Neurotransmitter: neuron; synaptic cleft; adjacent target; rapid response.
• Cytokine: immune or other cell; local or body-fluid signalling; coordinates immune responses.
• Ca²⁺: intracellular messenger; changes protein activity inside the responding cell.
• Hormones can be protein/peptide, steroid or amine; neurotransmitters include acetylcholine, amino acids, peptides, amines and nitric oxide.

A localized signal acts near its source, whereas an endocrine signal travels through circulation to distant target cells.

Paracrine and synaptic signals reach nearby cells with short delays and limited dilution. Endocrine hormones can reach the whole body but only cells with the correct receptor respond.

Locate the signal’s range:

• autocrine: same cell
• paracrine: nearby cells
• synaptic: across a synapse
• endocrine: distant through blood

A neurotransmitter acts across a tiny synaptic gap, while thyroxine circulates and affects distant tissues with thyroid-hormone receptors.

Circulation makes a signal widespread, not automatically effective in every tissue.

Whether a signalling ligand crosses the phospholipid bilayer determines whether it binds a transmembrane receptor or an intracellular receptor.

Oestradiol, progesterone and testosterone cross the membrane and bind intracellular receptors. The activated hormone–receptor complex binds specific DNA sequences and promotes transcription of target genes.

Receptor location follows ligand permeability and protein structure, not simply whether the signal is called a hormone. Protein hormones generally require surface receptors; steroid hormones use intracellular receptors.

Signal transduction is the chain of molecular events linking receptor activation to a change in cell behaviour.

The chain may include conformational changes, G proteins, kinases, second messengers, transcription factors and feedback. Amplification allows a few signal molecules to influence many targets.

Map a pathway:

• receptor state changes
• relay or second messenger activates
• target proteins or genes change
• response is terminated

One activated receptor can stimulate many downstream enzymes, amplifying a small external signal into a measurable metabolic response.

A pathway is not complete until the signal is switched off or degraded; otherwise the cell cannot respond accurately.

Acetylcholine binds a transmembrane receptor that is also a ligand-gated ion channel in the postsynaptic membrane.

Ligand binding changes the receptor's conformation and opens the channel. Positively charged ions diffuse into the postsynaptic cell down their electrochemical gradient, making the inside less negative and changing membrane potential.

Sequence: acetylcholine binds → receptor channel opens → positive ions enter → postsynaptic membrane depolarizes → the voltage change may trigger further events.

At a cholinergic synapse, opening acetylcholine-receptor channels produces a local depolarization; if combined depolarization reaches threshold, a postsynaptic action potential can follow.

Acetylcholine does not itself cross the membrane or carry charge into the cell. It opens a receptor channel; ion diffusion causes the voltage change.

A G protein-coupled receptor (GPCR) is a transmembrane receptor that conveys ligand binding into the cell by activating a membrane-associated G protein.

Ligand binding changes the receptor's conformation. The activated receptor promotes exchange of GDP for GTP on the G protein; an activated G-protein subunit then regulates an effector protein such as an enzyme or ion channel.

Many human receptors use this mechanism. Sequence: ligand binds GPCR → GDP is replaced by GTP on G protein → active subunit changes an effector → intracellular response follows.

A G-protein subunit can activate adenylyl cyclase, which generates the second messenger cAMP and thereby relays the external signal to intracellular targets.

The GPCR relays the signal without entering the cell. GTP is a switch on the G protein; it is not the extracellular ligand or the second messenger cAMP.

Adrenaline binds a GPCR and activates a cAMP pathway that rapidly promotes glycogen breakdown in target cells.

The receptor activates a stimulatory G protein, adenylyl cyclase makes cAMP, and a kinase cascade activates glycogen phosphorylase. Phosphodiesterase and receptor reset limit the response.

Follow the sequence:

• adrenaline binds receptor
• G protein activates adenylyl cyclase
• cAMP activates kinase
• glycogen becomes glucose units

One adrenaline molecule can trigger many cAMP molecules and therefore amplify glycogen breakdown during exercise.

Adrenaline does not directly split glycogen at the receptor; the kinase cascade mediates the response.

The insulin receptor is a transmembrane receptor with tyrosine kinase activity; insulin binding outside the cell causes phosphorylation of tyrosine residues inside the cell.

The phosphorylated receptor initiates a sequence of intracellular reactions. The syllabus endpoint is movement of vesicles containing glucose transporters to the plasma membrane.

Sequence: insulin binds receptor → intracellular tyrosines are phosphorylated → signalling reactions are activated → glucose-transporter vesicles move to and fuse with the plasma membrane → glucose uptake capacity increases.

In an insulin-responsive muscle or adipose cell, insertion of additional GLUT4 transporters increases facilitated diffusion of glucose from blood into the cell.

Insulin does not enter the cell or act as a glucose transporter. Keep this objective to receptor binding, tyrosine phosphorylation and transporter-vesicle movement; a Ras–MAP kinase growth-factor pathway is not the required example.

Oestradiol and progesterone are steroid hormones whose effects depend on intracellular receptors in specific target cells.

For oestradiol, the required target is receptor-containing cells in the hypothalamus that secrete gonadotropin-releasing hormone (GnRH). Oestradiol changes gene expression and thereby influences GnRH secretion.

For progesterone, the required target is receptor-containing cells in the endometrium. Progesterone-dependent gene expression maintains the endometrium in a state that can support pregnancy.

The same circulating hormone affects only cells with the matching receptor: hypothalamic GnRH-secreting cells respond to oestradiol, whereas endometrial cells are a specified target of progesterone.

Do not generalize these objectives to every reproductive effect of the hormones. The assessed target-cell examples are oestradiol in GnRH-secreting hypothalamic cells and progesterone in endometrium.

Feedback occurs when a response alters the signal pathway, either reducing further activation or amplifying it under defined conditions.

Negative feedback restores a variable toward a set range; positive feedback reinforces a change until a limiting event stops it. Receptors, hormones and downstream products can all participate.

Classify the loop:

• response opposes the original change: negative
• response reinforces it: positive
• identify the measured variable
• specify the stopping condition

Rising thyroid hormone suppresses hypothalamic and pituitary stimulation, whereas oxytocin-driven contractions can intensify until birth.

Calling any delayed response feedback is incomplete; the direction of the loop must be identified.

A neuron is a specialized cell of the nervous system that conducts electrical impulses along elongated nerve fibres.

The cell body contains cytoplasm and the nucleus. Multiple shorter dendrites usually conduct impulses toward the cell body, while one long axon conducts impulses away toward other neurons or effector cells.

Identify the nucleus and cytoplasm in the cell body, then distinguish the long single axon from the multiple shorter dendrites. Fibre length and branching vary with neuron function.

A motor neuron's long axon can carry an impulse from the central nervous system to a distant muscle, while its dendrites receive inputs from other neurons.

An axon and dendrites are cellular projections, not separate cells. Neuron shape varies, so use fibre number, length and direction of impulse rather than assuming every drawing has the same layout.

The resting potential is the negative voltage across a resting neuron's polarized plasma membrane, maintained by sodium and potassium ion gradients.

ATP supplies energy to the sodium–potassium pump, which moves Na⁺ out and K⁺ in opposite directions against their gradients. The membrane is more permeable to K⁺ at rest, so more positive charge diffuses out than enters and the inside remains negative relative to outside.

ATP-driven pumping establishes and maintains high Na⁺ outside and high K⁺ inside; selective leak channels create unequal ion movement; separated charge produces membrane polarization and a measurable membrane potential.

If ATP production stops, the pump can no longer maintain the gradients. Existing gradients may persist briefly, but continued leakage gradually erodes the negative resting potential.

The pump maintains the gradients over time; it does not directly create each action potential. A concentration gradient and a voltage gradient both influence ion movement.

An action potential is a brief, threshold-triggered reversal of membrane potential that propagates along an axon.

Voltage-gated sodium channels open rapidly during depolarization, then inactivate while potassium channels open to repolarize the membrane. Refractory periods prevent immediate re-firing in the same segment. The threshold is a condition, not a fixed promise: if it is not reached, the full action potential does not start.

Trace the phases:

• threshold
• Na⁺ influx and depolarization
• K⁺ efflux and repolarization
• hyperpolarization and recovery

A stimulus below threshold produces no full impulse, while a threshold stimulus produces an impulse with a similar amplitude.

Stronger stimuli are coded mainly by impulse frequency, not by a larger action-potential amplitude.

Impulse conduction speed increases with axon diameter and myelination because both allow local currents to influence the next excitable membrane region more rapidly.

Describe a positive correlation when conduction speed rises as axon diameter rises, and a negative correlation when one variable falls as the other rises. The correlation coefficient r gives direction and strength; R² estimates the proportion of variation in conduction speed explained by the fitted relationship.

If r = 0.90 for diameter and speed, the association is strong and positive; R² = 0.81 means about 81% of the observed variation in speed is explained by the fitted relationship, not that diameter is proven to be the sole cause.

Correlation does not by itself prove causation. Faster impulses do not have larger action-potential amplitudes; speed and signal strength are different properties.

A synapse is a junction where a presynaptic cell communicates with a postsynaptic cell, usually through neurotransmitter release.

The synaptic cleft separates membranes, creating a one-way delay in chemical synapses. Receptors on the postsynaptic membrane convert transmitter binding into ion-flow or intracellular effects. The synapse is a junction, so the signal must be converted before it can cross to the next cell.

Describe a chemical synapse:

• presynaptic terminal
• vesicles and cleft
• postsynaptic receptors
• signal termination

An action potential at a motor neuron terminal releases transmitter that binds receptors on a muscle fibre.

A synapse is not simply a physical gap; receptor type and transmitter removal determine the response.

Arrival of an action potential at a presynaptic terminal opens voltage-gated calcium channels and triggers vesicle fusion.

Ca²⁺ entry binds release machinery, causing exocytosis of neurotransmitter into the synaptic cleft. Enzymatic breakdown, reuptake or diffusion then terminates the signal. This one-way sequence explains why changing calcium entry can alter how strongly a synapse communicates.

Trace release:

• action potential arrives
• Ca²⁺ channels open
• vesicles fuse
• transmitter binds and is cleared

Blocking presynaptic calcium entry reduces transmitter release even if the action potential still reaches the terminal.

The electrical impulse triggers release; it does not cross the cleft as the same electrical current.

An excitatory postsynaptic potential (EPSP) is a graded depolarization that makes the postsynaptic neuron more likely to reach action-potential threshold.

Neurotransmitter released from the presynaptic terminal diffuses across the synaptic cleft and binds a transmembrane receptor. Acetylcholine can open ligand-gated channels, allowing net positive charge to enter and depolarize the postsynaptic membrane.

Sequence: transmitter release → diffusion across cleft → receptor binding → channel opening → local depolarization. Acetylcholine acts at many synapses, including neuromuscular junctions.

One acetylcholine input may produce a small EPSP below threshold; overlapping excitatory inputs can sum at the axon hillock until an action potential is triggered.

An EPSP is local and graded, not an all-or-none action potential. The neurotransmitter does not carry the electrical impulse across the cleft; receptor-controlled ion movement changes voltage.

Depolarization is the rising phase of an action potential as sodium conductance increases; repolarization returns the membrane toward negative values as potassium conductance rises.

Voltage-gated channels have distinct activation and inactivation timing. The overlap creates a refractory period and ensures the impulse moves forward rather than immediately backward. The changing voltage is evidence of ion movement across the membrane, not a flow of the whole neuron along its axon.

Read the voltage trace with channel states:

• Na⁺ activation: rising phase
• Na⁺ inactivation: peak
• K⁺ activation: falling phase
• delayed K⁺ closing: undershoot

A sodium-channel blocker removes the rapid rising phase, while a potassium-channel blocker prolongs repolarization.

The membrane does not become permanently positive; ion gradients and channel timing restore the resting state.

An action potential propagates along an axon when local currents from one depolarized region bring the adjacent membrane to threshold.

Na⁺ entering through open voltage-gated channels diffuses sideways inside the axon toward the next resting region. Compensating positive charge moves in the extracellular fluid, completing a local current circuit and depolarizing the next membrane segment to threshold.

The next segment opens its sodium channels and regenerates the action potential. The refractory region behind cannot immediately reopen sodium channels, so propagation normally proceeds forward.

Each newly depolarized segment supplies local current to the next segment, so a full-amplitude action potential is regenerated rather than gradually fading along the axon.

The action potential is not a single group of ions travelling the full axon length. Ions move short distances locally while sequential channel opening propagates the voltage change.

An oscilloscope trace displays voltage against time, allowing action-potential threshold, amplitude, duration and frequency to be compared.

The vertical scale reports membrane potential and the horizontal scale reports time. Repeated spikes indicate frequency; baseline shifts or noise must be distinguished from true action potentials. Reading the trace means linking each voltage change to a stage of channel opening or closing.

Read a trace in order:

• calibrate axes
• locate threshold and peak
• measure duration or interval
• calculate frequency from repetition

If five spikes occur in 0.1 seconds, the firing frequency is 50 Hz, not an amplitude of five units.

A taller trace is not automatically a stronger stimulus; check what the axis represents.

Saltatory conduction is rapid propagation in a myelinated fibre in which action potentials are regenerated from one node of Ranvier to the next.

Myelin insulates internodes and reduces current loss. Ion channels and pumps are clustered at the exposed nodes, so local current spreads quickly beneath myelin and brings the next node to threshold.

Internode: insulated rapid current spread. Node: concentrated voltage-gated channels generate the next action potential and pumps maintain the ion gradients. Fewer membrane regions need direct ion exchange.

Demyelination increases current leakage, so the next node may reach threshold later or not at all, slowing or blocking the impulse.

The impulse appears to jump, but electrical current still spreads through the axon between nodes. Action potentials are regenerated only at nodes rather than travelling through empty space.

Exogenous chemicals can alter synaptic transmission by acting on receptors or by changing how long neurotransmitter remains in the synaptic cleft.

If reuptake is blocked, a released neurotransmitter is cleared more slowly, so receptor stimulation continues after it would normally have ended.

The two examples act differently: neonicotinoids act at receptor sites, whereas cocaine blocks neurotransmitter reuptake. Do not infer a single behavioural outcome without considering dose, receptor distribution and organism.

Inhibitory neurotransmitters make a postsynaptic neuron less likely to reach action-potential threshold.

They may open chloride channels, increase potassium efflux or reduce excitatory channel activity. The resulting hyperpolarization or shunting inhibition opposes excitatory inputs. Inhibition is relational: the same transmitter can have different effects if receptor types differ, and the receiving circuit determines the outcome.

Identify inhibition by checking:

• receptor and ion channel
• current direction or conductance
• membrane-potential effect
• threshold probability

Opening chloride channels can stabilize the membrane near its resting potential and reduce the effect of a simultaneous EPSP.

Inhibition is not always a large negative voltage; increased conductance can also shunt excitatory current.

Summation combines graded postsynaptic potentials across space and time at the axon hillock.

Spatial summation adds inputs from different synapses; temporal summation adds repeated inputs from one synapse. The net potential reaches threshold only if excitation outweighs inhibition. The result depends on timing as well as size, because inputs that overlap can combine before the threshold decision.

Separate the two forms:

• spatial: different locations
• temporal: different times
• EPSPs add
• IPSPs subtract or shunt

Two small EPSPs arriving together at the axon hillock can reach threshold even though neither would trigger an action potential alone.

The neuron integrates potentials, not neurotransmitter names; receptor effects determine the sign and size.

Pain perception can begin at free sensory nerve endings in the skin that act as nociceptors for potentially damaging stimuli.

High temperature, acid or chemicals such as capsaicin open stimulus-sensitive channels for positively charged ions. Ion entry depolarizes the ending; if threshold is reached, action potentials travel along the sensory neuron to the brain, where pain is perceived.

Sequence: damaging stimulus → positive-ion channels open in free nerve ending → depolarization reaches threshold → impulses propagate to brain → conscious pain perception.

Capsaicin in chilli peppers activates ion channels in free nerve endings, producing depolarization and impulses that the brain interprets as burning pain even without high temperature.

Nociceptor activation is the neural input to pain, not pain perception itself. Conscious pain emerges after processing in the brain.

Consciousness is associated with coordinated activity across interacting brain networks rather than one isolated neuron or single region.

Arousal systems regulate wakefulness while cortical and subcortical networks integrate sensory information, memory and self-related processing. Evidence from sleep, anaesthesia and brain injury links changes in network connectivity to conscious state. Conscious awareness is therefore an emergent interpretation of coordinated neural activity, not a single switch in one neuron.

Separate the components:

• arousal and wakefulness
• information integration
• communication among brain regions
• reportable experience

During deep anaesthesia, widespread connectivity and coordinated responses fall, while local sensory circuits may still respond to stimulation.

A neural response alone does not prove a conscious experience; integration and reportability require additional evidence.

Integration joins specialised parts into a working whole.

In a multicellular organism, tissues, organs and organ systems exchange information and materials so the organism can coordinate one response. Nervous impulses are fast; hormones travel in blood and can act on distant targets.

Name the parts; identify the signal or material crossing between them; then state the coordinated outcome.

During exercise, receptors and the brain increase heart and ventilation activity while blood delivers oxygen to working muscle.

Integration is more than a list of organs: the explanation must include communication and a shared function.

Organisation builds new properties at each level.

Cells of one type form tissues, different tissues form organs, and organs form systems. Interactions between parts can produce an emergent capability that no isolated cell performs alone.

cell → tissue → organ → system → organism; at each step ask what cooperation adds.

Cardiac muscle, connective tissue and blood vessels combine in a heart that pumps blood; no one tissue performs the whole job.

Hierarchy is not just size ranking; it is a functional relationship between levels.

Animal organs are integrated by nervous and hormonal messages and by blood transport of materials and energy between organs.

During exercise, motor nerves stimulate skeletal muscles, epinephrine coordinates responses in several organs, and blood carries oxygen and glucose to muscle while transporting carbon dioxide away.

Blood is both a transport route for hormones and a carrier of non-message materials. Integration requires source, route, target and coordinated outcome—not just a list of organs.

The brain is a central integration organ that combines information from several inputs before coordinating responses.

Current sensory inputs can be compared with stored information. Changes in neural connections support learning, while stored and retrievable information supports memory; both influence how later inputs are interpreted and acted on.

Trace input from several receptors → combined processing in the brain → comparison with learned or remembered information → coordinated output. Detailed slow-acting neurotransmitter mechanisms are not required.

A learner sees a traffic signal, hears an approaching vehicle and recalls the crossing rule; the brain combines these inputs and memory before coordinating whether to step forward or wait.

Integration is not simple forwarding of one sensory message. The objective concerns combining inputs plus learning and memory, not memorizing detailed functions of many brain regions.

The spinal cord can integrate a rapid response.

The spinal cord links body and brain, but grey matter can connect sensory input to motor output without waiting for conscious brain processing. White matter carries impulses along longer pathways.

receptor → sensory neurone → spinal integration → motor neurone → effector.

Touching a hot surface triggers a withdrawal through a spinal reflex; the brain receives information as the movement is already beginning.

A reflex is not brain-free information: the spinal cord handles the immediate response, while the brain can receive the signal afterwards.

Sensory neurones carry transduced information to the CNS.

A receptor detects a stimulus and converts its energy into a change in membrane potential. If threshold is reached, an action potential travels along a sensory neurone to the CNS.

identify stimulus; receptor type; transduction; threshold; direction to CNS.

Salt ions entering taste-receptor channels depolarise the receptor cell, which then excites a sensory neurone carrying the signal to the brain.

A stronger stimulus is not encoded by a larger action potential; it can increase firing frequency or recruit receptors.

Motor neurones turn CNS decisions into muscle action.

Motor neurones carry action potentials from the CNS to an effector. At a neuromuscular junction, acetylcholine depolarises the muscle membrane and starts the calcium-controlled contraction sequence.

CNS signal → motor axon → acetylcholine release → muscle action potential → calcium release.

An impulse at a motor end plate opens ion channels; the resulting muscle action potential travels along the fibre and triggers calcium release from the sarcoplasmic reticulum.

The motor neurone does not contract the muscle directly; transmitter and membrane events link the two cells.