What is negative feedback and why is it central to Module 5?

Negative feedback is a control mechanism in which a deviation from a set point is detected by receptors and triggers a response, via a coordinator and effectors, that reverses the change and returns the factor towards normal. It is the principle behind almost every control system in this module: osmoregulation by ADH, blood glucose control by insulin and glucagon, and temperature control all use negative feedback. Recognising the receptor, coordinator, effector and corrective response in any scenario is the key skill the examiners test.

What is the difference between depolarisation and repolarisation in a neurone?

Depolarisation is the rapid change that makes the inside of the axon positive: voltage-gated sodium channels open and sodium ions diffuse in. Repolarisation restores the negative inside: the sodium channels close and voltage-gated potassium channels open, so potassium ions diffuse out. The resting potential (inside negative, about minus 70 millivolts) is maintained by the sodium-potassium pump, which moves three sodium ions out for every two potassium ions in. The action potential is all-or-nothing, and a stronger stimulus increases its frequency, not its size.

How is blood glucose concentration controlled?

By negative feedback through the islets of Langerhans in the pancreas. When blood glucose rises, beta cells release insulin, which increases glucose uptake by cells and its conversion to glycogen (glycogenesis), lowering glucose. When blood glucose falls, alpha cells release glucagon, which stimulates the breakdown of glycogen to glucose (glycogenolysis) and the formation of glucose from non-carbohydrates (gluconeogenesis), raising glucose. Adrenaline also raises glucose. Glucagon and adrenaline act through the second messenger model using cyclic AMP.

What are the two stages of photosynthesis and where do they happen?

The light-dependent stage happens in the thylakoid membranes: light excites electrons from chlorophyll, an electron transport chain pumps protons that drive ATP synthase (photophosphorylation), photolysis of water replaces the electrons and releases oxygen, and NADP is reduced. The light-independent stage (the Calvin cycle) happens in the stroma: carbon dioxide combines with RuBP to form GP, which is reduced to TP using the ATP and reduced NADP from the first stage, and most TP regenerates RuBP. The rate is set by limiting factors: light, carbon dioxide and temperature.

Why does oxidative phosphorylation make most of the ATP in respiration?

In oxidative phosphorylation the reduced NAD and reduced FAD made in glycolysis, the link reaction and the Krebs cycle are oxidised, and their electrons pass along the electron transport chain on the inner mitochondrial membrane. The energy released pumps protons into the intermembrane space, building a gradient, and the protons flow back through ATP synthase to make ATP by chemiosmosis. Because each reduced NAD and FAD yields several ATP this way, far more ATP is produced here than by the direct (substrate-level) phosphorylation in the earlier stages. Oxygen is the final electron acceptor, forming water.

EnglandBiology

OCR A-Level Biology Module 5 Communication, homeostasis and energy: nerves, hormones, the kidney, photosynthesis and respiration

A deep-dive OCR A-Level Biology guide to Module 5 Communication, homeostasis and energy. Covers homeostasis and the kidney, neuronal communication, hormonal communication and blood glucose, plant and animal responses, photosynthesis and respiration, with the exam patterns OCR repeats.

Generated by Claude Opus 4.822 min readOCR-H420-Module-5Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What Module 5 actually demands
Homeostasis and the kidney
Neuronal communication
Hormonal communication and blood glucose
Plant and animal responses
Photosynthesis and respiration
How Module 5 is examined
Check your knowledge

What Module 5 actually demands

Communication, homeostasis and energy is the largest content module in OCR A-Level Biology A, assessed mainly in Paper 1. It runs from how organisms detect and respond to change, through how the internal environment is kept stable, to how cells trap and release energy. You must master nervous and hormonal coordination, homeostasis and the kidney, plant and animal responses and muscle contraction, and the biochemistry of photosynthesis and respiration. The examiners reward precise process sequences (the action potential, the Calvin cycle, oxidative phosphorylation) and the ability to apply negative feedback and limiting-factor reasoning to data.

This guide ties together the six dot-point pages for Module 5 and sets out the exam patterns OCR repeats.

Homeostasis and the kidney

Homeostasis is the maintenance of a stable internal environment by negative feedback. The liver deaminates excess amino acids (forming urea) and detoxifies substances such as alcohol. In the nephron, ultrafiltration in the glomerulus uses high hydrostatic pressure to force small molecules through the basement membrane and podocytes, leaving cells and proteins behind; selective reabsorption in the proximal convoluted tubule recovers all glucose and amino acids by active transport, with water following by osmosis. Osmoregulation uses ADH: a fall in blood water potential is detected by hypothalamic osmoreceptors, more ADH makes the collecting duct permeable (more aquaporins), more water is reabsorbed, and concentrated urine forms.

Neuronal communication

The resting potential (inside negative, maintained by the sodium-potassium pump) is reversed by an action potential: at threshold, sodium channels open and sodium diffuses in (depolarisation), then potassium channels open and potassium diffuses out (repolarisation). Action potentials are all-or-nothing (a stronger stimulus raises their frequency), and the refractory period makes them discrete and one-way. Myelin gives saltatory conduction. At a cholinergic synapse, calcium entry triggers acetylcholine release, which depolarises the next neurone, and acetylcholinesterase breaks it down.

Hormonal communication and blood glucose

Hormonal coordination is slower, longer-lasting and more widespread than nervous. The pancreatic islets control blood glucose by negative feedback: insulin (beta cells) lowers glucose by uptake and glycogenesis; glucagon (alpha cells) raises it by glycogenolysis and gluconeogenesis. Glucagon and adrenaline use the second messenger model (cAMP from adenylyl cyclase, activating protein kinase). Type 1 diabetes is a failure to make insulin; type 2 is a loss of response to it.

Plant and animal responses

Auxin made at the shoot tip moves to the shaded side, promoting elongation there so the shoot bends towards light (positive phototropism). The mammalian nervous system divides into CNS and PNS, with somatic and autonomic (sympathetic and parasympathetic) motor outputs; the reflex arc gives fast protective responses and fight-or-flight uses the sympathetic system and adrenaline. Muscle contracts by the sliding filament mechanism: calcium exposes binding sites, myosin heads pull actin in (the power stroke), and ATP detaches them to repeat.

Photosynthesis and respiration

Photosynthesis has a light-dependent stage (photolysis, photophosphorylation, reduced NADP, oxygen released) in the thylakoids and the Calvin cycle (RuBP plus carbon dioxide to GP to TP, regenerating RuBP) in the stroma, limited by light, carbon dioxide and temperature. Respiration has four stages: glycolysis (cytoplasm), the link reaction and Krebs cycle (matrix), and oxidative phosphorylation (inner membrane), where the electron transport chain and chemiosmosis make most of the ATP, with oxygen as the final electron acceptor. Without oxygen, only glycolysis runs, regenerating NAD via lactate (animals) or ethanol (yeast).

How Module 5 is examined

A typical OCR profile for Communication, homeostasis and energy:

Multiple choice and short answer. Naming ion movements in an action potential, ordering the stages of respiration, matching a hormone to its effect.
Maths and graphs. Reading an action-potential trace, a glucose tolerance test, an oxygen dissociation or limiting-factor graph, and Calvin-cycle intermediate changes.
Applied and data questions. Explaining glucose in the urine, the effect of removing carbon dioxide on RuBP and GP, or the effect of a synapse poison.
Level-of-Response extended answers. Osmoregulation by ADH, synaptic transmission, the light-dependent stage and oxidative phosphorylation are all predictable.

OCR Module 5 Communication, homeostasis and energy covers homeostasis and negative feedback (the kidney, ultrafiltration, selective reabsorption and osmoregulation by ADH), neuronal communication (resting and action potentials, saltatory conduction and synapses), hormonal communication and blood glucose control (insulin, glucagon, the second messenger model and diabetes), plant and animal responses (auxin and phototropism, the reflex arc, fight-or-flight and the sliding filament mechanism of muscle), photosynthesis (the light-dependent stage and the Calvin cycle, with limiting factors) and respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, with anaerobic pathways). Master the process sequences and apply negative feedback and limiting-factor reasoning to data.

Check your knowledge

A mix of recall and application questions covering the whole of Module 5. Attempt them under timed conditions, then check against the solutions.

Describe ultrafiltration in the glomerulus. (3 marks)
Explain how ADH restores the water potential of the blood after sweating. (4 marks)
Describe how an action potential is generated at a point on an axon. (4 marks)
Explain how insulin lowers the blood glucose concentration. (3 marks)
Explain how auxin causes a shoot to bend towards light. (3 marks)
Describe the sliding filament mechanism of muscle contraction. (4 marks)
Describe the light-dependent stage of photosynthesis. (4 marks)
Explain how oxidative phosphorylation produces ATP. (4 marks)

Solutions

Q1: Blood enters the glomerulus through a wide afferent arteriole and leaves through a narrower efferent arteriole, creating a high hydrostatic pressure. This forces water, glucose, ions and urea out through the basement membrane and the gaps between podocytes into the Bowman's capsule, forming the filtrate. Blood cells and large plasma proteins are too big to pass and remain in the blood.
Q2: Osmoreceptors in the hypothalamus detect the fall in blood water potential. The posterior pituitary releases more ADH, which makes the collecting duct more permeable to water by inserting more aquaporins. More water is reabsorbed by osmosis into the blood, so a smaller volume of more concentrated urine is produced and the water potential rises back to normal (negative feedback).
Q3: A stimulus depolarises the membrane to the threshold (about minus 55 millivolts), opening voltage-gated sodium channels so sodium ions diffuse in, making the inside positive (depolarisation). The sodium channels then close and voltage-gated potassium channels open, so potassium ions diffuse out, returning the inside to negative (repolarisation), before the sodium-potassium pump restores the resting potential.
Q4: Insulin binds to receptors on liver and muscle cells, causing more glucose channels (GLUT4) to be inserted so cells take up more glucose, and activating enzymes that convert glucose to glycogen (glycogenesis). It also increases the use of glucose in respiration, so the blood glucose concentration falls.
Q5: Auxin is made at the shoot tip and moves to the shaded side when light comes from one side. Auxin promotes cell elongation, so the shaded side elongates more than the lit side, and the shoot bends towards the light.
Q6: A nerve impulse releases calcium ions from the sarcoplasmic reticulum; calcium binds troponin, moving tropomyosin to expose the myosin-binding sites on actin. Myosin heads attach to actin (cross-bridges) and flex (the power stroke), pulling actin past myosin so the sarcomere shortens. ATP binds the myosin head, detaching it, and the cycle repeats.
Q7: Light is absorbed by chlorophyll in the thylakoid membranes, exciting electrons that pass along an electron transport chain; protons are pumped into the thylakoid space and flow back through ATP synthase, making ATP (photophosphorylation). Photolysis of water replaces the lost electrons, releasing oxygen and providing protons. At the end of the chain, NADP is reduced.
Q8: Reduced NAD and reduced FAD are oxidised, and their electrons pass along the electron transport chain on the inner mitochondrial membrane. The energy released pumps protons into the intermembrane space, building a gradient. The protons flow back through ATP synthase, driving the synthesis of ATP (chemiosmosis). Oxygen is the final electron acceptor, combining with electrons and protons to form water.