Negative feedback and thermoregulation

Homeostasis maintains a constant internal environment despite external change, so that enzymes work at their optimum and cells are not damaged osmotically.

Positive feedback does the opposite — it amplifies the change. It is rarer, but real: the opening of sodium channels during an action potential, and the release of oxytocin during labour.

Thermoregulation (an endotherm): the hypothalamus monitors blood temperature.

• Too hot — vasodilation of arterioles near the skin surface (more blood flows through the surface capillaries, so more heat is radiated), sweating (evaporation requires a large amount of latent heat), hairs lie flat, and metabolic rate falls.
• Too cold — vasoconstriction, shivering (involuntary muscle contraction releases heat from respiration), hairs erected to trap an insulating layer of air, and increased metabolic rate (thyroxine, adrenaline).

Ectotherms rely on behaviour (basking, sheltering) and have a lower, more variable metabolic rate — which means they need far less food, but are inactive when it is cold.

The liver and the kidney

The liver. Excess amino acids cannot be stored, so they are deaminated: the amine group is removed to form highly toxic ammonia (the remaining keto acid enters respiration). Ammonia is then converted to the far less toxic, more soluble urea in the ornithine cycle, and excreted by the kidney. Liver cells (hepatocytes) also detoxify alcohol (using alcohol dehydrogenase) and hydrogen peroxide (using catalase).

The kidney.

• Ultrafiltration — the afferent arteriole is wider than the efferent, so blood in the glomerulus is under high hydrostatic pressure. Water, glucose, salts and urea are forced through the fenestrated endothelium and the basement membrane (the true molecular sieve) and past the podocytes. Blood cells and plasma proteins are too large.
• Selective reabsorption — in the proximal convoluted tubule, all the glucose and most salts are reabsorbed by active transport and co-transport with Na⁺. The cells have microvilli and many mitochondria. Water follows by osmosis.
• Loop of Henlé — a counter-current multiplier: Na⁺ and Cl⁻ are actively pumped out of the ascending limb (which is impermeable to water), lowering the water potential of the medulla; water then leaves the descending limb by osmosis. A longer loop creates a lower water potential deep in the medulla and therefore a more concentrated urine.
• ADH — osmoreceptors in the hypothalamus detect a fall in blood water potential; the posterior pituitary releases ADH, which inserts aquaporins into the collecting duct walls. More water is reabsorbed, and a small volume of concentrated urine is produced.

Kidney failure: treated by haemodialysis (blood flows counter-current to dialysis fluid across a partially permeable membrane; the fluid has normal plasma concentrations of glucose and salts, so only the excess and the urea diffuse out) or by transplant. Urine tests: glucose indicates diabetes; protein indicates damage to the basement membrane; hCG indicates pregnancy.

The action potential and the synapse

Resting potential (−70 mV): the sodium-potassium pump moves 3 Na⁺ out for every 2 K⁺ in, using ATP, and the membrane is more permeable to K⁺ (leak channels), so K⁺ diffuses out. The inside is therefore negative — the axon is polarised.

• Depolarisation — a stimulus opens voltage-gated Na⁺ channels. If the threshold (−55 mV) is reached, positive feedback opens many more, and the potential rises to about +40 mV.
• Repolarisation — Na⁺ channels close and voltage-gated K⁺ channels open; K⁺ leaves; the potential falls, overshooting into hyperpolarisation before the pump restores the resting potential.
• Refractory period — no new action potential can be fired, which makes impulses discrete, unidirectional, and limits the maximum frequency.
• All-or-nothing — a stronger stimulus does not give a larger action potential; it gives a higher frequency of identical ones. Saltatory conduction between the nodes of Ranvier makes myelinated neurones far faster.

The cholinergic synapse: the impulse opens voltage-gated Ca²⁺ channels; Ca²⁺ enters and causes vesicles to fuse with the presynaptic membrane, releasing acetylcholine. ACh diffuses across the cleft and binds receptors on the postsynaptic membrane, opening Na⁺ channels and depolarising it. Acetylcholinesterase then hydrolyses the ACh so the response stops. Synapses allow summation (spatial and temporal), inhibition, and ensure one-way transmission.

Your turn — conduction speed

Blocking the synapse

Blood glucose, adrenaline and diabetes

Hormones travel in the blood and act only on cells with complementary receptors — slow, long-lasting and widespread, in contrast to the fast, brief, targeted nervous system.

The adrenal glands: the cortex secretes steroid hormones (cortisol, aldosterone); the medulla secretes adrenaline.

Blood glucose control by the islets of Langerhans:

• β cells → insulin when glucose is too high. Insulin binds receptors on liver and muscle cells, causing more glucose transporter (GLUT4) proteins to be inserted into the membrane and activating enzymes for glycogenesis. Glucose uptake and respiration increase; blood glucose falls.
• α cells → glucagon when glucose is too low. It triggers glycogenolysis (glycogen → glucose) and gluconeogenesis (glucose made from amino acids and glycerol) in the liver.
• Adrenaline also raises blood glucose, preparing the body for action.

The second messenger model: adrenaline or glucagon binds a receptor on the cell surface; this activates adenylyl cyclase, converting ATP into cyclic AMP; cAMP activates protein kinase, which triggers the breakdown of glycogen. The hormone itself never enters the cell.

How insulin secretion is actually triggered: high blood glucose → more respiration in the β cell → more ATP → ATP-sensitive K⁺ channels close → the cell depolarises → voltage-gated Ca²⁺ channels open → Ca²⁺ influx → vesicles of insulin fuse with the membrane and are released by exocytosis. Type 1 diabetes: β cells destroyed (autoimmune) → insulin injections. Type 2: receptors become unresponsive → diet, exercise and weight loss.

Plant responses and muscle contraction

Plant hormones: auxin (IAA) promotes cell elongation in shoots (so a shoot bends towards light, as IAA accumulates on the shaded side) and inhibits elongation in roots (so a root bends downwards); it also maintains apical dominance. Gibberellin triggers seed germination — it stimulates the production of amylase in the aleurone layer, which hydrolyses the starch store into maltose for the embryo — and causes stem elongation. Ethene ripens fruit; abscisic acid closes stomata under water stress and maintains dormancy; cytokinins delay leaf senescence.

Muscle contraction — the sliding filament mechanism:

• Depolarisation travels down the T-tubules to the sarcoplasmic reticulum, which releases Ca²⁺.
• Ca²⁺ binds troponin, changing its shape and moving tropomyosin aside to expose the myosin-binding sites on actin.
• Myosin heads form cross-bridges with actin and perform the power stroke, pulling the actin past the myosin; ADP is released.
• ATP binds the myosin head, which detaches; its hydrolysis (by ATPase, activated by Ca²⁺) recocks the head for the next cycle.

The sarcomere on contraction: the I band and H zone shorten and the Z lines move closer together, but the A band does not change — because the filaments slide rather than shorten. Creatine phosphate in the sarcoplasm rapidly regenerates ATP from ADP at the start of intense exercise.

Gibberellin and germination

Pigments, the light-dependent and light-independent reactions

Chromatography separates the photosynthetic pigments (chlorophyll a, chlorophyll b, carotene, xanthophyll). Each has a characteristic R_f value in a given solvent, which identifies it.

Light-dependent reaction (thylakoid membranes): light excites electrons in chlorophyll (photoionisation); they pass along an electron transport chain, and the energy released pumps H⁺ into the thylakoid space. H⁺ then diffuse back through ATP synthase — chemiosmosis — making ATP (photophosphorylation). Photolysis of water (2H₂O → 4H⁺ + 4e⁻ + O₂) replaces the lost electrons and releases oxygen. NADP is reduced at the end of the chain. Cyclic photophosphorylation involves photosystem I only and makes ATP alone.

Light-independent reaction (the Calvin cycle, in the stroma): rubisco catalyses the fixation of CO₂ onto RuBP (5C); the unstable 6C intermediate splits into two GP (3C); GP is reduced to TP using ATP and reduced NADP; five out of six TP molecules regenerate RuBP, and one in six leaves the cycle. Six turns and six CO₂ are needed per hexose.

Limiting factors — light intensity, CO₂ concentration, temperature. If light is suddenly removed, GP rises (it cannot be reduced without ATP and reduced NADP) while TP and RuBP fall. If CO₂ is removed, GP falls and RuBP rises. Reason it out from the cycle rather than memorising it.

Your turn — Rf value

The four stages, and the respiratory quotient

• Glycolysis (cytoplasm): glucose is phosphorylated using 2 ATP, split into 2 triose phosphate, and oxidised to 2 pyruvate, producing 4 ATP (net 2) and 2 reduced NAD.
• Link reaction (mitochondrial matrix): pyruvate is decarboxylated and dehydrogenated to form acetyl coenzyme A, releasing CO₂ and reduced NAD.
• Krebs cycle (matrix): per turn — 2 CO₂, 3 reduced NAD, 1 reduced FAD, 1 ATP. Two turns per glucose.
• Oxidative phosphorylation (inner membrane/cristae): reduced NAD and FAD are oxidised; electrons pass along the chain, pumping H⁺ into the intermembrane space; H⁺ return through ATP synthase (chemiosmosis). Oxygen is the final electron acceptor, forming water.

Anaerobic respiration exists to regenerate NAD so that glycolysis can continue: pyruvate → lactate in animals, or → ethanol + CO₂ in yeast and plants. The yield is only the 2 ATP from glycolysis.

Why lipids give more energy per gram: they contain proportionately more hydrogen atoms, so their oxidation produces more reduced NAD and FAD, and therefore more ATP through oxidative phosphorylation. But they consume more oxygen per unit of CO₂ released — which is exactly why their RQ is low. A respirometer measures oxygen uptake with soda lime absorbing the CO₂ produced.

Your turn — respiratory quotient

Interpreting an RQ

Concentrating the urine

Which process?

Tap a statement, then tap the process it belongs to.

Hormone and effect

Tap an item on the left, then its partner on the right.

The big ideas to take away

That is the whole of OCR Module 5 — Communication, homeostasis and energy. Press Finish to see your score.