Cambridge International AS & A Level Biology 9700: Homeostasis – Revision Notes

These revision notes cover topic 14, Homeostasis, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: section 14.1 (outcomes 1–11) and section 14.2 (outcomes 1–4). This is A Level content, examined on Paper 4 alongside AS knowledge, and it may set the context for Paper 5. For full explanations and worked examples, use the homeostasis study guide.

Also useful: the homeostasis practice questions, the Cambridge A Level Biology hub, the printable 9700 checklist and the free A Level diagnostic. Nerves, synapses and muscle are in the control and coordination revision notes.

Key definitions

Term	What to write
Homeostasis	Keeping the internal environment constant, within narrow limits, despite internal and external changes
Negative feedback	A change triggers a response that reverses the change, returning the variable towards the set point
Receptor	Detects a stimulus (change from the set point)
Effector	Muscle or gland that carries out the response
Deamination	Removal of the amino group from excess amino acids in the liver; urea is produced
Ultrafiltration	Filtration under high hydrostatic pressure in the glomerulus and Bowman’s capsule
Selective reabsorption	Uptake of useful substances from filtrate back into the blood, mainly in the PCT
Osmoregulation	Control of the water potential of the blood
Second messenger	Molecule inside a cell that passes on a signal from a hormone bound outside (cAMP; Ca²⁺ in guard cells)

14.1.1–14.1.2 Principles

stimulus → receptor → coordination (nervous or endocrine) → effector → response
                                  ↑___________ negative feedback ___________|

• Controlled variables: core temperature, blood glucose concentration, blood water potential.
• Stimuli can be internal (blood glucose rises after a meal) or external (cold air on skin).
• Nervous: fast, short-lived. Endocrine: slower, longer-lasting.

14.1.3–14.1.5 Urea, kidney and nephron

• Urea is made in the liver by deamination of excess amino acids. State this; no cycle detail needed.
• Kidney, outside in: fibrous capsule → cortex → medulla → renal pelvis → ureter; renal artery and vein branch within the kidney.
• Nephron, in order of filtrate flow:

glomerulus → Bowman's capsule → proximal convoluted tubule → loop of Henle
           → distal convoluted tubule → collecting duct → renal pelvis

• Cortex: glomeruli, Bowman’s capsules, PCT, DCT. Medulla: loops of Henle and collecting ducts.

14.1.6–14.1.7 Forming urine

Ultrafiltration: method in steps

1. Afferent arteriole wider than efferent → high hydrostatic pressure in glomerulus.
2. Fluid forced through endothelium pores → basement membrane (the filter) → podocyte filtration slits.
3. Filtrate = water, glucose, amino acids, urea, ions. Not blood cells or most plasma proteins.

Selective reabsorption in the PCT: method in steps

1. Na⁺–K⁺ pump on the basal membrane moves Na⁺ out of the cell (uses ATP).
2. Low Na⁺ inside → Na⁺ enters from filtrate by co-transport with glucose or amino acids.
3. Glucose and amino acids leave the basal side by facilitated diffusion into the blood.
4. Water follows by osmosis; some urea diffuses back.

PCT feature	Link to function
Microvilli	Large surface area for uptake
Many mitochondria	ATP for active transport
Folded basal membrane	Large area for transport into blood
Tight junctions	Filtrate cannot leak between cells
Next to capillaries	Short diffusion distance

14.1.8 Osmoregulation

Blood water potential falls (dehydrated)	Blood water potential rises (drank a lot)
Osmoreceptors in hypothalamus stimulated	Osmoreceptors stimulated less
More ADH released from posterior pituitary	Less ADH released
More aquaporins inserted into luminal membrane of collecting duct cells	Aquaporins removed in vesicles
Collecting duct more permeable to water	Less permeable
More water reabsorbed by osmosis	Less water reabsorbed
Small volume of concentrated urine	Large volume of dilute urine

ADH route: made in the hypothalamus, stored and released by the posterior pituitary, acts on collecting duct cells.

14.1.9 Glucagon signalling: the six links

1. Glucagon binds receptor → conformational change
2. G-protein activated → stimulates adenylyl cyclase
3. ATP → cAMP (second messenger)
4. cAMP activates protein kinase A → starts enzyme cascade
5. Amplification: each enzyme phosphorylates and activates many of the next
6. Final enzyme (glycogen phosphorylase) catalyses glycogen → glucose

14.1.10 Blood glucose control

	Insulin	Glucagon
Made by	β cells, islets of Langerhans	α cells, islets of Langerhans
Released when	Blood glucose rises	Blood glucose falls
Acts on	Muscle and liver cells	Liver cells only
Effects	More GLUT4 transporters in muscle membranes → more glucose uptake; glycogenesis; more glucose respired	Glycogenolysis; gluconeogenesis; glucose released to blood

Both are negative feedback: the response stops the hormone’s own trigger.

14.1.11 Test strips and biosensors

glucose + O₂ —(glucose oxidase)→ gluconic acid + H₂O₂
H₂O₂ + colourless chromogen —(peroxidase)→ coloured product + H₂O

Test strip	Biosensor
Both enzymes + chromogen on a pad	Glucose oxidase immobilised on an electrode
Colour compared with a chart	Electric current → digital reading
Urine or blood	Blood
Semi-quantitative	Quantitative, present concentration

14.2 Stomata

• Trade-off: open for CO₂ uptake by diffusion; closed to reduce water loss by transpiration.
• Open: light, low CO₂ in the leaf. Close: dark, water stress.
• Daily rhythm: open by day, closed at night, about a 24-hour cycle.
• Guard cells: thick inner wall next to pore, thin outer wall; radial cellulose microfibrils; proton pumps, K⁺ channels, chloroplasts, mitochondria.

Opening: method in steps

1. Proton pumps (ATP) pump H⁺ out.
2. K⁺ channels open, K⁺ diffuses in.
3. Guard cell water potential falls; water enters by osmosis.
4. Cells turgid; uneven walls bend → stoma opens.

ABA closure: method in steps

1. Water stress → abscisic acid (ABA) made.
2. ABA binds receptors on guard cell membrane.
3. Ca²⁺ enters cytoplasm (second messenger); proton pumps inhibited.
4. K⁺ and anions leave; water potential rises; water leaves by osmosis.
5. Guard cells flaccid → stoma closes.

Worked reminders

Reading a glucose graph. After a meal, blood glucose rises, insulin concentration rises shortly afterwards, then glucose falls back towards the set point and insulin falls too. Describe the lag: the hormone changes after the glucose change, because the rise is the stimulus. Quote values with units (mmol dm⁻³) when you describe the trend.

Why is there no protein in the urine of a healthy person? Plasma proteins are too large to pass through the basement membrane, so they never enter the filtrate. Glucose is absent for a different reason: it is filtered, but it is all reabsorbed in the PCT.

Urine test versus biosensor. A urine strip shows glucose only if blood glucose was high enough for some to stay in the filtrate, and the urine may have collected over hours. A biosensor reads the blood concentration now. Use this when asked why two readings disagree.

Must-know distinctions

• Glucose (sugar) vs glycogen (storage polysaccharide) vs glucagon (hormone).
• Glycogenesis (make glycogen) vs glycogenolysis (break glycogen) vs gluconeogenesis (new glucose from non-carbohydrates).
• Ultrafiltration (pressure, not selective by need) vs selective reabsorption (active, needs ATP).
• Hormone (glucagon, ABA: first messenger, outside) vs second messenger (cAMP, Ca²⁺: inside).
• Posterior pituitary releases ADH; the hypothalamus makes it.

Quick self-test

1. State where urea is produced and from what.
2. Name the layer of the glomerulus–capsule barrier that acts as the filter.
3. Filtrate forms at 125 cm³ min⁻¹ and urine at 1.5 cm³ min⁻¹. Calculate the percentage of filtrate reabsorbed, to 3 significant figures.
4. Explain why glucose moves from filtrate into PCT cells against its concentration gradient.
5. Where are the receptors that detect blood water potential?
6. ADH secretion rises and urine flow falls from 5.6 cm³ min⁻¹ to 1.4 cm³ min⁻¹. Calculate the percentage decrease.
7. Name the enzyme that cAMP activates directly.
8. Why does one glucagon molecule lead to many glucose molecules being released?
9. Blood glucose rises from 4.5 to 7.2 mmol dm⁻³. Calculate the percentage increase.
10. Which enzyme produces the colour on a glucose test strip?
11. State the second messenger in guard cells responding to ABA.
12. Explain why guard cells bend apart when turgid.

Answers

1. In the liver, from deamination of excess amino acids.
2. The basement membrane.
3. (125 − 1.5) ÷ 125 × 100 = 98.8%.
4. Na⁺ is pumped out of the cell basally, so Na⁺ diffuses in from the filtrate and co-transporter proteins carry glucose in with it; the energy comes from the Na⁺ gradient set up using ATP.
5. Osmoreceptors in the hypothalamus.
6. (5.6 − 1.4) ÷ 5.6 × 100 = 75%.
7. Protein kinase A.
8. The enzyme cascade amplifies the signal: each active enzyme activates many molecules of the next.
9. (7.2 − 4.5) ÷ 4.5 × 100 = 60%.
10. Peroxidase (it uses the hydrogen peroxide from glucose oxidase to oxidise the chromogen).
11. Calcium ions (Ca²⁺).
12. The wall next to the pore is thicker and less stretchy than the outer wall, so the cells curve outwards and the pore opens.

Where marks are usually lost

• Writing “water level” or “water concentration” instead of water potential.
• Saying the anterior pituitary releases ADH.
• Saying ADH “reabsorbs water” or that water is actively transported. ADH increases permeability; water moves by osmosis.
• Leaving out the fusion of aquaporin vesicles with the luminal membrane.
• Missing a link in the glucagon chain, especially G-protein or adenylyl cyclase, or not saying the signal is amplified.
• Saying glucagon acts on muscle cells, or that insulin only acts on the liver.
• Confusing glycogenolysis with gluconeogenesis.
• Saying glucose is not filtered in the glomerulus.
• Stating that ABA itself enters the guard cell as the second messenger.
• In percentage change calculations, dividing by the final value instead of the starting value.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027 (Version 1), published by Cambridge University Press & Assessment (Cambridge International Education). Topic 14 Homeostasis: sections 14.1 Homeostasis in mammals and 14.2 Homeostasis in plants.