Cambridge International AS & A Level Biology 9700: Homeostasis – Study Guide

This study guide teaches topic 14, Homeostasis, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers section 14.1 Homeostasis in mammals (learning outcomes 1–11) and section 14.2 Homeostasis in plants (outcomes 1–4). Topic 14 is A Level content, not AS: it is examined on Paper 4 (A Level Structured Questions), which also expects AS knowledge, and it can supply the context for Paper 5 (Planning, Analysis and Evaluation).

Also for this topic: revision notes and practice questions. For the whole course: the Cambridge A Level Biology hub, the printable 9700 checklist and the free A Level diagnostic.

What this topic covers

Syllabus ref	What you must be able to do
14.1.1–14.1.2	Explain homeostasis and its principles, including negative feedback
14.1.3	State that urea comes from deamination of excess amino acids in the liver
14.1.4–14.1.5	Describe the kidney; identify the parts of a nephron
14.1.6–14.1.7	Explain ultrafiltration and selective reabsorption, linked to structure
14.1.8	Describe osmoregulation by ADH and aquaporins
14.1.9–14.1.10	Describe glucagon cell signalling; explain blood glucose control
14.1.11	Explain test strips and biosensors
14.2.1–14.2.4	Explain stomatal responses, rhythms, guard cells and abscisic acid

14.1 Homeostasis in mammals

What homeostasis is and why it matters

Homeostasis is the maintenance of a constant internal environment within narrow limits, despite changes inside or outside the body. The internal environment is the tissue fluid around your cells, kept steady by exchange with the blood (topic 8; see the transport in mammals practice set). In mammals the key variables are:

• Core temperature: enzymes work fastest near their optimum; too hot and they denature.
• Blood glucose concentration: cells need a steady supply for respiration; too much lowers the water potential of the blood.
• Blood water potential: otherwise cells lose water by osmosis and shrink, or gain it and may burst.

The principles: a control loop

Every homeostatic mechanism follows the same pattern:

stimulus (change) → receptor → coordination system → effector → response
       ↑                                                          │
       └──────────── negative feedback ───────────────────────────┘

• A stimulus can be internal (a rise in blood glucose after a meal) or external (a cold wind that cools the skin).
• Receptors detect the change from the set point.
• The nervous system (fast, electrical impulses) or endocrine system (slower, hormones in the blood) coordinates the response.
• Effectors are muscles and glands.
• Negative feedback: the response reverses the change, so the variable returns towards the set point and the corrective action then stops. The variable fluctuates within a narrow range.

Urea and deamination

Excess amino acids cannot be stored. In the liver their amino groups are removed (deamination) and converted to urea, which the blood carries to the kidneys. For 14.1.3 you only need to state this.

Structure of the kidney

In a longitudinal section, from outside in:

Part	Description
Fibrous capsule	Tough outer layer that protects the kidney
Cortex	Outer region; contains the glomeruli, Bowman’s capsules and convoluted tubules
Medulla	Inner region, arranged in pyramids; contains the loops of Henle and collecting ducts
Renal pelvis	Space where urine collects before leaving the kidney
Ureter	Tube carrying urine from the renal pelvis to the bladder
Renal artery and vein	Enter and leave at the hilum; their branches run between the pyramids

Parts of a nephron

Follow one nephron: blood enters the glomerulus (a knot of capillaries) through the wider afferent arteriole and leaves through the narrower efferent arteriole. The glomerulus sits in a cup, the Bowman’s capsule. The filtrate then passes along the proximal convoluted tubule (PCT), the loop of Henle (dipping into the medulla), the distal convoluted tubule (DCT), and into a collecting duct, which many nephrons share. In photomicrographs of the cortex, glomeruli appear as round, densely stained balls inside a clear capsule space.

Ultrafiltration in the Bowman’s capsule

Glomerular blood is at high hydrostatic pressure because the afferent arteriole is wider than the efferent. Water and small solutes are forced through three layers:

1. the capillary endothelium, which has gaps (pores) between its cells
2. the basement membrane, the mesh that acts as the filter
3. the podocytes of the capsule wall, whose extensions leave filtration slits

The glomerular filtrate contains water, glucose, amino acids, urea and ions. Blood cells and most plasma proteins stay in the blood, because they are too large to pass the basement membrane.

Selective reabsorption in the PCT

All the glucose, most of the water and ions, and some urea return to the blood from the PCT. The PCT cells are built for this:

Feature of PCT cell	How it helps
Microvilli on the luminal surface	Large surface area for uptake from the filtrate
Many mitochondria	Supply ATP for active transport
Folded basal membrane	Large surface area next to the blood capillaries
Tight junctions between cells	Stop filtrate leaking back between cells
Close contact with capillaries	Short diffusion distance

The mechanism, in order:

1. Na⁺–K⁺ pumps in the basal membrane actively move sodium ions out of the cell towards the blood, keeping the Na⁺ concentration inside the cell low.
2. Na⁺ diffuses into the cell from the filtrate through co-transporter proteins, each carrying glucose or an amino acid with it (co-transport).
3. Glucose and amino acids diffuse out of the basal side by facilitated diffusion into the blood.
4. Removing solutes raises the water potential of the filtrate, so water follows by osmosis.
5. Some urea diffuses back into the blood, as its concentration in the filtrate rises.

Worked example 1. Filtrate forms at 120 cm³ min⁻¹ and urine at 0.8 cm³ min⁻¹. Calculate the percentage of filtrate reabsorbed.

volume reabsorbed = 120 − 0.8 = 119.2 cm³ min⁻¹
percentage       = 119.2 ÷ 120 × 100 = 99.3 %  (3 s.f.)

If urea is 0.30 g dm⁻³ in plasma and 21 g dm⁻³ in urine, urine is 21 ÷ 0.30 = 70 times more concentrated in urea, mainly because water is reabsorbed and most urea is not.

Osmoregulation: ADH and aquaporins

If you sweat heavily and drink nothing, the water potential of your blood falls.

1. Osmoreceptors in the hypothalamus detect the fall.
2. ADH, made in the hypothalamus, is released from the posterior pituitary gland into the blood.
3. ADH binds to receptors on collecting duct cells, triggering a cAMP-based cascade.
4. Vesicles containing aquaporins fuse with the luminal membrane.
5. The duct wall becomes more permeable to water, so water moves by osmosis into the low-water-potential tissue fluid of the medulla, then into the blood.
6. A smaller volume of more concentrated urine forms. As blood water potential rises, less ADH is released (negative feedback).

Cell signalling: glucagon

Glucagon does not enter the liver cell; it uses a second messenger. Learn the six steps in order:

1. Glucagon binds to a receptor on the cell surface membrane, causing a conformational change in the receptor.
2. This activates a G-protein, which stimulates adenylyl cyclase.
3. Adenylyl cyclase converts ATP into cyclic AMP (cAMP), the second messenger.
4. cAMP activates protein kinase A, which starts an enzyme cascade.
5. At each step, one active enzyme phosphorylates and activates many molecules of the next enzyme, so the signal is amplified.
6. The final enzyme (glycogen phosphorylase) catalyses the breakdown of glycogen to glucose.

Worked example 2. In a model cascade, each active enzyme activates 50 molecules of the next, over three stages. One enzyme at the top gives 50 × 50 × 50 = 50³ = 125 000 active final enzymes. That is why a tiny glucagon concentration releases a lot of glucose quickly.

Negative feedback control of blood glucose

	Blood glucose too high	Blood glucose too low
Receptor and gland	β cells in the islets of Langerhans (pancreas)	α cells in the islets of Langerhans
Hormone	Insulin	Glucagon
Target cells	Liver cells and muscle cells	Liver cells (muscle cells lack glucagon receptors)
Effects	More glucose transporter proteins (GLUT4) inserted into muscle cell membranes, so uptake rises; in liver and muscle, glycogen synthesis from glucose (glycogenesis) is activated; more glucose used in respiration	Glycogenolysis (glycogen → glucose) and gluconeogenesis (glucose from amino acids and glycerol); glucose released into the blood
Result	Concentration falls towards the set point	Concentration rises towards the set point

Insulin and glucagon act antagonistically. Worked example 3: after a meal, blood glucose rises from 5.2 to 8.3 mmol dm⁻³. Percentage increase = (8.3 − 5.2) ÷ 5.2 × 100 = 59.6%. Always divide by the starting value.

Test strips and biosensors

glucose + oxygen  —glucose oxidase→  gluconic acid + hydrogen peroxide
hydrogen peroxide + colourless chromogen  —peroxidase→  coloured compound + water

• A test strip (for urine or blood) carries both enzymes and a chromogen, immobilised on a pad. More glucose gives a darker colour, matched to a chart.
• A biosensor has glucose oxidase immobilised on an electrode. The reaction produces a small electric current proportional to the glucose concentration, shown as a digital reading of the blood glucose concentration at that moment.

14.2 Homeostasis in plants

Why stomata open and close

Open stomata let carbon dioxide diffuse in for photosynthesis but also let water vapour diffuse out by transpiration. Stomata respond to environmental conditions: they tend to open in light and when the CO₂ concentration in the leaf is low, and close in darkness and during water stress. Regulating the aperture balances CO₂ uptake against water loss.

Stomata also have daily rhythms: they open during the day and close at night, a cycle of about 24 hours. Closing at night saves water when no photosynthesis can happen.

Guard cell structure

Feature	Function
Pair of guard cells around a pore	Change shape together to open or close the stoma
Thicker wall next to the pore, thinner outer wall	Cells bend outwards when turgid, opening the pore
Radially arranged cellulose microfibrils	Cells lengthen rather than swell in width
Proton pumps and K⁺ channels	Control ion movement and so water potential
Many mitochondria and chloroplasts	Supply ATP for the pumps

How stomata open and close

Opening:

1. Proton pumps use ATP to pump H⁺ out of the guard cells.
2. The inside becomes more negative, so K⁺ channels open and potassium ions diffuse in.
3. The water potential of the guard cells falls; water enters by osmosis from neighbouring cells.
4. Guard cells become turgid; the uneven walls make them curve apart and the stoma opens.

Closing is the reverse: the pumps stop, K⁺ leaves, the water potential rises, water leaves by osmosis and the guard cells become flaccid.

Abscisic acid and water stress

When a plant is short of water, abscisic acid (ABA) is made and reaches the guard cells.

1. ABA binds to receptors on the guard cell surface membrane.
2. This inhibits the proton pumps and causes calcium ions to enter the cytoplasm, from outside the cell and from the vacuole. Ca²⁺ acts as a second messenger.
3. Ca²⁺ opens channels that let K⁺ (and anions) leave the guard cells.
4. The guard cells’ water potential rises, water leaves by osmosis, the cells become flaccid and the stoma closes, cutting transpiration.

Common errors

• “Water level” of the blood. Write water potential.
• Anterior pituitary. ADH is released from the posterior pituitary.
• “Water is actively reabsorbed.” Water moves by osmosis; ADH only adds aquaporins.
• Saying glucose is not filtered. It is filtered, then all reabsorbed in the PCT.
• Mixing glucose, glycogen and glucagon. Check each spelling.
• Glucagon acting on muscle. Its target is the liver.
• No amplification. Say each enzyme activates many molecules of the next.
• Calling ABA the second messenger. ABA is the hormone; Ca²⁺ is the second messenger.

Where to go next

• Condense this page with the homeostasis revision notes, then try the practice set.
• Nerves, synapses and muscle continue in the control and coordination study guide.

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.