OCR A Level Biology A: Cell communication, homeostasis and the liver (H420) – Revision Notes

These notes condense outcomes 5.1.1(a) to 5.1.1(d) and 5.1.2(a) to 5.1.2(b) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. Read the full study guide for explanations, then test yourself with the practice questions. The course hub, the printable checklist and the free diagnostics show where this unit fits. This content is assessed in Biological processes (Component 01) and Unified biology (Component 03), according to the specification.

5.1.1(a) and (b): communication and cell signalling

Why communicate? Multicellular organisms need to:

• respond to external change (temperature, predators, light, touch) and internal change (glucose, pH, water potential, core temperature)
• coordinate organs, such as heart and lungs during exercise.

Cell signalling = a signal molecule binds to a complementary receptor on a target cell, changing what the cell does.

Type	Examples
Adjacent cells	Neurotransmitter across a synapse; membrane signals in cell-to-cell contact; gap junctions (animals); plasmodesmata (plants)
Distant cells	Hormones secreted into the blood by endocrine glands, acting on target cells with the right receptor

Nervous	Hormonal
Electrical impulse, then transmitter	Chemical in the blood
Fast, short-lived	Slower, longer-lasting

Peptide hormones bind to receptors on the surface membrane. Steroid hormones can cross the membrane and bind to receptors inside the cell.

5.1.1(c): principles of homeostasis

Homeostasis = keeping the internal environment within narrow limits around a set point.

Term	Meaning
Receptor	Senses a stimulus, meaning a departure from the set point
Effector	A muscle or gland that produces the response
Negative feedback	Response reverses the change and returns the factor towards the set point
Positive feedback	Response increases the change and moves the factor further away

Method in steps: explain a feedback loop

1. Name the stimulus (what changed and in which direction).
2. Name the receptor that detects it.
3. State how the signal travels (nerve impulses or a hormone).
4. Name the effector and say what it does.
5. State the effect on the factor, and say that it returns towards the set point (negative feedback).

Example loop: body too hot

1. Stimulus: core temperature rises above the set point.
2. Receptors: the hypothalamus detects warmer blood and skin receptors detect warmth.
3. Signal: nerve impulses from the hypothalamus.
4. Effectors: skin arterioles dilate and sweat glands secrete sweat.
5. Effect: more heat is lost, temperature falls, and the stimulus to the effectors reduces.

Must-know distinctions

• Negative feedback keeps a factor steady. Positive feedback is not homeostatic, as it moves the factor away from its start.
• Positive feedback examples: platelets at a wound attracting more platelets; uterine contractions and oxytocin during birth.
• Two opposing effectors give control in both directions.
• Excretion removes metabolic waste. Secretion releases a useful product, such as a hormone or bile salts. Egestion removes undigested food.
• Nervous signals reach specific cells. A hormone reaches every cell via the blood, but only cells with its receptor respond.

5.1.1(d): temperature control

	Endotherm	Ectotherm
Main heat source	Metabolism	Surroundings
Core temperature	Fairly constant	Varies with environment
Responses	Physiological and behavioural	Behavioural
Food needs	High	Lower

Endotherm control pathway: peripheral temperature receptors in the skin, plus the hypothalamus monitoring blood temperature, then effectors in the skin and muscles.

Effector	Too cold	Too hot
Skin arterioles	Vasoconstriction	Vasodilation
Sweat glands	Little or no sweat	Sweating; evaporation cools the skin
Skeletal muscle	Shivering releases heat	No shivering
Erector pili muscles	Hairs raised, trapping air	Hairs flat
Metabolic rate	Raised	Lowered

Behavioural examples (endotherms): seek shade, huddle, change posture, change clothing.

Behavioural examples (ectotherms): bask in the sun, orientate to the sun, flatten or spread out to gain heat, shelter in burrows or shade, change the time of activity.

Worked reminder: rate of temperature change

A lizard’s body temperature rises from 15.0 °C to 31.0 °C in 32 minutes.

• Rate = (31.0 − 15.0) ÷ 32 = 16.0 ÷ 32 = 0.50 °C per minute.

Worked reminder: heat loss by sweating

Assume that evaporation of 1.0 g of sweat removes 2.4 kJ of heat (data supplied). Evaporation of 150 g removes 150 × 2.4 = 360 kJ.

Worked reminder: percentage change

Percentage change = (new value − original value) ÷ original value × 100.

Skin blood flow falls from 20 to 6 cm³ min⁻¹ in the cold.

• Change = 6 − 20 = −14
• Percentage change = (−14 ÷ 20) × 100 = −70%

5.1.2(a): excretion

Excretion = getting rid of metabolic waste from the body. Egestion is removal of undigested material that was never absorbed.

Waste	Source	Why it must go	Organ
Carbon dioxide	Respiration	Lowers blood pH	Lungs
Ammonia, converted to urea	Deamination of amino acids	Ammonia is toxic	Liver forms urea; kidneys excrete it

Importance: protects metabolism (enzymes need suitable conditions) and contributes to homeostasis.

5.1.2(b): the liver

Blood vessels

Vessel	Carries
Hepatic artery	Oxygenated blood in
Hepatic portal vein	Nutrient-rich blood from the small intestine, in
Hepatic vein	Blood out, to the vena cava
Bile duct	Bile to the gall bladder and intestine

Histology of a lobule

• Roughly hexagonal; central vein (branch of hepatic vein) in the middle.
• Branches of the hepatic artery, portal vein and bile duct at the corners.
• Sinusoids: wide, leaky channels carrying mixed blood to the central vein.
• Hepatocytes: many mitochondria, endoplasmic reticulum and Golgi apparatus.
• Kupffer cells: macrophages in the sinusoid walls.
• Bile canaliculi: bile flows towards the bile duct, opposite to blood flow.

Drawing stained sections

• Low power: tissue plan only, with no individual cells.
• High power: a few cells, sharp pencil, single lines, no shading.
• Draw ruled label lines that never cross, and give a scale.

Roles

Role	Key points
Storage of glycogen	Excess glucose stored as glycogen; broken down when blood glucose falls
Detoxification	Ethanol broken down to ethanal and then ethanoate; hydrogen peroxide by catalase; drugs
Urea formation	Deamination releases ammonia; ammonia reacts with carbon dioxide in the ornithine cycle to form urea

The specification does not ask for details of the ornithine cycle. Urea is carried in the blood to the kidneys and leaves in urine.

Worked reminder: magnification

A sinusoid is 9 µm wide and its drawing is 27 mm wide.

• Magnification = image ÷ actual = 27 mm ÷ 0.009 mm = ×3000.

Quick self-test

1. State two reasons why multicellular organisms need communication systems.
2. Give one example of signalling between adjacent cells and one between distant cells.
3. Distinguish a receptor from an effector.
4. Why is positive feedback not used to keep body temperature steady?
5. Which region of the brain senses the temperature of the blood?
6. Describe two ways an ectotherm raises its body temperature.
7. Name the change in skin arterioles when an endotherm is cold, and say why it helps.
8. Define excretion.
9. State the overall change that forms urea in the liver.
10. Name the cell type in the sinusoid walls that engulfs foreign particles.
11. Calculate the actual width of a cell that measures 45 mm on an image at ×1500, in µm.
12. A lizard’s temperature rises from 20.0 °C to 28.0 °C in 16 minutes. Find the rate in °C per minute.

Answers

1. Respond to changes in the internal and external environment; coordinate the activities of different organs.
2. Adjacent: neurotransmitter across a synapse (or gap junctions). Distant: a hormone carried in the blood.
3. A receptor detects a stimulus. An effector (muscle or gland) carries out the response.
4. It increases a change rather than reversing it, so the temperature would move further from the set point.
5. The hypothalamus.
6. Any two: basking in the sun, orientating the body to the sun, flattening or spreading the body, moving to warmer ground.
7. Vasoconstriction. Less blood flows through the skin capillaries, so less heat is lost from the surface.
8. Getting rid of the waste products made by metabolism.
9. Ammonia reacts with carbon dioxide to form urea (the ornithine cycle).
10. Kupffer cells (macrophages).
11. 45 ÷ 1500 = 0.030 mm = 30 µm.
12. (28.0 − 20.0) ÷ 16 = 0.50 °C per minute.

Where marks are usually lost

• Saying the hypothalamus produces the response, rather than sending signals to effectors.
• Writing “capillaries move” or “vasoconstriction of capillaries” instead of arterioles changing diameter.
• Describing negative feedback without saying that the factor returns towards the set point.
• Writing “sweat cools the body” without saying that evaporation is what takes the heat away.
• Mixing up the hepatic artery and hepatic portal vein, or forgetting that the hepatic vein carries blood out.
• Writing that the liver “makes ammonia into urea” without mentioning carbon dioxide or deamination.
• Forgetting to convert units in magnification questions.
• Drawing cells in a low-power tissue plan, or using shading and overlapping lines.

Official syllabus

OCR, A Level in Biology A (H420), Version 4.1 (April 2026), first assessment 2025, Cambridge OCR. Sections 5.1.1 and 5.1.2.