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

This guide covers learning 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. They sit in Module 5, Communication, homeostasis and energy. The specification lists Biological processes (Component 01) as assessing modules 1, 2, 3 and 5, and Unified biology (Component 03) as assessing all six modules. Use the revision notes for recall and the practice questions to test yourself. The course hub, the printable checklist and the free diagnostics show the rest of the course. Microscopy basics are in the Foundations in Biology guide.

What this guide covers

Outcome	What you must be able to do
5.1.1(a)	Explain the need for communication systems in multicellular organisms, including responding to internal and external change and coordinating organs
5.1.1(b)	Describe cell signalling between adjacent cells and between distant cells
5.1.1(c)	Explain the principles of homeostasis: receptors, effectors, negative and positive feedback
5.1.1(d)	Describe physiological and behavioural temperature control in endotherms and behavioural control in ectotherms
5.1.2(a)	Define excretion and explain why removing carbon dioxide and nitrogenous waste matters
5.1.2(b)	Describe the gross structure, histology and functions of the liver: glycogen storage, detoxification and urea formation (ornithine cycle)

The specification does not require details of the ornithine cycle. Learn the overall change: ammonia reacts with carbon dioxide to form urea.

Why multicellular organisms need communication (5.1.1(a))

In a large multicellular organism most cells are far from the surface and are specialised, so they cannot sense or respond to everything themselves. Communication systems solve three problems.

• Responding to the external environment. Animals must react to temperature, predators and food. Plants must respond to light direction, gravity and touch.
• Responding to the internal environment. Blood glucose concentration, blood water potential, pH and core temperature must stay within limits that suit enzymes and membranes. Changes are detected and corrected.
• Coordinating organs. During exercise your muscles need more oxygen, so heart rate and breathing rate must rise together. Different organs must act as one system.

Animals use the nervous system, with electrical impulses along neurones and chemical transmitters between them, and the endocrine system, which releases hormones into the blood. Plants rely on chemical signals.

Cell signalling (5.1.1(b))

Cell signalling means one cell releasing or displaying a signal that changes another cell’s behaviour. The signal molecule binds to a receptor whose shape is complementary to it. Only cells with the right receptor, the target cells, respond. Cells without it ignore the signal.

Signalling between adjacent cells

• Across a synapse. A neurone releases neurotransmitter into the narrow gap. It diffuses a very short distance and binds to receptors on the next cell. The effect is fast and brief.
• Direct contact. Some cells carry signal molecules in their surface membranes, as when a T helper cell binds to an antigen-presenting cell.
• Channels between cells. Animal cells can pass small molecules through gap junctions. Plant cells are joined by plasmodesmata, which link cytoplasm through the cell walls.

Signalling between distant cells

• Hormones. An endocrine gland secretes a hormone into the blood. The blood carries it to every organ but only target cells with the matching receptor respond. Peptide hormones usually bind to receptors on the cell surface membrane. Steroid hormones can cross the membrane and bind to receptors inside the cell.

Feature	Nervous communication	Hormonal communication
Signal	Electrical impulses, then neurotransmitter at synapses	Hormone molecules in the blood
Speed	Very fast	Slower, as the blood must carry the hormone
Duration	Short; stops when the stimulus stops	Usually longer lasting

Principles of homeostasis (5.1.1(c))

Homeostasis is the maintenance of the internal environment within narrow limits, around a set point. It is a dynamic equilibrium: the value fluctuates slightly and is always being corrected. Enzymes work best in a narrow range of temperature and pH, and water potential must stay balanced for cells to avoid gaining or losing too much water.

A control loop has three parts: a receptor detects a stimulus (a change from the set point), information passes by nerve or hormone, and an effector (a muscle or gland) carries out a response. The receptors then detect the result, which closes the loop.

A receptor detects; an effector acts. A temperature-sensitive nerve ending in the skin is a receptor. A sweat gland is an effector.

Negative and positive feedback

Negative feedback reverses a change. If a factor rises above the set point, the response lowers it. If it falls, the response raises it. This keeps the factor near the set point, which is why negative feedback is the basis of homeostasis. Systems often have two opposing effectors, so the factor can be corrected in both directions. That gives finer control than one effector alone.

Positive feedback increases the change. The response makes the stimulus larger, so the factor moves further from its starting value. It does not hold a factor steady, but suits a rapid, one-off event. Platelets that stick at a wound release chemicals that attract more platelets, speeding up clot formation. Contractions of the uterus during birth stimulate release of oxytocin, which stimulates stronger contractions.

Temperature control (5.1.1(d))

An endotherm (for example a mammal or bird) produces most of its body heat by metabolism and keeps a fairly constant core temperature. An ectotherm (for example a lizard or a fish) gains most of its heat from the surroundings, so its body temperature changes with the environment.

Endotherms: receptors, hypothalamus, effectors

• Peripheral temperature receptors in the skin detect surface temperature.
• The hypothalamus in the brain monitors the temperature of the blood and receives input from the skin. It acts as the coordination centre and sends impulses to the effectors.
• Effectors in the skin and muscles bring about the response.

Response	When too cold	When too hot
Arterioles supplying skin capillaries	Vasoconstriction: skin receives less blood, so less heat escapes	Vasodilation: greater blood flow to the skin, so heat radiates away faster
Sweat glands	Little or no sweat	Sweat evaporates and takes heat from the skin
Skeletal muscle	Shivering: rapid contractions release heat from respiration	No shivering
Erector pili muscles	Hairs raised, trapping an insulating layer of air	Hairs lie flat
Metabolic rate	Increased, releasing more heat	Reduced heat production

Behavioural responses add to these. Endotherms may seek shade, change posture, huddle together, put on or remove clothing, or become less active in the heat.

Ectotherms: behavioural responses

Ectotherms control temperature by behaviour. They bask in the sun or press against warm ground to warm up. They orientate the body to the sun, or flatten and spread to increase the area exposed. They shelter in burrows or shade to cool down, and may be active at different times of day.

Endotherms need much more food, because heat production uses energy, but they stay active in the cold. Ectotherms need less food but may be sluggish when cold.

Worked example: comparing a lizard and a mammal

A lizard and a small mammal are placed in a chamber warmed from 18 °C. After 40 minutes the lizard’s body temperature has risen from 18.0 °C to 34.0 °C. In the same time the mammal’s body temperature changes from 37.0 °C to 37.4 °C.

1. Lizard: change = 34.0 − 18.0 = 16.0 °C. Rate = 16.0 ÷ 40 = 0.40 °C per minute.
2. Mammal: change = 37.4 − 37.0 = 0.4 °C. Rate = 0.4 ÷ 40 = 0.010 °C per minute.
3. The lizard’s temperature followed the environment because it relies on external heat. The mammal’s barely changed because negative feedback kept its core temperature near the set point.

Excretion (5.1.2(a))

Excretion means getting metabolic waste out of the body. It is different from egestion, which removes undigested food that has never entered body cells.

Two groups of waste matter most for this outcome.

• Carbon dioxide from respiration. It dissolves to form an acid in blood and lowers pH, which would disturb enzyme activity. The lungs excrete it.
• Nitrogenous waste. Excess amino acids cannot be stored. The liver removes the amino group, and the nitrogen leaves as ammonia, which is toxic. The liver converts ammonia to urea, which the kidneys remove in urine.

Removing waste protects metabolism, because enzymes need the right chemical conditions, and keeps blood composition stable.

The mammalian liver (5.1.2(b))

Gross structure

The liver is a large organ below the diaphragm. Two vessels bring blood in.

• The hepatic artery brings oxygenated blood from the heart.
• The hepatic portal vein brings blood rich in absorbed nutrients from the small intestine.

The hepatic vein takes blood away to the vena cava. The bile duct carries bile to the gall bladder and small intestine.

Histology

The liver is built from lobules, roughly hexagonal units about 1 mm across.

• Each lobule has a central vein, a branch of the hepatic vein, running through the middle.
• At the corners, a branch of the hepatic artery, a branch of the hepatic portal vein and a branch of the bile duct run side by side.
• Blood from the artery and portal vein mixes in channels called sinusoids that run between rows of liver cells towards the central vein. Sinusoids are wide, leaky capillaries with a slow flow.
• Hepatocytes are the liver cells. They line the sinusoids, so they exchange materials with the blood easily. They have many mitochondria, endoplasmic reticulum and Golgi apparatus, as they make and modify many substances.
• Kupffer cells are macrophages in the sinusoid walls. They engulf old red blood cells and foreign particles.
• Hepatocytes secrete bile into tiny canaliculi that lead to the bile duct. Bile flows the opposite way to the blood.

Drawing stained sections (5.1.2(b)(ii))

At low power, draw a tissue plan only: the lobule outline, central vein and corner tracts, with no individual cells. At high power, draw a few cells with a sharp pencil, clear single lines and no shading. Use ruled label lines that never cross, and state the magnification.

Worked example: magnification

A micrograph shows one hepatocyte measuring 36 mm across at a magnification of ×1500.

Actual size = image size ÷ magnification = 36 ÷ 1500 = 0.024 mm, which is 0.024 × 1000 = 24 µm.

Roles of the liver

• Storage of glycogen. Hepatocytes convert excess glucose to glycogen and break it down to glucose when blood glucose falls. The hormones that control this are covered in a later part of Module 5.
• Detoxification. The liver breaks down toxins from the diet or from metabolism into less harmful substances. Hepatocytes convert ethanol to ethanal and then ethanoate, using enzymes, and break down hydrogen peroxide using catalase. They also metabolise many drugs.
• Formation of urea. Amino acids not needed are deaminated. Ammonia is released and joins with carbon dioxide in the ornithine cycle to form urea. The blood carries urea to the kidneys.

Worked example: interpreting data

Ammonia in the blood of a patient is 0.24 mmol dm⁻³ in the hepatic portal vein and 0.03 mmol dm⁻³ in the hepatic vein.

1. Fall = 0.24 − 0.03 = 0.21 mmol dm⁻³.
2. Percentage fall = 0.21 ÷ 0.24 × 100 = 87.5%.
3. Explanation: hepatocytes take up ammonia and convert it to urea, so blood leaving the liver contains less.

Common errors

• Writing that the hypothalamus “sweats”. It is the coordination centre; glands and muscles are effectors.
• Saying vasoconstriction “moves capillaries” nearer the skin. Capillaries do not move. Arterioles change diameter.
• Confusing excretion with egestion.
• Mixing up the liver’s two blood supplies. Only the hepatic artery carries oxygenated blood in.
• Drawing cells at low power, or drawing shaded and overlapping lines.

Next steps

Try the practice questions, then use the revision notes and diagnostics to find gaps.

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.