OCR GCSE Biology: Coordination, Control and Homeostasis (B3)
OCR GCSE Biology: Coordination, Control and Homeostasis (B3)
Topic B3 (Organism-level systems) on the OCR Gateway Science A specification (J247) is about control — how an organism senses what is happening inside and around it, and responds to keep its internal conditions steady. It pulls together the nervous system, hormones, homeostasis, reproduction and plant responses. The good news for revision is that almost all of it follows a single recurring pattern. Once you can see that pattern, B3 becomes one of the most structured and predictable topics in the whole course.
This guide works through B3 at GCSE depth. For each system you will find the idea explained clearly, a diagram or table where it helps, the highest-yield exam points and the common misconceptions. The topic is examined on both the Foundation and Higher tiers; where a point is Higher only or appears mainly in separate (triple) Biology, it is flagged — typically with [H].
If you want structured practice alongside this guide, work through the LearningBro OCR GCSE Biology: Organism-level Systems course, which covers every idea below with exam-style questions that match the OCR format.
The Pattern Behind Everything in B3
Before the detail, learn the framework that runs through the entire topic. Almost every control system in B3 works through the same three stages:
flowchart LR
A[Stimulus<br/>change detected] --> B[Receptor<br/>detects the change]
B --> C[Coordination centre<br/>brain, spinal cord, pancreas]
C --> D[Effector<br/>muscle or gland]
D --> E[Response<br/>action that restores balance]
E -. negative feedback .-> A
A receptor detects a change (the stimulus); a coordination centre (such as the brain, spinal cord or pancreas) processes the information and decides on a response; and an effector (a muscle or a gland) carries out that response. When the response acts to cancel out the original change and return the body toward its normal level, that is negative feedback — the central idea of homeostasis. Keep this receptor → coordination centre → effector → negative-feedback loop in mind, and you can reconstruct almost any answer in B3 from first principles.
How B3 Is Examined on OCR J247
B3 sits on Paper 1 of J247 (which covers B1–B3), a paper lasting 1 hour 45 minutes and worth 90 marks. Expect short recall questions, "describe and explain" questions on how a control system works, data and graph interpretation (blood glucose traces are a favourite), and extended six-mark responses. As always, read the command word: "describe" wants the sequence of events, "explain" wants the reason, and "compare" wants similarities and differences.
The Nervous System
Neurones and the Reflex Arc
The nervous system lets you respond rapidly to your surroundings using electrical impulses carried by nerve cells called neurones. There are three types: sensory neurones carry impulses from receptors to the central nervous system (CNS); relay neurones connect neurones within the CNS; and motor neurones carry impulses from the CNS to effectors.
A reflex action is a fast, automatic response that does not involve conscious thought — for example pulling your hand off a hot object. Because it bypasses the conscious brain, it is quicker, which protects the body from harm. The pathway it follows is the reflex arc:
flowchart LR
A[Stimulus<br/>e.g. heat] --> B[Receptor<br/>in skin]
B --> C[Sensory neurone]
C --> D[Relay neurone<br/>in spinal cord]
D --> E[Motor neurone]
E --> F[Effector<br/>muscle]
F --> G[Response<br/>hand pulls away]
A reliable exam point is why reflexes are fast: the impulse passes straight through the spinal cord via the relay neurone without waiting for the brain to make a conscious decision.
Synapses
Neurones do not touch. The tiny gap between one neurone and the next is a synapse. When an impulse reaches the end of a neurone, it triggers the release of chemical neurotransmitters that diffuse across the gap and bind to receptors on the next neurone, starting a new impulse. This converts an electrical signal to a chemical one and back again.
Common misconception: the impulse does not "jump" the synapse as electricity. It crosses as a chemical, which is why there is a very short delay at each synapse.
The Brain and the Eye [separate / Higher]
These two sub-topics appear mainly in separate (triple) Biology and at Higher level, so check what your tier and course require.
The brain is the body's main coordination centre, made of billions of neurones. You should know three regions: the cerebral cortex (consciousness, intelligence, memory and language), the cerebellum (coordinating movement and balance) and the medulla (unconscious activities such as heart rate and breathing). Studying the brain is difficult because it is complex and delicate; scientists learn about it by studying patients with damage, by electrically stimulating regions, and by using MRI scans.
The eye is a sense organ containing receptors sensitive to light intensity and colour. Learn the parts and their jobs: the cornea and lens refract (bend) light to focus it; the iris controls how much light enters by changing the size of the pupil; the retina contains the light receptors; and the optic nerve carries impulses to the brain. Two adjustments are commonly examined:
- Accommodation — focusing on near or distant objects. For a near object the ciliary muscles contract, the suspensory ligaments slacken, and the lens becomes fatter (more curved). For a distant object the ciliary muscles relax, the ligaments pull tight, and the lens becomes thinner.
- Pupil reflex — in bright light the iris muscles narrow the pupil to protect the retina; in dim light they widen it to let more light in.
You may also meet two defects: myopia (short-sightedness), corrected with a concave lens, and hyperopia (long-sightedness), corrected with a convex lens.
The Endocrine System and Hormones
The second control system is hormonal. The endocrine system is made up of glands that secrete chemical messengers called hormones directly into the bloodstream, which carries them to target organs.
It is worth comparing the two coordination systems directly, because "compare" questions are common:
| Feature | Nervous system | Endocrine (hormonal) system |
|---|---|---|
| Signal type | Electrical impulses | Chemical hormones |
| Transmission | Along neurones | In the blood |
| Speed | Very fast | Slower |
| Duration of effect | Short-lived | Often longer-lasting |
Learn the main glands and at least one hormone each: the pituitary (the "master gland", releasing hormones that control other glands), the thyroid (thyroxine, controlling metabolic rate), the pancreas (insulin and glucagon, controlling blood glucose), the adrenal glands (adrenaline, the "fight or flight" response) and the ovaries and testes (the sex hormones oestrogen and testosterone).
Homeostasis and the Control of Blood Glucose
Homeostasis is the maintenance of a stable internal environment despite changes outside, and it depends on the negative-feedback loop from the start of this guide. The key examples are blood glucose, temperature and water balance.
Controlling Blood Glucose
The pancreas monitors and controls blood glucose concentration:
- When blood glucose is too high (after a meal), the pancreas releases insulin, which causes liver and muscle cells to take up glucose and store excess as glycogen. Blood glucose falls back to normal.
- When blood glucose is too low (during exercise or fasting), the pancreas releases glucagon [H], which causes the liver to convert glycogen back into glucose and release it. Blood glucose rises back to normal.
This is negative feedback in action: each hormone drives the level back toward normal, so the concentration oscillates around a set point rather than drifting.
Worked example: explaining a blood-glucose graph
A graph shows a person's blood glucose rising sharply after a meal, then falling back to normal over the next hour or two. Explain the fall.
After the meal, glucose is absorbed and blood glucose rises. The pancreas detects the high level and releases insulin. Insulin causes cells — especially in the liver and muscles — to take up glucose, and the liver converts the excess into glycogen for storage. As glucose is removed from the blood, the concentration falls back toward normal, at which point insulin secretion slows. A strong answer names insulin, names the pancreas as the source, and says glucose is stored as glycogen — not "glucagon", which is a different hormone.
Diabetes
Type 1 diabetes usually begins in childhood: the pancreas produces little or no insulin, so blood glucose can rise dangerously high. It is treated with insulin injections, along with attention to diet and exercise. Type 2 diabetes typically develops later in life: the body cells stop responding properly to insulin (insulin resistance), and obesity is a major risk factor. It is usually managed first by a controlled carbohydrate diet and exercise, with medication if needed.
Common misconception: Type 1 and Type 2 are often muddled. The clean distinction is production versus response — Type 1 cannot make enough insulin; in Type 2 the cells do not respond to it.
Thermoregulation
Body temperature is controlled by the thermoregulatory centre in the brain (the hypothalamus), which has receptors sensitive to the temperature of the blood, with extra information from temperature receptors in the skin.
- When you are too hot: blood vessels near the skin surface widen (vasodilation) so more heat is lost by radiation, and sweat is produced, which cools you as it evaporates.
- When you are too cold: skin blood vessels narrow (vasoconstriction) to reduce heat loss, sweating stops, and you shiver — muscle contractions that release heat through respiration.
Two terms reliably catch students out. Vasodilation does not mean blood vessels "move to the surface" — they cannot move; they widen. And sweating itself does not cool you; it is the evaporation of sweat that removes heat.
The Kidney and Osmoregulation [H]
The kidneys control the water and salt content of the blood and remove the waste product urea (made in the liver from excess amino acids). They work by filtering the blood and then reabsorbing the useful substances — all the glucose, the right amount of water and ions — while the rest leaves as urine.
Water balance is controlled by negative feedback using the hormone ADH (anti-diuretic hormone) [H], released by the pituitary gland:
- If the blood is too concentrated (not enough water), more ADH is released, the kidney tubules become more permeable, more water is reabsorbed, and a small volume of concentrated urine is produced.
- If the blood is too dilute (too much water), less ADH is released, less water is reabsorbed, and a large volume of dilute urine is produced.
You should also be able to discuss kidney failure and its treatments — dialysis (a machine that filters the blood) and kidney transplant — including the advantages and disadvantages of each, since evaluation questions appear here.
Hormones in Human Reproduction
At puberty, hormones trigger the development of secondary sexual characteristics: testosterone in males (from the testes) and oestrogen in females (from the ovaries).
The Menstrual Cycle
Four hormones control the roughly monthly menstrual cycle. You do not need exact day numbers, but you should know each hormone's role:
| Hormone | Source | Main role |
|---|---|---|
| FSH | Pituitary gland | Causes an egg to mature in the ovary; stimulates oestrogen release |
| Oestrogen | Ovaries | Repairs and thickens the uterus lining; stops FSH; triggers LH |
| LH | Pituitary gland | Triggers ovulation (release of the egg) at about day 14 |
| Progesterone | Ovaries | Maintains the uterus lining; inhibits FSH and LH |
The clearest way to remember the sequence is that FSH starts it, oestrogen builds the lining and triggers LH, LH releases the egg, and progesterone maintains the lining — if no fertilisation occurs, progesterone falls and the lining breaks down (the period), restarting the cycle.
Contraception
Be ready to compare hormonal and non-hormonal methods and to evaluate them:
- Hormonal methods — for example the combined pill (oestrogen and progesterone), which inhibits FSH so no egg matures. They are very effective but do not protect against sexually transmitted infections and can have side effects.
- Non-hormonal (barrier) methods — for example condoms and diaphragms, which physically stop sperm reaching the egg. Condoms also reduce the spread of infections.
You may also meet fertility treatments such as IVF, where FSH and LH are given to stimulate egg production, eggs are fertilised outside the body, and an embryo is implanted. Expect to weigh up the benefits against the emotional and physical stress, the low success rate and the cost.
Plant Hormones and Tropisms
Plants respond to their environment too, using hormones to control growth. A tropism is a growth response toward or away from a stimulus:
- Phototropism — growth in response to light. Shoots are positively phototropic (grow toward light), which helps the plant photosynthesise.
- Gravitropism (geotropism) — growth in response to gravity. Roots are positively gravitropic (grow downward toward water and anchorage); shoots are negatively gravitropic (grow upward).
The main plant hormone responsible is auxin. In a shoot bent toward light, auxin accumulates on the shaded side, where it makes cells elongate more, so that side grows faster and the shoot bends toward the light. Auxin's effect is the opposite in roots, which is why roots and shoots respond differently to the same hormone.
In separate (triple) Biology you should also know commercial uses of plant hormones, such as auxins in weedkillers and rooting powders, gibberellins to end seed dormancy and promote flowering, and ethene to ripen fruit.
Common misconception: auxin gathers on the shaded side of a shoot, not the lit side — and it makes cells grow longer, it does not "pull" the plant.
Common Mistakes Across B3
The same errors recur every year. Spotting them in advance protects easy marks.
- Impulses "jumping" the synapse. They cross as a chemical neurotransmitter, with a short delay.
- Confusing the two control systems. Nervous = fast, electrical, short-lived; hormonal = slower, chemical, longer-lasting.
- Storing glucose as "glucagon". The store is glycogen; glucagon is the hormone that releases it.
- Muddling Type 1 and Type 2 diabetes. Type 1 = no insulin produced; Type 2 = cells do not respond to insulin.
- Saying sweating cools you. It is the evaporation of sweat that removes heat.
- "Blood vessels move to the surface." They widen (vasodilation) or narrow (vasoconstriction); they do not move.
- Auxin on the wrong side of the shoot. It builds up on the shaded side.
Exam Technique for B3 on OCR J247
B3 is on Paper 1, and it rewards students who can reason through a control loop step by step.
- Use the receptor → coordination centre → effector framework. For any "explain how the body responds to..." question, name the stimulus, receptor, coordination centre, effector and response in order, then state the negative feedback.
- Name the hormone and its gland. Marks are often awarded specifically for naming insulin and the pancreas, or FSH and the pituitary — be precise.
- Read graphs closely. Blood-glucose and menstrual-cycle traces reward you for quoting the trend and linking it to the right hormone.
- Plan extended answers. Six-mark questions on homeostasis or the menstrual cycle are levels-marked for clear, logical sequencing — jot the steps first.
- Check your tier. The brain, the eye, the kidney and ADH lean toward Higher and separate Biology; make sure you know which your course needs.
Prepare with LearningBro
The LearningBro OCR GCSE Biology: Organism-level Systems course covers all of B3 — the nervous system and reflexes, the endocrine system, homeostasis and blood glucose, thermoregulation and the kidney, reproduction and the menstrual cycle, and plant hormones — with worked examples and exam-style questions that mirror the real OCR papers, plus immediate feedback.
To rehearse whole-paper strategy and the command words, work through the OCR GCSE Biology Exam Prep course. And for the wider picture of the whole subject, start with our OCR GCSE Biology complete revision guide.
Coordination and control is one of the most logical topics in GCSE Biology. Anchor yourself to the receptor → coordination centre → effector pattern, master one example loop in full, and the rest of B3 will fall into place. Good luck with your revision.