What you'll learn
Organism level systems are the systems that coordinate and control the whole body: the nervous system, which sends fast electrical impulses, the endocrine system, which sends slower chemical messages called hormones, and the homeostatic mechanisms that use both to keep internal conditions stable. In OCR GCSE Biology A (Gateway, J247) this is topic B3, and its ideas reappear throughout the course, from enzymes and temperature to inheritance of conditions such as diabetes. This guide covers the structure and function of the nervous system and reflex arcs, the brain and the eye, the main endocrine glands and hormones, the hormones of the menstrual cycle and their use in contraception and infertility treatment, negative feedback using thyroxine and adrenaline, plant hormones and their commercial uses, and homeostasis: temperature control, blood glucose control and diabetes, and water balance controlled by the kidneys and ADH. Some of this content (the brain, the eye, plant hormones and the kidney) is in the separate-science Biology specification only.
Key terms and definitions
Stimulus — A change in the environment, inside or outside the body, that is detected by receptors.
Receptor — A cell or group of cells that detects a stimulus, such as light receptors in the retina.
Effector — A muscle or gland that brings about a response.
Neurone — A nerve cell that carries electrical impulses. Sensory neurones carry impulses from receptors to the central nervous system (CNS); relay neurones carry impulses within the CNS; motor neurones carry impulses from the CNS to effectors.
Synapse — The small gap between two neurones, crossed by a chemical called a neurotransmitter.
Reflex — A fast, automatic response to a stimulus that does not involve the conscious part of the brain.
Hormone — A chemical messenger secreted by an endocrine gland directly into the blood, which carries it to a target organ.
Endocrine system — The system of glands that secrete hormones.
Negative feedback — A control mechanism in which a change from the normal level triggers a response that reverses the change.
Homeostasis — The maintenance of a constant internal environment, such as body temperature, blood glucose concentration and water content, at levels that allow cells and enzymes to work well.
Thermoregulation — The control of body temperature.
Osmoregulation — The control of the water content of the body.
Tropism — A growth response of a plant towards or away from a stimulus. Phototropism is a response to light; gravitropism (geotropism) is a response to gravity.
Core concepts
The nervous system
The nervous system lets an organism respond quickly to its surroundings. It is made of the central nervous system (the brain and spinal cord) and the peripheral nervous system (the nerves that connect the CNS to the rest of the body). The general pathway of a response is:
stimulus → receptor → sensory neurone → CNS (coordinator) → motor neurone → effector → response
Neurones are adapted to carry impulses quickly. They are long, so they can carry impulses over long distances; they have many branched endings (dendrites) to connect with many other neurones; and many are wrapped in a fatty myelin sheath that insulates them and speeds up the impulse.
At a synapse, the electrical impulse cannot jump the gap. Instead, the impulse arriving at the end of one neurone triggers the release of a neurotransmitter, which diffuses across the gap and binds to receptor molecules on the next neurone, starting a new impulse. Synapses make sure impulses travel in one direction and allow signals to be combined or blocked. Many drugs work by affecting synapses.
Reflex arcs
A reflex is fast and automatic, and it protects the body from harm, for example pulling a hand away from a hot object, blinking when something approaches the eye, or the pupil narrowing in bright light. In a reflex arc the impulse passes from the receptor along a sensory neurone to a relay neurone in the spinal cord (or an unconscious part of the brain), then along a motor neurone to the effector. Because the impulse does not travel to the conscious parts of the brain, the response is much faster than a voluntary action, which requires a decision.
The brain
The brain contains billions of interconnected neurones and controls complex behaviour. Its main regions include:
- the cerebrum, the largest part, which controls conscious thought, memory, language, intelligence and voluntary movement;
- the cerebellum, which coordinates muscle activity, movement and balance;
- the medulla, which controls unconscious activities such as heart rate and breathing rate.
The brain is difficult to study and treat because it is complex, delicate and protected by the skull. Scientists use methods such as scanning (for example MRI) and the study of people with brain damage to learn which areas control which functions. Damage to the brain or spinal cord is hard to repair because neurones do not easily regrow.
The eye
The eye is a sense organ containing receptors sensitive to light. Its key parts are:
- the cornea, a transparent layer that refracts light into the eye;
- the iris, which controls the size of the pupil and so the amount of light entering;
- the lens, which focuses light onto the retina;
- the retina, which contains light-sensitive receptor cells (rods, which work in dim light, and cones, which detect colour);
- the optic nerve, which carries impulses to the brain.
To focus on near objects, the ciliary muscles contract, the suspensory ligaments slacken and the lens becomes fatter, refracting light more. For distant objects, the ciliary muscles relax, the ligaments tighten and the lens becomes thinner. Short-sightedness (myopia) means distant objects are blurred because light focuses in front of the retina; it is corrected with a concave lens. Long-sightedness (hyperopia) means near objects are blurred because light focuses behind the retina; it is corrected with a convex lens. Colour blindness is caused by faulty cones and cannot be corrected.
The endocrine system
Hormones are carried in the blood plasma to target organs. Compared with nervous responses, hormonal responses are slower, act over a longer time and often affect more of the body.
| Gland | Hormone(s) | Main effect |
|---|---|---|
| Pituitary ("master gland") | TSH, FSH, LH, ADH and others | Controls other glands; ADH controls water reabsorption |
| Thyroid | Thyroxine | Controls basal metabolic rate |
| Adrenal glands | Adrenaline | Prepares the body for "fight or flight" |
| Pancreas | Insulin and glucagon | Control blood glucose concentration |
| Ovaries | Oestrogen and progesterone | Control the menstrual cycle and female characteristics |
| Testes | Testosterone | Controls sperm production and male characteristics |
Negative feedback: thyroxine and adrenaline
Thyroxine is controlled by negative feedback. When the level of thyroxine in the blood falls, the pituitary releases more TSH (thyroid-stimulating hormone), which stimulates the thyroid to release more thyroxine. When the thyroxine level rises, TSH release is reduced, so less thyroxine is produced. This keeps the metabolic rate steady.
Adrenaline is released by the adrenal glands in times of fear or stress. It increases heart rate and breathing rate, widens the airways and pupils, and causes the liver to release glucose, so that more oxygen and glucose reach the muscles for "fight or flight".
Hormones in human reproduction
The menstrual cycle lasts about 28 days and is controlled by four hormones:
- FSH (from the pituitary) causes an egg to mature in an ovary and stimulates the ovaries to produce oestrogen;
- oestrogen (from the ovaries) causes the lining of the uterus to thicken, inhibits FSH and stimulates LH release;
- LH (from the pituitary) triggers ovulation, the release of the egg, at around day 14;
- progesterone (from the empty follicle) maintains the uterus lining and inhibits FSH and LH. If no pregnancy occurs, progesterone falls and the lining breaks down (menstruation).
Hormonal contraception uses these hormones. The combined pill contains oestrogen and progesterone, which inhibit FSH so no eggs mature; progesterone-only methods (pills, implants and injections) thicken the mucus at the cervix and can stop ovulation. Non-hormonal methods include condoms and diaphragms (barriers that stop sperm reaching the egg and, for condoms, protect against sexually transmitted infections), spermicides, intrauterine devices, sterilisation and abstaining around ovulation.
Infertility can be treated with hormones. FSH and LH can be given to stimulate eggs to mature. In IVF, the woman is given FSH and LH so several eggs mature; the eggs are collected and fertilised with sperm in the laboratory, and the resulting embryos are placed into the uterus. IVF gives some couples a chance of having a baby, but success rates are low, it is expensive and emotionally stressful, and it can lead to multiple births.
Plant hormones
Plants respond to their environment using hormones. Auxins control growth at the tips of shoots and roots. In phototropism, auxin moves to the shaded side of a shoot, where it makes cells elongate more, so the shoot bends towards the light (positive phototropism). In gravitropism, auxin collects on the lower side; in shoots this makes them grow upwards (negative gravitropism), while in roots it slows growth so the roots grow downwards (positive gravitropism). Gibberellins end seed dormancy and promote germination, stem elongation, flowering and fruit growth. Ethene controls fruit ripening.
These hormones have commercial uses: auxins in weedkillers (causing broad-leaved weeds to grow too fast and die) and rooting powders for cuttings; gibberellins to make seeds germinate, to promote flowering and to grow larger, seedless fruit; and ethene to ripen fruit at the right time, such as bananas picked unripe for transport.
Homeostasis: body temperature
The thermoregulatory centre in the brain monitors blood temperature, and receptors in the skin detect skin temperature. The aim is to keep the body at about 37°C, the optimum temperature for enzymes. Above this, enzymes start to denature and reactions slow.
When the body is too hot: sweat glands release more sweat, which cools the skin as it evaporates, and the arterioles supplying skin capillaries dilate (vasodilation), so more blood flows near the surface and more heat is lost by radiation.
When the body is too cold: sweating stops, arterioles constrict (vasoconstriction), reducing blood flow near the surface, and muscles shiver, contracting rapidly so that respiration releases more heat. Hairs may also stand on end to trap an insulating layer of air. Note that blood vessels themselves do not move up or down in the skin.
Homeostasis: blood glucose and diabetes
Blood glucose concentration is monitored and controlled by the pancreas.
- When blood glucose is too high (for example after a meal), the pancreas releases insulin. Insulin causes body cells to take up glucose from the blood, and the liver and muscles to convert glucose into glycogen for storage.
- When blood glucose is too low, the pancreas releases glucagon, which causes the liver to convert glycogen back into glucose and release it into the blood.
This is an example of negative feedback using two hormones with opposite effects.
Type 1 diabetes is a disorder in which the pancreas produces too little or no insulin, so blood glucose can rise to dangerously high levels. It usually begins in childhood and is treated with insulin injections, matched to food intake and exercise, so blood glucose must be monitored regularly. Type 2 diabetes occurs when body cells no longer respond properly to insulin. Obesity and lack of exercise are major risk factors, and it is usually controlled by a carbohydrate-controlled diet, regular exercise and sometimes medication.
Homeostasis: water balance and the kidneys
The kidneys remove urea (made in the liver from excess amino acids), excess water and excess ions from the blood. In each kidney tubule (nephron), blood is filtered under pressure, then useful substances such as all the glucose, some ions and the right amount of water are reabsorbed into the blood; the remaining fluid leaves as urine.
Water reabsorption is controlled by ADH (anti-diuretic hormone) from the pituitary gland. When the blood is too concentrated (for example after sweating or not drinking), more ADH is released; it makes the collecting ducts more permeable to water, so more water is reabsorbed and a small volume of concentrated urine is produced. When the blood is too dilute, less ADH is released, so less water is reabsorbed and a large volume of dilute urine is produced. This is another example of negative feedback.
Worked examples
Example 1: Tracing a reflex arc
Question: Describe the pathway of the impulse when a person touches a hot pan and pulls their hand away. Heat receptors in the skin detect the stimulus. An impulse travels along a sensory neurone to the spinal cord, crosses a synapse to a relay neurone, then crosses another synapse to a motor neurone, which carries the impulse to the effector, a muscle in the arm. The muscle contracts and the hand is pulled away. The response is fast because the impulse does not pass through the conscious brain.
Example 2: Explaining blood glucose control
Question: Explain what happens to blood glucose concentration after a person eats a large meal containing carbohydrate. Digestion releases glucose, which is absorbed into the blood, so blood glucose rises. The pancreas detects the rise and releases insulin. Insulin causes cells to take up glucose and the liver to convert glucose into glycogen, so blood glucose falls back to normal.
Example 3: Comparing nervous and hormonal responses
A sprinter reacting to the starting gun uses a nervous response: impulses travel in milliseconds and the effect is brief. Their heart still pounding after the race is caused by adrenaline, a hormonal response that is slower to start but lasts longer, because the hormone remains in the blood until it is broken down.
Example 4: Applying ADH
On a hot day, a runner sweats heavily and does not drink. Their blood becomes more concentrated, so the pituitary releases more ADH. The kidney collecting ducts become more permeable, more water is reabsorbed into the blood, and the runner produces a small volume of dark, concentrated urine.
Common mistakes and how to avoid them
Do not say an impulse "jumps" across a synapse. A neurotransmitter diffuses across the gap.
Do not confuse insulin and glucagon. Insulin lowers blood glucose; glucagon raises it. Remember also that glycogen is the storage carbohydrate and glucagon is the hormone.
Do not write that blood vessels "move closer to the skin". The arterioles dilate or constrict, changing how much blood flows near the surface.
Do not say enzymes are "killed" at high temperatures. They are proteins and become denatured.
Do not mix up the menstrual cycle hormones: FSH matures the egg, LH triggers ovulation, oestrogen builds the lining and progesterone maintains it.
Do not reverse ADH: more ADH means more water reabsorbed and less, more concentrated urine.
Exam technique for "B3: Organism Level Systems"
When describing a response, use the full pathway: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response. For homeostasis questions, state the change, the detector, the hormone or response and the effect that reverses the change; this shows you understand negative feedback. Use precise words such as vasodilation, neurotransmitter, glycogen and permeable. In questions on contraception or IVF, give both benefits and drawbacks before reaching a judgement, and in data questions on hormone levels across the menstrual cycle, describe the pattern before explaining which hormone causes each change.
Quick revision summary
- Pathway: stimulus → receptor → sensory neurone → CNS → motor neurone → effector → response.
- Synapse: neurotransmitter diffuses across the gap. Reflexes bypass the conscious brain, so they are fast.
- Brain: cerebrum (thought), cerebellum (coordination), medulla (heart and breathing rate).
- Eye: iris controls light entering; lens focuses; myopia corrected by a concave lens, hyperopia by a convex lens.
- Hormones travel in the blood: slower and longer-lasting than nervous responses.
- Thyroxine is controlled by negative feedback via TSH; adrenaline prepares the body for fight or flight.
- Menstrual cycle: FSH, oestrogen, LH, progesterone. The pill inhibits FSH.
- Plant hormones: auxins (tropisms, weedkillers, rooting powder), gibberellins (germination, seedless fruit), ethene (ripening).
- Temperature: sweating and vasodilation when hot; shivering and vasoconstriction when cold.
- Blood glucose: insulin lowers it, glucagon raises it; type 1 diabetes = no insulin, type 2 = cells do not respond.
- ADH: more ADH, more water reabsorbed, more concentrated urine.