What you'll learn
Homeostasis and response is the unit of AQA GCSE Combined Science: Trilogy that explains how the body keeps its internal conditions steady and how it reacts to changes around it. Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, and it matters because enzymes and cells only work properly within narrow limits. The body achieves it through two control systems: the nervous system, which is fast and uses electrical impulses, and the endocrine system, which is slower and uses hormones carried in the blood. By the end of this unit you should be able to describe the reflex arc in order, explain how blood glucose concentration is controlled by insulin and glucagon, describe the roles of the main hormones in the menstrual cycle, compare hormonal and barrier methods of contraception, and explain negative feedback. This unit is assessed on Biology Paper 2 and includes the required practical on reaction time.
Key terms and definitions
Homeostasis — the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external change
Receptor — a cell that detects a stimulus
Effector — a muscle that contracts or a gland that secretes a hormone in response to a nervous impulse
Synapse — the junction between two neurones, across which a chemical diffuses to trigger an impulse in the next neurone
Reflex action — an automatic, rapid response that does not involve the conscious part of the brain
Hormone — a chemical messenger produced by a gland, carried in the blood to a target organ
Negative feedback — a control mechanism in which a change in a condition triggers a response that reverses that change
Insulin — the hormone released by the pancreas that causes glucose to move from the blood into the cells
Glucagon — the hormone released by the pancreas that causes the liver to convert glycogen back into glucose and release it into the blood
Type 1 diabetes — a disorder in which the pancreas fails to produce enough insulin, treated with insulin injections
Type 2 diabetes — a disorder in which the body cells no longer respond properly to insulin, with obesity as a risk factor
Core concepts
What homeostasis controls and why
In the human body, three conditions in particular are kept within narrow limits: blood glucose concentration, body temperature and water levels. All three matter for the same underlying reason. Enzymes control nearly every reaction in the body, and enzymes are denatured by temperatures or pH values outside their optimum range, while cells are damaged if too much or too little water moves across their membranes by osmosis.
Every control system has the same three parts. Receptors detect a change in the condition, called a stimulus. Coordination centres such as the brain, the spinal cord or the pancreas receive and process the information. Effectors, which are muscles or glands, then bring about a response that restores the optimum level.
The nervous system
The nervous system allows humans to react to their surroundings and coordinate behaviour. Information travels from receptors as electrical impulses along neurones to the central nervous system, which is the brain and spinal cord. The central nervous system coordinates the response, and impulses then travel along motor neurones to effectors.
The full pathway, which is worth memorising as a sequence, is stimulus, receptor, sensory neurone, coordinator, motor neurone, effector, response.
Between one neurone and the next there is a tiny gap called a synapse. The electrical impulse cannot jump the gap. Instead it causes a chemical to be released, which diffuses across the synapse and triggers a new electrical impulse in the next neurone. Because diffusion is slower than an electrical impulse, synapses slow the overall response slightly, and this is a favourite exam question.
Reflex actions
Reflex actions are automatic and rapid, and they do not involve the conscious part of the brain. That is precisely why they are so useful: they protect the body before you have had time to think, as when you pull your hand away from a hot object or blink when something approaches your eye.
In a reflex arc the impulse passes from the sensory neurone, through a relay neurone in the spinal cord, directly to the motor neurone, bypassing the conscious brain. The response is therefore faster than a considered action. Note the difference carefully: it is not that the brain is uninvolved, but that the conscious part of it is not.
Reaction time
Reaction time is the time taken to respond to a stimulus, usually a fraction of a second. It can be measured simply with the ruler drop test, in which one person drops a ruler between another's finger and thumb and the distance fallen before it is caught is converted to a time. Factors such as caffeine, tiredness and age affect it.
In the required practical, the control variables are what earn the marks: the same hand each time, the same starting position of the ruler, no warning of when it will be dropped, and repeated trials with a mean calculated.
The endocrine system
The endocrine system is made of glands that secrete hormones directly into the bloodstream. The blood carries the hormone to a target organ, where it produces an effect. Compared with the nervous system, the effects are slower to start but longer lasting, and they are more general rather than precisely targeted.
The pituitary gland in the brain is called the master gland because it secretes several hormones that in turn act on other glands to release further hormones. The other glands you need are the pancreas, the thyroid, the adrenal glands, the ovaries in women and the testes in men.
Controlling blood glucose
Blood glucose concentration is monitored and controlled by the pancreas. If the concentration is too high, the pancreas produces insulin. Insulin causes glucose to move from the blood into the liver and muscle cells, where it is converted into glycogen for storage, and the blood glucose concentration falls.
If the concentration is too low, the pancreas produces glucagon instead. Glucagon causes glycogen stored in the liver to be converted back into glucose and released into the blood, so the concentration rises.
These two hormones acting in opposite directions are a clear example of negative feedback: a rise triggers a response that lowers the level, and a fall triggers a response that raises it, so the level oscillates around an optimum rather than drifting.
In type 1 diabetes the pancreas does not produce enough insulin, so blood glucose rises uncontrollably. It is treated with insulin injections. In type 2 diabetes the body cells stop responding properly to the insulin that is produced. Obesity is a significant risk factor, and it is commonly treated with a carbohydrate-controlled diet and an exercise regime.
Hormones in human reproduction
At puberty, reproductive hormones cause secondary sexual characteristics to develop. The main hormone in men is testosterone, produced by the testes, which stimulates sperm production. The main hormone in women is oestrogen, produced by the ovaries.
Four hormones control the menstrual cycle. Follicle stimulating hormone, from the pituitary gland, causes an egg to mature in one of the ovaries. Luteinising hormone, also from the pituitary, stimulates the release of that egg, which is ovulation. Oestrogen and progesterone, from the ovaries, maintain the lining of the uterus so that it is ready to receive a fertilised egg.
Contraception
Hormonal methods work by preventing an egg from maturing or being released. Oral contraceptives contain hormones that inhibit follicle stimulating hormone production, so no egg matures. Injections, implants and skin patches release progesterone slowly to inhibit the maturation and release of eggs, and can last for months or years.
Non-hormonal methods work mechanically or otherwise. Barrier methods such as condoms and diaphragms prevent the sperm reaching the egg, and condoms alone also reduce the transmission of sexually transmitted infections. Intrauterine devices prevent the implantation of an embryo or release a hormone. Spermicidal agents kill or disable sperm. Abstaining from intercourse when an egg may be in the oviduct avoids the fertile window, though it depends on predicting the cycle accurately. Surgical methods of sterilisation are permanent.
Evaluation questions usually turn on effectiveness, side effects, whether protection against infection is offered, and whether the method is reversible.
Worked examples
Example 1: Describing a reflex arc (4 marks)
Describe the pathway of a reflex action when a person touches a hot object.
Receptors in the skin detect the heat, which is the stimulus, and an electrical impulse passes along a sensory neurone to the spinal cord. There the impulse passes across a synapse to a relay neurone and then across a second synapse to a motor neurone, without involving the conscious part of the brain. The impulse travels along the motor neurone to the effector, a muscle in the arm, which contracts to pull the hand away.
Example 2: Explaining blood glucose control (4 marks)
Explain what happens in the body after a person eats a meal rich in carbohydrate.
Digestion of the carbohydrate releases glucose, which is absorbed into the blood, so blood glucose concentration rises. The pancreas detects the rise and secretes insulin into the blood. Insulin causes glucose to move from the blood into liver and muscle cells, where it is converted into glycogen and stored. As a result the blood glucose concentration falls back towards its normal level.
Example 3: Evaluating contraceptive methods (4 marks)
Compare the contraceptive pill with the use of condoms.
The contraceptive pill is highly effective when taken correctly and is convenient because it does not interrupt intercourse, but it must be remembered daily, it can cause side effects such as headaches, and it gives no protection against sexually transmitted infections. Condoms are less effective in typical use and must be used every time, but they have no hormonal side effects and they do protect against sexually transmitted infections. A reasonable conclusion is that condoms are preferable where infection is a concern, and some couples use both.
Common mistakes and how to avoid them
The most frequent error is writing that reflex actions do not involve the brain at all. The specification's point is that they do not involve the conscious part of the brain; the spinal cord or unconscious regions still coordinate them.
Students often muddle insulin and glucagon. A reliable memory hook is that insulin puts glucose in to the cells. Glucagon does the opposite.
Another common slip is saying that insulin turns glucose into glycogen in the blood. The conversion happens inside liver and muscle cells, not in the bloodstream.
In synapse questions, many answers say the impulse jumps the gap. It does not; a chemical diffuses across and triggers a new impulse.
Finally, be precise with the two diabetes types. Type 1 is a failure to produce insulin and is treated with injections; type 2 is a failure of the cells to respond and is usually managed by diet and exercise. Saying type 2 is treated with insulin injections is a standard lost mark.
Exam technique for "Biology: Homeostasis and Response"
Any question that asks you to describe a control system is asking for the same skeleton: name the stimulus, the receptor, the coordination centre, the effector and the response, in that order. Writing the skeleton first and then filling in the context almost guarantees the marks.
Negative feedback questions want both directions. If you only explain what happens when the level is too high, you can score at most half. Always add the too-low case.
For hormone questions, name the gland and the target organ alongside the hormone. Writing that follicle stimulating hormone is produced by the pituitary gland and acts on the ovaries scores more than naming the hormone alone.
The required practical on reaction time often appears as a data question. Be ready to calculate a mean, to identify an anomalous result and exclude it before calculating, and to suggest improvements in terms of repeats and control variables.
Quick revision summary
Homeostasis maintains optimum internal conditions — blood glucose, temperature and water — so that enzymes and cells function. Every control system has receptors, a coordination centre and effectors. The nervous system is fast, electrical and precisely targeted; the endocrine system is slower, chemical, blood-borne and longer lasting. The reflex arc runs stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response, bypassing the conscious brain, and synapses slow it slightly because a chemical must diffuse. The pancreas controls blood glucose with insulin, which stores glucose as glycogen, and glucagon, which releases it again, an example of negative feedback. Type 1 diabetes is a lack of insulin treated with injections; type 2 is a failure to respond, with obesity as a risk factor. Follicle stimulating hormone, luteinising hormone, oestrogen and progesterone control the menstrual cycle, and contraception works either hormonally or by barrier and other non-hormonal means.