Homeostasis and response

1. Keeping your body ready for life

Your cells work best when conditions stay within a suitable range. Homeostasis means regulating conditions inside a cell or organism so it can work properly, even when things change inside or outside it.

Your body regulates temperature, blood glucose concentration and water levels. These conditions matter for enzyme activity and other cell functions. An optimum is the level at which something works best. Homeostasis does not mean a condition never changes.

Control happens automatically. It may use fast electrical signals in nerves or chemical signals called hormones. A warm room is an external change; glucose entering your blood after a meal is an internal change.

2. Detect, coordinate, respond

A control system has three main parts:

1. Receptors detect a stimulus — a change such as a rise in temperature.
2. A coordination centre receives and processes information. The brain, spinal cord and pancreas can act as coordination centres.
3. Effectors carry out the response: muscles contract and glands secrete substances.

For example, when body temperature rises, receptors detect the change. A coordination centre sends signals to sweat glands. Sweat evaporates from the skin, transferring energy away and helping temperature fall. This example helps explain control; you do not need detailed temperature-control mechanisms for this Trilogy topic.

Cold conditions slow enzyme reactions because particles move more slowly and collide successfully less often. Very high temperatures can change an enzyme's active site so the substrate no longer fits: the enzyme is denatured.

For question HR.10, use these original exercise results. The person exercises from minute 0 to minute 2, then rests.

The first 12 questions retain the introductory tasks. Any original 1.x reference in a hint refers to the corresponding HR.x question here. The rest of this worksheet extends the topic.

3. Your nervous system: fast messages

Imagine catching a falling ruler. Receptors in your eyes detect it moving. Messages travel along nerve cells called neurones as electrical impulses. Your central nervous system processes the information and sends impulses to muscles in your fingers, which contract.

The central nervous system (CNS) is the brain and spinal cord. Long neurones carry impulses between receptors, the CNS and effectors. Their long extensions help messages travel over a distance; many are insulated, helping impulses travel quickly.

A sensory neurone carries impulses from a receptor towards the CNS. A motor neurone carries impulses from the CNS to an effector. A relay neurone links neurones inside the CNS.

4. Reflexes protect you

A reflex is a rapid, automatic response that does not involve the conscious part of the brain. You do not have to decide to pull your hand away from something painfully hot. Acting quickly helps reduce damage.

1. A receptor in the skin detects the stimulus.
2. An impulse travels along a sensory neurone to the spinal cord.
3. The signal passes to a relay neurone, then to a motor neurone.
4. The motor neurone carries an impulse to a muscle. The muscle contracts and pulls the hand away.

A synapse is the tiny gap between neurones. When an impulse reaches the end of a neurone, a chemical is released. It diffuses across the gap and triggers an electrical impulse in the next neurone. Electrical impulses travel along neurones; chemicals carry the signal across synapses.

5. Required practical 6: reaction time

Aim: investigate how one factor affects human reaction time. Here we compare catching a ruler in quiet conditions with catching it while counting backwards aloud. Use willing volunteers who can stop at any time.

1. Seat the catcher with their forearm supported on a table and their hand beyond the edge. Use a plastic ruler.
2. Hold the ruler vertically, with the zero mark level with the top of the catcher's thumb. Keep the thumb and finger a fixed distance apart.
3. Without a countdown, release the ruler. The catcher closes their fingers as soon as they see it fall. Read the distance fallen at the top of the thumb.
4. Give the same number of practice attempts first. Take at least five measured readings in each condition.
5. Repeat with the counting task. Keep the same catcher, hand, ruler, starting position, dropper and room conditions.
6. Swap which condition is done first for different volunteers. This reduces effects of practice or tiredness. Include several volunteers and compare each person with themselves.

The independent variable is whether the catcher counts aloud. The dependent variable is catch distance, or reaction time found using a suitable distance-to-time conversion chart. A longer distance means a slower response. Distance and time are not directly proportional, so do not label centimetres as seconds.

Keep the ruler away from faces and feet; do not use a heavy metal ruler. Do not test effects of drugs, sleep deprivation or dangerous distractions. Do not collect private health information. Keep results anonymous.

Check an unusual result and record a reason before excluding it. Repeating trials helps reveal random variation. More volunteers improve how well the results represent a group. A bar chart is suitable for comparing the two named conditions; label the axis with the measured quantity and units.

6. Hormones: messages carried in blood

The endocrine system is a collection of glands. They release hormones straight into the bloodstream. Blood carries the hormones around the body, but only target cells with suitable receptors respond.

Hormonal responses are usually slower to start and last longer than nervous responses. Nerves send impulses to particular effectors; hormones travel in the blood.

The pituitary gland sits at the base of the brain. It is often called a master gland because some of its hormones stimulate other endocrine glands.

7. Keeping blood glucose under control

After a carbohydrate-containing meal, digestion supplies glucose and blood glucose concentration may rise. The pancreas monitors the concentration and releases insulin when it is too high.

Insulin causes glucose to move from the blood into body cells. In liver and muscle cells, excess glucose is converted to glycogen for storage. Blood glucose concentration falls towards its normal range.

8. Diabetes: two different problems

Diabetes affects control of blood glucose. Type 1 and Type 2 have different causes, so their management differs.

Use evidence carefully. A link between obesity and Type 2 diabetes is a risk, not a judgement about a person. Genetics and other factors matter too. Support should be practical and respectful, without blaming people.

To compare curves, use numbers and times. “B is higher” is less useful than “At 60 minutes, B is 12 mmol/L and A is 7 mmol/L”. The unit mmol/L describes concentration; you only need to read it correctly here.

9. Puberty and reproductive hormones

At puberty, reproductive hormones help the body become able to reproduce. They also cause secondary sex characteristics, such as facial hair, breast development or a deeper voice. Timing varies between people.

Testosterone is made by the testes and stimulates sperm production. Oestrogen is made mainly by the ovaries. Eggs begin to mature after puberty. Ovulation is the release of an egg from an ovary.

An egg enters an oviduct. If a sperm fertilises it, an embryo may later implant in the uterus lining. These are different events: releasing an egg is not the same as fertilisation.

10. The menstrual cycle, step by step

The menstrual cycle prepares the uterus for a possible pregnancy. A 28-day cycle is a useful example, but real cycles vary. Day 1 is the start of bleeding, when the uterus lining is shed.

1. An egg matures inside an ovary while the uterus lining builds up.
2. Ovulation releases the egg, around day 14 in a typical 28-day example.
3. The lining is kept ready for an embryo. If pregnancy does not occur, hormone levels fall and the lining is shed. A new cycle begins.

11. Preventing pregnancy: compare the options

Contraception reduces the chance of pregnancy. Hormonal methods change hormone signals; non-hormonal methods work in other ways.

Evaluate effectiveness, possible side effects, convenience, cost, access and STI protection. Science provides evidence about effects and risks; personal values and religious beliefs also influence choices. There is no single method that suits everyone.

12. Higher only: helping people have a baby

Some people have difficulty becoming pregnant. If eggs do not mature and release normally, fertility drugs containing FSH and LH may help. Fertilisation may then happen inside the body.

In vitro fertilisation (IVF) means fertilisation outside the body:

1. Hormones including FSH and LH stimulate several eggs to mature.
2. Eggs are collected and mixed with sperm in a laboratory.
3. Fertilised eggs divide to form embryos.
4. When an embryo is a small ball of cells, it is transferred into the uterus. Sometimes more than one is transferred.
5. If an embryo implants successfully, pregnancy can continue.

IVF can offer a chance of pregnancy, but success is not guaranteed. Treatment can be emotionally and physically stressful, expensive and time-consuming. Multiple pregnancies increase risks for the pregnant person and babies. Transferring fewer embryos reduces that risk.

People may disagree about unused embryos, cost and access. Evaluate arguments respectfully and separate evidence from personal values. Better microscopes and laboratory techniques helped make this treatment possible.

13. Higher only: adrenaline and thyroxine

Adrenaline is released by the adrenal glands in frightening or stressful situations. It increases heart rate and helps deliver more glucose and oxygen to the brain and muscles. This prepares the body for “fight or flight”.

Thyroxine is released by the thyroid gland. It increases basal metabolic rate — the rate of energy use for essential processes while at rest. It is also important for growth and development.

Thyroxine is controlled by negative feedback. When its level is low, signals from the brain and pituitary stimulate the thyroid to release more. A higher thyroxine level reduces this stimulation, so secretion falls. The response opposes the change.

14. Turn understanding into clear answers

For describe questions, state what happens. For explain, connect a cause to its effect. For evaluate, weigh evidence and limitations before giving a justified conclusion.

For a six-mark response, plan a logical route. A reflex explanation might follow the message from receptor to muscle. A practical plan should identify variables, explain the method, control other factors and describe how results will be processed.

Higher-only sections are labelled. Optional challenge questions at the end test application with less help; challenge does not automatically mean Higher-only.

Plan and evaluate required practical 6

Use the method and practice data from the reaction-time section. Explain how you would check whether counting aloud changes reaction time.

Your answer sheet

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One final thought

Use a scientific idea and a specific example in your reflection.