Maintaining a constant internal environment is called homeostasis. The nervous system and hormones are responsible for this.Body temperature is controlled by:

• controlling blood flow to the skin
• sweating
• shivering 
• This is controlled to protect cells by stopping too much water from entering or leaving them. The process is called osmoregulation.
• Hairs on the skin trap more warm air if they are standing up, and less if they are lying flat. Tiny muscles in the skin can quickly pull the hairs upright to reduce heat loss, or lay them down flat to increase heat loss.
• If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, to increase heat loss by evaporation. Sweat secretion stops when body temperature returns to normal.
• Blood vessels supplying blood to the skin can swell or dilate - vasodilation. This causes more heat to be carried by the blood to the skin, where it can be lost to the air. Blood vessels can shrink down again - vasoconstriction. This reduces heat loss through the skin once the body’s temperature has returned to normal.

Muscles can also receive messages from the brain when you are cold. They respond by shivering, which warms you up.

Plants need light and water for photosynthesis. They have developed responses called tropisms to help make sure they grow towards sources of light and water.

Positive phototropism in plant stems

There are different types of tropisms:

• positive tropism – towards the stimulus
• negative tropism – away from the stimulus
• phototropism – growth in response to the direction of light
• geotropism – growth in response to the direction of gravity

Responses of different parts of the plant

responsepart of plantdirection of growthadvantage positive phototropism stem tip growth towards light to get maximum light for photosynthesis negative phototropism root tip growth away from light less chance of drying out positive geotropism root tip towards gravity more chance of finding moisture negative geotropism stem tip away from gravity more chance of finding light

Auxin is made at the tips of stems and roots. It's moved in solution to older parts of the stem and root where it changes the elasticity of the cells. More elastic cells absorb more water and grow longer, causing bending in the stem or root. It's thought that light and gravity can interfere with the transport of auxin causing it to be unevenly distributed.

Selective weedkillers kill some plants but not others. 

Rooting powder makes stem cuttings quickly develop roots.

Some hormones slow the ripening of fruits and others speed it up. These hormones and their inhibitors are useful for delaying ripening during transport or when fruit is displayed in shops.

Dormancy stops seeds germinating until conditions are ideal for growth. Hormones can be used to remove the dormancy of a seed so it can germinate at all times of year. 

The nervous system allows the body to respond, through effectors, to changes in the environment detected by receptors. The process involves neurones and is usually coordinated by the brain. A reflex action is an extra-rapid response to a stimulus: this process also involves the nervous system but it bypasses the brain.

Receptors are groups of specialised cells. They can detect changes in the environment, which are called stimuli, and turn them into electrical impulses. Receptors are often located in the sense organs, such as the ear, eye and skin. Each organ has receptors sensitive to particular kinds of stimulus.

The central nervous system (CNS) in humans consists of the brain and spinal cord. When a receptor is stimulated, it sends a signal along the nerve cells - neurones - to the brain. The brain then co-ordinates the response.

An effector is any part of the body that produces the response. Here are some examples of effectors.

Neurones are nerve cells. They carry information as tiny electrical signals. There are three different types of neurones, each with a slightly different function:

1. sensory neurones carry signals from receptors to the spinal cord and brain.
2. relay neurones carry messages from one part of the CNS to another.
3. motor neurones carry signals from the CNS to effectors.

The diagram below shows a typical neurone: in this case, a motor neurone. It has tiny branches at each end (the dendron) and a long fibre carries the signals (theaxon).

A motor neurone

The axon is surrounded by a fatty layer known as the myelin sheath. This helps to protect the neurone and allow impulses to travel faster.

Synapses

Where two neurones meet, there is a tiny gap called a synapse. Signals cross this gap using chemicals released by a neurone. The chemical diffuses across the gap makes the next neurone transmit an electrical signal.

1. An electrical impulse travels along an axon.
2. This triggers the nerve-ending of a neuron to release chemical messengers called neurotransmitters.
3. These chemicals diffuse across the synapse (the gap) and bind with receptor molecules on the membrane of the next neuron.
4. The receptor molecules on the second neuron bind only to the specific chemicals released from the first neuron. This stimulates the second neuron to transmit the electrical impulse

Reflex actions

When a receptor is stimulated, it sends a signal to the central nervous system, where the brain co-ordinates the response. But sometimes a very quick response is needed, one that does not need the involvement of the brain. This is a reflex action.

Reflex actions are rapid and happen without us thinking. For example, you would pull your hand away from a hot flame without thinking about it. The animation below allows you to step through each stage of the reflex arc.

This is what happens:

1. receptor detects a stimulus - change in the environment
2. sensory neurone sends signal to relay neurone
3. motor neurone sends signal to effector
4. effector produces a response

The way the iris in our eye adjusts the size of the pupil in response to bright or dim light is also a reflex action.

In bright light:

• Radial muscles of the iris relax.
• Circular muscles of the iris contract.
• Less light enters the eye through the contracted pupil.

In dim light:

• Radial muscles of the iris contract.
• Circular muscles of the iris relax.
• More light enters the eye through the dilated pupil.

Drugs are chemicals that cause changes in the body. They can be divided into legal and illegal drugs. Drugs can also be medical (drugs taken to cure illness) or recreational (drugs taken because they have pleasing effects). Some drugs can be addictive – more and more is needed to achieve the same effect. Drugs can be separated into categories – solvents, painkillers, depressants and stimulants.

Stimulants include caffeine - found in fizzy drinks, tea and coffee, cannabis and amphetamines such as speed. They increase the transmission of signals from one nerve cell to the next, which then increases alertness, heart rate and breathing rate. 

Sedatives or depressants include alcohol and barbiturates (such as the prescribed drug amytal and the illegal GHB). Sedatives are also drugs prescribed by a doctor to help people sleep or to relieve the symptoms of stress. They slow down the nervous system and reactions.

Painkillers or analgesics include paracetamol, aspirin, heroin and morphine. They block nerve impulses from the painful part of the body, or block nerve impulses travelling to the part of the brain responsible for perceiving pain.Hallucinogens change the way our brains work, distorting our senses. This changes our response to what we see, feel and hear. LSD is an example of a hallucinogen.

Nicotine

Nicotine is the addictive substance in tobacco smoke. It reaches the brain within 20 seconds and creates a dependency so that smokers become addicted.

Carbon monoxide combines with the haemoglobin in red blood cells and so reduces the ability of the blood to carry oxygen. This puts extra strain on the circulatory system, and can cause an increased risk of heart disease and strokes.

Tar

Carcinogens are substances that cause cancer. Tobacco smoke contains many carcinogens, including tar. Smoking increases the risk of lung cancer, mouth cancer and throat cancer.

Short-term effects

Alcohol has short-term effects such as sleepiness and impaired judgment, balance and muscle control. This leads to blurred vision and slurred speech. Vasodilation occurs - blood vessels in the skin carry more blood - leading to heat loss.

Long-term effects

The long-term effects of alcohol include damage to the liver and brain. The liver removes alcohol from the bloodstream because it's a toxic chemical. Over time, alcohol consumption can lead to liver damage (cirrhosis).

Transplants

If an organ in the body has been damaged then it can be replaced by a healthy organ from a donor – someone who had healthy organs but very recently died from other causes.

A successful transplant has to have:

• similar tissues from donor to patient
• similar ages of donor and patient
• similar locations as organs deteriorate quickly

Organ donation can be an ethical issue especially as the supply of organs is limited. An ethical issue is one that has rights and wrongs. In an exam you will be expected to discuss the ethical issues involved in:

• liver transplants for alcoholics
• heart transplants for the clinically obese.

Pathogens are microorganisms that cause infectious disease. Pathogens are mostly bacteria but some are viruses, fungi and protoctists.

Bacteria

Bacteria are living cells and, in favourable conditions, can multiply rapidly. Once inside the body they release poisons or toxins that make us feel ill.Bacteria come in many shapes and sizes, but even the largest are only 10 micrometres long (10 millionths of a metre).

Viruses can only reproduce inside host cells, and they damage the cell when they do this. A virus can get inside a cell and, once there, take over and make hundreds of thousands of copies of itself. Eventually the virus copies fill the whole host cell and burst it open. The viruses are then passed out of the body in the bloodstream, the airways, or by other routes.

Examples of vehicle-borne transmission

VehicleBacterial diseaseViral disease droplets in the air tuberculosis (TB) colds, flu water cholera polio sharp objects tetanus HIV food Salmonella food poisoning hepatitis A

Antiseptics can be used to clean an open wound as well as surfaces on objects such as toilets.

Antibiotics are substances that kill bacteria or stop their growth. They do not work against viruses: it is difficult to develop drugs that kill viruses without also damaging the body’s tissues.

Antifungal agents kill fungi. An example of an antifungal is nyastatin which treats the fungus candida albicans.

How some common antibiotics work

AntibioticHow it works penicillin breaks down cell walls erythromycin stops protein synthesis neomycin stops protein synthesis vancomycin stops protein synthesis ciprofloxacin stops DNA replication