AQA Core Science: Human Biology

Pathogens are microorganisms - such as bacteria and
viruses - that cause disease. Bacteria release toxins, and viruses damage our cells. White blood cells can ingest and destroy pathogens. They can produce antibodies to destroy pathogens, and antitoxins to neutralise toxins.

In vaccination pathogens are introduced into the body in a weakened form. The process causes the body to produce enough white blood cells to protect itself against the pathogens, while not getting diseased. Antibiotics are effective against bacteria, but not against viruses. Some strains of bacteria are resistant to antibiotics.

Pathogens are microorganisms that cause infectious disease. Bacteria and viruses are the main pathogens.

Bacteria are microscopic organisms. They come in
many shapes and sizes, but even the largest are only 10 micrometres long - 10 millionths of a metre.  They are
living cells and, in favourable conditions, can multiply
rapidly. Once inside the body, they release poisons or
toxins that make us feel ill. Diseases caused by bacteria include: food poisoning, cholera, typhoid, whooping cough
and gonorrhoea - a sexually transmitted disease.

Viruses are many times smaller than bacteria. They are among the smallest organisms known and consist of a fragment of genetic material inside a protective protein coat.

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 in the bloodstream, the airways, or by other routes. Diseases caused by viruses include: influenza - flu, colds, measles, mumps, rubella, chicken pox and AIDS.

The body has different ways of protecting itself against pathogens. The first defence is passive immunity. This is aimed at stopping the pathogen getting into the body in the first place. The body’s passive immunity system includes the skin, mucus and cilia in the respiratory system, acid in the stomach, and enzymes in tears. If a pathogen still manages to get into the body, the second defence takes over. This is called active immunity, and the white blood cells have key functions in this.

White blood cells can:

• ingest pathogens and destroy them
• produce antibodies to destroy pathogens
• produce antitoxins that neutralise the toxins released by pathogens
• the pathogens are not the disease – they cause the disease
• white blood cells do not eat the pathogens – they ingest them
• antibodies and antitoxins are not living things – they are specialised proteins

There are several different types of white blood cells, each with different functions, but they can be put into two main groups: phagocytes or macrophages and lymphocytes. Pathogens contain certain chemicals that are foreign to the body and are called antigens.  

Phagocytes can easily pass through blood vessel walls into the surrounding tissue and move towards pathogens or toxins. They then either  ingest and absorb the pathogens or toxins
or release an enzyme to destroy them. Having absorbed a pathogen, the phagocytes may also send out chemical messages that help nearby lymphocytes to identify the type
of antibody needed to neutralise them.

Each lymphocyte carries a specific type of antibody - a protein that has a chemical 'fit' to a certain antigen. When a lymphocyte with the appropriate antibody meets the antigen, the lymphocyte reproduces quickly, and makes many copies of the antibody that neutralises the pathogen. Lymphocytes may also release antitoxins that stick to the appropriate toxin and stop it damaging the body.

Antibodies neutralise pathogens in a number of ways, either they bind to pathogens and damage or destroy them, coat pathogens, clumping them together so that they are easily ingested by phagocytes or bind to the pathogens and release chemical signals to attract more phagocytes.

People can be immunised against a pathogen through vaccination. Different vaccines are needed for different pathogens. Vaccination involves putting a small amount of an inactive form of a pathogen, or dead pathogen, into the body. Vaccines can contain:

• live pathogens treated to make them harmless
• harmless fragments of the pathogen
• toxins produced by pathogens
• dead pathogens

These all act as antigens. When injected into the body, they stimulate white blood cells to produce antibodies against the pathogen. Because the vaccine contains only a weakened or harmless version of a pathogen, the vaccinated person is not in danger of developing disease - although some people may suffer a mild reaction. If the person does get infected by the pathogen later, the required lymphocytes are able to reproduce rapidly and destroy it.

 
Vaccines in early childhood can give protection against many serious diseases. Sometimes more than one vaccine is given at a time, like the MMR triple vaccine against mumps, measles and rubella.

Sometimes vaccine boosters are needed, because the immune response 'memory' weakens over time. Anti-tetanus injections may need to be repeated every ten years.

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.

How some common antibiotics work:

Penicillin: breaks down cell walls.
Erythromycin: stops protein synthesis.
Neomycin: stops protein synthesis.
Vancomycin: stops protein synthesis.
Ciprofloxacin: Stops DNA replication.

The first antibiotic - penicillin - was discovered in 1928 by Alexander Fleming. He noticed that some bacteria he had left in a petri dish had been killed by naturally occurring penicillium mould. Since the discovery of penicillin, many other antibiotics have been discovered or developed. Most antibiotics used in medicine have been altered chemically to make them more effective and safer for humans.

Bacterial strains can develop resistance to antibiotics. This happens because of natural selection. In a large population of bacteria, there may be some cells that are not affected by the antibiotic. These cells survive and reproduce, producing even more bacteria that are not affected by the antibiotic.

MRSA (methicillin-resistant staphylococcus aureus) is very dangerous because it is resistant to most antibiotics. It is important to avoid over-use of antibiotics, so we can slow down, or stop, the development of other strains of resistant bacteria.

 
One simple way to reduce the risk of infection is to maintain personal hygiene and to keep hospitals clean. In the 19th century, Ignaz Semmelweis realised the importance of cleanliness in hospitals. However, although his ideas were successful, they were ignored at the time because people did not know that diseases were caused by pathogens.

Regular exercise and a balanced diet are needed to keep
the body healthy. Too little food leads to a person being underweight and prone to illness, while too much food and
not enough exercise leads to a person being overweight and prone to other illnesses. Excess cholesterol increases the
risk of heart disease, and excess salt causes high blood pressure and increases the risk of heart disease and stroke.

A mixture of different types of food in the correct amounts is needed to maintain health. The main food groups are:

Carbohydrates/Grain: potatoes, pasta, bread, bananas, sugar and rice required by our bodies for a source of energy for other life processes. Sometimes referred to as fibre, which is actually just one - very common - type of carbohydrate.

Meat/Protein: meat, fish and eggs required by our bodies for growth and repair.

Fruit/Vegetables: wholemeal bread, fruit, vegetables and pulses required by our bodies as the fibre or roughage in our diet is not digested, but is important because it allows the muscles in our intestines to move food through our system by peristalsis (wave-like muscular contractions in the smooth wall of the gut).  

Dairy/Fat: cheese, butter and oils required by our bodies to make cell membranes and to insulate our bodies as they contain fat-soluble vitamins.

A healthy diet contains all the different nutrients in the correct amounts, and provides the right amount of energy for each individual. An unbalanced diet can lead to a person becoming malnourished. They may be too thin or too fat as a result, and they may suffer from deficiency diseases.

Respiration is the chemical reaction that allows cells to release energy from food. The metabolic rate is the speed at which such chemical reactions take place in the body. It varies because of several factors, including:

• age
• gender - male or female
• the proportion of muscle to fat in the body
• the amount of exercise and other physical activity
• genetic traits

The metabolic rate increases as we exercise and stays high for a while afterwards.

If you don't eat enough food, you will become too thin and may suffer from health problems. These include:

• irregular periods in women
• reduced resistance to infection
• deficiency diseases

Deficiency diseases include rickets - which affects proper growth of the skeleton and is caused by insufficient vitamin D - and kwashiorkor - which causes a swollen abdomen and is a result of insufficient protein.

Problems such as these are more likely to affect people in the developing world, where it can be more difficult to get enough food.

In warm weather, or when you don't do much exercise, you do not need to eat as much food as when it is cold or when you have exerted yourself physically. If you eat too much food without taking enough exercise, you will become overweight. Very fat people are described as obese. Overweight people may suffer from health problems, including:

• diabetes - an illness in which the body is unable to control the amount of sugar in the blood
• arthritis - an illness in which the joints become worn, inflamed and painful
• high blood pressure
• heart disease

The heart is an organ that needs its own supply of blood to keep it working. If the blood supply is reduced, the heart muscle will not work properly and will become weaker. A heart attack happens when part of the heart does not get any blood because of a blocked artery.

Cholesterol is a substance found in the blood. It is made in the liver and is needed for healthy cell membranes. However, too much cholesterol in the blood increases the risk of heart disease, and of diseased arteries.The bloodstream transports cholesterol around the body attached to proteins. The combination of cholesterol and protein is called lipoprotein, and there are two types: Low-density lipoproteins - LDLs - carry cholesterol from the liver to the cells and High-density lipoproteins - HDLs - carry excess cholesterol to the liver.

LDLs are often called 'bad' cholesterol because they lead to fat building up on artery walls, which causes heart disease. HDLs are often called ‘good’ cholesterol because they help to stop fat building up in the arteries.

A high proportion of HDLs to LDLs is good for a healthy heart. Monounsaturated and polyunsaturated oils - as found in vegetable oils - help to reduce cholesterol levels in the blood, and also increase the proportion of HDLs compared with LDLs. Check your understanding of such oils by looking at Vegetable oils. There are also drugs that can improve high blood pressure and high cholesterol levels.

Table salt is sodium chloride. Too much salt in the diet can lead to high blood pressure, which in turn leads to an increased risk of heart disease and strokes.

Salt is found naturally in many kinds of food, but many people add even more than the original amount added when they are eating. Processed foods often have a high proportion of salt and fat. Salt added to food during processing accounts for about two-thirds of the average salt intake.

On average, men in the UK eat about 10g of salt a day and women about 8g a day. However, food scientists recommend no more than 6g of salt a day in the diet. Reducing the amount of salt in the diet will help to reduce the risk of heart disease, and to a greater extent the risk of suffering a stroke.

There is some evidence that the food industry has persuaded government bodies to set higher targets for the recommended amount of dietary salt in our food than doctors might wish.

Drugs are substances that change chemical reactions in the body. Medical drugs relieve disease and illness, and are extensively tested before being used. Thalidomide is a drug that caused unexpected and damaging side effects to babies in the last century.

Recreational drugs such as alcohol and tobacco are taken by people because they like the effects they have on their bodies, but they are addictive. Heroin and cocaine are illegal recreational drugs that are very addictive.

Drugs are substances that cause changes to the body. Some drugs can help the body, but others can harm it. Some drugs can be extracted from natural sources and their existence has been known about for a long time. For example, willow bark is known to have been used by the ancient Greeks to help cure fevers and pains. It was later discovered that the active ingredient was salicylic acid. This was modified by chemists into the substance we call aspirin, which is less irritating to the stomach than salicylic acid.

New medical drugs have to be tested before they can be prescribed for patients. They are tested in a laboratory to check that they are not toxic, and later they are trialled using human volunteers. At this point, any potential side effects should show themselves. Most substances do not pass all the tests and trials, so drug development is expensive and takes a long time.

Medical drug trials are not without risk. Sometimes very severe and unexpected side-effects appear.

Thalidomide is a drug that was prescribed during the late 1950s and early 1960s. It was developed as a sleeping pill, but it was also thought to be useful for easing morning sickness in pregnant women. Unfortunately, it had not been tested for use in this way.

By 1960, thalidomide was found to damage the development of unborn babies, especially if it had been taken in the first four to eight weeks of pregnancy. The drug led to the arms or legs of the babies being very short or incompletely formed. More than 10,000 babies were affected around the world. As a result of this disaster, thalidomide was banned.

Thalidomide is now used as a treatment for leprosy and bone cancer. Its use is heavily regulated, however, to prevent a repeat of the problems it caused in the last century.

About 114,000 people die every year as a result of smoking-related illnesses. All cigarettes sold now carry a prominent health warning. Cigarettes contain about 4,000 different chemicals, many of which are harmful to the body.

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.

Smoking during pregnancy is very dangerous. It reduces the amount of oxygen available to the growing fetus. This leads to an increased risk of

• miscarriage and premature birth
• low weight of babies at birth

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.

  
   A healthy lung          A smoker's lung

The images above compare a healthy lung to a smoker’s lung. Tar deposits can be seen clearly in the smoker’s lung.

The alcohol in alcoholic drinks such as wines, beer and spirits is ethanol. It is a depressant. This means that it slows down signals in the nerves and brain.

Small amounts of alcohol help people to relax, but greater amounts lead to a lack of self-control. Drinkers of alcohol may not realise how much they are consuming, and fall ill as a result. They may become unconscious, and may even fall into a coma.

Long-term effects of alcohol include damage to the liver and brain. Alcohol may also cause weight gain, and it is addictive.

Legal drugs are drugs that are prescribed to a patient by their doctor or bought over the counter. Illegal drugs include prescription drugs that have been dangerously modified and substances that are banned by law.

Recreational drugs are taken by people to alter their mood.

• alcohol and tobacco are legal recreational drugs
• heroin and cocaine are illegal recreational drugs

Many drugs alter the chemical processes in the body in such a way that the person taking them becomes addicted to them. They feel they have to take the drugs, and they suffer unpleasant withdrawal symptoms if they stop taking them. All four drugs listed above are addictive. Heroin and cocaine are very addictive, and may cause permanent mental problems.

All drugs have the potential to damage our health, because they change chemical processes in the body. Addictive recreational drugs may damage our health indirectly by reducing the amount of money available to buy food, and by placing users in dangerous situations. Injecting any drug with a needle and syringe that someone else has used may lead to a number of diseases from infected blood, including HIV and hepatitis. Users of illegal drugs may turn to crime to pay for their habit, and this affects the lives of other people.

A research report in 2006 classified various drugs according to their health and social risks, without reference to whether they are legal or not. In the report, heroin and cocaine are the most dangerous two drugs, alcohol is the fifth most dangerous and tobacco is ninth.

Hormones are chemical substances that help to regulate processes in the body. Hormones are secreted by glands and travel to their target organs in the bloodstream. Several hormones are involved in the female menstrual cycle. Hormones can be used to control human fertility and have advantages and disadvantages.

Hormones are chemicals secreted by glands in the body. Different hormones affect different target organs. The bloodstream transports hormones from the glands to the target organs.

The adrenal gland produces an adrenalin hormone that targets vital organs such as the heart and liver and prepares the body for action - 'fight or flight'. The ovary produces an oestrogen hormone that targets the ovaries, uterus and pituitary gland and controls puberty and the menstrual cycle in females, stimulates production of LH and suppresses the production of FSH in the pituitary gland. It also produces a hormone known as progesterone that targets the uterus and maintains the lining of the womb - suppresses FSH production in the pituitary gland. The pancreas produces the hormone insulin that targets the liver and controls blood sugar levels within our bodies. The pituitary gland produces three different hormones. Anti-diuretic hormone (ADH) which targets the kidney and controls blood water level by triggering the uptake of water, follicle stimulating hormone (FSH) which targets the ovaries and triggers egg ripening and oestrogen production in ovaries and luteinising hormone (LH) which also targets the ovaries and triggers egg release and progesterone production in ovaries.The testes produces the hormone testosterone that targets the male reproductive organs and controls puberty in males.

It is important that the internal environment of the body is controlled. Maintaining a constant internal environment is called homeostasis. The nervous system and hormones are responsible for this. These are some of the internal conditions that are controlled.

The water content of the body is controlled to protect cells by avoiding too much water entering or leaving them. Water content is controlled by water loss from:

• the lungs when we exhale
• the skin by sweating
• the body, in urine produced by the kidneys

The ion (salts) content of the body is controlled to protect cells by avoiding too much water entering or leaving them. Ion content is controlled by loss of ions from:

• the skin by sweating
• the body in urine produced by the kidneys

The temperature of the body is controlled to maintain the temperature at which enzymes work best. Body temperature is controlled by:

• controlling blood flow to the skin
• sweating
• shivering

The blood sugar levels are controlled to provide cells with a constant supply of energy. Blood sugar level is controlled by the release and storage of glucose controlled by insulin.

The menstrual cycle in women is a recurring process in which the lining of the uterus - womb - is prepared for pregnancy, and if pregnancy does not happen, the lining is shed at menstruation. Several hormones control this cycle, which includes controlling the release of an egg each month from an ovary, and changing the thickness of the uterus lining. These hormones are secreted by the ovaries and pituitary gland.

The hormone FSH is secreted by the pituitary gland. FSH causes two things to happen: an egg to mature in an ovary and the ovaries to release the hormone oestrogen.

The hormone oestrogen is secreted by the ovaries. Oestrogen makes two things happen: it stops FSH being produced - so that only one egg matures in a cycle and stimulates the pituitary gland to release the hormone LH.

The hormone LH causes the mature egg to be released from the ovary. Progesterone is another hormone secreted by ovaries: it maintains the lining of the uterus and stays high during pregnancy.

Human fertility is controlled by hormones. This means that knowledge of hormones can be used to decide to increase, or reduce, the chances of fertilisation and pregnancy.

The oral contraceptive, 'the pill', greatly reduces the chances of mature eggs being produced. The pill contains oestrogen, or oestrogen and progesterone. These hormones inhibit the production of FSH, which in turn stops eggs maturing in the ovaries.

Some women have difficulty becoming pregnant because they don't produce enough FSH to allow their eggs to mature. 'Fertility drugs' contain FSH, which stimulates eggs to mature in the ovary. This is known as fertility treatment.

Oral contraceptives allow couples to choose the time they start a family, and choose the time they stop having children.

However, some women who take the pill may suffer from changes in weight, mood and blood pressure, as a result of the hormones in it. There is also a chance of an increased risk of developing blood clots. On the other hand, there is evidence of a decreased risk of developing cancer of the uterus or ovaries.

Fertility treatments increase a woman's chance of becoming pregnant, although the treatment may not always work. On the other hand, because the treatment boosts the production of mature eggs, multiple conceptions sometimes occur, with twins or triplets being expected. This increases the risk of complications in pregnancy and childbirth, and may lead to premature or underweight babies.

If a couple are having difficulty conceiving a child because the quantity or quality of the man’s sperm is poor then in vitro fertilisation - or IVF - can be used. This is where the egg is fertilised outside the woman’s body and then implanted back into her uterus. As FSH can also be used to encourage the production of several mature eggs at once, it is used as part of IVF to increase the number of eggs available for fertilisation.

Some people worry about the ethical implications of IVF, and are concerned that couples may only want fertilised eggs with 'desirable' qualities. For example, they may want a girl if they have lots of boys in the family, or they may wish to avoid producing a baby with an inherited defect.

The nervous system allows the body to respond to changes in the environment. This is a process usually coordinated by the brain. Reflex actions are extra-rapid responses to stimuli (things that set off a reaction in the nervous system), and this process also involves the nervous system, but 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 skin is sensitive to touch, pressure, pain and temperature, the tongue is sensitive to chemicals in food, the nose  is sensitive to chemicals in the air, the eyes are sensitive to light and the ears are sensitive to sound, the position of the head and balance.

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: a muscle contracting to move the arm, a muscle squeezing saliva from the salivary gland, a gland releasing a hormone into the blood.

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.

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

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 - a rapid reflex action that happens without us thinking.

1. the receptor detects a stimulus - change in the environment
2. the sensory neurone sends signal to relay neurone
3. the motor neurone sends signal to effector
4. the 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 the radial muscles of the iris relax. The circular muscles of the iris contract and less light enters the eye through the contracted pupil.

In dim light radial muscles of the iris contract, circular muscles of the iris relax and more light enters the eye through the dilated pupil.