Animal tissues, organs and organ systems

The digestive system is an example of an organ system in which several organs work together to digest and absorb food. Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.

Enzymes catalyse specific reactions in living organisms due to the shape of their active site. Each enzyme has an active site of a particular shape, and only a substrate that fits it will react — which is why one enzyme does one job.

Enzymes are not only digestive. Metabolism is the sum of all the reactions in a cell or in the body, and enzymes catalyse those reactions too — building new carbohydrates, lipids and proteins from the products of digestion, and breaking glucose down in respiration. The digestive enzymes here are one visible corner of it.

You are expected to use the lock and key theory as a simplified model to explain enzyme action. Simplified is the operative word — it explains specificity and it explains denaturing, and it is not a picture of what a real enzyme looks like. If a question offers you another model, use that one on its own terms.

Temperature and pH matter because both change the shape of the active site. Raising the temperature speeds the reaction up at first, because molecules collide more often; past the optimum the enzyme denatures, the active site loses its shape, and the substrate no longer fits. The wrong pH does the same thing. A denatured enzyme has not been "killed" — it was never alive — its shape has changed.

The products of digestion are used to build new carbohydrates, lipids and proteins, and some glucose is used in respiration. Word equations are enough here — no chemical symbol equations are required.

Bile is made in the liver and stored in the gall bladder. It does two things, and questions usually want both. It is alkaline, so it neutralises hydrochloric acid from the stomach, giving the enzymes of the small intestine the alkaline conditions they work best in. It also emulsifies fat into small droplets, which increases the surface area. Together, the alkaline conditions and the large surface area increase the rate of fat breakdown by lipase.

The heart is an organ that pumps blood around the body in a double circulatory system. The right ventricle pumps blood to the lungs, where gas exchange takes place; the left ventricle pumps blood around the rest of the body.

"Double" means blood passes through the heart twice on one full circuit. The advantage is pressure: blood leaving the lungs is slowed by the capillaries there, so returning it to the heart lets it be pumped out to the body at high pressure again.

The named vessels are a fixed list: the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries. The names of the heart valves are not required.

Knowledge of the lungs is restricted to the trachea, bronchi, alveoli and the capillary network surrounding the alveoli. The alveoli are the exchange surface — millions of them, each with a wall one cell thick, surrounded by capillaries, and ventilated by breathing. Those are the four features of an effective exchange surface described under transport in cells, in one organ.

The natural resting heart rate is controlled by a group of cells in the right atrium that act as a pacemaker. Artificial pacemakers are electrical devices used to correct irregularities in the heart rate.

The body contains three types of blood vessel — arteries, veins and capillaries — and you should be able to explain how the structure of each relates to its function.

Arteries carry blood away from the heart at high pressure, so they have thick muscular and elastic walls and a narrow lumen. Veins return blood at low pressure, so the wall is thin and the lumen wide, and they contain valves to stop blood flowing backwards. Capillaries are where exchange happens, so their walls are one cell thick and permeable, giving a short diffusion path.

Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended. You should know the function of each component, and be able to recognise the different cells in a photograph or diagram and explain how they are adapted to their functions.

Blood products — transfused blood, plasma or platelets — save lives, and they carry risks worth being able to evaluate: an infection can be passed on if donated blood is not screened, and a patient given the wrong blood group can have a serious immune reaction. Screening and cross-matching reduce both, which is why the risk in practice is very small. An answer that weighs the benefit against the managed risk is what an evaluation question is asking for.

In coronary heart disease, layers of fatty material build up inside the coronary arteries, narrowing them. This reduces the flow of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle.

The chain matters: narrowed artery → less blood → less oxygen → less respiration in the heart muscle. Questions ask for the steps, not just the outcome.

Stents are used to keep the coronary arteries open. Statins are widely used to reduce blood cholesterol levels, which slows down the rate of fatty material deposit.

In some people heart valves may become faulty, preventing the valve from opening fully, or the valve might develop a leak. A valve that will not open fully restricts blood flow; one that leaks lets blood flow backwards, so the heart works harder to move the same amount. Faulty heart valves can be replaced using biological or mechanical valves.

In the case of heart failure, a donor heart, or heart and lungs, can be transplanted. Artificial hearts are occasionally used to keep patients alive while waiting for a transplant, or to allow the heart to rest as an aid to recovery.

Health is the state of physical and mental well-being. Diseases, both communicable and non-communicable, are major causes of ill health, but other factors including diet, stress and life situations may have a profound effect on both physical and mental health.

Different types of disease may interact, and the four examples in the specification are worth learning as a set:

• Defects in the immune system mean an individual is more likely to suffer from infectious diseases. • Viruses living in cells can be the trigger for cancers. • Immune reactions initially caused by a pathogen can trigger allergies such as skin rashes and asthma. • Severe physical ill health can lead to depression and other mental illness.

This section is unusually mathematical for biology. You are expected to translate disease incidence information between graphical and numerical forms, construct and interpret frequency tables, bar charts and histograms, and use a scatter diagram to identify a correlation between two variables.

A scatter diagram shows correlation: as one variable rises the other tends to rise (positive), tends to fall (negative), or shows no pattern. Correlation is not proof of cause — that distinction is the whole point of the next section.

You also need the principles of sampling as applied to scientific data, including epidemiological data. A sample must be large enough that chance variation evens out, and representative of the population being studied — a survey of one hospital's patients tells you about that hospital, not about the country. Whenever you are asked why a conclusion is unreliable, sample size and sample bias are the two places to look first.

Risk factors are linked to an increased rate of a disease. They can be aspects of a person's lifestyle, or substances in the person's body or environment. A causal mechanism has been proven for some risk factors, but not for others — and many diseases are caused by the interaction of a number of factors.

That sentence is the examiner's favourite trap. A risk factor being linked to a disease does not by itself show it causes it; a causal mechanism has to be demonstrated separately.

You are also asked to discuss the human and financial cost of these non-communicable diseases — to an individual, a local community, a nation or globally — and to explain the effect of lifestyle factors including diet, alcohol and smoking on their incidence at each of those levels.

Human cost is the illness and shortened life of the patient and the strain on the people around them. Financial cost is treatment and care, lost earnings for the individual, and for a country the cost to its health service plus the working days lost. A good answer names the level it is talking about rather than treating cost as one undifferentiated thing.

The table below does one risk factor at all four levels. In an exam you are usually given the data and asked to interpret it, so the skill is naming which level you are talking about and keeping human cost and financial cost apart.

Cancer is the result of changes in cells that lead to uncontrolled growth and division.

Benign tumours are growths of abnormal cells which are contained in one area, usually within a membrane. They do not invade other parts of the body.

Malignant tumour cells are cancers. They invade neighbouring tissues and spread to different parts of the body in the blood, where they form secondary tumours.

Scientists have identified lifestyle risk factors for various types of cancer, and there are also genetic risk factors for some cancers.