Animal tissues, organs and organ systems — AQA GCSE Combined Science: Trilogy

Revision notes

20 short notes, in the order of the specification. Each one in short:

1. The digestive system

   The digestive system is an organ system in which several organs work together to digest and absorb food. Large molecules of carbohydrate, protein and lipid are too big to pass into the blood, so digestive enzymes convert them into small soluble molecules. Digestion is finished in the small intestine, where these molecules are absorbed into the bloodstream.

2. Enzymes

   Enzymes are large protein molecules that catalyse specific reactions in living organisms. Each has an active site whose shape fits only one substrate, as in the lock and key theory. The rate rises with temperature up to the optimum, but too high a temperature or an extreme pH changes the shape of the active site, so the enzyme is denatured.

3. Amylase, proteases and lipases

   Amylase, a carbohydrase, breaks down starch into simple sugars. Protease breaks down proteins into amino acids, and lipase breaks down lipids into glycerol and fatty acids. Amylase is made in the salivary glands, pancreas and small intestine; protease in the stomach, pancreas and small intestine; and lipase in the pancreas and small intestine.

4. Bile

   Bile is an alkaline substance made in the liver, stored in the gall bladder and released into the small intestine. It neutralises hydrochloric acid from the stomach and emulsifies fat into small droplets, which increases the surface area of the fat. Both effects increase the rate of fat breakdown by lipase. Bile is not an enzyme.

5. Required practical: food tests

   Practical

   Food tests use qualitative reagents to show whether a food molecule is present. Benedict's solution, heated in a hot water bath, turns from blue to green, yellow, orange or brick-red with sugars. Iodine turns from orange-brown to blue-black with starch. Biuret reagent turns from blue to purple with protein, and Sudan III gives a red-stained oily layer with lipids.

6. Required practical: the effect of pH on amylase

   Practical

   This required practical measures how pH affects the rate at which amylase digests starch. Starch, a buffer and amylase are kept at a constant temperature, and every 30 seconds a sample is added to iodine on a spotting tile. When the iodine stays orange-brown, all the starch is digested. The rate, 1 ÷ time, is highest at the optimum pH.

7. The heart and the double circulatory system

   The heart is an organ that pumps blood around a double circulatory system, so blood passes through it twice on each complete circuit. The right ventricle pumps blood to the lungs for gas exchange, and the left ventricle pumps blood around the rest of the body. The left ventricle has a thicker wall, and valves stop blood flowing backwards.

8. Controlling the heart rate

   The natural resting heart rate is controlled by a group of cells in the right atrium that act as a pacemaker. If the heart rate becomes irregular, an artificial pacemaker, which is an electrical device, can be used to correct it. A common exam mistake is to say that the pacemaker is the whole heart or the brain.

9. The lungs and gas exchange

   Gas exchange in the lungs happens at the alveoli. Air passes down the trachea into two bronchi, which branch into tubes ending in alveoli. Oxygen diffuses into the blood in the surrounding capillaries and carbon dioxide diffuses out. Millions of alveoli give a large surface area, and their walls are one cell thick, giving a short diffusion distance.

10. Blood vessels

    Arteries carry blood away from the heart at high pressure, so they have thick walls with muscle and elastic fibres. Veins carry blood back to the heart at low pressure and have thinner walls, a wide lumen and valves to stop backflow. Capillaries have walls one cell thick, giving a short diffusion distance for exchanging substances with body cells.

11. Blood

    Blood is a tissue made of plasma, in which red blood cells, white blood cells and platelets are suspended. Plasma transports substances such as carbon dioxide, soluble products of digestion, urea and hormones. Red blood cells carry oxygen using haemoglobin, white blood cells defend the body against pathogens, and platelets help the blood to clot at a wound.

12. Coronary heart disease

    Coronary heart disease is a non-communicable disease in which layers of fatty material build up inside the coronary arteries, narrowing them. This reduces blood flow through the coronary arteries, so the heart muscle gets less oxygen. Stents are used to keep the arteries open, and statins reduce blood cholesterol, slowing down the rate of fatty material deposit.

13. Treating cardiovascular disease

    Cardiovascular disease can be treated with drugs, mechanical devices or transplants. Faulty heart valves that do not open fully or that leak can be replaced with biological or mechanical valves. In heart failure, a donor heart, or heart and lungs, can be transplanted, and artificial hearts are occasionally used while a patient waits for a transplant.

14. Health and disease

    Health is the state of physical and mental well-being. Communicable diseases are caused by pathogens and can spread from person to person; non-communicable diseases cannot. Diseases can interact: immune system defects make infection more likely, viruses can trigger cancers, immune reactions can trigger allergies, and severe physical ill health can lead to depression.

15. Handling data about disease

    Disease incidence is how often a disease occurs in a population. You should be able to move it between numbers and graphs using frequency tables, bar charts, histograms and scatter diagrams, which can show a correlation. Data come from a sample, and a larger sample that is representative of the population gives more reliable data.

16. Risk factors and causes

    A risk factor is an aspect of a person's lifestyle, or a substance in their body or environment, that is linked to an increased rate of a disease. A correlation does not prove that the risk factor causes the disease; a causal mechanism must explain how. Many diseases, such as coronary heart disease, are caused by several interacting factors.

17. The effect of lifestyle on disease

    Lifestyle factors affect the incidence of non-communicable diseases. Diet, smoking and lack of exercise increase the risk of cardiovascular disease. Obesity is a risk factor for Type 2 diabetes. Too much alcohol damages the liver and affects the brain. Smoking causes lung disease and lung cancer, and carcinogens, including ionising radiation, can cause cancer.

18. The cost of non-communicable diseases

    The cost of non-communicable diseases is both human and financial, for an individual, a local community, a nation and the world. Human costs include pain, loss of independence and a shorter life. Financial costs include lost income and health service spending on treatment, drugs and operations. Prevention through diet and lifestyle can reduce these costs.

19. Using data about risk factors

    Using risk factor data means interpreting charts, tables and scatter diagrams that link a risk factor to a disease. First describe what the data show, then say whether there is a correlation. A correlation alone does not prove that the risk factor causes the disease; you also need a causal mechanism, and a large, representative sample.

20. Cancer

    Cancer is the result of changes in cells that lead to uncontrolled growth and division, forming a tumour. Benign tumours are contained in one area, usually within a membrane, and do not invade other parts of the body. Malignant tumours are cancers: they invade neighbouring tissues and spread in the blood to form secondary tumours.