What you'll learn
Organisation is the unit where AQA GCSE Combined Science: Trilogy moves up from single cells to whole organisms. The central idea is a hierarchy: cells of the same type form tissues, different tissues working together form an organ, and organs working together form an organ system. This unit applies that idea to three systems in detail — the digestive system and its enzymes, the heart and circulatory system, and the lungs — and then looks at how health can break down through non-communicable disease. You should be able to describe how enzymes work and how temperature and pH affect them, name the parts of the heart and the blood vessels and explain how each is adapted, describe the components of blood and their functions, explain how the alveoli are adapted for gas exchange, and discuss risk factors for diseases such as cardiovascular disease and cancer. This unit is assessed on Biology Paper 1 and includes the required practicals on food tests and on the effect of pH on amylase.
Key terms and definitions
Tissue — a group of cells with a similar structure and function working together
Organ — a collection of tissues performing a specific function
Organ system — a group of organs working together to perform a particular role in the body
Enzyme — a biological catalyst, made of protein, that speeds up a reaction without being used up
Substrate — the molecule an enzyme acts on
Active site — the region of an enzyme with a shape complementary to its substrate
Denatured — the state of an enzyme whose active site has changed shape so the substrate no longer fits
Bile — an alkaline liquid made in the liver and stored in the gall bladder that neutralises stomach acid and emulsifies fats
Coronary heart disease — a condition in which fatty material builds up in the coronary arteries, narrowing them and reducing blood flow to the heart muscle
Benign tumour — a growth of abnormal cells contained in one place and not invading other tissues
Malignant tumour — a cancerous growth that invades neighbouring tissue and spreads in the blood to form secondary tumours
Core concepts
Levels of organisation
The hierarchy runs cells, tissues, organs, organ systems, organism. Muscular tissue contracts, glandular tissue produces enzymes and hormones, and epithelial tissue covers surfaces. The stomach illustrates the idea neatly: it contains muscular tissue to churn the contents, glandular tissue to produce digestive juices, and epithelial tissue lining the inside and outside. Those tissues together make an organ, and the stomach in turn is one organ of the digestive system.
Enzymes and the lock and key model
Enzymes are large protein molecules folded into a precise shape. Somewhere in that shape is the active site, into which only a substrate of a complementary shape can fit. This is the lock and key model. Once the substrate is in place the reaction proceeds, the products leave, and the enzyme is free to work again.
Because the active site's shape depends on the folding of a protein, anything that changes the folding destroys the enzyme's function. Raising the temperature increases the rate at first, because molecules move faster and collide more often, but above the optimum the bonds holding the protein shape break, the active site changes shape and the enzyme is denatured. The same happens at a pH far from the optimum. Denaturing is permanent; cooling a denatured enzyme does not restore it. Every enzyme has an optimum temperature and an optimum pH, and these differ between enzymes, which is why stomach protease works best in acid while pancreatic enzymes work best in alkaline conditions.
Digestive enzymes and where they act
Three groups of enzymes matter. Carbohydrases break carbohydrates into simple sugars; amylase is the example, breaking starch to maltose and then to glucose. Proteases break proteins into amino acids. Lipases break lipids into fatty acids and glycerol.
Amylase is produced in the salivary glands, the pancreas and the small intestine. Protease is produced in the stomach, where it is called pepsin, and in the pancreas and small intestine. Lipase is produced in the pancreas and small intestine. The products of digestion are small enough to be absorbed into the bloodstream and are then used to build new carbohydrates, lipids and proteins, or respired to release energy.
Bile is not an enzyme. It is made in the liver, stored in the gall bladder and released into the small intestine, where it does two jobs. It neutralises the hydrochloric acid that arrived from the stomach, providing the alkaline conditions the pancreatic enzymes need, and it emulsifies fat into small droplets, greatly increasing the surface area for lipase to work on. Both effects increase the rate of fat digestion.
The heart and the circulatory system
Humans have a double circulatory system: blood passes through the heart twice on each complete circuit. One loop carries deoxygenated blood to the lungs and back, the other carries oxygenated blood to the rest of the body and back. The advantage is that blood can be pumped to the body at high pressure after passing through the lungs, so oxygen is delivered quickly.
The heart has four chambers. Deoxygenated blood from the body enters the right atrium through the vena cava, passes into the right ventricle and is pumped out through the pulmonary artery to the lungs. Oxygenated blood returns through the pulmonary vein into the left atrium, passes into the left ventricle and is pumped out through the aorta to the body. The left ventricle has a much thicker muscular wall than the right because it pumps blood all the way round the body rather than just to the lungs. Valves between the chambers and in the vessels prevent backflow. The natural resting rate of about 70 beats per minute is set by a group of cells in the right atrium acting as a pacemaker; an artificial pacemaker can be fitted to correct an irregular heartbeat.
The coronary arteries supply the heart muscle itself with blood. If fatty material builds up in them, the arteries narrow, blood flow falls and the heart muscle receives less oxygen, which is coronary heart disease. Stents can be inserted to hold the artery open, and statins can be taken to reduce blood cholesterol and slow the build-up.
Blood vessels and blood
Arteries carry blood away from the heart at high pressure, so they have thick muscular and elastic walls and a narrow lumen. Veins carry blood back to the heart at low pressure, so they have thinner walls, a wider lumen and valves to stop backflow. Capillaries are one cell thick, which gives a very short diffusion path so substances can pass easily between blood and tissue.
Blood is a tissue with four components. Plasma is the straw-coloured liquid that transports dissolved substances including carbon dioxide, urea, glucose and hormones. Red blood cells carry oxygen; they have no nucleus, leaving more room for haemoglobin, and a biconcave disc shape that increases surface area. White blood cells defend against pathogens by engulfing them, producing antibodies or producing antitoxins. Platelets are cell fragments that help the blood to clot at a wound.
The lungs and gas exchange
Air passes through the trachea, which divides into two bronchi, then into smaller bronchioles ending in air sacs called alveoli. The alveoli are the exchange surface. Oxygen diffuses from the alveolus into the blood in the surrounding capillaries, and carbon dioxide diffuses the opposite way.
The alveoli are adapted in exactly the ways an exchange surface should be. There are millions of them, giving an enormous total surface area. Their walls are one cell thick, as are the capillary walls, so the diffusion path is very short. A dense capillary network carries oxygen away and brings carbon dioxide, maintaining steep concentration gradients, and breathing constantly refreshes the air inside.
Non-communicable disease and risk factors
Non-communicable diseases are not passed between people. Risk factors are things that increase the chance of developing one. Some are aspects of lifestyle, such as smoking, diet, exercise and alcohol; others are substances in the body or the environment, such as ionising radiation.
You should know several established links. Smoking causes lung disease and lung cancer, and also increases the risk of cardiovascular disease. Obesity is a risk factor for type 2 diabetes. Alcohol affects liver function and brain function. Smoking and alcohol during pregnancy affect the developing baby. Carcinogens, including ionising radiation, increase the risk of cancer.
An important point of scientific reasoning here is the difference between correlation and cause. A risk factor is shown to cause a disease only when a mechanism is understood, not simply because the two occur together in a population.
Worked examples
Example 1: Explaining the effect of pH on amylase (4 marks)
A student finds that amylase breaks down starch fastest at pH 7 and not at all at pH 3. Explain these results.
At pH 7 the active site of amylase has the shape that is complementary to starch, so substrate molecules fit and the reaction proceeds quickly; pH 7 is the optimum. At pH 3 the change in conditions breaks the bonds holding the enzyme's shape, so the active site changes shape and the substrate can no longer fit. The enzyme has been denatured, so no starch is broken down.
Example 2: Explaining ventricle wall thickness (3 marks)
Explain why the wall of the left ventricle is thicker than the wall of the right ventricle.
The left ventricle pumps oxygenated blood through the aorta to the whole body, whereas the right ventricle pumps blood only as far as the lungs. Pumping blood round the entire body requires a higher pressure, so more muscle is needed to contract with greater force.
Example 3: Evaluating stents and statins (4 marks)
Compare the use of stents with the use of statins for coronary heart disease.
A stent is a mechanical device inserted into a narrowed coronary artery to hold it open, so it works immediately and lasts a long time, but fitting it requires surgery, which carries a risk of infection and of a blood clot forming near the stent. Statins are drugs taken regularly that reduce blood cholesterol, slowing the build-up of fatty material in all arteries rather than just one, but they must be taken for life, they act slowly and they can cause side effects such as muscle pain. A reasonable conclusion is that stents treat an existing blockage while statins reduce the risk of further ones.
Common mistakes and how to avoid them
The single most common error in this unit is writing that a high temperature kills an enzyme. Enzymes are molecules, not organisms; they are denatured. Use that word.
Many students say the active site is destroyed. It changes shape, which is not the same thing, and examiners are strict about it.
Bile is regularly described as an enzyme that digests fat. Bile emulsifies fat; lipase digests it. Keep the two verbs separate.
In circulation questions, arteries and veins are often defined by the blood they carry. They are defined by direction: arteries carry blood away from the heart, veins carry it back. The pulmonary artery carries deoxygenated blood, which is why the direction definition is the only safe one.
Finally, students often list risk factors without connecting them to a disease. A mark is awarded for the link, so write that smoking increases the risk of cardiovascular disease rather than simply naming smoking.
Exam technique for "Biology: Organisation"
Enzyme questions almost always want the same chain of reasoning: shape of active site, complementary substrate, effect of the condition on that shape, consequence for rate. Writing that chain in order will collect the marks even when the context is unfamiliar.
For the food tests required practical, learn the reagent and the positive result together: Benedict's solution heated in a water bath turns brick red for sugars, iodine solution turns blue-black for starch, biuret solution turns purple for protein, and the ethanol emulsion test turns cloudy white for lipids.
Where a question gives data on heart rate, blood flow or lung volume, look first at the units and then at what is being compared. Calculations often involve rate, which is a quantity divided by a time, and cardiac output, which is stroke volume multiplied by heart rate.
Evaluation questions on treatments, lifestyle or risk expect both sides and a judgement. Two advantages and two disadvantages followed by a one-sentence conclusion is a reliable structure.
Quick revision summary
Cells form tissues, tissues form organs, organs form organ systems. Enzymes are proteins with an active site complementary to their substrate, and they are denatured by temperatures or pH values away from the optimum. Carbohydrases, proteases and lipases break down carbohydrates, proteins and lipids, while bile neutralises stomach acid and emulsifies fat to increase surface area. The double circulatory system passes blood through the heart twice; the left ventricle is thickest because it supplies the whole body. Arteries carry blood away from the heart under high pressure, veins return it with the help of valves, and capillaries are one cell thick for exchange. Blood is plasma, red cells, white cells and platelets. The alveoli provide a large surface area, a short diffusion path and a maintained concentration gradient. Non-communicable diseases have risk factors, and a risk factor is established as a cause only when a mechanism is understood.