Organisation — AQA Combined Science: Trilogy

Living organisms are organised into increasingly complex structures. This unit covers that hierarchy, enzymes and digestion, the human circulatory system, health and disease, and transport in plants.

Levels of organisation

Cells → tissues → organs → organ systems → organism.

A tissue is a group of similar cells working together (e.g. muscle tissue).
An organ is a group of tissues performing a function (e.g. the stomach).
An organ system is a group of organs working together (e.g. the digestive system).

Enzymes

Enzymes are biological catalysts — proteins that speed up reactions without being used up. Each enzyme has a specific shape with an active site that fits a particular substrate. This is the "lock and key" model.

If the temperature or pH is wrong, the active site changes shape and the enzyme denatures, so the substrate no longer fits.

Temperature: rate increases with temperature up to an optimum (around 37 °C in humans), then falls sharply as the enzyme denatures.
pH: each enzyme has an optimum pH. Stomach protease (pepsin) works best in acidic conditions; enzymes in the small intestine prefer alkaline conditions.

Required practical: investigating the effect of pH on the rate at which amylase breaks down starch, using iodine solution to test for starch.

Digestive enzymes

Carbohydrases (e.g. amylase) break carbohydrates into simple sugars.
Proteases break proteins into amino acids.
Lipases break lipids (fats) into fatty acids and glycerol.

Bile is made in the liver, stored in the gall bladder and released into the small intestine. It neutralises stomach acid (providing alkaline conditions) and emulsifies fats (breaking large fat droplets into smaller ones to increase surface area for lipase).

Food tests

Starch — iodine solution turns blue-black.
Sugars (reducing) — Benedict's solution turns brick-red on heating.
Protein — Biuret reagent turns purple/lilac.
Lipids — Sudan III stains the fat layer red, or the emulsion test turns cloudy white.

The human digestive system

Food is broken down (digested) so molecules are small enough to be absorbed into the blood. Key organs: mouth, stomach, small intestine (digestion and absorption), large intestine (water absorption), plus the pancreas and liver which produce enzymes and bile.

The circulatory system

Humans have a double circulatory system: one loop carries blood to the lungs, the other to the rest of the body. The heart is the pump.

Right side pumps deoxygenated blood to the lungs.
Left side pumps oxygenated blood around the body (its wall is thicker because it pumps at higher pressure).
The heart's resting rate is controlled by a group of cells (pacemaker) in the right atrium. Artificial pacemakers correct irregularities.

Blood vessels

Arteries — carry blood away from the heart at high pressure; thick, elastic walls.
Veins — carry blood back to the heart at low pressure; have valves to prevent backflow.
Capillaries — tiny vessels with walls one cell thick for efficient exchange of substances.

Blood

Blood is a tissue made of:

Plasma — liquid that transports dissolved substances (carbon dioxide, urea, nutrients).
Red blood cells — contain haemoglobin to carry oxygen; no nucleus; biconcave shape.
White blood cells — defend against pathogens.
Platelets — fragments that help blood clot.

Coronary heart disease (CHD)

In CHD, layers of fatty material build up inside the coronary arteries, narrowing them and reducing blood flow to the heart muscle. Treatments:

Stents — keep arteries open (effective, but risk of complications).
Statins — drugs that reduce blood cholesterol, slowing fatty build-up (must be taken long term; possible side effects).
Faulty valves can be replaced with biological or mechanical valves.
Heart failure may need a donor heart/lungs transplant or an artificial heart (avoids rejection but can lead to blood clots).

You should be able to evaluate the advantages and disadvantages of these treatments.

Health and disease

Health is a state of physical and mental wellbeing. Diseases are a major cause of ill health.

Communicable diseases can be spread (caused by pathogens).
Non-communicable diseases cannot be spread (e.g. CHD, many cancers).

Different types of disease may interact: e.g. a weakened immune system makes a person more likely to catch communicable diseases; some viruses can trigger cancers.

Risk factors

A risk factor is anything linked to an increased rate of a disease. Examples: smoking (lung disease and cancer), poor diet and lack of exercise (CHD, type 2 diabetes), alcohol (liver disease), carcinogens and ionising radiation (cancer). A causal mechanism has been proven for some but not all risk factors — be careful to distinguish correlation from cause.

Cancer

Cancer results from uncontrolled cell division forming a tumour.

Benign tumours stay in one place and are not usually dangerous.
Malignant tumours invade tissues and spread in the blood to form secondary tumours — these are cancers.

Plant tissues and transport

Plant organs include roots, stems and leaves. Key tissues:

Epidermis — covers the plant.
Palisade mesophyll — packed with chloroplasts for photosynthesis.
Spongy mesophyll — air spaces for gas exchange.
Xylem — carries water and minerals up from the roots (transpiration).
Phloem — carries dissolved sugars around the plant (translocation).

Transpiration is the loss of water vapour from the leaves through tiny pores called stomata, controlled by guard cells. The transpiration rate increases with higher temperature, more air movement, lower humidity and brighter light. The flow of water from roots to leaves is the transpiration stream.

Exam tips

Link structure to function: thick left ventricle wall, valves in veins, capillary walls one cell thick.
Be precise about enzyme denaturing — the active site changes shape, the enzyme is not "killed".
Distinguish xylem (water, upwards) from phloem (sugars, both directions).
Practise evaluating CHD treatments — these are common 4–6 mark questions.

Biology: Organisation