Organisation: GCSE Biology

Organisation is the second topic of AQA GCSE Biology and Combined Science. It starts with how living things are built, from cells to tissues, organs and organ systems, then uses three big examples: the digestive system, the heart, blood and lungs, and the transport system of plants. It also covers diseases that are not passed from person to person, such as coronary heart disease and cancer. This page summarises each part and links to a full guide for each.

Levels of organisation

Cells become specialised for their jobs as organisms develop; see specialised cells.

The digestive system and enzymes

The digestive system is an organ system in which several organs work together to digest and absorb food. Digestive enzymes turn large food molecules into small soluble ones:

Enzymes work because only one kind of molecule fits their active site (the lock and key model), and their activity depends on temperature and pH. Bile, made in the liver and kept in the gall bladder, neutralises stomach acid and emulsifies fat. The two required practicals here are food tests (Benedict's, iodine and Biuret) and the effect of pH on amylase. Full details: the digestive system and enzymes.

The heart, blood vessels and lungs

The heart pumps blood round a double circulatory system: the right ventricle pumps blood to the lungs for gas exchange, and the left ventricle pumps it round the rest of the body. A group of cells in the right atrium acts as a natural pacemaker that sets the resting heart rate; artificial pacemakers correct irregular rhythms. AQA limits the vessels you must name to the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries, and the lungs to the trachea, bronchi, alveoli and their capillaries. See the heart and alveoli adaptations.

Arteries, veins and capillaries

Sort the three types of vessel by their features in this question from the Human Physiology course.

Blood

Blood is a tissue: a liquid called plasma carrying three kinds of cell or cell fragment. Red cells carry oxygen, white cells defend against pathogens, platelets help clotting, and plasma carries everything else. See the white blood cell diagram.

Coronary heart disease and other heart problems

• Coronary heart disease: fatty deposits gradually narrow the coronary arteries from the inside, so the heart muscle gets less blood and too little oxygen. Stents hold the arteries open, and statins reduce cholesterol in the blood, so the deposits form more slowly.
• Faulty valves may not open fully or may leak; they can be replaced with biological or mechanical valves.
• Heart failure can be treated with a donor heart, or heart and lungs. Artificial hearts are occasionally used while a patient waits for a transplant, or to let the heart rest.

You may be asked to weigh up treatment by drugs, mechanical devices or transplant: for each, give a benefit and a risk.

Health, lifestyle and non-communicable diseases

Health is the state of physical and mental well-being. Non-communicable diseases cannot be passed from person to person, and their risk factors are aspects of lifestyle, or substances in the body or environment, linked to a higher rate of the disease. AQA lists examples including diet, smoking and exercise for cardiovascular disease, obesity for Type 2 diabetes, alcohol for liver and brain function, smoking for lung disease and lung cancer, and carcinogens, including ionising radiation, for cancer. For some risk factors a causal mechanism has been proven; for others it has not, so a correlation in data is not proof of cause.

Cancer

• Cancer begins when changes in a cell make it grow and divide out of control.
• Benign tumours stay in one place, usually inside a membrane, and do not spread.
• Malignant tumours are cancers: they invade neighbouring tissues and spread in the blood to form secondary tumours.

Plant tissues and transport

A leaf is a plant organ. Its tissues include the epidermis, palisade mesophyll, spongy mesophyll, xylem and phloem, with guard cells round the stomata; meristem tissue at the tips of shoots and roots makes new cells. The roots, stem and leaves form the plant's transport organ system:

The rate of transpiration changes with temperature, humidity, air movement and light intensity. More in xylem and phloem and leaf structure.

Worked examples

Example 1: a rate of blood flow

2,700 cm³ of blood passes through a blood vessel in 40 s. Calculate the rate of blood flow.

• Rate = volume ÷ time = 2,700 ÷ 40 = 67.5 cm³/s. The cardiac output calculator works with flow from the heart.

Example 2: a rate of transpiration

In a potometer, an air bubble moves 18 mm along the capillary tube in 6 minutes. Calculate the rate of water uptake.

• Rate = distance ÷ time = 18 ÷ 6 = 3 mm per minute. Strictly this is the rate of water uptake by the shoot, which stands in for the transpiration rate.

Common mistakes

• Calling the heart a tissue. It is an organ, made of several tissues.
• Saying the left ventricle pumps blood to the lungs. The right ventricle does; the left pumps to the rest of the body.
• Saying veins have thick walls. Arteries do, because they carry blood at high pressure; veins have valves.
• Mixing up benign and malignant. Only malignant tumours invade tissues and spread to form secondary tumours.
• Saying phloem carries water. Xylem carries water and mineral ions; phloem carries dissolved sugars.

Practise with a short timed paper

Organisation is in Paper 1 of AQA GCSE Biology and Combined Science. The daily GCSE Biology Higher and GCSE Combined Science Higher papers are short timed papers of 8 questions, with a new one every day. For the next big topic, see infection and response.

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