The human digestive system

This section assumes knowledge of the digestive system studied in Key Stage 3 science.

The digestive system is an example of an organ system in which several organs work together to digest and absorb food.

Students should be able to relate knowledge of enzymes to Metabolism (4.2.3).

Students should be able to describe the nature of enzyme molecules and relate their activity to temperature and pH changes.

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Students should be able to carry out rate calculations for chemical reactions.

Enzymes catalyse specific reactions in living organisms due to the shape of their active site.

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Students should be able to use the ‘lock and key theory’ as a simplified model to explain enzyme action.

Students should be able to recall the sites of production and the action of amylase, proteases and lipases.

Students should be able to understand simple word equations but no chemical symbol equations are required.

Digestive enzymes convert food into small soluble molecules that can be absorbed into the bloodstream.

Carbohydrases break down carbohydrates to simple sugars. Amylase is a carbohydrase which breaks down starch.

Proteases break down proteins to amino acids.

Lipases break down lipids (fats) to glycerol and fatty acids.

The products of digestion are used to build new carbohydrates, lipids and proteins. Some glucose is used in respiration.

Bile is made in the liver and stored in the gall bladder. It is alkaline to neutralise hydrochloric acid from the stomach. It also emulsifies fat to form small droplets which increases the surface area. The alkaline conditions and large surface area increase the rate of fat breakdown by lipase.

The heart and blood vessels

Students should know the structure and functioning of the human heart and lungs, including how lungs are adapted for gaseous exchange.

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.

Knowledge of the blood vessels associated with the heart is limited to the aorta, vena cava, pulmonary artery, pulmonary vein and coronary arteries. Knowledge of the names of the heart valves is not required.

Knowledge of the lungs is restricted to the trachea, bronchi, alveoli and the capillary network surrounding the alveoli.

The natural resting heart rate is controlled by a group of cells located 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 different types of blood vessel:
- arteries
- veins
- capillaries.

Students should be able to explain how the structure of these vessels relates to their functions.

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Students should be able to use simple compound measures such as rate and carry out rate calculations for blood flow.

Blood

Blood is a tissue consisting of plasma, in which the red blood cells, white blood cells and platelets are suspended.

Students should know the functions of each of these blood components.

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Students should be able to recognise different types of blood cells in a photograph or diagram, and explain how they are adapted to their functions.

Coronary heart disease: a non-communicable disease

Students should be able to evaluate the advantages and disadvantages of treating cardiovascular diseases by drugs, mechanical devices or transplant.

In coronary heart disease layers of fatty material build up inside the coronary arteries, narrowing them. This reduces the low of blood through the coronary arteries, resulting in a lack of oxygen for the heart muscle. 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 heart valve might develop a leak. Students should understand the consequences of faulty valves. 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 whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.

Health issues

Students should be able to describe the relationship between health and disease and the interactions between different types of disease.

Health is the state of physical and mental well-being.

Diseases, both communicable and non-communicable, are major causes of ill health. 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.
- Defects in the immune system mean that 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.

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Students should be able to translate disease incidence information between graphical and numerical forms, construct and interpret frequency tables and diagrams, bar charts and histograms, and use a scatter diagram to identify a correlation between two variables.

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Students should understand the principles of sampling as applied to scientific data, including epidemiological data.

The effect of lifestyle on some non-communicable diseases

Students should be able to:
- discuss the human and financial cost of these non-communicable diseases to an individual, a local community, a nation or globally
- explain the effect of lifestyle factors including diet, alcohol and smoking on the incidence of non-communicable diseases at local, national and global levels.

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Risk factors are linked to an increased rate of a disease.

They can be:
- aspects of a person’s lifestyle
- substances in the person’s body or environment.

A causal mechanism has been proven for some risk factors, but not in others.
- The effects of diet, smoking and exercise on cardiovascular disease.
- Obesity as a risk factor for Type 2 diabetes.
- The effect of alcohol on the liver and brain function.
- The effect of smoking on lung disease and lung cancer.
- The effects of smoking and alcohol on unborn babies.
- Carcinogens, including ionising radiation, as risk factors in cancer.

Many diseases are caused by the interaction of a number of factors.

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Students should be able to understand the principles of sampling as applied to scientific data in terms of risk factors.

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Students should be able to translate information between graphical and numerical forms; and extract and interpret information from charts, graphs and tables in terms of risk factors.

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Students should be able to use a scatter diagram to identify a correlation between two variables in terms of risk factors.

Cancer

Students should be able to describe cancer as 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. There are also genetic risk factors for some cancers.