AQA AS Biology - Chapter 1

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  • Created on: 03-04-13 15:31

1.1 Pathogens (1)

Pathogens - "Micro organisms which cause disease"

For a micro organism to be considered a pathogen it must:

  • Gain entry to the host
  • Colonise the host's tissues
  • Resist the defences of the host
  • Cause damage to the host tissues

If a pathogen gets into the host and colonises its tissue an infection results. Disease occurs when an infection leads to recognisable symptoms in the host. When a pathogen is transferred from one individual to another, it is known as transmission.

To help prevent the entry of pathogens, the body has a number of natural defences including:

  • A mucous layer that covers exchange surfaces and forms a sticky, thick barrier that is difficult to penetrate
  • The production of enzymes that break down the pathogens
  • The production of stomach acid which kills micro organisms
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1.1 Pathogens (2)

Interface - "A surface or boundary linking two systems"

Micro organisms get into the body by penetrating one of the organism's interfaces with the environment, in this case the interface linking the external environment to the internal environment of the body. e.g. the skin. As the skin is a thick, continuous layer, it is an effective barrier to pathogens. Therefore invasion only occurs through the skin when it is broken e.g. cuts, abrasions, insect bites.

Other interfaces have evolved to allow material exchange (diffusion) between the internal and external environments, and so the body linings at these points are thin, moist, have a large surface area, and have a good blood supply. Therefore, entry is not only easy for molecules, but also for pathogenic micro organisms.

These common points of entry include:

  • The gas-exchange system - Pathogens that cause influenza, tuberculosis & bronchitis enter through here.
  • The digestive system - Food and water may carry pathogens into the stomach and intestines via the mouth. e.g. cholera and typhoid bacteriums.
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1.1 Pathogens (3)

Pathogens affect the body in two main ways:

  • By damaging host tissue - Sometimes the sheer number of pathogens causes damage, e.g. preventing tissues functioning properly. Viruses inhibit the synthesis of DNA, RNA and proteins by the host cells. Many pathogens break down the membranes of the host cells.
  • By producing toxins - Most bacterial pathogens produce toxins, e.g. the cholera bacterium produces a toxin that leads to excessive water loss from the lining of the intestines. Hence the classic symptom of diarrhoea.

How quickly a pathogen causes damage, and hence the onset of symptoms is related to how rapidly the pathogen divides. e.g. Pathogens causing gastroenteritis divide about every 30 minutes, so symptoms of vomiting and diarrhoea become apparent within 24 hours.

N.B. Oral antibiotics are not normally used to treat gastroenteritis and other diarrhoeal diseases because the symptoms of vomiting and diarrhoea mean that the antibiotic is unlikely to remain in the body long enough to be absorbed.

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1.2 Data and Disease

Epidemiology - "The study of the incidence (number of cases) and pattern of a disease with a view to finding the means of preventing and controlling it"

Correlation occurs when a change in one of two variables is reflected by the change in the other variable.

Although a graph may show a correlation between two variables, without experimental evidence we cannot conclude that the two variables are directly linked. Therefore:

Correlation does not mean there is a causal link

When deciding how reliable data is, consider the following:

  • Has the right factor been measured and have the correct questions been asked?
  • How was the data gathered, were methods reliable, and was the right apparatus used?
  • Has the study been repeated, with the same results, by other people?
  • Are there still unanswered questions?
  • Do those collecting the data have a vested interest (a personal stake) in the outcome of the research?
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1.3 Lifestyle and Health (1)

Risk - "A measure of the probabilty that damage to health will occur as a result of a given hazard"

The concept of risk has two elements:

  • The probability that a hazardous event will occur.
  • The consequences of that hazardous event.

Risk can be measured from a value of 0% (no harm will occur), to a value of 100% (harm will definitely occur).

Health risks need a timescale. To tell someone they will die is meaningless - we all die. To tell someone that their risk of dying in the next month is 100% has a completely different meaning.

Risk is often relative - It is measured by comparing the likelihood of harm occuring in those exposed to a hazard with those who are not exposed to it, e.g. smokers and non-smokers.

Misleading statistics are often single figures which give the impression of applying to everyone, when this is often not the case.




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1.3 Lifestyle and Health (2)

We cannot control our age or genetic factors, but our lifestyle - something we do control - can expose us to environmental and carcinogenic factors that put us at risk of developing cancer.

Specific lifestyle factors that contribute to cancer include:

  • Smoking. As well as smokers themselves, those who passively breathe tobacco smoke (second-hand smoke) also have an increased risk of getting cancer.
  • Diet. There is strong evidence that a low-fat, high-fibre diet, rich in fruit and vegetables reduces the risk of contracting cancer.
  • Obesity
  • Physical Activity
  • Sunlight. The more someone is exposed to sunlight or light from sunbeds, the greater the risk of skin cancer.

Some epidemiological evidence linking smoking to disease includes:

  • Smokers make up 98% of emphysema sufferers.
  • A regular smoker is 3x more likely to die prematurely than a non-smoker.
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1.3 Lifestyle and Health (3)

Coronary Heart Disease - Largest cause of death in the UK.

Risk factors that we can control:

  • Smoking. Giving up smoking is the single most effective way of increasing life expectancy.
  • High blood pressure. Causes include certain diets, prolonged stress and lack of exercise.
  • Blood cholesterol levels. Can be kept lower by including fewer saturated fats in the diet.
  • Obesity. A BMI of over 25 brings an increased risk of CHD.
  • Diet. High levels of salt raise blood pressure, while fibre lowers blood cholesterol levels.
  • Physical activity. Aerobic exercise can lower blood pressure and blood cholesterol.

Measures that we can take to reduce our chances of getting both cancer and CHD include:

  • Giving up or not taking up smoking,
  • Avoiding becoming overweight,
  • Reducing salt intake in the diet,
  • Taking regular aerobic exercise,
  • Increasing the intake of dietary fibre and antioxidants in the diet.
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