Biology AQA




Microorganism is a general term for single celled organism eg bacteria and viruses.

What is a disease?

Disease suggests a malfunction of the body or mind which has an adverse affect on good health.

Microorganisms as pathogens

For a microorganism to be considered a pathogen it must:

Gain entry to host

colonise the tissue of the host

resist defences of the host and

cause damage to the host tissue.

Disease occurs when an infection leads to recognisable symptoms.

When a pathogen is transferred from one to another it is called transmission.

How do microorganisms get into the body?

Normally get in by penetrating one of the organisms interfaces with the environment. An interface is a surface or boundary linking two systems i.e the the external environment and the internal environment of the body.

An example of an interface is the skin. The skin forms a thick, continuous layer so it is an effective barrier. Invasion can only happen when the skin is broken eg by a cut.

Some interfaces of the body have evolved to allow exchange of material so the body linings at these points are thin, moist, large surface area and a good blood supply. These features makes it easy for entry of pathogens.

Common entry points are the gas exchange system eg TB or bronchitis. Or the digestive system eg cholera

Human body has natural defences to prevent penetration eg a mucuous layer that covers exchange surfaces. Enzymes that break down pathogens and stomach acid that kills microorganisms.

How do pathogens cause disease?

In two ways. One: By damaging hosts tissue i.e preventing tissue functioning properly. Viruses inhibit synthesis od DNA , RNA and proteins. Or they can break down membranes.

Two: by producing toxins i.e the cholera bacterium produces toxin that leads to excessive water loss from the lining of the intestines.

How quickly pathogen causes damage depends on how rapidly it divides.

Correlations and casual relationships

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

Looking critically at data

Has the right factor been measured?

How was the data gathered? Is it reliable?

Has the study been repeated with same outcomes?

What is risk?

Its a measure of the probability that damage to health will occur as a result of a hazard. High probability and high risk causes concern.

Risk can be measured on a scale of 0 to 100

Lifestyle choices and cancer

Smoking- increased risk of cancer

Diet- Low fat, high fibre, rich in fruit and vegetables reduce risk of cancer

Obesity- overweight increases risk of cancer

Exercise- regular exercises lowers risk

Sunlight- more sunlight exposure increases risk of skin cancer

Factors we can control

Smoking- smokers more likely to suffer from CHD

High blood pressure- Stress, diet and lack of exercise increase risk of CHD

Blood cholesterol levels- kept lower by including fewer saturated fatty acids in diet


Diet- high salt levels raises blood pressure and high levels of saturated…


Jodie Horne


thank you! this will help lots for my revision for the january biology exam :)



this is so good thank you



great, but where is the heart, and the cardiac cycle?