AQA GCSE Biology: Cancer
A clear GCSE Biology revision guide to cancer: how tumours form, the difference between benign and malignant tumours, and the risk factors for cancer.
Cancer is caused by cells dividing out of control. This guide explains how tumours form, the difference between benign and malignant tumours, the lifestyle and genetic factors that raise the risk, and why a risk factor is not the same as a cause. The distinction to master is benign versus malignant.
How tumours form
Cells normally divide only when the body needs them to, and the cell cycle controls when that happens. Changes in the genes that control the cycle can break that control, so a cell divides again and again when it should not. The result is uncontrolled growth and division, producing a mass of abnormal cells called a tumour.
Not every tumour is cancer, which is what the next section turns on.
Benign and malignant tumours
| Benign | Malignant | |
|---|---|---|
| Cancerous? | No | Yes |
| Growth | Slower | Faster |
| Boundary | Contained, usually inside a membrane | Invades surrounding tissues |
| Spread | Stays in one place | Cells break away and spread |
Benign tumours are not cancerous. They grow slowly, stay in one place, usually contained within a membrane, and do not invade nearby tissues or spread elsewhere.
Malignant tumours are cancerous. They grow faster, invade the surrounding tissues, and can metastasise: cells break away, travel in the bloodstream, and form secondary tumours in other parts of the body. It is the spreading that makes malignant tumours so dangerous, because a secondary tumour can start growing somewhere far harder to treat.
One thing worth knowing: benign does not mean harmless. A benign tumour growing inside the skull has nowhere to expand and can press on the brain. What makes a tumour benign is that it stays where it is, not that it does no damage.
Risk factors for cancer
The risk of cancer is raised by both lifestyle and genetic factors. Questions often name a factor and ask which cancer it is linked to, so it is worth learning the pairs.
- Smoking is strongly linked to lung cancer, and also to cancers of the mouth and throat.
- Obesity raises the risk of several cancers, including bowel and liver cancer.
- Ultraviolet light from the sun or sunbeds raises the risk of skin cancer.
- Ionising radiation, such as X-rays and gamma rays, damages DNA and raises the risk of several cancers.
- Alcohol is linked to cancers of the liver, mouth and throat.
- Some viruses raise the risk, and infection with certain strains of HPV is linked to cervical cancer.
- Genetic factors: some people inherit faulty genes that make particular cancers far more likely.
A substance that causes cancer is called a carcinogen.
Risk factors are not the same as causes
This is the part that catches people out. A risk factor increases the chance of a disease; it does not guarantee it. Most smokers do not get lung cancer, and some people who never smoked do.
Showing that smokers get lung cancer more often than non-smokers is a correlation. Showing that the chemicals in tobacco smoke damage the DNA in lung cells gives a causal mechanism, and that is what turns a correlation into a cause. If a question gives you data and asks whether one thing causes the other, that is the distinction it is testing.
How this fits together
Cancer completes the human part of Topic 2. The topic then turns to plants, starting with the tissues that make up a leaf, covered in the plant tissues guide.