Cancer can occur in any tissue of the body. It results from a change in certain cells that do not follow normal growth limits and begin to multiply at an erratic rate. A tumour forms when the rate of cell multiplication is greater than that of cell death. The reasons for this uncontrolled cell multiplication are not known. Some factors are known to increase changes found in tumour cells. The process by which normal cell becomes cancerous in nature is called carcinogenesis. The agents that lead to this are called carcinogens.
Examples of carcinogens:
Environmental agents such as chemicals, irradiation and oncogenic viruses.
6. Nickel compounds
7.Some fuel oils
8. Vinyl chloride.
X-rays, radioactive isotopes, environmental radiations and ultraviolet rays in sunlight.
Viruses, some consisting of DNA and some of RNA can cause mutation and thereby cancer.
Substances are transported through the cell membrane by 2 major processes.
Diffusion : This is the physical process involved in the 'downhill' movement of substances, i.e. from a higher concentration on one side to a lower concentration on the other, without the use of energy.
Active transport : This is the 'uphill' transport of substances across the membranes, i.e. from a lower concentration on one side to a region of higher concentration with the use of energy.