How do normal cells sometimes become concerous? Why are malignant tumors so feared?

OR

(a) What does high ‘Biological Oxygen Demand’ (BOD) of a water body indicate?

(b) Explain the changes in BOD level of the primary effluent during its secondary treatment in sewage treatment tanks.

Normal cells in the body have an age and a time period to live after which they degenerate or divide and differentiate into newer cells. And during this division and differentiation if the cell has some abnormal changes and loses its contact inhibition then it starts to multiply uncontrollably and in a haphazard manner leading to formation of a tumour which may either be benign or malignant (cancerous).

Cells may also become cancerous after coming in contact with a carcinogen which may be external or internal.

OR

a) High Biological Oxygen Demands of a water body indicates that the water body is highly polluted and it contains high quantity of bacteria.

As the amount of bacteria increases the B.O.D. of water also increases.

b)The B.O.D. of primary effluents during secondary treatment in sewage treatment tanks decreases by 10-15% as the bacterial masses are held together by the use of flocs and the microbes digest a lot of organic matter.