Metabolic activities occur all the time in a cell. Some of the By products of such activities are poisonous and must be removed from the cell and eventually from the organism. Sometimes a substance is present in excess amounts in a cell. This too must be removed and got rid of if it cannot be stored. The removal of such substances or wastes is know as excretion.
The main wastes are
• carbon dioxide and water formed as by products during cellular respiration;
• oxygen in an actively photosynthesizing cell;
• nitrogenous wastes produced during the break down of excess amino acids; and
• excess water , salts and other unnecessary substances that enter the cell during its metabolic activities.
Most wastes are present as an aqueous solution and are readily excreted through the cell membrane by diffusion or active transport. In an aquatic protozoan like, the amoeba excess water enter the cell all the time by osmosis. Such a cell usually gets rif of the excess water by means of a contractile vacuole.
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