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SUBJECT: BIOLOGY                                                                                     



(a) Excretory systems

  • (i) Contractile vacuole
  • (ii) Flame cells
  • (iii) Malpighian tubules
  •  (iv) Kidney
  • (v) Stomata and Lenticels.


Definition: Excretory systems refer to all the cells and organs of any living organism which enables it to eliminate harmful substances away from the body.

Excretion is therefore the removal of toxic wastes of metabolism from the body of living organisms.


Waste products produced in the body by excretion must not be allowed to remain in the because of the following reasons:

  1. Waste products when not removed can interfere with normal metabolic activities of the body.
  2. Excretion helps to maintain salt and water balance i.e. homeostasis in the body.
  3. Some products are poisonous and must never be allowed to accumulate within the body.
  4. The excretory products are harmful to the body and so must be removed.
  5. All animals are heterotrophic and may eat some materials as food which is not needed by their bodies. These substances and unwanted products of the chemical activities of the body is excretion.
  6. Proteins are used for growth and repair but any excess cannot be stored. The initial waste products of the biochemical processes involving protein is ammonia gas. This compound is very poisonous and must be excreted quickly.


Contractile vacuole is an organelle or a device found in Amoeba and other unicellular organisms or protozoans. It is a simple device, a small sac lined with a membrane lying freely in the cytoplasm. The cell membrane surrounding the amoeba is a semi permeable to water and because the osmotic pressure inside the animal is greater than outside the animal, water enters the cell by osmosis. To counter this, water is secreted into the contractile vacuole as fast as it enters the body. As this happens, the contractile vacuole expands and bursts, thus discharging its contents to the exterior through a small pore in the cell membrane after which the whole process is repeated. The energy required by the contractile vacuole is produced by the mitochondria found near the vacuole. The contents of the contractile vacuole are; water, carbon (iv) oxide and nitrogenous wastes.


The paramecium is a single-celled organism that propels itself by minute, hairlike projections called cilia. Cilia also create currents that sweep food particles toward the paramecium’s gullet for ingestion. The paramecium gets rid of excess water with the help of a contractile vacuole, which pumps water out of the cell.


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