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TOPIC: EXCRETORY SYSTEM I
CONTENT:
(a) Excretory systems
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- (i) Contractile vacuole
- (ii) Flame cells
- (iii) Malpighian tubules
- Â (iv) Kidney
- (v) Stomata and Lenticels.
EXCRETORY SYSTEMS
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.
NEEDS FOR EXCRETION
Waste products produced in the body by excretion must not be allowed to remain in the because of the following reasons:
- Waste products when not removed can interfere with normal metabolic activities of the body.
- Excretion helps to maintain salt and water balance i.e. homeostasis in the body.
- Some products are poisonous and must never be allowed to accumulate within the body.
- The excretory products are harmful to the body and so must be removed.
- 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.
- 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 VACUOLES
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.
Paramecium
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.
FLAME CELLS IN FLATWORM
These are two longitudinal excretory canals, which open onto the dorsal surface of the flat worm by a number of minute pores. The main canals give off numerous branches, which ramify among the parenchyma cells. The final branches end in flame cells. These are cells, which have intracellular cavities and ducts, with numerous cytoplasmic branches penetrating between the parenchyma cells. Projecting into the cavity of the cell is a bundle of long cilia which arise from basal granules in the cytoplasm. These cilia are characterised by their flickering movement which give use to the name ‘flame cells’.
MALPIGHIAN TUBULES IN INSECTS
Generally, insects are very successful group of animals because of their ability to conserve water, wings to fly, metamorphosis, and small body structure. Â On the other hand, they have an extremely efficient excretory system. The malpighian tubules are the excretory organs. They are found between the midgut and the rectum. One end of each tubule opens into the gut, while the other free end floats in the haemoloel of insects. They are long and extremely slender and penetrate among the visera over. The greater part of the horux and abdomen. In the cockroach for example, they are in six groups with about twelve tubules in each group.
Generalized Anatomy of an Insect
All adult insects have three main body parts—the head, which holds an insect’s primary sense organs; the thorax, which is the attachment site for the legs and wings; and the abdomen, which contains the organs for digestion and reproduction. All of the insect’s soft inner body parts are protected by an external skeleton, or exoskeleton, made of semirigid plates and tubes.
EVALUATION
- Write down the excretory products of insects, and paramecium.
- Explain the excretory mechanism in Amoeba.
KIDNEYS IN VERTEBRATES
All vertebrates have a pair of kidneys, which form a part of the excretory system. In mammals, the kidneys are bean-shaped, dark red in colour and surrounded by fat. The right kidney is slightly lower in the body than the left. If a kidney is cut longitudinally into two, it will be seen to consist of two main regions: an outer dark coloured cortex and an inner lighter coloured medulla. When viewed under a microscope, a kidney consists of blood vessels, kidney tubules or nephrons and connective tissues. Each nephron begins in the cortex as a tiny cup-shaped structure known as Bowman’s capsule, which surrounds capillaries called the glomerulus. The Bowman’s capsule leads into a coiled tube called the proximal tubule. This goes down to form a U-shaped structure, the Henle’s loop. Finally, it coils again to form the distal tubul, which twists and  empties into a collecting duct. Renal artery, which branches from the products and oxygenated blood to the kidney so that nitrogenous wastes products (urea) can be removed.
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