Monday, February 29, 2016


Excretionis the elimination of wastes that are formed during the metabolism of various substances in the body.

The main substances excreted are nitrogenous substances like Ammonia, Urea and Uric acid. Nitrogenous wastes are formed in the metabolism of amino acids and nucleic acids.

Along with these excess water, certain salts and organic substances are also excreted. The main purpose of excretion is to maintain ionic balance in the body.

If the metabolic wastes are not discharged from the body, they become toxic and harm the body.

 All the animals do not excrete the type of nitrogenouswastes.
Some animals, like crabs excrete ammonia, while frogs and man excrete urea, insects, reptiles and birds excrete uric acid.

The type of the product excreted may change during different stages of development and availability of water.

Aquatic animals prefer to excrete ammonia while animals like reptiles and birds, to whom water is very scarce, excrete uric acid.

You have learnt that a variety of end products are formed during metabolism and of these nitrogenous wastes are important. Plants do not have any specific organ to excrete these wastes. As in animals, carbon dioxide, water, ammonia and other nitrogenous wastes are also formed in plants.
In the plants, carbon dioxide released during respiration is sent out through stomata of leaves. Due to this, the heat in the plant body is reduced.

 Even when leaves are absent, as in the germinating seeds. Carbon dioxide formed during respiration is released into the atmosphere. Plants have the capacity to utilize the by-products of one metabolic activity as raw materials for another metabolic activity.

oxygen released during photosynthesis is utilized for respiration. Carbon dioxide released during respiration is utilized for photosynthesis.
Plants can convert nitrogen and ammonia into nitrates. However, several compounds are synthesized by the plants for their own use, especially for defence.

 As plants are sessile(do not have the capacity to move), they cannot defend from the predators by running away.
Majority of plants do not have organs for protection against the herbivores. Hence, several plants prepare chemicals and store them in roots, leaves, seeds and even in stem.

Most of these chemicals are unpleasant to taste and hence herbivores usually do not prefer to eat such plants. Some of the chemicals are toxic and may even kill the animal that eats them.

 Some of the plants secrete chemicals when injured. These chemicals  seal the wound and help the plant to recover from the injury.

Though plants produce these chemicals for their use, man has found other uses for these chemicals and benefited immensely from these chemicals.

 Some of the plants release attractants for other Organisms which will help the plant in pollination, seed dispersal or even in their nutrition.
For example, plants having root nodules, secrete chemicals to attract rhizobia in to the surroundings of the roots and form a symbiotic relationship with the rhizobium. These compounds are called secondary metabolites.
 The biochemical substances produced in plants are of two types-primary metabolites and secondary metabolites, Food materials like carbohydrates, fats and proteins are the primary metabolites.

 Alkaloids, Tannins, Resins, Gums, Latex etc., are some of the secondary metabolites in plants.

These are nitrogenous by products and poisonous. These are stored in different parts of the plant. Some of the alkaloids are used in medicines. Common alkaloids, the plant and the part in which they occur are give below……..

Tannins are carbon compounds. These are stored in different parts of a plant of a plant and are deep brown in colour. Tannins are used in tanning of leather and in medicines e.g. cassia, acacia.

Occur mostly in gymnosperms in passages called resin passages. These are used in varnishes-e.g pinus.

Plants like Neem, Acacia secrete a sticky substance called gum when branches are cut. The gums swell absorbing water and help in the healing of cut parts of a plant. Economically, gums are valuable being used as adhesive and binding agents, in the preparation of medicines, foods etc.

Latex is the milky white substance by plants belonging to Euphorbiaceae, Apocyanaceae. Asclepiadaceae and papvaraceae. Latex is stored in latex cells or latex vessels. From the latex of Hevea braziliensis (rubber plant) rubber is prepared.

 Nitrogenous substances are also found in the walls of pollen grains. These are protein substances and when they enter our body, they cause allergy. Hence, they are called allergins. They cause skin allergy(urticaria) asthma etc.

 Aromatic oils and scents occur in the leaves of lemon grass, ocimum, eucalyptus etc.


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