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.
EXCRETION AND RELEASE OF SUBSTANCES IN PLANTS:
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.
Alkaloids:
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:
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.
Resins:
Occur
mostly in gymnosperms in passages called resin passages. These are used in
varnishes-e.g pinus.
Gums:
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:
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.
THANKYOU,