Excretion is elimination of metabolic waste, which is an essential process in all . In , this is primarily carried out by the lungs, kidneys, and skin.
During life activities such as cellular respiration, several chemical reactions take place in the body. These are known as metabolism. These chemical reactions produce waste products such as carbon dioxide, water, salts, urea and uric acid. Accumulation of these wastes beyond a level inside the body is harmful to the body. The excretory organs remove these wastes. This process of removal of metabolic waste from the body is known as excretion.
Processes across various types of life
Plants
Green
plants excrete carbon dioxide and water as respiratory products. In green plants, the
carbon dioxide released during respiration gets used during photosynthesis.
Oxygen is a byproduct generated during
photosynthesis, and exits through
stomata, root cell walls, and other routes. Plants can get rid of excess water by transpiration and
guttation. It has been shown that the leaf acts as an 'excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion. Other waste materials that are exuded by some plants —
resin, saps,
latex, etc. are forced from the interior of the plant by hydrostatic pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, they act passively. However, during the pre-abscission phase, the metabolic levels of a leaf are high.
Plants also excrete some waste substances into the soil around them.
[http://www.tutorvista.com/content/science/science-ii/excretion/excretion-plants ]
Animals
In animals, the main excretory products are
carbon dioxide,
ammonia (in
),
urea (in
),
uric acid (in
),
guanine (in
Arachnida), and
creatine. The liver and kidneys clear many substances from the
blood (for example, in renal excretion), and the cleared substances are then excreted from the body in the
urine and
feces.
usually excrete ammonia directly into the external environment, as this compound has high solubility and there is ample water available for dilution. In terrestrial animals, ammonia-like compounds are converted into other nitrogenous materials, i.e. urea, that are less harmful as there is less water in the environment and ammonia itself is toxic. This process is called detoxification.
Birds
excrete their nitrogenous wastes as
uric acid in the form of a paste. Although this process is metabolically more expensive, it allows more efficient
water retention and it can be stored more easily in the egg. Many avian species, especially
, can also excrete salt via specialized nasal salt glands, the saline solution leaving through nostrils in the
beak.
Insects
In
, a system involving Malpighian tubules is used to excrete
metabolic waste. Metabolic waste diffuses or is actively transported into the tubule, which transports the wastes to the intestines. The metabolic waste is then released from the body along with fecal matter.
The excreted material may be called ejecta.
See also
External links
