BRIEF ACCOUNT ON EXCRETION IN PLANTS AND ANIMALS

EXCRETION:

The removal of nitrogenous wastes from the body is called excretion or the elimination of wasteful metabolites, mainly of the nitrogenous nature is called as excretion.

Excretory Products in Plants:

In plants, the mechanism of excretion is different from animals. Plants in their autotrophic mode of life produce oxygen and in metabolism produce CO₂ and H₂O as the excretory products. Plants also produce several organic and inorganic compounds which are stored for various purposes and are also removed when necessary.

Excretion in Plants:

In plants, oxygen could be a waste product of as a result of photosynthesis; carbon dioxide a waste product of respiration and water as the waste product of both.

Mechanisms of Excretion in Plants:

Following are the mechanisms adopted by plants for excretion:

Transpiration:

Water is lost through transpiration or just used for maintaining the turgidity in the cells.

Vacuoles:

Plant cells have large vacuoles; these can be used for either storage of useful compounds, or the storage of waste substances. These may accumulate at the concentrations that lead to crystal formation in the vacuoles.

Leaves:

Rotten Leaf

Plants produce certain wastes of inorganic and organic nature, which are stored in the organs. The leaves are the prominent organs for this purpose. These leaves are destined to fall off, as is the case of autumn leaves in the plants or die off as happens in the leaves and stalk of certain bulb e.g. bluebell leaving the bulb underground. This is the reason gardener find rotten autumn leaves a good source of minerals. The falling of yellow leaves in autumn is the seasonal time for the plants to get rid of the accumulated wastes and because of this reason leaves are said to be excretophore.

According to an explanation the change in the color in these leaves is not due to removal of chlorophyll as the microscopic examination of autumn leaves shows that leaves are loaded with pigmented compounds prior to falling off and many toxic materials like heavy metals increase sharply as the yellowing proceeds.

Branches and Trunks:

Some trees deposit strange chemicals in their branches and trunks, especially in old xylem which is no longer used for water transport. This takes place in Ebony (common name of a flowering plant) which produces very black wood in the centre. These are considered to be waste materials by plant physiologists.

Release of Wastes into Soil:

Some plants will actively secrete waste compounds into the soil, occasionally using them as chemical weapons against other competing plants e.g. conifers.

Excretion in Animals:

Among the assimilated nutrients in animals, carbohydrates and lipids are metabolized to CO₂ and H₂O. Metabolism of proteins and nucleoproteins produce waste nitrogen in various forms (ammonia, urea or uric acid) in different animals. The waste nitrogen proves toxic if it is concentrated in the cell, therefore, must be removed from the body.

Keeping in view the definition of excretion, several products may be included in the list of excretory products.

Main Types of Excretory Products in Animals:

Following are the main types of excretory products in animals:

Water:

Water due to its removal in hyperosmotic environment is labeled as an excretory product in these specific conditions.

Salts:

Salts removed by animals of hypertonic environment are the excretory products for these animals.

Nitrogenous Wastes:

Nitrogenous waste metabolites constitute the major excretory products. Primarily, in the catabolism of amino acids, the amino group (NH₂) is released (deamination) or transferred to another molecule for removal or reuse. Amino group which is not reused for recycling of amino acids is essentially dissolved in water and excreted to avoid toxic rise in the plasma. Elevated levels of these wastes can cause conv ulsions, coma and eventually death. Mostly excess nitrogen is excreted by animals as ammonia, urea or uric acid. Lower quantities of nitrogen are excreted in the form of other compounds such as creatinine, creatine or trimethylamine oxide and in very small quantities as amino acids, purine and pyrimidine. Metabolism of purine and pyrimidine bases produces significant amount of nitrogenous wastes of hypoxanthine, xanthine, uric acid, allantoin, urea and ammonia.

Structure of Amino Acid, Ammonia, Urea and Uric Acid

Following is the comparison between different types of excretory products:

Waste	Advantages	Disadvantages	Habitat	Excreted by
Ammonia	Produced with little energy	Toxic in concentrated solution. Excretion took place in lot of water	Water	Marine and fresh water invertebrates, bony fishes, amphibian
Urea	Less toxic than ammonia. Less water is needed to excrete it	Requires little more energy to produce it	Land, Sea	Adult amphibians, turtles, mammals and bony fishes
Uric Acid	Very little water is used for its excretion	Requires considerable energy to produce it	Land	Reptiles, birds, insects, land, gastropods
Guanine	Relatively non toxic. Less water is used to excrete	More energy is needed	Arid Habitat	Scorpions, Spiders