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Excretion In Plants And Animals


A large number of waste products are formed during metabolic activities in the body. These products become toxic or harmful if it is retained inside. Large amounts of carbon dioxide and water are produced by metabolism of carbohydrates, fats and proteins. Nitrogenous wastes such as ammonia, urea, uric acid, etc. are formed from proteins and other complex nitrogenous compounds.

The process by which metabolic wastes are eliminated from the body is called excretion.

Excretion in Plants

  • Plants do not have specialised structures like nephrons for excretion of wastes.
  • Oxygen, which is a waste product of photosynthesis, and carbon dioxide, which is a waste product of respiration, diffuse out through the stomata in leaves and lenticels in stems.
  • In land plants, excess water which is a waste product of respiration is removed by evaporation or transpiration.
  • Transpiration is the loss of water in the form of vapour from the aerial parts of a plant, particularly the leaves.
  • All the water that is absorbed by the plants is not utilised. Nearly 95% of the absorbed water is lost either in the form of vapour or in the form of liquid.
  • There are three types of transpiration namely cuticular, lenticular and stomatal transpiration. Out of these, most transpirations occur in leaves through stomata. The number and distribution of stomata varies in different types of plants.
  • Rate of transpiration in a dorsiventral leaf is greater at the lower surface because more stomata are situated on the lower surface.
  • Transpiration in plants will be lowest when there is high humidity in the atmosphere. The process of transpiration keeps the plant cool.
  • Potometer is the device used to determine the rate of transpiration.
  • Synthetic substances such as antitranspirants reduce the rate of transpiration without affecting photosynthesis.
  • Phenylmercuric acetate, aspirin and salicylic acid are some of the metabolic inhibitors which are used as antitranspirants.
  • Loss of water in a liquid form from uninjured parts during night and early mornings along the margins of leaves of herbaceous plants through hydathodes is called guttation. Root pressure is said to be the cause of guttation.
  • Plants also get rid of their wastes by combining them with inorganic salts to form insoluble crystals which are harmless. These are stored in different parts of the plant body. Wastes stored in leaves are lost when the leaves fall in autumn. Examples of plant wastes are gum, resin, rubber and tannin.

Excretion in Animals

  • In unicellular animals such as amoeba, the excretory products of metabolism diffuse out over the whole cell surface. The contractile vacuoles assist in osmoregulation.
  • Coelenterates do not have any special excretory organs. The nitrogenous waste matter is diffused out through the general surface of the body. The solid waste matter escapes through the mouth.
  • In flatworms, excretory units are single cells called flame cells.
  • The excretory organs of earthworms are tubular in structure called nephridia.
  • The excretory system in human comprises a pair of kidneys and the urinary tract.
  • Nitrogenous wastes excreted by animals are in the form of ammonia, urea or uric acid. Organisms excreting ammonia as their nitrogenous wastes are called ammonotelic organisms, e.g., aquatic organisms
  • such as fish and tadpole of frog. Organisms excreting urea as their nitrogenous wastes are called ureotelic organisms, e.g., terrestrial animals such as mammals and few fishes. Organisms excreting uric acid as their nitrogenous wastes are called uricotelic organisms, e.g., reptiles and birds.
  • Animals which face shortage of water generally excrete uric acid as the principal nitrogenous waste product because uric acid is highly insoluble in water and its elimination minimises water loss.
  • Excretory product of cockroach is uric acid. Excretion of uric acid is helpful in conserving body water.
  • A marine bony fish continuously must acquire water and get rid of salt whereas a freshwater fish must continuously get rid of water and acquire salt.
  • A nephridium of an earthworm removes waste materials directly from the coelom.
  • In insects, Malpighian tubules drain materials directly from the haemocoel.
  • Apart from kidneys, excretion is also carried out by liver. The conversion of ammonia into urea occurs mainly in liver.
  • The kidney in mammals is to excrete extra urea, salts and excess water.

Excretory System of Man The excretory system of man includes two kidneys. The ureters that start from kidneys open into the sac-like urinary bladder that collects the urine. The urinary bladder opens outside by urethra.

Kidneys are dark red, bean-shaped structures, located at the back of abdominal cavity one on either side of the backbone. Left kidney is placed at a higher level than the right one.

  • There is a depression in each kidney called helum. At this point, the renal artery enters into the kidney and renal vein and ureter exit from the kidney. Each kidney is covered with a tough connective tissue capsule called the renal capsule.
  • Human kidney shows three main zones, namely outer cortex, middle medulla and an inner cavity called renal pelvis.
  • The basic unit of a vertebrate kidney is the nephron. There are approximately one million nephrons in each human kidney.
  • Nephrons are connected with excretory system.
  • The glomerules of mammalian kidney are concerned with urine formation through blood filteration.
  • Glomerular filtration occurs because afferent glomerular arteriole is wider than the efferent arteriole.
  • Bowman’s capsule and glomerulus lie in the cortex of the kidney and represent the filtering unit.
  • Most of the water that filters into Bowman’s capsule is reabsorbed.
  • The loop of Henle is important in concentrating the urine. It is well developed in mammals. The ascending limb of the loop of Henle is least permeable to water.
  • Reabsorption of glucose in a nephron occurs in the first half of proximal tube.
  1. Osmoregulation: Osmoregulation refers to balancing of the osmotic pressure between the extracellular fluid (blood) and the intracellular fluid. Osmotic pressure of the blood depends upon the amount of water and other substances present in the blood.
  2. Functions of Kidney:
  • The role of kidney in osmoregulation lies in the removal or retention of water, salts, etc.
  • Kidney plays an important role in regulating the composition of blood by maintaining the mineral balance, exact proportion of water and acid–alkali balance.
  • It excretes wastes such as urea.
  1. Diseases and disorders of kidney:
  • Antidiuretic hormone is secreted by pituitary gland. Lack of antidiuretic hormone (ADH) results in excessive urination.
  • Kidney stones are formed by the accumulation of calcium oxalate. Some stones are formed by hardening of uric acid. Now-a-days the stones are broken into smaller pieces by a surgery called lithopsy.
  • If the kidney is unable to perform its filteration, reabsortion and secretion, the condition is called kidney failure. It may be due to the infection of glomerulus. Excess of urea in blood resulting from kidney failure is called uraemia.
  • Nephritis is the inflammation of kidney which often results from an infection of the throat or the skin.
  • Many kidney problems can be treated by dialysis, where a machine acts as a kidney. Kidney transplants are an alternative to dialysis.
  1. Other organs of excretion:
  • The lungs excrete carbon dioxide and a considerable amount of water vapour.
  • The skin also excretes wastes in the form of sweat. Water and wastes diffuse from the blood into the sweat glands present in the skin and escape the body as sweat through pores in the skin.

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