BIOLOGY FORM THREE TOPIC 4: EXCRETION

Concept of Excretion
The Concept of Excretion
Explain the concept of excretion
Chemical reactions occur in the cells of living organisms all the time to carry out the life processes. The sum of these reactions is called metabolism. Metabolism produces useful products as well as toxic (poisonous) by-products.
These toxic substances have to be removed as they are harmful if allowed to accumulate. The removal of metabolic waste products from the body of an organism is known as excretion.
The major excretory products are carbon dioxide, excess water, and nitrogenous compounds like ammonia, urea, uric acid, etc. Carbon dioxide and water are produced in the process of tissue respiration. Nitrogenous compounds are formed from the breakdown of proteins and amino acids. Water and salts in excess of the body’s needs are also excreted.
Other excretory products include chemicals from medicines, toxic substances, and circulating hormones that have already served their purpose. We will learn how metabolic wastes get eliminated.
In concise, excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism.
Examples of Excretory Products Eliminated by Organisms
Give examples of excretory products eliminated by organisms
Living organisms excrete various excretory products of diverse chemical nature. The following are examples of excretory products excreted by living organisms:
  • Carbon dioxide:This is a by-product of respiration of both plants and animals. It is excreted through the pores of the stomata in plants (some of the carbon dioxide produced by respiration is used in photosynthesis). In man, carbon dioxide is eliminated from the body by lungs.
  • Water:The concentration of water in cells must be kept within narrow limits. Too little or too much water can have a negative effect on the osmotic condition in and around the cell. Therefore, it has to be regulated. Plant cells are protected from bursting by their cell walls. Animals do not have cell walls, and will burst if they have too much water. Excess water is lost from the surface of gaseous exchange in both plants and animals. In mammals, water is also lost through sweat and through osmoregulation controlled by the kidneys.
  • Urea:This is a compound produced in mammals from the breakdown of excess amino acids. Amino acids cannot be stored because their accumulation is toxic. They are therefore converted into a less toxic substance. This process occurs in the liver and is called de-amination.Ammonia is converted to urea by the liver. Urea is transported by blood to the kidneys where they are excreted. The kidneys are also used to remove uric acid, water, excess salts, excess hormones and bile pigments.
  • Calcium oxalate:This is a waste material produced by plants and is stored as an insoluble crystalline structure in the cells. Calcium oxalate is stored in aging leaves, stems and roots, flowers or fruits.
  • Oxygen:Through the process of photosynthesis, oxygen is produced as a by-product. Some of the oxygen is used for respiration, and the remainder is excreted through the stomata of the leaves.In plants, some waste substances are stored in parts of the plant that are dead. Examples of this are the tannin in the bark of trees such as mangroves and the dyes in the heartwood of trees such as logwood. The purpose of the storage of waste material ranges from protection to a decreased risk of being consumed.

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