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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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