OCR A Level Biology

Revision Notes

5.2.1 The Importance of Excretion

The Importance of Excretion

  • Excretion is the process by which toxic waste products of metabolism and substances in excess of requirement are removed from the body
  • For example:
    • The lungs excrete the waste product carbon dioxide by gas exchange and the act of breathing out (exhalation)
    • The kidneys produce urine that contains the waste product urea in solution
  • Excretion is a key process in homeostasis and is important in maintaining metabolism, as metabolic waste can have serious negative consequences on the body if allowed to accumulate

Metabolic waste

  • As mammals are active, warm-blooded animals, they have high metabolic rates, which means they also produce relatively large amounts of metabolic waste
  • This metabolic waste includes:
    • Carbon dioxide
    • Nitrogenous waste (ammonia, urea and uric acid)
    • Bile pigments (produced during the breakdown of haemoglobin)
  • Carbon dioxide is produced from the decarboxylation of respiratory substrates
  • Ammonia is produced from the deamination of excess amino acids
  • If these two waste products are not excreted properly, they can accumulate and change the cytoplasm and body fluid pH, which can cause enzymes to work less efficiently
  • The effects that different metabolic wastes can have on the body if allowed to accumulate are summarised in the table below
    • Note that the liver is a key organ in producing all of these excretory substances (except for carbon dioxide)

Metabolic Wastes Table

Metabolic wastes table, downloadable AS & A Level Biology revision notes


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