Edexcel IGCSE Biology

Revision Notes

2.9.1 Excretion

The Need for Excretion in Humans

  • Many of the necessary metabolic reactions that take place within the cells of organisms produce waste products
  • Excretion is the removal of the waste substances of metabolic reactions, toxic materials and substances in excess of requirements
  • Metabolic wastes produced by the human body include:
    • Carbon dioxide and water from aerobic respiration in cells
    • Urea produced by the breakdown of excess proteins (amino acids) in the liver
    • Other substances in excess such as medical drugs (e.g. antibiotics) and dietary minerals (e.g. sodium)

The danger of waste products

  • If waste products are allowed to build up they can have a range of negative effects on the body:
    • Toxicity – waste products can have toxic effects if they are allowed to reach high concentrations
      •  Carbon dioxide dissolves in water easily to form an acidic solution which can lower the pH of cells. This can reduce the activity of enzymes in the body which are essential for controlling the rate of metabolic reactions
    • Osmotic effect – body fluids can become more concentrated due to higher amounts of waste products
      • Concentrated body fluids can cause water to move out of cells, changing their water potential and preventing them from carrying out essential reactions
    • Using up necessary storage – space within an organism is limited and is required for the storage of more useful molecules

Exam Tip

Be careful not to get confused between excretion and egestion, they are two very different things!

Egestion is the expulsion of food that has never been absorbed from the body (as faeces) via the anus.

The Need for Excretion in Plants

  • Within plant cells there is a range of metabolic reactions taking place producing waste products
  • Some of these waste products can be used up in other processes in the plant while some must exit the plant via the leaf organ
  • Waste products or substances in excess within a plant can include:
    • Oxygen
    • Carbon dioxide
    • Water/water vapour
    • Other unwanted chemical substances

Oxygen and carbon dioxide

  • Oxygen and carbon dioxide can be both reactants and waste products within a plant
  • The amount or intensity of light affects the waste products within plants
  • During the day, when there is sufficient light:
    • The rate of photosynthesis is higher than the rate of respiration
    • More oxygen is released than used in respiration
    • Less carbon dioxide is released than used in photosynthesis
    • Net effect – oxygen is in excess and a waste product
  • During the night, when there is insufficient light:
    • There is no photosynthesis, only respiration
    • Oxygen is used in respiration and carbon dioxide is produced
    • No photosynthesis means that no carbon dioxide is used
    • Net effect – carbon dioxide is in excess and a waste product
  • Whichever gas is in excess diffuses out of the plant via the leaf organ
    • The gases exit through the stomata
  • As the excretion of gases in plants occurs via diffusion it is technically not an active process

Water vapour

  • The majority of water vapour lost from a plant is not a waste product of metabolism, but instead water that has been drawn up from the roots in the transpiration stream

Excretion in plants, downloadable AS & A Level Biology revision notes

Excretion in plants during the daytime compared to night time

Chemical substances

  • Plant cells can break down molecules into chemical substances no longer required by the plant
  • Some of these substances cannot be converted into another useful compound and so must be removed from the plant
  • Chemical waste materials such as this can be stored in the dying tissues of a plant
  • When the dying tissue falls off the plant the substances are removed
    • E.g. in autumn the leaves of deciduous trees turn a variety of colours due to the presence of chemical waste products
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