Gas exchange in single-celled organisms and insects

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  • Gas exchange in single-celled organisms and insects
    • Gas exchange in single-celled organisms
      • Single-celled organisms are small and therefore have a large surface area to volume ratio
      • Oxygen is absorbed by diffusion across their body which is covered only by a cell surface membrane
      • Carbon dioxide diffuses out across their body surface
    • To reduce water loss they have waterproof coverings over their body surfaces this is a rigid outer skeleton that is covered with a waterproof cuticle and they have a small surface area to volume ratio to minimise the area water can be lost
    • Cannot use body surface to exchange gases so have developed internal network of tubes called tracheae. Supported by strengthened rings to stop them collapsing.
      • Divide into smaller tubes called tracheoles. These extend throughout all the body tissues of the insect This brings oxygen straight to the respiring tissues
    • Gases move in and out the tracheal system in two ways
      • Along a diffusion gradient
      • Ventilation. The movement of muscles in insects can create mass movements of air in and out of the trachea. This further speeds up the exchange of respiratory gases
    • Gases enter and leave trachea through tiny pores, called spiracles on the body surface. They open and close with a valve.
    • When the spiracles are open water can evaporate from them. For much of the time they keep these closed to prevent water loss


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