OCR AS Biology

Revision Notes

3.1.6 Gas Exchange in Fish & Insects

Adaptations of Gas Exchange Surfaces

  • Effective exchange surfaces in organisms have:
    • A large surface area
    • Short diffusion distance
    • Concentration gradient (maintained)

Across the Body Surface of a Single-celled Organism

  • Chlamydomonas is a single-celled organism that is found in fresh-water ponds. It is spherical in shape and has a diameter of 20μm. Oxygen can diffuse across the cell wall and cell surface membrane of Chlamydomonas
  • The maximum distance that oxygen molecules would have to diffuse to reach the centre of a Chlamydomonas is 10μm, this takes 100 milliseconds
  • Diffusion is an efficient exchange mechanism for Chlamydomonas

Tracheal System of an Insect

  • All insects possess a rigid exoskeleton with a waxy coating that is impermeable to gases
  • Insects have evolved a breathing system that delivers oxygen directly to all the organs and tissues of their bodies
  • A spiracle is an opening in the exoskeleton of an insect which has valves
    • It allows air to enter the insect and flow into the system of tracheae
  • Tracheae are tubes within the insect respiratory system which lead to tracheoles (narrower tubes)
    • There are rigid rings of cartilage that keep the tracheae open
  • A large number of tracheoles run between cells and into the muscle fibres – the site of gas exchange
  • For smaller insects, this system provides sufficient oxygen via diffusion

Tracheal System of Insect, downloadable AS & A Level Biology revision notes

Image showing the structure of the tracheal system of an insect

  • Very active, flying insects need a more rapid supply/intake of oxygen. They create a mass flow of air into the tracheal system by:
    • Closing the spiracles
    • Using muscles to create a pumping movement for ventilation
  • Also, during flight the water found at the narrow ends of the tracheoles is drawn into the respiring muscle so gas diffuses across quicker

Gills of Fish

  • Oxygen dissolves less readily in water
    • A given volume of air contains 30 times more oxygen than the same volume of water
  • Fish are adapted to directly extract oxygen from water
  • Structure of fish gills in bony fish:
    • Series of gills on each side of the head
    • Each gill arch is attached to two stacks of filaments
    • On the surface of each filament, there are rows of lamellae
    • The lamellae surface consists of a single layer of flattened cells that cover a vast network of capillaries
  • Mechanism:
    • The capillary system within the lamellae ensures that the blood flow is in the opposite direction to the flow of water – it is a counter-current system
    • The counter-current system ensures the concentration gradient is maintained along the whole length of the capillary
    • The water with the lowest oxygen concentration is found adjacent to the most deoxygenated blood

Gills in a Fish (1), downloadable AS & A Level Biology revision notesGills in a Fish (2), downloadable AS & A Level Biology revision notes

Image showing the structure of fish gills and the counter-current system within gills.

Leaves of Dicotyledonous Plants

  • In order to carry out photosynthesis, plants must have an adequate supply of carbon dioxide
  • There is only roughly 0.036% CO2 in the atmosphere, so efficient gas exchange is necessary
  • Leaves have evolved adaptations to aid the uptake of carbon dioxide
  • Structure of a leaf:
    • Waterproof cuticle
    • Upper epidermis – layer of tightly packed cells
    • Palisade mesophyll layer – layer of elongated cells containing chloroplasts
    • Spongy mesophyll layer – layer of cells that contains an extensive network of air spaces
    • Stomata – pores (usually) on the underside of the leaf which allow air to enter
    • Guard cells – pairs of cells that control the opening and closing of the stomata
    • Lower epidermis – layer of tightly packed cells
  • Mechanism:
    • When the guard cells are turgid (full of water) the stoma remains open allowing air to enter the leaf
    • The air spaces within the spongy mesophyll layer allows carbon dixoide to rapidly diffuse into cells
    • The carbon dioxide is quickly used up in photosynthesis by cells containing chloroplasts – maintaining the concentration gradient
    • No active ventilation is required as the thinness of the plant tissues and the presence of stomata helps to create a short diffusion pathway

Leaf Structure, downloadable AS & A Level Biology revision notes

Image showing the structure of a leaf from a dicotyledonous plant

Adaptations of Gas Exchange Surfaces

Adaptations of Gas Exchange Surfaces, downloadable AS & A Level Biology revision notes

Exam Tip

Make sure you know how and why each system above is adapted for efficient gas exchange.

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