Adaptations of Gas Exchange Surfaces (AQA AS Biology): Revision Note

Adaptations of gas exchange surfaces

• Effective gas exchange needs to take place in order to, e.g.:

  • supply oxygen for respiration

  • remove waste carbon dioxide from respiration

• The features of exchange surfaces ensure that gas exchange can take place at a sufficient rate

Single-celled organisms

• Single-celled organisms, e.g. amoeba, carry out gas exchange at the cell surface by simple diffusion

• Their high SA:V ratio means that:

  • diffusion occurs at a high rate over their relatively large surface area

  • the diffusion distance from the surface to all parts of the cell is short

Specialised gas exchange systems

Insects

• Gas exchange in insects occurs via the tracheal system

• Air enters the bodies of insects via openings in the exoskeleton known as spiracles

• Air flows into tracheae tubes, and then into narrower tubes called tracheoles

• Many tracheoles lead to the muscle fibres, where their endings provide a large surface area for gas exchange

• Movement of gases in the tracheal system mainly relies on diffusion gradients

  • Oxygen moves down its concentration gradient from the air into the respiring muscle cells

  • Carbon dioxide moves down its concentration gradient from the respiring muscle cells into the air

• Active insects may need a more rapid supply of oxygen, which they gain using rapid contractions of the abdominal muscles to draw oxygen into the tracheae down a pressure gradient

Fish

• Fish are adapted to extract oxygen from water

• Fish have gills to maximise surface area for gas exchange

  • There are a 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

• Gas exchange in the gills is maximised by a counter-current system

  • The blood in the capillary system flows in the opposite direction to the flow of water as it passes over the fills

  • This ensures that the concentration gradient is maintained along the whole length of the capillary

    • The water that enters the capillary has the highest oxygen concentration, and this flows adjacent to the blood that is already partially oxygenated

    • The water that exits the capillary has the lowest oxygen concentration, and is adjacent to the most deoxygenated blood

Dicotyledonous plants

• Plants need carbon dioxide for photosynthesis, and oxygen for respiration, and the leaves are adapted to maximise exchange of these gases

• Leaf adaptations for gas exchange include:

  • the spongy mesophyll layer

    • Air flows into and around the air spaces

    • The surfaces of the spongy mesophyll cells come into contact with the air spaces, creating a large surface area for gas exchange

  • the stomata

    • These are pores on the underside of most leaves which allow air to enter and exit the leaf

    • Guard cells control the opening and closing of the stomata

  • the shape of leaves

    • Leaves are flat and thin, reducing the diffusion distance for gases

• Gases move in and out of the cells of the leaf due by diffusion down their concentration gradients