Gas exchange in plants

HideShow resource information
View mindmap
  • Gas exchange in Plants
    • Respiration: Glucose + Oxygen -> CO2 + H2O + ATP
    • Photosynthsis: CO2 + H2O ->(Light) Glucose + Oxygen
    • Plants require CO2 as well as light for photosynthesis. They have leaves which are adapted for both light absorption and gas exchange
    • Plants rely entirely on diffusion for the exchange of gases
    • Structure of an (angiosperm) leaf
      • The cuticle is impermeable to water, so reduces water loss (but also limits gas exchange)
      • The epidermis prevents mechanical damage
      • The palisade mesophyll is the main site of photosynthesis
      • Spongy mesophyll allows the circulation of gases and is the main site of gas exchange
      • Vascular bundle transports water and mineral ions in the xylem/sucrose in the phloem
      • The air spaces allows circulation of water and gas needed in exchange
      • The stomata permit the entry and exit of gases (and allow water through)
      • Guard cells open and close the stomata
    • Movement of Gases
      • Day
        • The rate of photosynthesis greater than the rate of respiration so overall, more oxygen is released
      • Night
        • The rate of respiration is smaller than the rate of photosynthesis so overall, more carbon dioxide is released by the plant
    • Adaptations of the leaf for light absorption
      • Leaves are flat and have a large surface area to absorb as much light as possible
      • Leaves can grow towards the light and expose a greater surface area
      • Leaves are thin to allow light to penetrate the lower layers of cells
      • Cuticle and epidermis are transparent to allow light to penetrate the the mesophyll tissue
      • Palisade cells are elongated and to reduce the total number of cell walls that would absorb the light and prevent it from getting to the chloroplasts
      • Palisade cells contain many chloroplasts. These can move around within the cells to gain the best position for light absorption
    • Adaptations for gas exchange/CO2 absorption
      • Leaves are thin to reduce the absorption distance
      • The spongy mesophyll;
        • Has a large surface area for gas exchange
        • Contains air spaces to allow the circulation of gases and reduce the diffusion pathway of CO2 into cells
        • Is moist for the absorption of CO2
      • Stomatal pores allow the entry of gases into the air spaces
    • Adaptations to reduce water loss from gas exchange surfaces
      • Leaves have a waxy cuticle on the upper epidermis to reduce water loss by evaporation
      • Most water is lost from the plant via the stomatal pores
        • The stomatal pores are located on the lower epidermis which helps reduce water loss via evaporation
      • Guard cells surrounding the stomatal pores can change shape
        • They open the stomatal pores during the day to allow gas exchange and close them at night to reduce water loss
    • Mechanism of stomatal opening
      • The stomata are bordered by two guard cells
        • Guard cells are unusual in having chloroplasts and unevenly thickened walls with the inner wall being thick and the outer wall thin
          • These can change shape to open and close the stomata to control the amount of water and gases that move in or out of the cell
      • In the light, stomata open because:
        • Photosynthesis occurs, which results in increased production of ATP and a lower CO2 concentration in the guard cells
          • ATP is used to actively transport potassium ions (K+) into the guard cells surrounding the epidermal cells
            • The lower CO2 concentration triggers the conversion of insoluble starch to soluble malate
              • The K+ and malate lower the water potential of the guard cells
                • Water flows into the guard cells by osmosis down the water potential gradient
                  • The guard cell expands and becomes turgid
                    • The inner wall of the guard cell is thick and inelastic, so the pairs of cells curve away from each other
                      • The pore opens
    • Xerophytes
      • These are plants adapted to dry or desert habitats
      • Open stomata at night instead of the day to conserve water
      • For the same reason, other plants may also close their stomata during the day in conditions of drought


No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Cellular processes resources »