Gas exchange surface leaves

• Describe the gas exchange surface of a leaf

  Leaves are thin so no cell is far from external

  air. The spongy mesophyll provides a large surface area for gas exchange when the stomata are open. Stomata are small pores that can open and close. Stomata can open and close to reduce water loss or increase the surface area for gas exchange. The spongy mesophyll has air spaces that interconnect to allow gases to come into contact with the mesophyll cells.
• Describe how carbon dioxide in the air outside a leaf reaches mesophyll cells inside the leaf

  Carbon dioxide is used by mesophyll cells for photosynthesis so the concentration in the cell is low. Carbon dioxide enters via stomata; Stomata are opened by guard cells; Carbon dioxide diffuses through air spaces; Down concentration gradient to the cells.
• Describe and explain how leaves are adapted as gas exchange surfaces

  Leaves are thin so there is a short diffusion pathway. The spongy mesophyll has a large surface for diffusion. Stomata can open and close to reduce water loss, or increase the surface area for diffusion. The spongy mesophyll has air spaces that interconnect to allow gases to come into contact with the mesophyll cells.
• Describe and explain how xerophytic plants can be adapted to reduce water loss

  Hairs so 'trap' water vapour and water potential gradient is decreased; Stomata are in pits so 'trap' water vapour and water potential gradient is decreased; There is a thick waxy cuticle layer so increases diffusion distance; Waxy cuticle so reduces evaporation; Rolled leaves so 'trap' water vapour and water potential gradient is decreased;

  Spines/needles so reduces surface area to volume ratio;
• Stomata close when there is no light explain the advantage to the plant

  Water is lost through stomata; So closure reduces water loss; Maintain water content of cells. The cells do not need

  carbon dioxide as no photosynthesis can occur in the dark.