plants

• Opening and closing stomata for gas exchange

  1. In light, chloroplasts in guard cells photosynthesise and produce ATP

  2. The ATP is used for the active transport of potassium ions into guard cells

  3. Starch is converted to malate

  4. Malate and potassium ions lower water potential of guard cells and water is drawn in by osmosis

  5. Uneven thickening of guard cell inner walls causes them to bend as they swell, opening the stomatal pore

  6. The opposite occurs when there is no light closing the pore
• Leaves adapted for photosynthesis

  • They have a large surface area and orientate perpendicular to the sun for maximum light absorption
  • They are thin with a transparent cuticle and upper epidermis for light to penetrate into the leaf and also for efficient diffusion of gases from the stomata, up through the gas spaces to the photosynthetic layers
  • They have palisade cells packed with chloroplasts; cells are densely stacked with long axes perpendicular to leaf surface
• Spongy mesophyll is surrounded by air spaces for easy diffusion of gases. The mesophyll cell membranes are the site of gas exchange.
• Cuticle is a waxy transparent layer that allows light to pass through to the photosynthetic palisade mesophyll below but reduces water lost by evaporation through the top surface of the leaf.
• Upper epidermis is transparent for light to easily penetrate to photosynthetic layers.
• Stomata


  • The stomatal pores allow the exchange of gases down a concentration gradient
  • The gases diffuse through intracellular spaces to and from the photosynthetic cells where they dissolve in the moist lining and diffuse into the cells
  • Guard cells open and close the stomata
  • The opening of the stomata during the day allows carbon dioxide to enter the air spaces and then the cells to be used in photosynthesis
  • The closing of the stomata during the night reduces water loss
• Vascular bundle contains xylem (carrying water from roots), phloem (to carry sucrose to other parts of the plant) and bundle sheath parenchyma.
• Palisade mesophyll is the main photosynthetic layer packed with chloroplasts.
• Lower epidermis
• Open

  Turgid guard cells bend due to thickened inner walls, opening stomatal pore
• Closed


  Flaccid guard cells meet in the middle, closing the stomatal pore