Leaf Structures and Functions

• How the leaf is adapted to photosynthesis (part 1)

  • waxy cuticle on upper and lower epidermis reduces water loss through evaporation from the leaf
  • stomata present in epidermal layers. Stomata open in the presence of light, allowing carbon dioxide to diffuse in and oxygen to diffuse out of the leaf
  • chloroplast containing chlorophyll in all mesophyll cells. Chlorophyll absorbs energy from light and transfers it to chemical stores of energy in glucose molecules
• How the leaf is adapted to photosynthesis (part 2)

  • more chloroplasts in upper palisade tissue than spongy tissue allow more light to be absorbed near the upper leaf surface
  • interconnecting system of air spaces in the spongy mesophyll allow rapid diffusion of carbon dioxide and oxygen into and out of mesophyll cells
  • veins connecting xylem and phloem situated close to mesophyll cells
  • thin film of moisture covering the spongy mesophyll cells allows carbon dioxide to dissolve and diffuse into cell
• Guard Cells in sunlight

  • in sunlight, guard cells manufacture glucose by photosynthesis, the increase in glucose concentration lowers the water potential of the cell sap in the cell
  • water enters the guard cell by osmosis
  • guard cells become turgid and more curved since the cell wall around the stoma is thicker than other parts of the guard cell
  • the guard cell curves around the stoma and the stoma opens

  
• Guard cells on a hot sunny day

  • When the leaf loses too much water in strong sunlight, the guard cells become flaccid
  • the stomata closes to prevent excessive loss of water

  
• How Carbon Dioxide enters the leaf

  • in daylight, when photosynthesis occurs, carbon dioxide in the leaf is rapidly used up
  • carbon dioxide concentration in the leaf decreases and becomes lower than carbon dioxide concentration in the atmospheric air, creating a diffusion gradient
  • carbon dioxide diffuses from the surrounding air through the stomata into the intercellular air spaces in the leaf
  • the surfaces of the mesophyll are covered in a thin film of moisture, allowing carbon dioxide to dissolve in it
  • the dissolved carbon dioxide diffuses into the cells

  
• How water enters the leaf

  • the xylem transports water and dissolved mineral salts to the leaf from the roots
  • the veins in the leaf form fine branches which end among the mesophyll cells. The veins contain the xylem and the phloem
  • once out of the veins, the water and mineral slats move from cell to cell right through the mesophyll of the leaf