B6 Organisation in plants

• Plants have defences against pathogens, including physical barriers and chemical defences
• Mesophyll


  • Waxy cuticle makes the leaf waterproof
  • Upper epidermis is a single layer of cells that protects against water loss
  • Transparent to allow light to pass through
• Palisade mesophyll


  • Tightly packed cells
  • Lots of chloroplasts to absorb light for photosynthesis
• Spongy mesophyll


  • Spherical cells
  • Lots of air spaces to allow gases to diffuse quickly
  • Large surface area-to-volume ratio to increase gas exchange
• Stomata


  Tiny openings on the lower surface of the leaf that allow gases to move into and out of the leaf
• Guard cells


  • Control the opening and closing of the stomata
• Stomata opening and closing


  1. When plant has plenty of water, guard cells become turgid and curve away from each other, opening the stoma

  2. When plant is flaccid, guard cells become flaccid and close the stoma
• Stomata are placed on the underside of leaves to reduce water loss through evaporation
• Stomata function


  • Allow diffusion of carbon dioxide into the plant for photosynthesis
  • Allow diffusion of oxygen out of the plant