Organisation in plants

• Tissues in leaves


  Leaves are organs because they contain many tissues that work together to perform photosynthesis
• Waxy cuticle


  • Makes the leaf waterproof
• Upper epidermis

  • Single layer of cells
  • 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
• Lower epidermis

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

  Tiny openings in the undersides of leaves that allow gases to move into and out of the leaf
• Stomata

  • Placement reduces water loss through evaporation
  • Control gas exchange and water loss from leaves
  • Allow diffusion of carbon dioxide into the plant for photosynthesis
  • Allow diffusion of oxygen out of the plant
• Guard cells

  Used to open and close the stomata
• Opening and closing of stomata

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

  2. When guard cells are flaccid, the stomata close
• Key terms

  • cuticle
  • epidermis
  • translocation
  • flaccid
  • transpiration
  • guard cell
  • mesophyll
  • transpiration stream
  • phloem
  • stomata
  • xylem
• Transpiration


  Water is lost through the stomata by evaporation, pulling water up from the roots through the xylem
• Xylem


  • Specialised tissues for one-way transport of water and minerals
  • Made of dead cells, joined together with no end walls between them
  • Thick walls stiffened with lignin
• Translocation


  Movement of dissolved sugars from the leaves to the rest of the plant through the phloem
• Phloem


  • Substances transported in both directions
  • Cells have end walls with small holes to allow substances to flow through
• Factors affecting the rate of transpiration
• Root hair cells


  • Increase absorption of water and mineral ions into the root by increasing the root surface area
  • Contain lots of mitochondria to transfer energy, which is used to take in mineral ions by active transport