Edexcel IGCSE Biology 复习笔记 2.7.2 Leaf Structure

• The structure of the leaf is adapted to carry out both photosynthesis and gas exchange
• The different cell types (palisade mesophyll, spongy mesophyll etc.) and tissues are arranged in a specific way to facilitate these processes

The cross-section of a leaf

Leaf structure and gas exchange

• The leaf is specifically adapted to maximise gas exchange
• There are 3 key gases which we must consider
  • Carbon dioxide - released in respiration but used in photosynthesis
  • Oxygen - released in photosynthesis but used in respiration
  • Water vapour - released in respiration and transpiration

   

• The route of diffusion of carbon dioxide into the leaf can be seen in the diagram below
• Gases will always diffuse down a concentration gradient (from where there is a high concentration to where there is a low concentration)

Pathway of carbon dioxide from atmosphere to chloroplasts by diffusion.

atmosphere → air spaces around spongy mesophyll tissue → leaf mesophyll cells → chloroplast

Adaptations of the whole leaf for gas exchange

• Adaptations of leaves to maximise gas exchange:
  • They are thin which gives a short diffusion distance
  • They are flat which provides a large surface area to volume ratio
  • They have many stomata which allow movement of gases in and out of the air spaces inside the leaf to maintain a steep concentration gradient

   

• Other adaptations of the internal leaf structure/tissues include:
  • Air spaces to allow gas movement around the loosely packed mesophyll cells
  • Many stomata in the lower epidermis open in sunlight to allow gas movement in and out of the leaf
  • Thin cell walls allow gases to move into the cells easily
  • Moist air which gases can dissolve into for easier movement into and out of cells
  • The close contact between the cells and the air spaces allows efficient gas exchange for photosynthesis and respiration

   

• Stomata are spaces found between two guard cells predominantly on the lower epidermis of the leaf
• The guard cells are responsible for the opening and closing of the stomatal pore which controls gas exchange and water loss
• Stomata open when water moves (by osmosis) into the guard cells causing them to become turgid
  • This allows gases to diffuse in and out of the leaf through the stomatal pore
  • Stomata tend to open when there is plenty of water and sunlight

   

• Stomata close when the guard cells lose water (by osmosis) to the neighbouring epidermal cells and they become flaccid
  • This prevents any diffusion into or out of the leaf
  • Stomata tend to close due to low water availability or low sunlight

   

The guard cells control the opening and closing of the stomata