Edexcel IGCSE Biology 复习笔记 2.7.2 Leaf Structure

Edexcel IGCSE Biology 复习笔记 2.7.2 Leaf Structure

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

 

 

 

Leaf-Structure

 

 

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)

 

 

How-photosynthesising-cells-obtain-carbon-dioxide

 

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

 

  • 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

     

 

 

 

Guard-cells-and-stomata-1Guard-cells-and-stomata-2_1

 

The guard cells control the opening and closing of the stomata

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