Edexcel IGCSE Biology

Revision Notes

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, downloadable AS & A Level Biology revision notes

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, IGCSE & GCSE Biology revision notes

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 1, downloadable IGCSE & GCSE Biology revision notes
The guard cells control the opening and closing of the stomata

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Ruth graduated from Sheffield University with a degree in Biology and went on to teach Science in London whilst also completing an MA in innovation in Education. With 10 years of teaching experience across the 3 key science disciplines, Ruth decided to set up a tutoring business to support students in her local area. Ruth has worked with several exam boards and loves to use her experience to produce educational materials which make the mark schemes accessible to all students.
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