leaf tissue and gas exchange
Post on 23-Feb-2016
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Leaf Tissue and Gas Exchange
Dermal Tissue or Epidermis
• Stomata are tiny openings on the underside of the leaf
• Stomata are formed in the dermal tissue or epidermis
• Stomata regulate the movement of gases in and out of the cell
• Carbon dioxide can therefore enter or leave the leaf by diffusion at any time
• The direction of the movement of these gases depends on their concentration gradients
• The guard cells that surround the stomata control whether they are open or closed
• Light striking the leaf stimulates the guard cells to accumulate potassium ions by active transport (requires ENERGY to move from [low] to [high] )
• As a result the number of particles present in the guard cells increases, water enters by osmosis, and the guard cells swell up
• Swelling up causes the guard cells to bulge outwards and open the stomata
• Look at it: http://www.youtube.com/watch?v=IlmgFYmbAUg
• Guard cells also function to protect the leaves from losing too much water
• Because all gases must dissolve in water before they pass across the cell membranes, when they pass through, water is lost
• This water loss out the stomata is known as transpiration
• If the stomata were open continuously water would be continuously being lost, therefore the guard cells are very important.
• The number of stomata in the epidermis is sensitive to environmental conditions
• In hot dry climates with low humidity, plants have adapted to having fewer stomata, so they lose less water through transpiration
• In places of high humidity water loss is not a problem and plants may have many stomata
Ground tissue
• Between the upper epidermis and the lower epidermis of the leaf there are specialized ground tissues called mesophyll.
Mesophyll types
• Palisade tissue cells- are just below the upper epidermis which are responsible for holding chloroplasts and therefore photosynthesis, they are tightly packed together
• Spongy tissue cells- loosely spaced mesophyll cells in the leaf; the increased distance between cells allows for the primary function of these cells; gas exchange by diffusion
Vascular tissue
• The xylem and phloem tissues are bunched together like a handful of straws in a structure called a vascular bundle
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