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