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Biology 2014-2015

Leaves
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Section 23-4

Leaf Structure

       Leaves of a plant are its main organs of photosynthesis

       The blade is a thin flattened section used to collect sunlight.

       A petiole is a thin stalk that attached the blade to the stem.

       Leaves have three main tissues:

         Dermal = epidermis covered by the cuticle (waterproof barrier)

         Vascular = xylem and phloem in the veins

         Ground = parenchyma and sclerenchyma

 

Leaf Functions

       The leaf is a system specialized for photosynthesis.

       Subsystems include tissues that bring gases, water, and nutrients to the cells that carry out photosynthesis.

 

Photosynthesis

       The mesophyll is ground tissue that makes up the bulk of the leaves and the site of photosynthesis.

       The carbohydrates produced move into phloem vessels and carry it to the rest of the plant.

       Palisade mesophyll is just under the upper epidermis and absorbs light that enters the leaf.

       Spongy mesophyll is a loose tissue with many air spaces that connect with the exterior through stomata.

       Stomata are porelike openings on the underside of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf.

       Guard cells control the opening and closing of stomata by responding to changes in water pressure.

 

Transpiration

       The spongy mesophyll needs to be kept moist so that gases can enter and leave the cells easily.

       Transpiration is the loss of water through its leaves.

       Lost water is replaced by water drawn into the leaf through xylem vessels.

 

Gas Exchange

       Leaves take in carbon dioxide and give off oxygen during photosynthesis.

       The gases are exchanged through the air spaces in the spongy mesophyll and the exterior by opening their stomata.

       If stomata were kept open all the time, water loss would be too great and the plant would not survive.

       Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose excess water.

       Guard cells control the stomata by opening and closing in response to water pressure within the guard cells.

       Guard cells respond to conditions in the environment, such as wind and temperature, and maintain homeostasis within a leaf.

       Stomata are open during the daytime, when photosynthesis is active, and closed at night, when open stomata would lead to water loss.

       Stomata may even be closed during bright sunlight under hot, dry conditions in which water conservation is a matter of life and death.