Photosynthesis and leaf structure
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Transcript of Photosynthesis and leaf structure
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Photosynthesis and leaf structure
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Photosynthesis and leaf structure• Leaves are a plant’s main photosynthetic
organs. Leaves must:1. Permit carbon dioxide access to the
photosynthetic cells2. Prevent water from diffusing out3. Allow the waste product oxygen escape
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Internal structure of a leaf
A leaf consists of following layers:• Cuticle• Upper epidermis• Palisade mesophyll
or palisade layer• Spongy mesophyll
or spongy layer• Lower epidermis• Pores also known
as stoma /stomataStoma
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Cuticle• The outer thick waxy
covering of the plants and leaves.
• Cuticle protects plant from drying out by reducing water loss.
• Does not let oxygen or carbon dioxide pass through it.
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Upper epidermis• Upper epidermis
consists of a single layer of cells covered by cuticle.
• The cells are quite transparent and permit most of the light pass through to the underlying cells
• Sometimes it contains pores which allow gas or water molecule to pass through.
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Palisade mesophyll• Just beneath the
epidermis there are column shaped cells containing chloroplast, they are known as Palisade mesophyll.
• Mesophyll: Middle leaf• The cells are filled with
chloroplasts and carry on most of the photosynthesis in the leaf
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Spongy mesophyll• It is composed of
loosely packed irregular shaped cells surrounded by air spaces.
• Although they contain a few chloroplasts, their main function is to temporarily store sugar and aid in the exchange of gases between the leaf and the environment
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Spongy mesophyll• During the day, these
cells give off oxygen and water vapor to the air space that surround them
• They also pick up carbon dioxide from the air spaces
• Together, the palisade and spongy layers make up the mesophyll
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Mesophyll• Special structure in
the mesophyll layer: Vascular bundles (veins)
• Vascular bundle consists of :
- Xylem: To supply water and minerals to the mesophyll
- Phloem: To supply food (glucose) from the mesophyll to other parts of the plant
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Lower epidermis• Most of the stomata
are located in the lower epidermis.
• Sunlight causes water molecules to evaporate faster causing plants to be dehydrated faster
• By having most stomata on the bottom of the a leaf, there will be less water loss.
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Stomata • The pore like
openings in the underside of the leaf is called stoma (singular) and stomata (plural).
• Each stoma consists of two guard cells which control the opening and closing of the stomata
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Structure and function of stomata
• Stomata consists of two guard cells: When there is high water pressure guard cells open. Low water pressure in the guard cells cause stomata to close.
• Stomata allow carbon dioxide, oxygen and water vapor to diffuse into and out of the leaf by opening and closing .
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Do you think stomata are open all the time?
• Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose excessive amount of water.
• In general, stomata are open during the day time when photosynthesis are active, and closed at night.
• However, stomata may be closed even in the bright sunlight under hot, dry conditions in which water conservation is a matter of life and death.