Photosynthesis and leaf structure • Leaves are a plant’s main photosynthetic organs. Leaves must: • Permit carbon dioxide access to the photosynthetic cells • Prevent water from diffusing out • Allow the waste product oxygen escape
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 /stomata Stoma
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.
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.
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
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
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
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
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.
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
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 .
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.