2 Photosynthesis and leaf structure Leaves are a plant’s main photosynthetic organs. Leaves must:Permit carbon dioxide access to the photosynthetic cellsPrevent water from diffusing outAllow the waste product oxygen escape
3 Internal structure of a leaf A leaf consists of following layers:CuticleUpper epidermisPalisade mesophyll or palisade layerSpongy mesophyll or spongy layerLower epidermisPores also known as stoma /stomataStoma
4 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.
5 Upper epidermisUpper 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 cellsSometimes it contains pores which allow gas or water molecule to pass through.
6 Palisade mesophyllJust beneath the epidermis there are column shaped cells containing chloroplast, they are known as Palisade mesophyll.Mesophyll: Middle leafThe cells are filled with chloroplasts and carry on most of the photosynthesis in the leaf
7 Spongy mesophyllIt 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
8 Spongy mesophyllDuring the day, these cells give off oxygen and water vapor to the air space that surround themThey also pick up carbon dioxide from the air spacesTogether, the palisade and spongy layers make up the mesophyll
9 MesophyllSpecial structure in the mesophyll layer: Vascular bundles (veins)Vascular bundle consists of :Xylem: To supply water and minerals to the mesophyllPhloem: To supply food (glucose) from the mesophyll to other parts of the plant
10 Lower epidermisMost of the stomata are located in the lower epidermis.Sunlight causes water molecules to evaporate faster causing plants to be dehydrated fasterBy having most stomata on the bottom of the a leaf, there will be less water loss.
11 StomataThe 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
12 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 .
13 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.