Photosynthesis and leaf structure

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

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

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.