Biology - B4

Cell Structure

The leaf contains many layers these include:

Wax Cuticle - This is the top layer of the leaf and prevents loss of water vapor escaping through the epideris.

Upper Epidermis - This is a single layer of cells containing few or no chloroplasts. The cells are quite transparent and permit most of the light that strikes them to pass through to the underlying cells. 

Palisade layer - This consists of one or more layers of cylindrical cells oriented with their long axis perpendicular to the plane of the leaf. The cells are filled with chloroplasts and carry out most of the photosynthesis in the leaf.

Cell Sturcture Cont.

Spongy layer - Lying beneath the palisade layer, its cells are irregular in shape and loosely packed. Although they contain a few chloroplasts, their main function seems to be the temporary storage of sugars and amino acids synthesized in the palisade layer. They also aid in the exchange of gases between the leaf and the environment. During the day, these cells give off oxygen and water vapor to the air spaces that surround them. They also pick up carbon dioxide from the air spaces. 

Lower Epidermis - Most of the stomata are located in the lower epidermis. Although most of the cells of the lower epidermis resemble those of the upper epidermis, each stoma is flanked by two sausage-shaped cells called guard cells. These differ from the other cells of the lower epidermis not only in their shape but also in having chloroplasts. The guard cells regulate the opening and closing of the stomata which reguluates the control of gas's in and out of the leaf. 

Diffusion

Diffusion is the passive movement of particles from and area of high concentration to an area of low concentration.

Osmosis

Osmosis is a speacial type of diffusion (which only happens with water) were the particles travel from an area of higher concentration to an area of lower concentration through a partially permeable membrane. 

Partially permable membrane - this is a membrane with very small in it that alllow small molecule through like water but not substances that are larger molecules like glucose.