Plant leaves are the primary site of photosynthesis. Their flat surface maximizes the surface area exposed to sunlight. They also store food and water, and function in transportation --- the loss of water vapor from the plant to the atmosphere. Leaf structure and shape varies according to climate, the availability of light, humidity and temperature.
Leaf Tissue Structure
A leaf cross-section reveals a cuticle layer and epidermal cells on the underside and the top surface. Epidermal cells secrete a waxy substance known as the cuticle that aids in protection and keeps water from evaporating. Its epidermis gives the leaf structure, support and protection. The specialized stomata cells function as gate keepers, allowing carbon dioxide to enter and oxygen to escape. They are layered just above the epidermis on the bottom side of the leaves. Cells containing chloroplasts make up the central mesophyll layer. Some mesophyll cells contain as many as 50 chloroplasts.
Plants produce their own food through the chemical reactions of photosynthesis in the leaves. Chlorophyll, the green pigment, is located in cell organelles --- chloroplasts --- that reside in plant cells.
Photosynthesis has two phases: the light reaction and the dark reaction. The daylight process converts solar energy to chemical energy and stores it as sugars. The requirements are light, carbon dioxide and water. The reaction produces oxygen and sugar. The dark phase occurs at night and makes use of the energy produced during the day to convert carbon dioxide to sugar.
Pores called stomata on the underside of the leaf are formed by a pair of guard cells that regulate the size of the openings during gas exchange. Guard cells are usually open during the day and closed at night.
Air containing carbon dioxide enters through a stoma and once inside the leaf, the mesophyll cells use it in photosynthesis and respiration. Photosynthesis produces oxygen that exits the cell through the stomata, and water vapor is released into the atmosphere through these pores in the transpiration cycle.
Respiration is the major form of gas exchange in living organisms. On the cellular level, diffusion is the movement of molecules from a region of greater concentration to one with a smaller concentration of molecules until equilibrium is reached.
Plants respire when they absorb carbon dioxide and release oxygen through the stomata in the leaves. During transpiration, the leaves release water vapor in the same manner. The number of stomata present on the leaves varies according to temperature, humidity and light intensity.