Problem : Describe the basic structure of a leaf.

In general, a leaf contains mesophyll cells, made up of a palisade layer and a spongy layer, sandwiched between two layers of epidermal cells. Photosynthesis takes place in the mesophyll. The epidermal cells provide the leaf with a waxy, nearly impermeable cuticle that protects against water loss. Stomata, small pores on the underside of the leaf, allow gases in and out of the leaf.

Problem : Describe the mechanism of the guard cells. How are they necessary to the stomata and to the leaf?

The two guard cells that surround each stoma can open and close depending on environmental conditions. When moisture is plentiful, the guard cells swell with water, forcing the the stoma open and allowing gas exchange to occur. When the plant loses too much water or water in the environment becomes less plentiful, the guard cells deflate, closing the stoma and preventing further water loss or gas exchange. Guard cells are vital to the plant because they allow the gases necessary for photosynthesis in and out while keeping the plant from losing too much water.

Problem : What gases pass in and out of the stomata?

Carbon dioxide is taken in (for photosynthesis), and oxygen (a byproduct of photosynthesis) is released back into the environment. In addition, water vapor passes out of the leaf through the stomata.

Problem : What is transpiration? How does the plant make up for it?

Transpiration occurs when the plant loses water (via the stomata) to the surrounding air through evaporation. To make up for this water loss, additional water is drawn in from the soil by the roots and passed upward through the plant by the xylem.

Problem : What is the purpose of the moist outer membranes of mesophyll cells?

The moist membranes allow gases to dissolve easily, thus facilitating gas exchange (carbon dioxide for photosynthesis is allowed into the mesophyll, and oxygen is passed out).