Leaf
Leaves contain chlorophyll and are the sites of
photosynthesis in plants. Their
broad, flattened surfaces gather energy from sunlight while apertures on the
their undersides bring in carbon dioxide and release oxygen. The cells of a
leaf are sandwiched in between two layers of epidermal cells, which provide
the leaf with a waxy, nearly impermeable cuticle that protects against water
loss. The only way for gases to diffuse in and out of the leaf is though small
openings on the underside of the leaf, the stomata. These stomata can open
and close according to the plant's needs. The tissues of the leaf in between
the epidermal cells, into which gases diffuse from the stomata, are called
mesophyll.
The mesophyll can be further broken down into two layers, the palisade layer
and the spongy layer, both of which are packed with
chloroplasts, the factories of
photosynthesis. In the palisade layer, chloroplasts are lined in columns just
below the epidermal cells, to facilitate the capture of light. The spongy layer
is a tissue that also contains chloroplasts and other parenchyma cells, but
the cells are less ordered and spread out, leaving large intracellular spaces. These intracellular spaces, along with the moist surface of mesophyll cells, facilitate the exchange of carbon dioxide and oxygen.
Figure 4.1: Cross-Section of a Leaf
Overall, it is to the plant's advantage to maximize the gas
exchange and sunlight trapping surface while keeping leaf thickness to a minimum
so that gases can diffuse easily throughout the cells of the leaf (a process
that occurs readily only when there are only a few layers of cells present).
Stomata and Gas Exchange
Stomata, as mentioned above, are the structures through which gas exchange
occurs in leaves. Each stoma is surrounded by two guard cells, which can
open and close depending on environmental conditions. When moisture is
plentiful, the guard cells swell with water, forcing the opening of 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.
Figure 4.2: Stoma and Guard Cells
When the stomata are open, the plant can take in carbon dioxide from the air for
photosynthesis and release oxygen (a byproduct of photosynthesis) back into the
environment. While doing so, the plant also loses an enormous amount of water
by evaporation. This process is called transpiration. 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.
