Page 145 - Biology Form Two

Biology for Secondary Schools

The guard cells become turgid, and the stomata open as Figure 6.9 (b) shows.
Air from the atmosphere enters into the air spaces in the spongy mesophyll. The
cells next to the air spaces have more oxygen that is produced by the cells during
photosynthesis and less carbon dioxide as it is used up during photosynthesis.
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Nucleus

Chloroplast

Vacuole

Cell wall

Guard cells
Stoma

(a) Closed stoma (b) Opened stoma

Figure 6.9: Structure of a stoma

Carbon dioxide and oxygen diffuse Oxygen diffuses into the plant cells

in opposite directions, depending on while carbon dioxide diffuses out into
their concentration-gradients. Carbon the intercellular spaces and eventually
dioxide diffuses into neighbouring into the atmosphere through the
cells until it reaches the site of partially opened stomata.
photosynthesis. Oxygen moves out Gas exchange through the lenticels
through the open stomata to the Lenticels are made up of loosely
atmosphere.
packed cork cells located on some parts
At night, there is no sunlight. Therefore, of the bark of woody stems and roots,

light reaction of photosynthesis ceases. as shown in Figure 6.10. These loosely
This means that little or no glucose is arranged cells create many small pores
produced hence the guard cells do not through which gas exchange occurs.
absorb water by osmosis. Therefore, The air spaces between the cells are
the stomata remain partially closed. On called intercellular air spaces.
the other hand, respiration continues
during the night. The partially open
stomata allow in a small amount of air
to accumulate in the air spaces.

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