Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
Stomata show daily rhythms of opening and closing.
Even when kept in constant light or constant dark, these
rhythms persist (Figure 14.30). Opening during the day
maintains the inward diffusion of carbon dioxide and
the outward diffusion of oxygen. However, it also allows
the outward diffusion of water vapour in transpiration
(pages 134 –136). The closure of stomata at night when
photosynthesis cannot occur reduces rates of transpiration
and conserves water.
Stomata respond to changes in environmental
conditions. They open in response to:
■■
■■
increasing light intensity
low carbon dioxide concentrations in the air spaces
within the leaf.
When stomata are open, leaves gain carbon dioxide
for photosynthesis, but tend to lose much water in
transpiration.
a
6
4
2
b
0
c
Stomatal opening / arbitrary units
6
4
2
0
321
6
4
2
0
24 12 24 12 24 12 24 12 24 12
Time of day (12 = midday, 24 = midnight,
= darkness)
Figure 14.30 a The opening of stomata was measured in leaves of Tradescantia over several days to reveal a
daily rhythm of opening and closing. This rhythm persisted even when the plants were kept in constant light b;
and in long periods of constant dark c.