Cambridge International A Level Biology Revision Guide

Chapter 12: Energy and respiration
QUESTION
12.9 Outline the steps you would take to investigate the
effect of temperature on respiration rate.
Adaptations of rice for wet
environments
Although rice can grow in dry conditions, it is often grown
in ‘paddies’ – fields where the ground is intentionally
flooded. Rice can tolerate growing in water, whereas most
of the weeds that might compete with it are not able to do
so (Figure 12.20).
Most plants cannot grow in deep water because their
roots do not get enough oxygen. Oxygen is required for
aerobic respiration, which provides ATP as an energy
source for active transport and other energy-consuming
processes such as cell division. Nor, if the leaves are
submerged, can photosynthesis take place, because there
is not enough carbon dioxide available. This happens
because gases diffuse much more slowly in water than they
do in air. Moreover, the concentrations of dissolved oxygen
and dissolved carbon dioxide in water are much less than
they are in air. This is especially true in rice paddies, where
the rich mud in which the rice roots are planted contains
large populations of microorganisms, many of which are
aerobic and take oxygen from the water.
Some varieties of rice respond to flooding by growing
taller. As the water rises around them, they keep growing
upwards so that the top parts of their leaves and flower
spikes are always held above the water. This allows oxygen
and carbon dioxide to be exchanged through the stomata
on the leaves.
The stems of the rice plants contain loosely packed cells
forming a tissue known as aerenchyma (Figure 12.21).
Gases are able to diffuse through the aerenchyma to other
parts of the plant, including those under the water. This is
supplemented by air that is trapped in between the ridges
of the underwater leaves. These leaves have a hydrophobic,
corrugated surface that holds a thin layer of air in contact
with the leaf surface.
Nevertheless, the cells in the submerged roots do
still have to use alcoholic fermentation at least some of
the time. Ethanol can therefore build up in the tissues.
Ethanol is toxic, but the cells in rice roots can tolerate
much higher levels than most plants. They also produce
more alcohol dehydrogenase, which breaks down ethanol.
This allows the plants to grow actively even when oxygen
is scarce, using ATP produced by alcoholic fermentation.
QUESTION
12.10 List the features that make rice adapted to grow
when partly submerged in water.
vascular tissues
air space
aerenchyma
cells of the cortex
281
Figure 12.20 Rice growing in Madagascar. The blocks of rice
were planted at different times and are at different stages
of growth.
Figure 12.21 Photomicrograph of a cross-section of a rice stem
near its tip, with a leaf base around it. Lower down, the stem is
completely hollow (× 140).