Cambridge International A Level Biology Revision Guide

Chapter 13: Photosynthesis
BOX 13.2: Investigating the rate of photosynthesis using an aquatic plant
Elodea, or other similar aquatic plants, can be used to
investigate the effect on the rate of photosynthesis of
altering the:
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light intensity – by altering the distance, d, of a small
light source from the plants (light intensity is
proportional to 1 d 2)
wavelength of light – by using different colour filters,
making sure that they each transmit the same light
intensity
concentration of carbon dioxide – by adding different
quantities of sodium hydrogencarbonate (NaHCO 3
) to
the water surrounding the plant
temperature of the water surrounding the plant
– using a large container, such as a beaker, to help
maintain the chosen temperatures.
The aquatic plant needs to be well illuminated before use
and the chosen stem needs to be cut cleanly just before
putting it into a test tube (Figure 13.10).
The bubbles given off are mostly oxygen, but contain
some nitrogen. To prevent these gases from dissolving in
the water, rather than forming bubbles, the water needs to
be well aerated (by bubbling air through it) before use.
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Figure 13.10 Investigating the rate of photosynthesis
using an aquatic plant.
C4 plants
In the light independent stage of photosynthesis, you may
remember that carbon dioxide combines with RuBP to
form a six-carbon compound, which immediately splits to
form two three-carbon molecules (page 290). Plants that
do this are called C3 plants.
However, maize and sorghum plants – and most
other tropical grasses – do something different. The first
compound that is produced in the light independent
reaction contains four carbon atoms. They are therefore
called C4 plants.
Avoiding photorespiration
Why do tropical grasses need to do something different
from other plants in the light independent stage of
photosynthesis? The reason is a problem with the enzyme
rubisco. This enzyme catalyses the reaction of carbon
dioxide with RuBP. But, unfortunately, it can also catalyse
the reaction of oxygen with RuBP. When this happens,
less photosynthesis takes place, because some of the
RuBP is being ‘wasted’ and less is available to combine
with carbon dioxide. This unwanted reaction is known
as photorespiration. It happens most readily in high
temperatures and high light intensity – that is, conditions
that are found at low altitudes in tropical parts of
the world.
Tropical grasses such as maize, sorghum and sugar cane
have evolved a method of avoiding photorespiration. They
keep RuBP and rubisco well away from high oxygen
concentrations. The cells that contain RuBP and rubisco
are arranged around the vascular bundles, and are called
bundle sheath cells (Figures 13.11, 13.12 and 13.13). They
have no direct contact with the air inside the leaf.
Carbon dioxide is absorbed by another group of
cells, the mesophyll cells, which are in contact with
air (Figure 13.13). The mesophyll cells contain an enzyme
called PEP carboxylase, which catalyses the combination
of carbon dioxide from the air with a three-carbon
substance called phosphoenolpyruvate, or PEP. The
compound formed from this reaction is oxaloacetate
(Figure 13.14).