Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
292
These two experiments illustrate two important points.
Firstly, from other research we know that photochemical
reactions are not generally affected by temperature.
However, these experiments clearly show that temperature
affects the rate of photosynthesis, so there must be two sets
of reactions in the full process of photosynthesis. These
are a light dependent photochemical stage and a light
independent, temperature dependent stage. Secondly,
Blackman’s experiments illustrate the concept of
limiting factors.
Limiting factors
The rate of any process which depends on a series of
reactions is limited by the slowest reaction in the series.
In biochemistry, if a process is affected by more than
one factor, the rate will be limited by the factor which is
nearest its lowest value.
Look at Figure 13.9. At low light intensities, the limiting
factor governing the rate of photosynthesis is the light
intensity; as the intensities increase so does the rate. But
at high light intensity, one or more other factors must be
limiting, such as temperature or carbon dioxide supply.
As you will see in the next section of this chapter, not
all wavelengths of light can be used in photosynthesis. This
means that the wavelengths of light that reach a plant’s
leaves may limit its rate of photosynthesis (Figure 13.16b,
page 295).
Rate of photosynthesis
Light intensity
experiment 3
25 °C; 0.4% CO 2
experiment 1
25 °C; 0.04% CO 2
experiment 2
15 °C; 0.04% CO 2
Figure 13.9 The rate of photosynthesis at different
temperatures and different carbon dioxide concentrations.
(0.04% CO 2
is about atmospheric concentration.)
QUESTION
13.4 Examine Figure 13.9, which shows the effect of various
factors on the rate of photosynthesis, and explain the
differences between the results of:
a experiments 1 and 2
b experiments 1 and 3.
At constant light intensity and temperature, the rate
of photosynthesis initially increases with an increasing
concentration of carbon dioxide, but again reaches a
plateau at higher concentrations. A graph of the rate of
photosynthesis at different concentrations of carbon
dioxide has the same shape as that for different light
intensities (Figure 13.9). At low concentrations of carbon
dioxide, the supply of carbon dioxide is the rate-limiting
factor. At higher concentrations of carbon dioxide,
other factors are rate-limiting, such as light intensity or
temperature.
The effects of these limiting factors on the rate of
photosynthesis are easily investigated by using an aquatic
plant such as Elodea or Cabomba in a simple apparatus
as shown in Figure 13.10. The number of bubbles of gas
(mostly oxygen) produced in unit time from a cut stem
of the plant can be counted in different conditions.
Alternatively, the gas can be collected and the volume
produced in unit time can be measured. This procedure
depends on the fact that the rate of production of oxygen is
a measure of the rate of photosynthesis.
Growing plants in protected
environments
An understanding of the effect of environmental factors
on the rate of photosynthesis allows their management
when crops are grown in protected environments, such
as glasshouses. The aim is to increase the yield of the
crop concerned.
For example, many hectares of tomato plants are grown
in glasshouses. In the most sophisticated of these, sensors
monitor the light intensity, the humidity of the atmosphere
and the concentration of carbon dioxide around the
plants. The plants grow hydroponically – that is, with their
roots in a nutrient solution whose nutrient content can be
varied at different stages of the plants’ growth. All of these
factors are managed by a computer to maximise the yield
of the crop.
Such glasshouse-grown crops have the added advantage
that insect pests and fungal diseases are more easily
controlled than is possible with field-grown crops, further
improving yield.