Cambridge International A Level Biology Revision Guide

Chapter 13: Photosynthesis
QUESTION
13.5 Some of the most productive crop plants in the
world are C4 plants. However, rice grows in tropical
regions and is a C3 plant. Research is taking place
into the possibility of producing genetically modified
rice that uses the C4 pathway in photosynthesis.
Explain how this could increase yields from rice.
Trapping light energy
Chloroplasts contain several different pigments, and
these different pigments absorb different wavelengths of
light. The photosynthetic pigments of higher plants form
two groups: the chlorophylls (primary pigments) and the
carotenoids (accessory pigments) (Table 13.1).
Group Pigment Colour
chlorophylls
carotenoids
chlorophyll a
chlorophyll b
β carotene
xanthophyll
yellow-green
blue-green
orange
yellow
Table 13.1 The colours of the commonly occurring
photosynthetic pigments.
Chlorophylls absorb mainly in the red and blueviolet
regions of the light spectrum. They reflect green
light, which is why plants look green. The structure of
chlorophyll a is shown in Figure 13.15. The carotenoids
absorb mainly in the blue-violet region of the spectrum.
CH CH 3
CH 2
CH 2 CH 3
An absorption spectrum is a graph of the absorbance of
different wavelengths of light by a pigment (Figure 13.16a).
An action spectrum is a graph of the rate of
photosynthesis at different wavelengths of light
(Figure 13.16b). This shows the effectiveness of the
different wavelengths, which is, of course, related to their
absorption and to their energy content. The shorter the
wavelength, the greater the energy it contains.
a
Absorbance
Rate of photosynthesis
chlorophyll a
chlorophyll b
carotenoids
400 450 500 550 600 650 700
Wavelength of light / nm
b
295
head
tail
H 3 C
H 3 C
N N
Mg
N N
(CH 2 ) 2
C O C O
O O
CH 2 CH 3
CH
C CH 3
(CH 2 ) 3
CH CH 3
(CH 2 ) 3
CH CH 3
(CH 2 ) 3
CH CH 3
CH 3
O
CH 3
400 450 500 550 600 650 700
Wavelength of light / nm
Figure 13.16 a Absorption spectra of chlorophylls a and b,
and carotenoid pigments. b Photosynthetic action spectrum.
QUESTION
13.6 Compare the absorption spectra shown in
Figure 13.16a with the action spectrum shown
in Figure 13.16b.
a Identify and explain any similarities in the
absorption and action spectra.
b Identify and explain any differences between the
absorption and action spectra.
Figure 13.15 Structure of chlorophyll a. You do not need to
learn this molecular structure.