Horticulture Principles and Practices

called rubisco. Because this enzyme is abundant in chloroplasts, rubisco is said to
be the most abundant protein in nature. The overall process can be summarized
by the following equation:
6CO 2 12NADPH 12H + 18ATP → 1 glucose 12NADP + 18ADP 18P i 6H 2 O
The intermediate product is glyceraldehyde 3-phosphate.
Glucose is indicated in the preceding summary equation, but in practice,
photosynthesizing cells generate a minimal amount of this sugar. Most of the
fixed carbon dioxide is either converted to sucrose (which is the principal form in
which sugar is transported in plants) or stored in the form of starch.
2. The four-carbon pathway. Many plants are known to be able to fix carbon dioxide
by a pathway whose first product is a four-carbon substance. This pathway is also
called the C 4 pathway, and plants that photosynthesize by this pathway are called C 4
plants. First, a CO 2 molecule is bonded to phosphoenol pyruvate (PEP), a threecarbon
acceptor compound, resulting in the production of oxaloacetate. The reaction
is catalyzed by the enzyme PEP carboxylase (Figure 5–6).
C 4 plants are less efficient than C 3 plants in terms of the energy requirements
in fixing CO 2 . To fix one molecule of CO 2 ,C 4 plants need five ATPs,
whereas C 3 plants need only three. However, C 4 plants have higher photosynthetic
rates than C 3 plants and also are able to continue photosynthesizing under
conditions such as high temperatures and high light intensity when C 3 plants
cannot (Figure 5–7). Generally, C 4 plants are adapted to tropical conditions.
Select examples of both categories of plants are presented in Table 5–1. Under
hot, sunny skies, C 3 plants undergo a process called photorespiration (lightdependent
production of glycolic acid in chloroplasts and its subsequent
oxidation in peroxisomes). C 4 plants use CO 2 more efficiently and hence are able
to function at only partially closed stomata, as occurs on hot, sunny days.
C 3 and C 4 plants are different structurally. The bundle sheath cells of C 3 leaves
have small chloroplasts. Photosynthesis occurs only in the mesophyll cells.
However, the bundle sheath cells of C 4 plants are large and contain large chloroplasts.
These chloroplasts exhibit the Calvin cycle, while the mesophyll cells exhibit
the C 4 pathway. All plants known to use the C 4 pathway are flowering plants. In
FIGURE 5–5 The Calvin cycle
or C 3 pathway of carbon dioxide
fixation.
12 NADP +
12
12 NADPH 2
6ATP
GA3P
12 ADP + 12P
2
GA3P
CALVIN
CYCLE
10
GA3P
12 ATP
2
3PGA
6
RuBP
6ADP
START
6
CO 2
162 Chapter 5 Plant Physiology