Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

786 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
3 ADP
3 ATP
2 P i
3 × ribulose
1,5-bisphosphate
5C
5 × glyceraldehyde
3-phosphate
3C
3 × CO 2
1C
Rubisco CARBOXYLATION
6 × 3-phosphoglycerate
CARBON-FIXATION
(CALVIN) CYCLE
NET RESULT:
For every 3 molecules of CO 2
that enter the cycle, 1 molecule
of glyceraldehyde 3-phosphate is
produced and 9 molecules of ATP
+ 6 molecules of NADPH are
consumed
3C
6 ATP
6 ADP
6 × 1,3-bisphosphoglycerate
3C
6 × glyceraldehyde
3-phosphate
3C
6 NADPH
6 NADP +
6 P i
REDUCTION
Figure 14–41 The carbon-fixation
cycle. This central metabolic pathway
allows organic molecules to be produced
from CO 2 and H 2 O. In the first stage
of the cycle (carboxylation), CO 2 is
added to ribulose 1,5-bisphosphate, as
shown in Figure 14–40. In the second
stage (reduction), atp and NADPH are
consumed to produce glyceraldehyde
3-phosphate molecules. In the final stage
(regeneration), some of the glyceraldehyde
3-phosphate produced is used to
regenerate ribulose 1,5-bisphosphate.
Other glyceraldehyde 3-phosphate
molecules are either converted to starch
and fat in the chloroplast stroma, or
transported out of the chloroplast into the
cytosol. The number of carbon atoms in
each type of molecule is indicated in yellow.
There are many intermediates between
glyceraldehyde 3-phosphate and ribulose
5-phosphate, but they have been omitted
here for clarity. The entry of water into the
cycle is also not shown (but see Figure
14–40).
REGENERATION
1 MOLECULE OF
GLYCERALDEHYDE 3-PHOSPHATE
LEAVES THE CYCLE
H
H
C O
C OH
CH 2 O
P
sugars, fats,
amino acids
glyceraldehyde 3-phosphate
glycolytic pathway (see Figure 2–46), where it is converted to pyruvate. Both that
pyruvate and the fatty acids can enter the plant cell mitochondria and be fed into
the citric acid cycle, ultimately leading to the production of large amounts of ATP
by oxidative phosphorylation (Figure 14–42). Plants use this ATP in the same way
that animal cells and other nonphotosynthetic organisms do to power a variety of
metabolic reactions.
The glyceraldehyde 3-phosphate exported from chloroplasts into the cytosol
can also be converted into many other metabolites, including the disaccharide
sucrose. Sucrose is the major form MBoC6 in e14.40/14.41
which sugar is transported between the cells
of a plant: just as glucose is transported in the blood of animals, so sucrose is
exported from the leaves to provide carbohydrate to the rest of the plant.
The Thylakoid Membranes of Chloroplasts Contain the Protein
Complexes Required for Photosynthesis and atp Generation
We next need to explain how the large amounts of ATP and NADPH required for
carbon fixation are generated in the chloroplast. Chloroplasts are much larger and
less dynamic than mitochondria, but they make use of chemiosmotic energy conversion
in much the same way. As we saw in Figure 14–38, chloroplasts and mitochondria
are organized on the same principles, although the chloroplast contains
a separate thylakoid membrane system in which its chemiosmotic mechanisms
occur. The thylakoid membranes contain two large membrane protein complexes,
called photosystems, which endow plants and other photosynthetic organisms
with the ability to capture and convert solar energy for their own use. Two
other protein complexes in the thylakoid membrane that work together with the
photosystems in photophosphorylation—the generation of ATP with sunlight—
have mitochondrial equivalents. These are the heme-containing cytochrome