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

784 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
electrochemical proton gradient drives the synthesis of ATP in the stroma.
As the final step in this series of reactions, electrons are loaded (together
with H + ) onto NADP + , converting it to the energy-rich NADPH molecule.
Because the positively charged chlorophyll in the first reaction center
quickly regains its electrons from water (H 2 O), O 2 gas is produced as a
by-product. All of these reactions are confined to the chloroplast.
2. The carbon-fixation reactions do not require sunlight. Here the ATP and
NADPH generated by the light reactions serve as the source of energy and
reducing power, respectively, to drive the conversion of CO 2 to carbohydrate.
These carbon-fixation reactions begin in the chloroplast stroma,
where they generate the three-carbon sugar glyceraldehyde 3-phosphate.
This simple sugar is exported to the cytosol, where it is used to produce
sucrose and many other organic metabolites in the leaves of the plant. The
sucrose is then exported to meet the metabolic needs of the nonphotosynthetic
plant tissues, serving as a source of both carbon skeletons and
energy for growth.
Thus, the formation of ATP, NADPH, and O 2 (which requires light energy
directly) and the conversion of CO 2 to carbohydrate (which requires light energy
only indirectly) are separate processes (Figure 14–39). However, they are linked
by elaborate feedback mechanisms that allow a plant to manufacture sugars only
when it is appropriate to do so. Several of the chloroplast enzymes required for
carbon fixation, for example, are inactive in the dark and reactivated by light-stimulated
electron-transport processes.
Carbon Fixation Uses ATP and NADPH to Convert CO 2 into
Sugars
We have seen earlier in this chapter how animal cells produce ATP by using the
large amount of free energy released when carbohydrates are oxidized to CO 2 and
H 2 O. The reverse reaction, in which plants make carbohydrate from CO 2 and H 2 O,
takes place in the chloroplast stroma. The large amounts of ATP and NADPH produced
by the photosynthetic electron-transfer reactions are required to drive this
energetically unfavorable reaction.
LIGHT
2 H 2 O
IN
O 2
OUT
O 2
thylakoid membrane
STROMA
inner membrane
H +
outer membrane
e –
LIGHT
thylakoid space
H + H+
NADP + NADPH ATP ADP
carbon-fixation
cycle
IN CO 2
OUT
sugars
amino acids
fatty acids
sugars
amino acids
fatty acids
LIGHT
REACTIONS
DARK
REACTIONS
Figure 14–39 A summary of the energyconverting
metabolism in chloroplasts.
Chloroplasts require only water and
carbon dioxide as inputs for their lightdriven
photosynthesis reactions, and
they produce the nutrients for most other
organisms on the planet. Each oxidation of
two water molecules by a photochemical
reaction center in the thylakoid membrane
produces one molecule of oxygen, which
is released into the atmosphere. At the
same time, protons are concentrated
in the thylakoid space. These protons
create a large electrochemical gradient
across the thylakoid membrane, which is
utilized by the chloroplast atp synthase to
produce atp from ADP and phosphate.
The electrons withdrawn from water
are transferred to a second type of
photochemical reaction center to produce
NADPH from NADP + . As indicated, the
NADPH and atp are fed into the carbonfixation
cycle to reduce carbon dioxide,
thereby producing the precursors for
sugars, amino acids, and fatty acids. The
CO 2 that is taken up from the atmosphere
here is the source of the carbon atoms for
most organic molecules on Earth.
In a plant cell, a variety of metabolites
produced in the chloroplast are exported to
the cytoplasm for biosyntheses. Some of
the sugar produced is stored in the form of
starch granules in the chloroplast, but the
rest is transported throughout the plant as
sucrose or converted to starch in special
storage tissues. These storage tissues
serve as a major food source for animals.