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

CHLOROPLASTS AND PHOTOSYNTHESIS
787
LIGHT
(A)
H 2 O CO 2
CYTOSOL
NADPH carbonfixation
+
sugars sugars
ATP cycle
starch
chloroplast
O 2
metabolites
O 2
citric
oxidative
acid phosphorylation
cycle
mitochondrion
CO 2 ATP
(B)
10 µm
Figure 14–42 How chloroplasts and mitochondria collaborate to supply cells with both metabolites and ATP. (A)The
inner chloroplast membrane is impermeable to the ATP and NADPH that are produced in the stroma during the light reactions
of photosynthesis. These molecules are therefore funneled into the carbon-fixation cycle, where they are used to make sugars.
The resulting sugars and their metabolites are either stored within the chloroplast—in the form of starch or fat—or exported to
the rest of the plant cell. There, they can enter the energy-generating pathway that ends in ATP synthesis linked to oxidative
phosphorylation inside the mitochondrion. Unlike the chloroplast, mitochondrial membranes contain a specific transporter
that makes them permeable to ATP (see Figure 14–34). Note that the O 2 released to the atmosphere by photosynthesis
in chloroplasts is used for oxidative phosphorylation in mitochondria; similarly, the CO 2 released by the citric acid cycle in
mitochondria is used for carbon fixation in chloroplasts. (B) In a leaf, mitochondria (red) tend to cluster close to the chloroplasts
(green), as seen in this light micrograph. (B, courtesy of Olivier Grandjean.)
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b 6 -f complex, which both functionally and structurally resembles cytochrome
c reductase in the respiratory chain; and the chloroplast ATP synthase, which
closely resembles the mitochondrial ATP synthase and works in the same way.
Chlorophyll–Protein Complexes Can Transfer Either Excitation
Energy or Electrons
The photosystems in the thylakoid membrane are multiprotein assemblies of a
complexity comparable to that of the protein complexes in the mitochondrial
electron-transport chain. They contain large numbers of specifically bound chlorophyll
molecules, in addition to cofactors that will be familiar from our discussion
of mitochondria (heme, iron–sulfur clusters, and quinones). Chlorophyll,
the green pigment of photosynthetic organisms, has a long hydrophobic tail that
makes it behave like a lipid, plus a porphyrin ring that has a central Mg atom and
an extensive system of delocalized electrons in conjugated double bonds (Figure
14–43). When a chlorophyll molecule absorbs a quantum of sunlight (a photon),
the energy of the photon causes one of these electrons to move from a low-energy
molecular orbital to another orbital of higher energy.
The excited electron in a chlorophyll molecule tends to return quickly to its
ground state, which can occur in one of three ways:
1. By converting the extra energy into heat (molecular motion) or to some
combination of heat and light of a longer wavelength (fluorescence); this
is what usually happens when light is absorbed by an isolated chlorophyll
molecule in solution.
2. By transferring the energy—but not the electron—directly to a neighboring
chlorophyll molecule by a process called resonance energy transfer.
3. By transferring the excited electron with its negative charge to another
nearby molecule, an electron acceptor, after which the positively charged
chlorophyll returns to its original state by taking up an electron from some
other molecule, an electron donor.
Figure 14–43 The structure of chlorophyll. A magnesium atom is held in a
porphyrin ring, which is related to the porphyrin ring that binds iron in heme
(see Figure 14–15). Electrons are delocalized over the bonds shaded in blue.
H 3 C
H
H 3 C
H
3
CH 2 CH 3
CH H CH
C C C
C C C C
CH 2
C N N C
C Mg C H
C N N C
C C C C
CH
C
C
CH 2 H C C
CH 2 C O O
C O O
O CH 3
CH 2
CH
C CH 3
CH 2
CH 2
CH 2
HC CH 3
CH 2
CH 2
CH 2
HC CH 3
CH 2
CH 2
CH 2
CH
CH 3 CH 3
hydrophobic
tail region
CH 3