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

792 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
light
2H 2 O O 2 + 4 H + 2 H +
photosystem II
pC pC
THYLAKOID SPACE
P 680 QH 2 QH 2
P 700
Q Q
2 H + Fd Fd FNR NADPH
cytochrome
b 6 -f complex
photosystem I
one at a time to the mobile electron carrier plastocyanin (a small copper-containing
protein that takes the place of the cytochrome c in mitochondria), which will
transfer them to photosystem I (Figure 14–50). As we discuss next, photosystem
I then harnesses a second photon of light to further energize the electrons that it
receives.
light
STROMA
Figure 14–50 Electron flow through the
cytochrome b 6 -f complex to NADPH.
The cytochrome b 6 -f complex is the
functional equivalent of cytochrome c
reductase (the cytochrome b-c 1 complex)
in mitochondria (see Figure 14–22). Like its
mitochondrial homolog, the b 6 -f complex
receives its electrons from a quinone and
engages in a complicated Q cycle that
pumps two protons across the membrane
(details not shown). It hands its electrons,
one at a time, to plastocyanin (pC).
Plastocyanin diffuses along the membrane
surface to photosystem I and transfers
the electrons via ferredoxin (Fd) to the
ferredoxin-NADP + reductase (FNR), where
they are utilized to produce NADPH. P 700 is
a special pair of chlorophylls that absorbs
light of wavelength 700 nm.
Photosystem I Carries Out the Second Charge-Separation Step
in the Z Scheme
Photosystem I receives electrons from plastocyanin in the thylakoid space and
transfers them, via a second charge-separation reaction, to the small protein
ferredoxin on the opposite membrane surface (Figure 14–51). Then, in a final
step, ferredoxin feeds its electrons to a membrane-associated enzyme complex,
the ferredoxin-NADP + reductase, which uses the electrons to produce NADPH
from NADP + (see Figure 14–50).
The redox potential of the NADP + /NADPH pair (–320 mV) is already very low,
and reduction of NADP + therefore requires a compound with an even lower redox
potential. This turns MBoC6 out n14.327/14.50
to be a chlorophyll molecule near the stromal membrane
surface of photosystem I that has a redox potential of –1000 mV (chlorophyll A 0 ),
making it the strongest known electron donor in biology. The reduced NADPH is
released into the chloroplast stroma, where it is used for biosynthesis of glyceraldehyde
3-phosphate, amino acid precursors, and fatty acids, much of it to be
exported to the cytoplasm.
pC
plastocyanin
e –
Fd
ferredoxin
PQ
iron–sulfur
clusters
A 0
A 0
P 700
PQ
Figure 14–51 Structure and function
of photosystem I. At the heart of the
photosystem I complex assembly is the
electron-transfer chain shown. At one
end is a special pair of chlorophylls called
P 700 (because it absorbs light of 700 nm
wavelength), receiving electrons from
plastocyanin (pC). At the other end are the
A 0 chlorophylls, which hand the electrons
on to ferredoxin via two plastoquinones
(PQ; purple) and three iron–sulfur clusters.
Even though the roles of photosystems I
and II in photosynthesis are very different,
their central electron-transfer chains are
structurally similar, indicating a common
evolutionary origin (see Figure 14–53).
Note that in photosystem I both branches
of the electron-transfer chain are active,
unlike in photosystem II (see Figure 14–48).
(PDB code: 3LW5.)