Essential Cell Biology 5th edition

Chloroplasts and Photosynthesis
483
THYLAKOID
SPACE
thylakoid
membrane
STROMA
LIGHT
e –
photosystem I
H + +
Fd
ferredoxin
FNR
ferredoxin-
NADP + reductase
Figure 14–36 Photosystem I transfers
high-energy electrons to an enzyme that
produces NADPH. When light energy
is captured by photosystem I, a highenergy
electron is passed to a mobile
electron carrier called ferredoxin (Fd), a
small protein that contains an iron–sulfur
center. Ferredoxin carries its electrons to
ferredoxin-NADP + reductase (FNR), the final
protein in the electron-transport chain that
catalyzes the production of NADPH.
NADP +
NADPH
Oxygen Is Generated by a Water-Splitting Complex
Associated with Photosystem II
The scheme that we have thus far described for photosynthesis has
ignored a major chemical conundrum. When a mobile electron carrier
removes an electron from a reaction center (whether in photosystem I or
photosystem II), it leaves behind a positively charged chlorophyll special
pair (see Figure 14–34). To reset the system and allow photosynthesis to
proceed, this missing electron ECB5 must e14.35/14.36 be replaced.
For photosystem II, the missing electron is replaced by a special manganese-containing
protein complex that removes the electrons from water.
The cluster of manganese atoms in this water-splitting enzyme holds
onto two water molecules from which electrons are extracted one at a
time. Once four electrons have been removed from these two water molecules—and
used to replace the electrons lost by four excited chlorophyll
special pairs—O 2 is released (Figure 14–37). It is by this means that all of
the O 2 in our atmosphere—all of the O 2 we breathe—is produced. Life on
Earth would be a very different affair without the water-splitting enzyme
of photosystem II.
QUESTION 14–9
Both NADPH and the related carrier
molecule NADH are strong electron
donors. Why might plant cells have
evolved to rely on NADPH, rather
than NADH, to provide the reducing
power for biosynthesis?
2 H 2 O O 2
THYLAKOID
SPACE
STROMA
(A)
LIGHT
2 H 2 O O 2
e –
Q
photosystem II
reaction center
+ 4
H +
manganese cluster in
water-splitting enzyme
special pair of
chlorophylls
4 H +
e –
e –
e –
antenna
complex
thylakoid
membrane
water-splitting enzymes
reaction center
photosystem II
antenna
complex
10 nm
(C)
e –
plastoquinone
(B)
Figure 14–37 The reaction center of photosystem II includes a water-splitting enzyme that catalyzes the extraction of electrons
from water. (A) Schematic diagram showing the flow of electrons through the reaction center of photosystem II. When light energy
excites the chlorophyll special pair, an electron is passed to the mobile electron carrier plastoquinone (Q). An electron is then returned
to the special pair by a water-splitting enzyme that extracts electrons from water. The manganese (Mn) cluster that participates in the
electron extraction is shown as a red spot. Once four electrons have been withdrawn from two water molecules, O 2 is released into
the atmosphere. (B) The structure and position of some of the electron carriers involved. (C) Structure of a membrane-embedded
photosystem II (PSII) complex, including a reaction center and several light-harvesting antenna complexes. This structure, obtained
from spinach, was determined by cryoelectron microscopy (see Panel 4–6, pp. 168–169). Note that this complex exists as a dimer in the
membrane, and thus contains two copies of the water-splitting enzyme.
ECB5 e14.36/14.37