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

790 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
redox potential (mV)
–1200
–1000
–800
–600
–400
–200
0
200
400
600
800
1000
1200
light
produces
charge
separation
O 2
Mn
plastocyanin
e – cytochrome
b 6 -f complex
e – Figure 14–47 Changes in redox potential
photosystem I
during photosynthesis. The redox
and NADPH
potential for each molecule is indicated
by its position along the vertical axis.
Photosystem II passes electrons derived
from water to photosystem I, which in turn
ferredoxin- passes them to NADP + through ferredoxin-
NADP + reductase NADP + reductase. The net electron flow
through the two photosystems is from
ferredoxin
water to NADP + , and it produces NADPH
as well as an electrochemical proton
an electrochemical
gradient. This proton gradient is used
gradient is formed
light
by the atp synthase to produce atp.
that generates ATP
produces
NADPH Details in this figure will be explained in the
Q
charge
separation
NADP +
subsequent text.
H +
+ H +
plastoquinone
pC
+
light energy harnessed to produce
ATP
4H +
photosystem II
+
2H 2 O
water-splitting enzyme
direction of electron flow
The Z scheme is necessary to bridge the very large energy gap between water
and NADPH (Figure 14–47). A single quantum of visible light does not contain
enough energy both to withdraw electrons from water, which holds on to its electrons
very tightly (redox potential +820 mV) and therefore is a very poor electron
donor, and to force them on to NADP + , which is a very poor electron acceptor
(redox potential –320 mV). The Z scheme first evolved in cyanobacteria to enable
them to use water as a universally MBoC6 available m14.49/14.47 electron source. Other, simpler photosynthetic
bacteria have only one photosystem. As we shall see, they cannot use
water as an electron source and must rely on other, more energy-rich substrates
instead, from which electrons are more readily withdrawn. The ability to extract
electrons from water (and thereby to produce molecular oxygen) was acquired by
plants when their ancestors took up the endosymbiotic cyanobacteria that later
evolved into chloroplasts (see Figure 1–31).
Photosystem II Uses a Manganese Cluster to Withdraw Electrons
From Water
In biology, only photosystem II is able to withdraw electrons from water and to
generate molecular oxygen as a waste product. This remarkable specialization of
photosystem II is conferred by the unique properties of one of the two chlorophyll
molecules of its special pair and by a manganese cluster linked to the protein.
These chlorophyll molecules and the manganese cluster form the catalytic core of
the photosystem II reaction center, whose mechanism is outlined in Figure 14–48.
Water is an inexhaustible source of electrons, but it is also extremely stable;
therefore a large amount of energy is required to make it part with its electrons.
The only compound in living organisms that is able to achieve this feat after its ionization
by light, is the chlorophyll special pair called P 680 (P 680 /P
+ 680 redox potential
= +1270 mV). The reaction 2H 2 O + 4 photons → 4H + + 4e – + O 2 is catalyzed by
its adjacent manganese cluster. The intermediates remain firmly attached to the
manganese cluster until two water molecules have been fully oxidized to O 2 , thus