Essential Cell Biology 5th edition

Chloroplasts and Photosynthesis
485
redox potential (mV)
–1200
–1000
–800
–600
–400
–200
0
200
400
600
light
produces
charge
separation
in reaction
center
photosystem II
proton gradient
is used to
generate ATP
light
produces
charge
separation
in reaction
center
photosystem I
Q
H +
cytochrome
b 6 -f complex
e –
e – plastocyanin
plastoquinone
pC
+
ferredoxin
NADP +
+
H +
FNR
NADPH
Figure 14–39 The combined
actions of photosystems I and II
boost electrons to the energy level
needed to produce both ATP and
NADPH. The redox potential for each
molecule is indicated by its position
on the vertical axis. Electron transfers
are shown with non-wavy blue arrows.
Photosystem II passes electrons from
its excited chlorophyll special pair to an
electron-transport chain in the thylakoid
membrane that leads to photosystem I
(see Figure 14–38). The net electron
flow through these two photosystems
linked in series is from water to NADP + ,
to form NADPH.
800
4H +
1000
1200
O 2
2H 2 O
+
water-splitting enzyme
light energy harnessed to produce NADPH +
ATP
direction of electron flow
that they cannot be exported directly to the cytosol. To provide energy and
reducing power for the rest of the cell, the ATP and NADPH are instead
used within the chloroplast stroma to produce a simple three-carbon
sugar that can be exported to the cytosol by specific carrier proteins in
the chloroplast inner membrane. This production of sugar from CO 2 and
water, which occurs during stage ECB5 e14.38/14.39
2 of photosynthesis, is called carbon
fixation.
In the central reaction of photosynthetic carbon fixation, CO 2 from
the atmosphere is attached to a five-carbon sugar derivative, ribulose
1,5-bisphosphate, to yield two molecules of the three-carbon compound
3-phosphoglycerate. This carbon-fixing reaction, which was discovered in
1948, is catalyzed in the chloroplast stroma by a large enzyme called ribulose
bisphosphate carboxylase or Rubisco (Figure 14–40). Rubisco works
much more slowly than most other enzymes: it processes about three
molecules of substrate per second—compared with 1000 molecules per
second for a typical enzyme. To compensate for this sluggish behavior,
CH 2 O
P
CH 2 O
O C O + C O
carbon
dioxide
H C OH
H C OH
CH 2 O
P
P
Rubisco
O
_ O
C
CH 2 O P
C
C
OH
O
H C OH
CH 2 O P
H 2 O
H
C
C OO _ + H +
COO _ OH
H C OH
CH 2 O P
ribulose 1,5-bisphosphate intermediate 2 molecules of
3-phosphoglycerate
Figure 14–40 Carbon fixation is
catalyzed by the enzyme ribulose
bisphosphate carboxylase, also
called Rubisco. In this reaction, which
takes place in the chloroplast stroma,
a covalent bond is formed between
carbon dioxide and an energy-rich
molecule of ribulose 1,5-bisphosphate.
This union generates a chemical
intermediate that then reacts with water
(highlighted in blue) to generate two
molecules of 3-phosphoglycerate.