Fundamentals of
Biochemistry
Third Edition
Donald Voet • Judith G. Voet •
Charlotte W. Pratt
Chapter 19
Photosynthesis
Copyright © 2008 by John Wiley & Sons, Inc.
The basic equations
• Overall: 6 CO2 + 6 H2O  C6H12O6 + 6 O2
• Two stages:
• The dark reaction is also known as the
Calvin cycle or the reductive pentose
phosphate cycle (Melvin Calvin, James Bassham
and Andrew Benson, UC Berkeley (1950), “The path of
carbon in photosynthesis”, J. Biol. Chem. 185)
The chloroplast is the site of the light reaction. This organelle
is similar to the mitochondrion in having its own DNA.
The lumen is on the inside of the thylakoid.
Figure 19-1
Chlorophyll is the principle
photoreceptor. It has a
heme-analogous
heterocycle, with a
magnesium ion stabilizing
the structure. The various
chromophores (R groups)
alter the individual
pigment’s maximum
absorption wavelength.
Figure 19-2
Other pigments “fill in” absorption holes of chlorophyll
Figure 19-3
Photosynthetic antenna complex
located on the thylakoid
membrane
Dark green circles are
the reaction centers where
Photosystem I and II are present
Figure 19-4
A light-harvesting
complex (LHC)
that combines
proteins (red and
blue) with a
chlorophyll
pigment (green)
and a lycopene
carotenoid
pigment (yellow)
Figure 19-5a
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Much of the point of
the LHC is to excite
electrons within the
pigment, thus
increasing the energy
of the molecule. This
energy can then be
transferred to other
molecules.
Recall that E = h c/λ
and that ν = c/λ.
Figure 19-6
Er, yeah.
Figure 19-7
Purple photosynthetic bacterial reaction complexes (PbRC)
Figure 19-8
The structure of the PbRC allows the absorption of a particular
wavelength of light, e.g., P960
Figure 19-9
The energy cycle of a PbRC
3 picoseconds
Transfers within the LHC
Membranebound
ubiquinone
s
Figure 19-10
This process does not result in a net oxidationreduction, since the excited electrons return to
P960. The energy is used to translocate protons
across the thylakoid membrane using QB.
In plants and cyanobacteria, the light reaction is not cyclic.
There are two photosystems (I and II), where electrons from
II into I.
This herbicide blocks that flow and causes
photosynthetic O2 production to cease.
Page 650
The thylakoid membrane protein
complexes are similar to those found
in the mitochondrial inner membrane.
There are proton translocators, and an ATP
synthase, as well as electron transporters.
Figure 19-11
How the electron
transporters work
Page 651
Energy level diagram of the light reaction in plants and cyanobacteria
Figure 19-12
Bottom line: about 4 ATP and 2 NADPH (total of 9 ATP equivalents)
are made for each oxygen molecule evolved, which translates to about
one ATP per photon absorbed.
Box 19-1
Dark reaction keys
off of this molecule
Page 663
1 carbon
2 times 3 carbons
5 carbons
Figure 19-25
Sugars
Figure 19-26
Note that step 2 will be the
rate-determining,
irreversible step
Figure 19-26 part 1
Hey, you can mess with concentrations to
make reactions more favorable!
Table 19-1
Figure 19-26 part 2
Steps 7 and 10 also have a
large negative ΔG, and so are
rate-determining steps as well.
Figure 19-26 part 3
If the Calvin cycle is allowed to
complete, it converts five 3carbon sugars (3PG or GAP) into
three 5-carbon sugars (Ru5P).
Figure 19-26 part 4
Calvin cycle is controlled
by light, indirectly,
through changes in pH,
[Mg2+] and the synthesis
of a transition-state analog
2-carboxyarabinitol-1phosphate.
Figure 19-27c
The enzyme ribulose-1,5,bisphosphate carboxylase (step 2)
“fixes” carbon. It has a very low
catalytic efficiency but there are a
lot of these molecules.
Figure 19-27
The mechanism of Ru5P carboxylase
involves base catalysis (first step)
Cleavage of the β-keto acid is
exergonic and drives the
reaction.
Figure 19-28
The “products” of
photosynthesis (basically,
GAP) can be converted to
F6P, then G1P, then starch
(as shown). Note the
similarity to glycogen
synthesis.
Figure 19-29
Sucrose is the major
transport sugar in
plants to nonphotosynthesizing
cells.
Synthesized in cytosol, so
precursor transport out of the
chloroplast is needed.
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By regulating FBPase (step
7) and SBPase (step 10)
using the mechanism to the
right – note the light
sensitivity – the Calvin cycle
can be turned off when there
is darkness, which prevents
the Calvin cycle from using
ATP and NADPH produced
by glycolysis.
Figure 19-30
Plants have an alternative method of generating CO2 and using O2
distinct from respiration – it is called photorespiration; O2 was found to
compete as a substrate for RuBP carboxylase (step 2).
Figure 19-31
The peroxisome is the organelle where
the oxygen is used during
photorespiration to yield water. This
process, due to its repeated reductions
and phosphorylations, uses up ATP and
NADPH made by the light reaction,
and thus qualifies as a futile cycle. The
metabolic reason for photorespiration
is unknown.
The existence of RuBP oxygenase
activity (NE Tolbert (1971), Annual
Review of Plant Physiolology 22) was
shown by the use of the then-new gas
phase IR spectrometer.
Figure 19-32
Since photorespiration is futile, most plants have evolved a way of concentrating CO2 so
that O2 doesn’t compete for RuBP carboxylase. These plants are called C4 plants because
they use a four-carbon intermediate (incorporating a CO2 from the atmosphere, as
opposed to C3 plants that use only GAP. C4 plants are found more in the tropics, where
heat and light are more prevalent, so photorespiration would be more of a threat.
Figure 19-33