Download From lec. 2, Kluyver and van Niel proposed that all photosynthetic

LECTURE 7
PHOTOSYNTHESIS & CARBON FIXATION
From lec. 2, Kluyver and van Niel proposed that all
photosynthetic reactions could be summarized with:
Photosynthesis is the conversion of light energy
to chemical energy, usually using only CO2 as a
carbon source.
Photosynthesis exists in all 3 domains of life,
although eukaryotes are photosynthetic only via
the presence of symbiotic organelles.
CO2 + 2 H2A ----> CH2O (cell material) + H2O + 2 A
CO2 is being reduced
Phototrophy is wide spread in bacteria and appears
to be a very ancient trait ---
H2A is being oxidized
fossil stromatolites in rock over 3 billion years old
show photosynthetic prokaryotes. Fig. 10.14. Modern day
stromatolites in Australia
Table 6.9 Five main groups of Bacteria are
photosynthetic:
• Purple bacteria (sulfur and non-sulfur)
• Green sulfur bacteria
• Green nonsulfur bacteria
• Heliobacteria
• Cyanobacteria
2 parts to photosynthesis:
• light reactions - light energy is trapped by
molecules and converted to chemical
energy (ATP + NADPH)
• dark reactions - this chemical energy is
used to reduce CO2.
Photosynthesis is perhaps the most important
biological process on the planet.
•Produces most organic carbon on
Earth
•These organisms completely transformed
the planet.
The earliest photosynthesis was anoxygenic - did
not produce O2 gas.
Oxygenic photosynthesis created our
atmosphere, including the ozone layer which
allowed life to evolve onto land.
Need molecule that is capable of absorbing
light.
**CHLOROPHYLLS**
Similar in structure to cytochromes from the
ETC, but instead of iron in their center, they
have a magnesium atom.
Photosynthetic
bacteria also
have
chlorophylls,
though there is
a much greater
diversity in the
types of
molecules.
Purple bacteria - chlorophylls a and b
Green sulfur bacteria - chlorophylls c, d, and e
Green nonsulfur bacteria - chlorophyll cS
Heliobacteria - chlorophyll g
For example,
purple bacteria
have bacteriochlorophyll a…
In prokaryotes, the chlorophylls are intergrated
into internal membrane systems in different
places in different bacteria:
• purple bacteria - invagination of
the membrane
• heliobacteria - the membrane itself
• green bacteria - both the membrane
and in membrane-enclosed structures
called chlorosomes
In addition to chlorophylls, organisms may have
other pigments such as carotenoids and
phycobiliproteins.
These accessory pigments absorb light in other
areas of the spectrum and can transfer the
trapped energy to chlorophyll.
• cyanobacteria - thylakoid membranes
Chlorophylls and accessory pigments are arranged in
arrays called antennas (fig. 6.26).
Fig. 6.26. Light energy being used to excite an electron
Remember that as electrons flow down an
electrochemical gradient, energy is released.
A similar process occurs in photosynthetic
organisms, i.e. they use membrane-bound
proteins to pass electrons down a gradient,
generating a proton motive force.
This is called photophosphorylation.
Because electrons are being driven out of
chlorophyll using light energy, they must be
replaced.
• oxygenic photosynthesis - the source
is water. O2 is produced as a byproduct.
Oxygenic Photosynthesis
Eukaryotes, cyanobacteria and some other bacteria
Use water as an electron source and produce
oxygen as a final product.
• Anoxygenic photosynthesis - the source
is reduced sulfur compounds, organic
compounds, hydrogen gas or Fe(II)
Oygenic photosynthesis involves 2 distinct, but
interconnected reactions:
The products of these light reactions then are:
• Photosystem I - chlorophyll P700 and
absorbs light at long wavelengths (far red).
• ATP (non-cyclic photophosphorylation)
• Photosystem II - chlorophyll P680 and
absorbs light at shorter wavelengths (near
red).
•O2 (from splitting of water)
• NADH (Where the electrons end up..)
Fig. 6.27. The path of electron flow looks like a Z turned on its
side, so it is often called the “Z scheme”.
The process we just talked about was oxygenic photosynthesis
and was used by cyanobacteria and eukaryotes.
Other bacteria use anoxygenic photosynthesis,
i.e. harvest light energy to synthesize ATP without
water as an electron source (no O2 is produced).
In these organisms, light energy also excites a
pigment molecule.
Electrons also cascade down a series of
molecules, to drive photophosphoryllation to
generate ATP.
If enough carbon is present - cyclic electron flow
= cyclic photophosphorylation
Green Sulfur Bacteria
Use H2S or
S2O32- as
electron
donors, thus no
O2 produced,
often do cyclic
photophosphorylation.
Table 6.9
The most common way that organisms fix
carbon is via the Calvin cycle. This is used by
cyanobacteria, purple bacteria, algae, and
some Archaea. See fig. 6.28
At the end of one turn of the Calvin cycle,
we have:
• regenerated our 1,5-ribulose biphosphate
Those were all light reactions. The next steps,
called the dark reactions, involve the fixation of
inorganic carbon using the ATP and NADH
generated in the light reactions.
In order to form 6-carbon sugars such as glucose or
fructose, the Calvin cycle must turn 6 times.
The incorporation of one CO2 into an organic
compound requires 3 ATP and 2 NADPH.
Therefore, the formation of glucose requires 18
ATP and 12 NADPH.
• incorporated 1 CO2
• freed a carbon for biosynthesis
Thus, we can sum up the equation for the
formation of glucose as follows:
6 CO2 + 12 NADPH + 18ATP -> C6H12O6 + 12
NADP+ + 18 ADP
Green sulfur bacteria do not use the Calvin cycle.
They just run the TCA cycle backwards - called
reductive TCA.