Transcript Photosynthesis - California Science Teacher

Photosynthesis
Overall Reaction
6CO2 + 6H2O --> C6H12O6 + 602
Overall Reaction
Which oxygen gets incorporated into the sugar,
the one from water or the one from CO2?
What type of reaction is this?
What is the purpose of water?
Water as a Source of Electrons
Why is H2O used as a donor of electrons for so
many plants? After all, there are many other
compounds that can give up electrons, such as
SH2.
Light Reactions
The Z-Scheme
Z-scheme
What's the function?
Z-scheme: function
Harness sunlight energy to excite an electron
(taken from water) to a higher energy state.
Use the energy to maintain a proton gradient (for
ATP synthase) and create reducing molecules that
can do chemical work.
Z-scheme
Why not two separate photosystems (why link
them together)?
Z-scheme: linkage
Save room.
Instead of having oxygen evolving complexes for
each photosystem type, there is just one for both.
Efficiency: the photosystems are linked together
so that the electrons from the first are passed to
next.
Z-scheme: linkage
Another question:
If it is efficient to link the two systems, why not join
them physically to save even more room?
Z-scheme: linkage
One photosystem is easier to excite than the
other. If the two were physically linked together,
excitons from the antenna system would
preferentially go to one photosystem, leaving one
underexcited. This imbalance would defeat the
whole purpose of having the two systems
together.
Z-scheme
Why so many intermediate carrier molecules?
Z-scheme: carrier molecules
Photosystems can deal with only one electron at a
time. Also, remember that the two photosystems
can't be too close together.
“Dark” Reactions
Is dark reactions a good name for the second part
of photosynthesis?
What observation(s) would render this name
misleading?
Carbon Assimilation Reactions
CO2 is incorporated into a 3 carbon sugar.
Uses molecules generated during the light
reactions.
Ribulose-1,5-bisphosphate carboxylase
oxygenase (aka Rubisco) catalyzes the
incorporation of CO2.
Ribulose-1,5-bisphosphate
carboxylase oxygenase
Rubisco catalyzes not only the carboxylation but
also the oxygenation of ribulose 1,5-bisphosphate
(sugar with 5 carbons).
In other words, when rubisco binds O2, it is
counter-productive.
Oxygenation leads to photorespiration, which
produces no ATP and leads to a net loss of carbon
and nitrogen.
Ribulose-1,5-bisphosphate
carboxylase oxygenase
CO2 and O2 compete for the same active sites on
the Rubisco molecule.
Oxygenation consumes significant amounts of
ribulose 1,5- bisphosphate in vivo.
What's the deal with rubisco?
Rubisco
Rubisco evolved in an environment relatively
devoid of O2. It did not need a high specificity for
CO2 over O2.
But wouldn't selection favor a rubisco that is
better, now that there is more O2 in the
atmosphere?
Building a Better Wheel
Using the “wheel” provided, build a better wheel.
Building a Better Wheel
Rubisco is relatively unchanged. Because it is a
central pathway, it is sensitive to tinkering.
It's not a good idea to work on building a better
airplane while it is flying.
Solution: peripheral components enhance
rubisco's efficiency (eg CAM, C4).
Solar Panels
Plant Leaves
Why Aren't Leaves Black?
Why Aren't Leaves Black?
An excess of NADPH disrupts chemical balance
(highyly reducing environment)
Light in large quantities generates reactive
chemical species, such as superoxides and
radicals. An increase in these reactive chemical
species damages the plant, and results in
photoinhibition
Why Aren't Leaves Black?
Black may result in overheating, which may
denature plant enzymes and destroy vital
processes.
Most plants can get plenty of sunlight, and thus
the amount of light is not the factor that limits the
rate of photosynthesis.
Black Photosynthetic Organisms
Probably Exist
Where would you expect to find these kinds of
organisms?
Evolutionary Constraint
Rubisco
Black Plants