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