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Photosynthesis - The Light Reactions

The Light Reactions


How does chlorophyll transfer its excited state into useful biosynthetic energy? The solution to this problem closely parallels oxidative phosphorylation. A system of membrane-bound enzymes transfers the electrons taken by chlorophyll from water down an energy pathway, pumping hydrogen ions across the thylakoid membrane . This generates a charge separation across the membrane, due to the concentration of positive charge on the side to which the the hydrogen has been transferred. This charge separation is alleviated by allowing the hydrogen ions to pass back through the membrane through an ATP-ase pump that generates ATP with the electric potential produced by the charge separation. The electrons are finally transferred to NADP, producing NADPH which can be used as a source of reductive power for biosyntheses.

Non-Cyclic Photophosphorylation

The form of photosynthesis with which we are most familiar is non-cyclic photophosphorylation. It consists of two sets of pigments to excite. They are called PS1, or photosystem 1, and PS2, or photosystem 2. PS1 is better excited by light at about 700 nm, and is thus sometimes called P-700. PS2 cannot use photons of wavelength longer than 680 nm, and is thus sometimes called P-680.

Energy enters the system when PS2 becomes excited by light. Electrons are shed by the excited PS2 (oxidation), which grabs electrons from water, producing a molecule of oxygen gas for every two waters split. PS2 thus returns it to its unexcited state (reduction) . The electrons are passed through a chain of oxidation-reduction reactions. Each arrow in the diagram above actually represents a reaction like this one:

Each element in the pathway is reduced by the electrons, and turns right around to reduce its neighbour in the pathway by giving it the electrons, thus becoming reoxidized and ready for the next electrons to pass through the photsystem.

Here is a diagram of what is actually happening in the thylakoid membrane:

As you can see, PS2 passes on the energy to move the electron through the redox chain, thus pumping protons through the membrane to generate ATP. PS1, on the other hand, passes on the energy required to reduce NADPH. This division of labour between the two photsystems becomes important when we look at cyclic photophosphorylation.

Cyclic Photophosphorylation

Sometimes an organism has all the reductive power (NADPH) thatit needs to synthesize new carbon skeletons, but still needs ATP to power other activities in the chloroplast. Many bacteria can shut off PS2, allowing the production of ATP in the absence of glucose . A proton gradient is generated across the membrane using the mechanisms of photosynthesis. This type of energy generation is called cyclic photophosphorylation.

This may seem counter-intuitive. It appeared from noncyclic phtotphosphorylation that PS1 was responsible for NADPH production, while in cyclic photophosphorylation it is important for ATP production. This apparent dichotomy can be resolved when we understand what makes PS1 both a good candidate for noncyclic photophosphorylation and for NADPH production. PS1 is very good at transferring an electron, whether it be to NADP or to ferredoxin (fd). It is a powerful reductant. PS2, on the other hand, is better at grabbing electrons from water to transfer them to quinone (Q). It is a good oxidant.

As you can see, the electron transferred is not derived from water, but from PS1 itself. It therefore must be recycled to PS1.


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