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Photosynthesis: The Role of Light

The heart of photosynthesis as it occurs in most autotrophs consists of two key processes:

The second process involves a cyclic series of reactions named (after its discoverer) the Calvin Cycle. It is discussed in Photosynthesis: Pathway of Carbon Fixation. The details of the first process is our topic here.

A description of some of the experiments that led our understanding of these processes are described in Discovering the Secrets of Photosynthesis.

The electrons (e) and protons (H+) that make up hydrogen atoms are stripped away separately from water molecules.

2H2O -> 4e + 4H+ + O2

The electrons serve two functions: The protons also serve two functions:

The removal of electrons from water molecules and their transfer to NADP+ requires energy. The electrons are moving from a redox potential of about +0.82 volt in water to −0.32 volt in NADPH. Thus enough energy must be available to move them against a total potential of 1.14 volts. Where does the needed energy come from? The answer: Light.

The Thylakoid Membrane

Chloroplasts contain a system of thylakoid membranes surrounded by a fluid stroma.
Link to page on chloroplast structure.
Six different complexes of integral membrane proteins are embedded in the thylakoid membrane. The exact structure of these complexes differs from group to group (e.g., plant vs. alga) and even within a group (e.g., illuminated in air or underwater). But, in general, one finds:

1. Photosystem I

The structure of photosystem I in a cyanobacterium ("blue-green alga") has been completely worked out. It probably closely resembles that of plants as well.

It is a homotrimer with each subunit in the trimer containing:
View structures of chlorophyll a, chlorophyll b, and beta-carotene, a carotenoid.

2. Photosystem II

Photosystem II is also a complex of

3. & 4. Light-Harvesting Complexes (LHC)

These LHCs also act as antenna pigments harvesting light and passing its energy on to their respective photosystems.

The LHC-II of spinach is a homotrimer, with each monomer containing

5. Cytochromes b6 and f

6. ATP synthase

How the System Works

The sawtooth shifts in redox potential as electrons pass from P680 to NADP+ have caused this system to be called the Z-Scheme (although as I have drawn the diagram, it looks more like an "N"). It is also called noncyclic photophosphorylation because it produces ATP in a one-way process (unlike cyclic photophosphorylation and pseudocyclic photophosphorylation described below).

More on redox potentials and how they are exploited in photosynthesis.
Link to page analyzing the energy changes that occur during photosynthesis.

Chemiosmosis in Chloroplasts

The energy released as electrons pass down the gradient between photosystem II and plastocyanin (PC) is harnessed by the cytochrome b6/f complex to pump protons (H+) against their concentration gradient from the stroma of the chloroplast into the interior of the thylakoid (an example of active transport). As their concentration increases inside (which is the same as saying that the pH of the interior decreases), a strong diffusion gradient is set up. The only exit for these protons is through the ATP synthase complex. As in mitochondria, the energy released as these protons flow down their gradient is harnessed to the synthesis of ATP. The process is called chemiosmosis and is an example of facilitated diffusion.

Link to a description of two experimental tests of chemiosmosis in chloroplasts.

Cyclic Photophosphorylation

In cyclic photophosphorylation, This process is truly cyclic because no outside source of electrons is required. Like the photocell in a light meter, photosystem I is simply using light to create a flow of current. The only difference is that instead of using the current to move the needle on a light meter, the chloroplast uses the current to help synthesize ATP.

Pseudocyclic Photophosphorylation

Another way to make up the deficit is by a process called pseudocyclic photophosphorylation in which some of the electrons passing to ferredoxin then reduce molecular oxygen back to H2O instead of reducing NADP+ to NADPH.

At first glance, this might seem a fruitless undoing of all the hard work of photosynthesis. But look again. Although the electrons cycle from water to ferredoxin and back again, part of their pathway is through the chemiosmosis-generating stem of cytochrome b6/f.

Here, then, is another way that simply by turning on a light, enough energy is imparted to electrons that they can bring about the synthesis of ATP.

Antenna Pigments

Chlorophylls a and b differ slightly in the wavelengths of light that they absorb best (although both absorb red and blue much better than yellow and green — View). Carotenoids help fill in the gap by strongly absorbing green light. The entire complex ensures that most of the energy of light will be trapped and passed on to the reaction center chlorophylls.

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12 October 2013