One of the simplest methods for capturing light and transforming it into chemical energy is used by the archaea Halobacterium halobium.

Halobacteria are extreme halophiles or salt-loving archaea that live in waters that contain up to 5M NaCl.

Our atmosphere allows mainly visible light from the sun to reach the earth's surface. Most biological molecules do not absorb visible light.

Organisms must therefore use special molecules, known as pigments, to capture light.

Light acts as both a wave and a particle. The wavelength of a photon determines its color and the amount of energy it contains.

A pigment's color is due to the photons it does not absorb. These are reflected and can be seen.

To absorb light, H. halobium uses a membrane protein, bacteriorhodopsin.

A cross section of a H. halobium

Bacteriorhodopsin consists of two components, a polypeptide, known generically as an opsin, and a prosthetic group, the pigment retinal, a molecule derived from vitamin A.

A ribbon diagram of bacteriorhodopsin.

When a photon of light is absorbed by the retinal group, the retinal undergoes a photoisomerization reaction

The end result is the movement of a H⁺ across the membrane.

This H⁺ gradient is tapped by ATP synthase to make ATP.

When the light goes off, the H⁺ gradient dissipates.

More complex photosystems

Most bacteria and plants use a more complex system to capture and utilize light.

Light harvesting complexes act as antennas to increase the amount of light the organism can capture.

Plants are often in intense competition with one another for access to light.

A photon absorbed by a light harvesting complex leads to the excitation of an electron.

The antennal system is highly organized and passes this excited electron to a specialized protein-chlorophyll complex, the reaction center .

From the reaction center, the high energy electron is passed to a membrane-bound, electron transport chain.

As the electron moves through the electron transport chain, it looses free energy, some of which is used to pump H⁺ across the membrane and so generate a H⁺ gradient.

Removing an electron from the reaction center creates a highly reactive free radical.

If left in this state, the reaction center could be damaged.

In the cyclic reaction, after the electron has lost its energy it is returned to the reaction center.

The molecular machinery that carries out the cyclic reaction is known as photosystem I.

These electrons are transferred along an electron transport chain which extracts some of their energy and uses it to pump H⁺ across the membrane.

The system is non-cyclic in that these original electrons are not returned to the photosystem II reaction center, rather they are passed on to the photosystem I reaction center. Here the absorption of a second photon excites them again.

These excited electrons are then used to reduce NADP, a phosphorylated form of NAD⁺, to form NADPH.

This generates a free radical at the photosystem II reaction center.

To regenerate the photosystem, a water molecule is oxidized. Two electrons derived from water pass to the reaction center, 2 H⁺ are released and contribute to the proton gradient across the membrane, and 1/2O₂ molecular oxygen, is released as a waste product.

O₂ is essential for many organisms, particularly larger eukaryotes like us.

Photophosphorylation and Photosynthesis

Both the cyclic and non-cyclic reactions are known as Photophosphorylation.

In these reactions, the energy captured from light is used to form H⁺ gradients which in turn are used to drive the phosphorylation of ADP to form ATP.

Photosynthesis is a distinct process that refers to the reaction by which carbon dioxide, CO₂, is used to synthesize carbohydrates, generally the sugar glucose.

The formula for this reaction is 6H₂0 + 6CO₂ C₆H₁₂O₆ + 6O₂.

NADPH, produced by the non-cyclic light reaction, is used to drive the synthesis of carbohydrate. Photosynthesis is glycolysis run backward.

ATP and NADPH are produced as long as there is light, so this part of the photosynthesis is known as the light reaction.

The fixation of CO₂ into carbohydrate occurs, at least for awhile, in the dark and is known as the dark reaction.

Using a radioactive isotope of carbon, carbon-14, a by-product of the American effort to develop atomic weapons during World War II, Melvin Calvin and co-workers defined the steps in the photosynthetic dark reaction, which is now known as the Calvin cycle in his honor.

Evolutionary collaborations

The appearance of photosynthesis coupled to the release of molecular oxygen was a key event in the evolutionary history of life on earth.