0.23 Bis2a 06.3 photophosphorylation: the light reactions in  (Page 4/33)

Photophosphorylation a brief synopsis

Photophosphorylation is the process of converting light into chemical energy, ATP and NADPH. Photosynthesis is the integration of these light-reactions driving the reduction of CO ₂ to sugars, specifically Glyceraldehyde-3-Phosphate, a triose. In this module we will focus on the first part of photosynthesis, the light-reactions or the generation of ATP and NADPH from light.

First and foremost it is important to realize that photophosphorylation and photosynthesis are very ancient sets of reactions. When we think of photosynthesis we mainly think of green plants; taking up CO ₂ and giving off O ₂ . But this is a special, and from an evolutionary perspective, relatively new form of photophosphorylation. While extremely efficient and complicated, oxygenic photophosphorylation , the form of photophosphorylation that produces O ₂ as a product, is only part of the picture of the evolution of photophosphorylation.

Photophosphorylation has its roots in the anaerobic world, between 3 billion and 1.5 billion years ago, when life was abundant in the absence of molecular oxygen. Photophosphorylation probably evolved relatively shortly after electron transport chains and anaerobic respiration began to provide metabolic diversity. Think of photophosphorylation this way: it is simply a form of an electron transport chain. The major difference is that instead of electrons being donated by a very strong reducing compound, such as NADH, light energy is used to "energize" an electron into a "high energy state". This "energized" electron can be donated to an electron transport chain, and as it decays, that is, as it passes from one electron carrier to another via red/ox reactions protons are pumped across a membrane. The pumping of these protons across a membrane leads to the generation of a PMF, which in turn results in the production of ATP. If enough light energy or photons can be absorbed and transferred to electrons, and if those electrons can have a lower (that is a more negative) reduction potential than NADP/NADPH, then they can be used reduce NADP to form NADPH. Therefore, photophosphorylation requires a compound that can absorb light energy or photons, use that energy to excite an electron and then donate that excited electron to NADPH. That compound is chlorophyll or bacteriochlorophyll. The final piece of the photophosporylation story is finding something to reduce the oxidized bacteriochlorophyll, under anaerobic conditions, reduced sulfur compounds such as SH ₂ and even elemental S ⁰ are excellent electron donors (look at the redox tower provided in figure 9 to see more potential electron donors).

These early, simple anoxygenic photophosphorylation pathways could either make NADPH, in a process called noncyclic photophosphorylation or ATP, in a processes called cyclic photophosphorylation per donated electron. At some point, about 1.5 billion years ago, a chlorophyll molecule evolved that when oxidized (when a photon of light was absorbed and transferred to the electron which is ejected) had a higher (more positive) reduction potential than O ₂ . Which meant that the oxidized form of chlorophyll could be reduced by water and generate molecular oxygen. That event changed the shape of the planet for ever. That event, the great oxygen event now begins to accumulate molecular oxygen, a toxic, highly corrosive and reactive compound, into the environment and life on this planet would change forever.