PHOTOSYNTHESIS

The reaction that makes the world green is just one of many variants

When the sun shines, green plants break down water to get electrons and protons, use those particles to turn carbon dioxide into glucose, and vent out oxygen as a waste product. That process is by far the most complex and widespread of the various known versions of photosynthesis, all of which turn the light of particular wavelengths into chemical energy. (Studies have even suggested that certain single-celled fungi can utilize the highly energetic gamma rays: colonies of such fungi have been found thriving inside the postmeltdown nuclear reactor at Chernobyl.) Using water as a photosynthetic reactant instead of scarcer substances such as hydrogen sulfide eventually enabled life to grow and thrive pretty much everywhere on the planet. Water-splitting photosynthesis was “invented” by the ancestors of today’s cyanobacteria, also known as blue-green algae. The organisms that now do this type of photosynthesis, including plants, green algae and at least one animal (the sea slug Elysia chlorotica), carry organelles called chloroplasts that appear to be the descendants of what once were symbiotic cyanobacteria.

All of them use some form of the pigment chlorophyll, sometimes in combination with other pigments. Photosynthesis starts when arrays of chlorophyll molecules absorb a photon and channel its energy toward splitting water. But water is a uniquely hardy molecule to be involved in photosynthesis. Taking electrons from water and giving them enough energy to produce glucose requires two separate assemblies of slightly different chlorophyll molecules (and an apparatus of more than 100 different types of proteins). Simpler forms of photosynthesis use one or the other version, but not both. The mystery is, Which one appeared first in evolution, and how did the two end up combined? “It’s a question we don’t really know the answer to,” says Robert Blankenship of Washington University in St. Louis. Scientists also do not know when cyanobacteria learned to split water. Some evidence suggests that it may have been as early as 3.2 billion years ago. It surely must have happened at least 2.4 billion years ago, when oxygen shifted from being a rare gas to being the second most abundant one in the atmosphere—a change without which complex multicellular animals that can formulate scientific questions could never have existed. —Davide Castelvecchi

Source of Information : Scientific American September 2009