Photosynthesis is the process plants use to make wood from air and light. Although that sounds suspiciously similar to magic, it's not that hard to understand. Solar cells after all, convert light to electricity - or rather, transform the energy of light into more energetic electrons. The difference for a plant is that it needs to store the energy as chemicals, just like batteries.

Stage One
As with respiration, the 'machine' which caries out the conversion is a membrane studded with proteins. This uses membrane potential to drive synthesis of ATP, and the absorbed light drives the formation of the membrane potential. This doesn't sound like the formula above, but this is only stage one - harnessing the light. This is where the oxygen gets made from water in a process called photolysis (water-splitting)1:

ADP + Pi + H2O --light--> O2 + ATP

So photolysis stores light energy at the expense of water - which is part of the reason plants need so much water. Well, maybe this is negligible compared to evapotranspiration, but water is essential. Actually, there are organisms that use other reducing agents (H2S - Hydrogen Sulphide for example) and they produce the corresponding oxidised product (elemental sulphur, for example).

Stage Two
Now the plant (or micro-organism) is free to use the generated ATP2 to make sugars; the sugars can then be made into almost anything else - carbohydrates, amino acids' skeletons, fats...This is the 'synthesis' part of the process, and it starts with carbon dioxide - a gas found in the air at a fraction of a percent3. An extremely important protein called RUBISCO uses the CO2 to make a 5-carbon sugar into a six carbon sugar4. In other words it 'fixes' the carbon into a larger molecule. The ATP produced in stage one provides the energy (ultimately from the sunlight) to convert these sugars into a common intermediate called GAP (glyceraldehyde-3-phosphate) which is also an intermediate in glycolysis. The whole fixation process is cyclical, in that the 5-Carbon sugar (RuBP) has to be regenerated to be able to fix more carbon. The wheels of this conveyor belt that brings carbon into the plant is turned by light and greased by ATP2.

1: The stoichiometry isn't right, but it gives the idea.
2 : And NAD(P)H, but that's just detail :)
3: For the moment...
4 : Which then breaks down immediately to two 3-C sugars.

Pho`to*syn"the*sis (?), n. (Plant Physiol.)

The process of constructive metabolism by which carbohydrates are formed from water vapor and the carbon dioxide of the air in the chlorophyll-containing tissues of plants exposed to the action of light. It was formerly called assimilation, but this is now commonly used as in animal physiology. The details of the process are not yet clearly known. Baeyer's theory is that the carbon dioxide is reduced to carbon monoxide, which, uniting with the hydrogen of the water in the cell, produces formaldehyde, the latter forming various sugars through polymerization. Vines suggests that the carbohydrates are secretion products of the chloroplasts, derived from decomposition of previously formed proteids. The food substances are usually quickly translocated, those that accumulate being changed to starch, which appears in the cells almost simultaneously with the sugars. The chloroplasts perform photosynthesis only in light and within a certain range of temperature, varying according to climate. This is the only way in which a plant is able to organize carbohydrates. All plants without a chlorophyll apparatus, as the fungi, must be parasitic or saprophytic. -- Pho`to*syn*thet"ic (#), a. -- Pho`to*syn*thet"ic*al*ly (#), adv.


© Webster 1913

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