Photosynthesis is the process plant
s 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.
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).
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.