…we have glucose…
Well, I promised you Photosynthesis Part II, and here it is. I have to say, I was most disappointed that it didn’t involve Voldemort, or a dark lord of any kind. Not even the Sith.
Anyway. The dark reactions are so called not because they take place in the dark, necessarily, but because they take place independently of the light – and the only place they happen is within the stroma of the chloroplast.
The light reactions gave us ATP and NADP.2H, which are used to drive the dark reactions. ATP provides energy for the process, while NADP.2H reduces (adds hydrogen to) carbon dioxide to a carbohydrate – a process also known as carbon fixing. So, if you like, ATP gives a plant enough energy to get its carbon fix.
The natural world is great at recycling – REALLY great at it. As NADP.2H is reducing carbon dioxide to a carbohydrate, it is, itself, being oxidised back to NADP – ready to be reused as an electron acceptor in the light reactions.
The whole process of the dark reactions is known as the Calvin cycle, after its discoverer – Melvin Calvin, whose parents had a terrible sense of humour when it came to baby names. I find it quite astonishing that back in 1945, scientists were able to delve this deeply into a plant cell and find out exactly what was going on.
A sugar phosphate with three carbon atoms as its backbone is the first product of the Calvin cycle, and it requires quite a lot of energy to make:
3CO2 + 9ATP + 6NADP.2H → 3C sugar phosphate + 9ADP + 8Pi + 6NADP
Some of the sugar phosphate is used as energy in the cytosol of the cell; the rest is converted into glucose phosphate and fructose phosphate, both of which are 6C sugars. These then combine to form sucrose, and lose their phosphate groups. Sucrose is transported around the plant for energy.
Photosynthesis is extremely well regulated and very efficient. Not to mention the fact that the light reactions are a truly renewable energy source – scientists are looking at their mechanisms, and wondering how to use the key components in artificial, light-driven fuel cells.
This is a brilliant idea, and I would suggest that any youth with an interest in photosynthesis, plant biology, and industry should get themselves on the rung of that ladder. It’s not just a career with a future; you may well be able to save our planet. And THAT is priceless.
This has been an exercise in ensuring that I understand photosynthesis; it’s rather complicated, you see. And it doesn’t make terribly interesting reading – so I promise that is the last long, boring explanation of a biological process there will be in this blog.