Photosynthesis: Calvin Cycle
You probably know that plants take in carbon dioxide and give out oxygen, but as we saw in the last article, that isn’t a neat exchange, turning O2 into CO2. Rather, oxygen is created as a byproduct of splitting water, and CO2 is consumed by being turned into sugar. This happens in the Calvin Cycle.
In the Calvin Cycle, carbon dioxide, NADPH, and ATP are put in, and a sugar called G3P comes out. There are three steps to create this sugar: carbon fixation, reduction, and regeneration. Note that none of these steps needs direct light!
The first step is carbon fixation. CO2 is taken in from the atmosphere around the plant, added to a 5-carbon sugar called RuBP (ribulose bisphosphate), and thus turned into 3-phosphoglycerate, an organic molecule. This process is catalysed by an enzyme called Rubisco—basically, it recognises CO2 and pairs it with the “CO2 acceptor”, RuBP. For every “turn” of the Calvin Cycle, three CO2 molecules are fixed into two 3-phosphoglycerate molecules.
In the second step, reduction, the cycle takes in 6 NADPH and 6 ATP (from the light reactions) to convert these molecules into glyceraldehyde 3-phosphate (G3P). The “reducing power” of NADPH is used to add electrons to the molecules, and the ATP gives them phosphate groups.
Then in the last stage, regeneration, 3 more ATP molecules are used to turn five molecules of G3P back into RuBP, the CO2 acceptor, so it can be used again at the start of the cycle. What’s leftover—a single G3P—is the output of the cycle. It’s the overall goal of photosynthesis: a sugar molecule that can then be used in cellular respiration to create energy for living cells to use.
So, a roundup of the cycle:
- We put in 9 ATP, 6 NADPH, and 3 CO2.
- We get out 9 ADP, 6 NADP+, and 1 G3P (plus 3 RuBP molecules).
- The ADP and NADP+ are then recycled back to the light reactions, and photosynthesis begins over again.
Body images sourced from Wikimedia Commons
Further resources: 3D video or Video from Crashcourse