Nature's smallest factory: The Calvin cycle - Cathy Symington
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For a great introduction to photosynthesis, check out this TED-Ed lesson by Amanda Ooten: The simple story of photosynthesis and food. It will start to set the scene with an overview of the process and how it relates to us.
Photosynthesis; a two-act tale.
Photosynthesis is a complex process that happens in two distinct parts, somewhat like a play in two acts. The Calvin cycle, as seen in this lesson, is the second of these two acts.
This over-all reaction of photosynthesis gives a summary of the process:
6CO2 + 12H2O --> C6H12O6 +6H2O + 6O2
In the first act, the light dependent reaction, many of the components needed for the Calvin cycle are generated. The hydrogens needed for glucose synthesis are made available by splitting H2O and the energy needed to power the Calvin cycle (in the form of ATP) is also produced. Hydrogens are attached to biological electron carriers (NADP - which, by the way, is vitamin B3!) so they can be safely delivered to the stroma where the Calvin cycle happens, and oxygen gas is produced and released from this stage as a by-product:
12H2O + 12NADP + 18ADP +18Pi --> 6O2 + 12NAPDH2 + 18ATP
The light dependent reactions occur at the thylakoid membrane of the chloroplasts as this is where the light-capturing chlorophyll molecules are anchored and where the proteins involved in ATP generation are embedded. Note: It is the fact that all 6O2 that are released come only from the splitting of water that shows that writing 12H2O in the reactants with 6 H2O produced rather than 6 H2O in the reactants and zero in the products is technically correct.
The Calvin cycle (as represented in this lesson) happens in the stroma of the chloroplast, where all the enzymes involved are in solution (these enzymes are soluble, globular proteins). This is where carbon fixation occurs; constructing glucose from the carbon and oxygen of CO2, and hydrogen from H2O:
6CO2 + 12NAPDH2 + 18ATP --> C6H12O6 + 6H2O +12NADP + 18ADP +18Pi
What comes after the end of the tale, after the “Happily Ever After”?
The end of the Calvin Cycle brings us to the point of G3P production. It might seem like it’s the end of the story, the “happily ever after” part, and in some ways it is (it’s the end of the Calvin cycle at least!). But like many stories, what happens after the “happily ever after” can get super interesting. If formation of the G3P molecules and their use in producing glucose is the “happily every after” of the Calvin Cycle, what is it that comes after?
G3Ps have quite a range of possible destinations:
i.Two G3Ps can be connected together to form glucose or fructose (monosaccharides) outside the chloroplasts in the cytosol of the cell. ii.Glucose and fructose can be connected to form sucrose – the disaccharide transport form of sugars in plants,iii.Chains of glucose can be connected together to form starch by condensation reactions in the stroma, or chains of cellulose to become part of cell walls, oriv.They can be used in synthesis of lipids and amino acids.Chloroplasts sometimes store some of their own energy in the form of starch. Why they might do that? Do you think starch would accumulate in chloroplasts during the day or during the night?
C3 vs C4
Did you know that there’s not just one way that photosynthesis can happen? C3 and C4 are just two of the different photosynthesis pathways that exist.
C3 plants (wheat, legumes, apples, oranges, potatoes, etc.) are from more moderate environments; not too hot and not too dry such as temperate grasslands or woodlands. They get the name “C3” because the Calvin cycle product molecule, G3P, is a 3-carbon molecule; C3. Their Calvin cycle happens exactly as described here in this lesson, in the mesophyll cells where the chloroplasts are located.
C4 plants (corn, rice, sorghum, sugar cane, etc.) live in hotter, drier climates such as tropical regions or in deserts. These plants tend to keep their stomata closed during the day to minimize the rate of transpiration. While this is very effective for minimising water loss (particularly important in deserts), it is equally effective in minimizing gas exchange – something of a (potential) problem for photosynthesis given the reliance on supply of gaseous CO2 for construction of glucose.
Evolution has produced a brilliant solution! Where C3 plants have the Calvin cycle happening in their mesophyll cells (the same place where the light dependent reactions are happening), C4 plants separate the location of these two steps of photosynthesis. The light-dependent reactions happen in the mesophyll cells just like in the C3 plants, but the Calvin cycle happens in photosynthesizing vascular bundle cells (something that C3 plants don’t have). Operating the Calvin cycle deeper in the leaf tissue and further away from the gas exchange surface of the spongy mesophyll establishes a stronger concentration gradient of CO2 from outside the leaves to the location of the Calvin cycle. (more here)
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