CAM PLANT PHOTOSYNTHESIS ANIMATION

To learn about the light/dark reactions of Photosynthesis:
• Photosynthesis: Light-...
To learn about C4 plants:
• C4 CYCLE
The Calvin Cycle employs the enzyme RuBisCo in its first step, producing the 3-phosphoglyceric acid (PGA). C3 plants directly fix the CO2 in the atmosphere, and so this 3 carbon compound is the first stable product of their dark reactions. However, RuBisCo evolved when there wasn’t much oxygen around. So plants didn’t care that it would sometimes bind to oxygen instead of CO2. Today there IS oxygen, but plants all already have this enzyme, so now they just have to deal with this problem!
C3 plants take the "lazy" approach - losing a hefty percentage of their photosynthetic efficiency to photorespiration. But they can only afford to do this because they live the luxury life climate-wise. They wouldn’t last long in a hot, dry climate, where stomata must remain closed to prevent water loss. Keeping stomata closed means (NO GAS EXCHANGE - CO2 in, O2 out) that oxygen produced during the light-dependent reactions accumulates - which means rising oxygen concentrations around RuBisCo! And RuBisCo gets more likely to choose oxygen over CO2 as the ratio of oxygen vs CO2 increases and as temperature increases.
The REAL tough guys are C4 and CAM plants! They can limit how much oxygen can get to RuBisCo, which gives them an edge when lack of water and hot conditions force them to close their stomata. These plants are similar in that they both use the enzyme PEP carboxylase to combine CO2 with a 3-carbon compound (PEP) to make a four-carbon compound. The four-carbon compound stores the CO2 until it can get to a RuBisCo present in low oxygen concentrations.
The difference between C4 and CAM plants is how they limit oxygen’s access to RuBisCo - C4 plants employ spatial isolation, and CAM plants employ temporal isolation. C4 plants fix CO2 in their mesophyll cells and then hand it to RuBisCo in their bundle sheath cells, which lack oxygen.
Meanwhile, CAM plants, living in the harshest deserts, are not as concerned about the efficiency of RuBisCo as they are about water loss. They use temporal isolation - basically overwhelming RuBisCo temporarily with high levels of CO2.
At high temperatures, usually during the day, CAM plants keep stomata closed and don’t let water escape. At low temperatures, usually at night, stomata open, allowing CO2 in and oxygen out. CAM plants have very large vacuoles. They store the CO2 collected at night in these vacuoles in the form of malic acid, which has four carbons. During the day, the carbon is released to the Calvin cycle, with the CO2 concentrated around the enzyme RuBisCo. Remember, the light-dependent reactions take place during the day, providing the Calvin Cycle with the ATP and NADPH it needs to proceed - that’s why the CO2 can’t get passed along at night.
What happens during crassulacean acid metabolism, or CAM photosynthesis? It’s nighttime and stomata are open. CO2 diffuses into the spongy mesophyll cells. In the cytoplasm, PEP carboxylase combines CO2 with the 3 carbon compound phosphoenolpyruvate (PEP) to produce oxaloacetate (OAA). Oxaloacetate is converted into malate by NAD-malate dehydrogenase, using up an NADH. Now, instead of being passed on to the Calvin Cycle, this malate is shuttled to vacuoles for later use. Malate can only be shuttled into vacuoles efficiently at low temperatures, and that’s what we’ve got! Lack of malate in the cytoplasm means that expression of PEP carboxylase kinase (PEP-C kinase) is no longer inhibited. PEP-C kinase phosphorylates its target enzyme, PEP carboxylase - PEP-C - and makes it much more efficient at catalyzing the formation of oxaloacetate. It’s not just PEP-C kinase that is inhibited during the day. Although PEP carboxylase is constantly being produced, it is inhibited during the day by dephosphorylation thanks to PEP-C phosphatase and the binding of malate.
Anyway, once malate has been shuttled into the vacuole, it is converted to malic acid, a nonionic form of malate, for storage. Again, the plant can’t directly pump the CO2 into the Calvin Cycle because there isn’t ATP and NADPH available at night from the light-dependent reactions. Hence, the vacuoles continue to accumulate malic acid.
It’s daytime!! The sun is shining and stomata are closed. The malic acid is transported out of the vacuole into the stroma of chloroplasts. There, depending on the plant species, either malic enzyme or PEP carboxykinase cleave it into pyruvate and CO2, which then enters the Calvin Cycle. The pyruvate can be broken down in the mitochondrial Kreb’s cycle, providing additional CO2 molecules for the Calvin Cycle. Otherwise, it can be used to make more PEP via pyruvate phosphate dikinase. This requires ATP and an additional phosphate. The PEP gets exported into the cytoplasm the following night to fix more CO2.
Some images from PixaBay