Plants C3, C4 y CAM

We can classify plants, based on their metabolism, into C3, C4 and CAM plants.
85% of the vascular plants (higher plants) of the planet are C3 plants, that is, the luminous phase and the first reaction of the dark phase of photosynthesis (CALVIN CYCLE) occur in the same type of cells (and simultaneously); the cells of the mesophile, and a molecule with three carbon atoms (3-PGA or 3-phosphoglyceric acid) is produced. Hence the problem that occurs when the CO2/O2 ratio decreases to a certain value: photorespiration.
But there are other plants, the C4 plants that have solved this to some extent, physically separating the reactions of the luminous phase and the Calvin Cycle; the former occur in the leaf mesophile (spongy tissue of the leaf center) and the Calvin Cycle occurs in vascular bundle cells (special cells found near leaf nerves). This carries a higher energy expenditure (ATP) but is compensated: first, atmospheric CO2 is fixed in the mesophile cells to form a simple 4-carbon organic acid (oxaloacetate). This step is carried out by a non-RUBISCO enzyme, the enzyme PEP CARBOXYLASE, which does not tend to bind to O2. The oxaloacetate is then converted into a similar molecule, malate, which can be transported to vascular bundle cells. Within these, the malate breaks down and releases a CO2 molecule. RUBISCO then fixes co2 and converts it into sugars through the Calvin cycle, exactly as in the photosynthesis of C3 plants. This strategy minimizes photorespiration. The C4 pathway is used by 3% of the total vascular plants on the planet. These plants are characteristic of warm and little rainy areas. This mechanism has no reason to exist in cool areas.
Finally, plants such as cacti and pineapple have gone a little further and separated the reactions of the luminous phase and the Calvin Cycle in time; they open the stomata at night and fix CO2 in oxaloacetate using the enzyme PEP CARBOXYLASE. This oxaloacetate is transformed into malate and other organic acid and stored in the vacuoles of cells until the next day. Once the day has come, and with the stoma closed, the acids stored in the vacuoles are extracted from them and decomposed to release a CO2 molecule which is captured by rubisco, starting the Calvin Cycle. The CAM pathway needs ATP in several steps (not shown), so like C4 photosynthesis it is not an energetic “gift”. However, plant species that use CAM photosynthesis not only prevent photorespiration, but also use water very efficiently. Their stomata only open at night, when the humidity tends to rise and the temperature to drop, and both factors reduce the loss of water from the leaves. They are CAM plants (Acid Metabolism of Crassulaceae) and usually predominate in very hot and dry areas, such as deserts.


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