Question

What is Crassulacean acid metabolism?

Answer 1

CRASSULACEAN ACID METABOLISM

Reference: http://plantphys.info/plant_physiology/images/cam.gif

1. Crassulacean acid metabolism generally takes place in the plants which are adapted to arid conditions, in which loss of water during photosynthesis creates a major problem for plants.
2. So, to overcome the loss of water during photosynthesis, plant assimilates CO₂ at the night and stored as four carbon molecule called Malate.
3. This pathway is known as CAM (Crassulacean acid metabolism) photosynthesis which was first discovered in the family of Crassulacean in late 1940s.
4. In this type of plants, CO₂  passes from “stomata” which are formed by two guard cells, containing specific cells having chloroplast.
5. Here after passing through stomata CO₂ reach to the photosynthetic cells.
6. There are two effects seen during CAM pathway: one is during day time and second is during night time.
7. To minimize water loss during day time, stomata of these plants are closed.
8. Whereas, during night CO₂ is taken up by mesophyll cells through open stomata.
9. This taken up CO₂ is fixed by PEP carboxylase enzyme that converts phosphoenol pyruvate to oxaloacetate that is further reduced to form malate in presence of NAD malate dehydrogenase enzyme.
10.  Produced malate is stored in the central vacuole that helps in the maintenance of neutral pH during night which tries to reach acidic pH.
11.  During day time in presence of sunlight malate released from the vacuoles and decarboxylated.
12. Here during decarboxylation of malate the leaf stomata is tightly closed to restrict the loss of water and as well as of CO₂ from the cell.
13. At the same time formation of ATP and NADPH takes place by photosynthesis.
14. By the decarboxylation of malate, phosphenol pyruvate is formed that is further converted to starch by the process of gluconeogenesis.
15. This produced starch is stored in chloroplasts.
16. The main function of this pathway is to regulate level and inhibition of PEP carboxylase by production of malate and by lowering the pH.
17. PEP carboxylase is mostly inhibited during day time when the level of malate is high and pH is very low.