Question

The carbon dioxide acceptor in CAM plants is
(a) Malic acid
(b) Oxalo-acetic acid
(c) Pyruvic acid
(d) Phosphoenol pyruvic acid

Answer 1

Hint: CAM pathway stands for Crassulacean Acid Metabolism as it was first observed in the Crassulaceae plants. They undergo CAM photosynthesis to adapt to the arid climatic conditions. Pineapple, bryophyllum, agave, aloe, etc are the examples of the CAM plants.

Complete answer
-In CAM plants, carbon fixation is carried out at night, not in the daytime. During the day, the stomata in the leaves are closed to prevent transpiration by evaporation of water. PEP stands for the three-carbon compound phosphoenolpyruvate. This compound is the primary ${ CO }_{ 2 }$ acceptor in ${ C }_{ 4 }$ and CAM plants.
- In CAM, after carbon dioxide is fixed at night, it is stored as the four-carbon compound malate. Carbon dioxide is released during daylight when the light-dependent reactions of photosynthesis take place. During the night, carbon dioxide enters through the open stomata and is fixed by PEPc as oxaloacetate. This is again converted to malate which accumulates in cell vacuoles.
-PEPcase is phosphoenolpyruvate carboxylase, an enzyme that catalyzes the addition of bicarbonate (${ HCO }_{ 3 }^{ - }$) to phosphoenolpyruvate (PEP) to form the four-carbon compound oxaloacetate and inorganic phosphate.
- The enzyme carries out carbon fixation in CAM and ${ C }_{ 4 }$ plants.
- ${ C }_{ 4 }$ and CAM metabolism is similar in their function. Both pathways concentrate ${ CO }_{ 2 }$ around the RuBisCO enzyme.

So, the correct answer is ‘Phosphoenol pyruvic acid’.

Note:
These plants generally have thick cuticles and finely divided leaves or no leaves and also the reduced stomata. They also have succulents and protective coverings of color, wax, hair, or spines are their water storage adaptations. To adapt to xerophytic conditions the plants increase the amount of water intake and lower their expenditure.