Question

$C_4$ and $C_3$ pathways of CAM plants are separated by
A)Bundle sheath
B)Mesophyll and bundle sheath cells
C)Mesophyll and bundle sheath chloroplasts
D)Time

Answer 1

Hint: Some plants, especially succulents that grow under highly xeric conditions, fix carbon dioxide in the dark. Such plants are known as CAM (Crassulacean Acid Metabolism) plants. Stomata remain open at night but closed throughout the day in these plants (in light).

Complete answer:
The $C_3$ pathway is named so because the first molecule is named 3-phosphoglyceric acid which is formed in the cycle (a 3-carbon molecule). The $C_3$ pathway is used by about 85 percent of the plants on Earth to fix carbon through the Calvin Cycle.
The $C_4$ method is also referred to as the Hatch-Slack pathway and is named so because of the formation of 4-carbon intermediate molecules, malic acid or aspartic acid.

1. In the presence of PEP carboxylase like $C_4$ plants, combine PEP with this $CO_2$ to form OAA.
2. In the presence of NADH, OAA converted to malic acid. Malic acid is formed as a result of acidification in the dark.
3. Malic acid is converted to pyruvic acid and during the day when stomata are closed, $CO_2$ forms. This mechanism is known as deacidification. The $CO_2$ which is released by deacidification is fixed to carbohydrates by $C_3$ cycle after being accepted by RuBP.

Option A,B and C: These are incorrect options.
Option D: Time is the correct option.

Hence, the correct answer is option (D)

Note: 1.CAM photosynthesis is a process that concentrates $CO_2$ using a PEP carboxylation. This is followed by the $C_4$ cycle where decarboxylation of $C_4$ acid takes place to concentrate $CO_2$ around Rubisco.
2.There is temporal separation of PEP carboxylation and Rubisco carboxylation in CAM plants.