Question

In $C_3$ plants
A. Carbon dioxide fixation takes place with the help of RuBP carboxylase
B. Product of fixation is Oxaloacetic acid
C. Are 95% of total green plants on earth
D. Are mainly warm season plants
Choose the correct answer from the options given below:

• A. A and B only
• B. A and C only
• C. B and C only
• D. C and D only

Hint:

To remember this:
- $C_3$ pathway $\rightarrow$ first product has 3 carbons (3-PGA) $\rightarrow$ uses Rubisco enzyme.
- $C_4$ pathway $\rightarrow$ first product has 4 carbons (OAA) $\rightarrow$ uses PEP carboxylase enzyme.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Plants are classified into $C_3$, $C_4$, and CAM pathways based on how they fix atmospheric carbon dioxide during photosynthesis.
The $C_3$ pathway is the most common carbon fixation pathway.

Step 2: Detailed Explanation:
Let us evaluate each statement:
- Statement A: In $C_3$ plants, carbon dioxide is directly fixed by the enzyme Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco or RuBP carboxylase) in the mesophyll cells. This statement is correct.
- Statement B: The first stable product of CO\(_2\) fixation in $C_3$ plants is 3-phosphoglyceric acid (3-PGA), which is a 3-carbon compound. Oxaloacetic acid (OAA, a 4-carbon compound) is the first stable product in $C_4$ plants. This statement is incorrect.
- Statement C: Approximately 95% of the plant species on Earth use the $C_3$ photosynthetic pathway, including crops like rice, wheat, soybeans, and barley. This statement is correct.
- Statement D: $C_3$ plants are adapted to cool, wet, and temperate climates. They show lower photosynthetic efficiency in warm, dry environments due to photorespiration. $C_4$ plants are better adapted to warm, sunny, and dry conditions. This statement is incorrect.
Therefore, only statements A and C are correct.

Step 3: Final Answer:
The correct option is (B).