Question

The carbon dioxide concentration at which the carbon dioxide fixed by photosynthesis and respiration equal each other is called _________.

• A. Saturation point
• B. Ecological efficiency
• C. Carbon dioxide balance
• D. Carbon dioxide compensation point

Hint:

- \(\text{C}_4\) plants have a much lower \(\text{CO}_2\) compensation point than \(\text{C}_3\) plants, which allows them to continue photosynthesizing efficiently even under low \(\text{CO}_2\) conditions (e.g., when stomata are partially closed under water stress).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Photosynthesis consumes carbon dioxide (\(\text{CO}_2\)) to produce carbohydrates, while cellular respiration (and photorespiration) releases \(\text{CO}_2\).
The rate of these two opposing processes is highly dependent on environmental parameters, including light intensity and atmospheric \(\text{CO}_2\) concentration.

Step 2: Detailed Explanation:
- The carbon dioxide (\(\text{CO}_2\)) compensation point is defined as the specific concentration of ambient \(\text{CO}_2\) at which the rate of photosynthetic carbon assimilation exactly equals the rate of respiratory (and photorespiratory) \(\text{CO}_2\) release.
- At this point, the net exchange of carbon dioxide between the plant and the atmosphere is zero.
- Let us compare typical values for different plant types:
- In \(\text{C}_3\) plants, due to the high rate of photorespiration, the \(\text{CO}_2\) compensation point is relatively high, ranging from 40 to 100 ppm.
- In \(\text{C}_4\) plants, which have an efficient carbon-concentrating mechanism (Kranz anatomy) that suppresses photorespiration, the \(\text{CO}_2\) compensation point is very low, ranging from 0 to 10 ppm.
- Other options like "Saturation point" refer to the light intensity beyond which further increases do not increase photosynthesis, while "Ecological efficiency" describes energy transfer between trophic levels.

Step 3: Final Answer:
The point of equality between fixed and released carbon dioxide is the Carbon dioxide compensation point (Option D).