If CO2 concentration increases in C3 plants at moderate temperatures, what is the typical effect on photosynthesis?

If CO2 concentration increases in C3 plants at moderate temperatures, what is the typical effect on photosynthesis?

• A. Usually increases the rate by reducing photorespiration and enhancing carboxylation, with saturation at higher CO2 levels.
• B. Significantly decreases the rate due to feedback inhibition.
• C. Has no effect on the rate of photosynthesis.
• D. Causes chlorophyll degradation.

Answer: (B)

When CO2 rises in C3 plants at moderate temperatures, photosynthesis generally increases. The reason is that Rubisco, the enzyme that fixes carbon, can either carboxylate CO2 or oxygenate O2. At these temperatures, increasing CO2 shifts the balance toward carboxylation and away from photorespiration, so more CO2 is fixed into sugars and less energy is wasted. As a result, the net photosynthetic rate goes up, up to a point where other factors (like light, enzyme capacity in the Calvin cycle, or the plant’s ability to use or store the produced sugars) become limiting and the rate levels off. Chlorophyll content isn’t driven down by higher CO2, and the effect isn’t a decrease or no effect at moderate levels.

When CO2 rises in C3 plants at moderate temperatures, photosynthesis generally increases. The reason is that Rubisco, the enzyme that fixes carbon, can either carboxylate CO2 or oxygenate O2. At these temperatures, increasing CO2 shifts the balance toward carboxylation and away from photorespiration, so more CO2 is fixed into sugars and less energy is wasted. As a result, the net photosynthetic rate goes up, up to a point where other factors (like light, enzyme capacity in the Calvin cycle, or the plant’s ability to use or store the produced sugars) become limiting and the rate levels off. Chlorophyll content isn’t driven down by higher CO2, and the effect isn’t a decrease or no effect at moderate levels.