Effect of CO2 Concentration on the Rate of Photosynthesis
Biology · Photosynthesis in Higher Plants · NEET
Carbon dioxide is the major limiting factor of photosynthesis because its level in air is very low (only 0.03-0.04%). Raising CO2 up to about 0.05% increases the rate of CO2 fixation, but higher levels become harmful over long periods. Memory hook: "CO2 is low, so it holds photosynthesis back the most" - that is why greenhouses pump in extra CO2 to grow more tomatoes.
At high light, both C3 and C4 plants raise photosynthesis with more CO2, but C4 plants saturate early (~360 uL/L) while C3 plants keep responding and saturate only beyond ~450 uL/L. So today's CO2 is limiting to C3 plants.
Your doubts, answered
Is CO2 or light the major limiting factor for photosynthesis?
CO2 is called the major limiting factor. Light saturation happens at only about 10% of full sunlight, so except in shade or dense forests light is rarely limiting in nature. But CO2 stays very low in air (0.03-0.04%), so it is usually the factor that limits the rate. NEET often frames CO2 as the 'major limiting factor' - remember that exact phrase.
Why does raising CO2 increase photosynthesis only up to about 0.05%?
An increase in CO2 concentration up to about 0.05% can raise CO2 fixation rates. Beyond this level, CO2 becomes damaging to the plant over longer periods, so the rate does not keep rising. So the safe useful ceiling in NCERT is 0.05%.
Do C3 and C4 plants respond the same way to high CO2?
No. At low light, neither group responds to high CO2. At high light intensity both increase, but C4 plants get saturated at about 360 uL/L of CO2, while C3 plants keep responding and saturate only beyond 450 uL/L. So C3 plants can still be pushed higher by extra CO2.
Why is the current CO2 level limiting for C3 plants but not for C4 plants?
Because C4 plants already saturate at about 360 uL/L, which is close to present-day air CO2, so extra CO2 barely helps them. C3 plants need over 450 uL/L to saturate, so today's CO2 is below what they can use - meaning the current CO2 level limits C3 plants. This is why raising CO2 boosts C3 crops.
Why do greenhouses add CO2 for tomato and bell pepper?
Tomato and bell pepper are C3 greenhouse crops. Since C3 plants respond to higher CO2 with a higher rate of photosynthesis and higher productivity, growers keep them in a CO2-enriched atmosphere to get higher yields. This is a direct NCERT example NTA loves to test.
⚠️ The NEET trap ✗ C4 plants respond to increased CO2 and saturate only beyond 450 uL/L, while C3 plants saturate at 360 uL/L. ✓ It is the reverse: C4 plants saturate at about 360 uL/L, and C3 plants respond further and saturate only beyond 450 uL/L. Current CO2 is therefore limiting to C3 plants. 🧠 C4 saturates EARLY (360), C3 keeps going (450+). Whoever saturates later (C3) is the one still limited by today's CO2.
Real NEET questions
NEET 2017
With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct?
A · Light saturation for CO2 fixation occurs at 10% of full sunlight
B · Increasing atmospheric CO2 concentration upto 0.05% can enhance CO2 fixation rate
C · C3 plants responds to higher temperatures with enhanced photosynthesis while C4 plants have much lower temperature optimum ✓
D · Tomato is a greenhouse crop which can be grown in CO2 - enriched atmosphere for higher yield
Solution: The question asks for the incorrect statement. NCERT states the reverse of option C: C4 plants respond to higher temperatures with enhanced photosynthesis, while C3 plants have a much lower temperature optimum. So option C is wrong and is the answer. Options A (light saturation at 10% of full sunlight), B (CO2 boost up to 0.05%) and D (CO2-enriched greenhouse crops like tomato) are all correct NCERT statements about CO2 and photosynthesis.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
