Effect of Light Intensity on the Rate of Photosynthesis
Biology · Photosynthesis in Higher Plants · NEET
At low light, the rate of photosynthesis rises in a straight line as light intensity increases. At higher light, the rate stops rising because other factors become limiting; this flat point is called light saturation, and NCERT says it happens at only 10 per cent of full sunlight. Memory hook: "10 percent of sun is enough" — so in nature, light is rarely the limiting factor.
At low light the rate rises linearly with intensity; it then flattens at the light saturation point, which NCERT places at about 10% of full sunlight, after which CO2 or temperature becomes the limiting factor.
Your doubts, answered
Does increasing light intensity always increase the rate of photosynthesis?
No. At low light intensity there is a linear (straight-line) relationship — more light means more CO2 fixation. But at higher intensities the graph flattens because some other factor (like CO2 or temperature) becomes limiting. So beyond a point, adding light gives no extra rate. NEET loves this word 'linear' for the low-light part.
What is the light saturation point?
It is the light intensity beyond which the rate of photosynthesis does not increase further, even if you add more light. At this point light is no longer the limiting factor — CO2 or temperature is. NCERT states light saturation occurs at only 10 per cent of full sunlight.
Why is light rarely a limiting factor in nature?
Because light saturation is reached at just 10% of full sunlight, and normal daylight is far brighter than that. So plants almost always get more than enough light. NCERT notes the only exceptions are plants in shade or in dense forests, where light can limit the rate.
What happens if light intensity is increased too much?
Very high light does not help — and it can harm. NCERT clearly says that increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis. So the rate can actually drop at extreme intensities. This is a favourite NEET trap.
What is the difference between light quality, light intensity and duration?
Light quality means the wavelength/colour of light, light intensity means how bright the light is, and duration means how long the leaf is exposed. NCERT says all three must be distinguished when discussing light as a factor. Light intensity is the one that shows the linear-then-saturation curve.
⚠️ The NEET trap ✗ More light always means more photosynthesis, so the rate keeps rising with intensity. ✓ The rate rises linearly only at low light. It saturates at about 10% of full sunlight, and beyond a point very high light breaks down chlorophyll and decreases photosynthesis. 🧠 NCERT line: 'light saturation occurs at 10 per cent of full sunlight' — and 'increase in incident light beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.'
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 respond 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 wrong statement. Option A is correct — NCERT states light saturation occurs at 10% of full sunlight. Options B and D are also correct. Option C is reversed: it is the C4 plants that respond to higher temperatures with enhanced photosynthesis, while C3 plants have the lower temperature optimum. So C is the incorrect statement and is the answer.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
At what percentage of full sunlight does light saturation occur?
At 10 per cent of full sunlight, according to NCERT Class 11. This is why light is rarely a limiting factor in open sunlight.
Is the relationship between light and photosynthesis linear?
Only at low light intensities. There the graph is a straight line. At higher intensities it flattens into a plateau (saturation) as other factors limit the rate.
Can too much light reduce photosynthesis?
Yes. NCERT states that light beyond a point causes breakdown of chlorophyll and a decrease in photosynthesis.
When is light actually a limiting factor?
For plants in shade or in dense forests, where the light received is below the saturation level. In open sunlight, CO2 is usually the main limiting factor.
Which is the major limiting factor if light is not?
CO2 concentration. NCERT calls CO2 the major limiting factor for photosynthesis because atmospheric CO2 (0.03–0.04%) is very low.
