Biology · Photosynthesis in Higher Plants · NEET
CO2 is the major limiting factor in nature, not light. NCERT says light saturation happens at only 10% of full sunlight, so except for plants in shade or dense forests, light is rarely limiting. CO2 stays very low in air (0.03-0.04%), so it usually limits the rate. This is a common NEET trap.
Light saturation for CO2 fixation occurs at just 10 per cent of full sunlight. Below this, there is a linear (straight-line) relationship between light and CO2 fixation. Above 10%, the rate stops rising because other factors (like CO2) become limiting. Remember the exact figure: 10%, not 100%.
No. C4 plants saturate at about 360 microlitre per litre CO2, while C3 plants keep responding and saturate only beyond 450 microlitre per litre — so current CO2 levels limit C3 plants. For temperature, C4 plants respond to higher temperatures with a higher rate, while C3 plants have a much lower temperature optimum. Mixing these up is the exact NEET 2017 trap.
Even though water is a reactant in the light reaction, its effect is mostly indirect. Water stress closes the stomata, which reduces CO2 entry, and it makes leaves wilt, reducing leaf surface area and metabolic activity. So water acts more through the plant than directly on the reaction.
Because of Blackman's Law of Limiting Factors: when a process depends on many factors, its rate is set by the factor nearest its minimum value. Once light is plentiful (past 10% sunlight), CO2 or temperature becomes the limiting factor, so adding more light does nothing. The rate rises only if you raise the truly limiting factor.
With reference to factors affecting the rate of photosynthesis, which of the following statements is not correct?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Internal (plant) factors include the number, size, age and orientation of leaves, the number of mesophyll cells and chloroplasts, the internal CO2 concentration, and the amount of chlorophyll. These depend on the plant's genes and its growth.
The four external factors are light (quality, intensity and duration), CO2 concentration, temperature and water. Remember them as L-C-T-W. All act at the same time, but usually one limits the rate.
CO2 is the major limiting factor because its concentration in air is very low (0.03-0.04%). Raising CO2 up to about 0.05% can increase the fixation rate, which is why greenhouse crops like tomato and bell pepper are grown in CO2-enriched air.
Blackman (1905) stated that if a chemical process is affected by more than one factor, its rate is decided by the factor nearest to its minimum value. Changing that limiting factor directly changes the rate. This is covered on the next concept page.
The dark (biosynthetic) reactions are enzymatic, so they are strongly temperature controlled. The light reactions are also temperature sensitive but affected much less. Tropical plants have a higher temperature optimum than temperate ones.
