Photosynthesis rate depends on several factors. Blackman’s law of limiting factors states that the rate is controlled by the factor in shortest supply. CBSE Class 11 and NEET test this with graph-based questions.
Core Concepts
Light intensity
Rate increases with light up to saturation. Beyond that, additional light has no effect — another factor becomes limiting. At very high intensity, photooxidation damages the chloroplast.
The light response curve is one of the most tested graphs in NEET:
- At low light, rate increases linearly (light is limiting)
- At moderate light, the curve bends (approaching saturation)
- At high light, rate plateaus (CO2 or temperature becomes limiting)
- At very high light, rate may actually decrease (photoinhibition — excess light energy damages photosystem II)
Light compensation point: The light intensity at which the rate of photosynthesis equals the rate of respiration. Below this point, the plant consumes more O2 than it produces. Shade plants have a lower compensation point than sun plants, which is why they can survive under forest canopy.
CO2 concentration
Atmospheric CO2 is about 0.04%. Photosynthesis saturates around 0.05 to 0.1% for C3 plants and higher for C4. CO2 is often the limiting factor in field conditions.
Why C4 plants handle high light and low CO2 better: C4 plants (maize, sugarcane) have a CO2-concentrating mechanism using PEP carboxylase in mesophyll cells. PEP carboxylase has a much higher affinity for CO2 than RuBisCO, so it can capture CO2 even at low concentrations and shuttle it to bundle sheath cells where RuBisCO operates at high local CO2 levels.
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First CO2 acceptor | RuBP (5C) | PEP (3C) |
| First stable product | 3-PGA (3C) | OAA (4C) |
| CO2-fixing enzyme | RuBisCO only | PEP carboxylase + RuBisCO |
| Photorespiration | Significant (wastes energy) | Minimal (CO2 concentrated) |
| Optimum temperature | 20-25°C | 30-40°C |
| CO2 compensation point | 50-100 ppm | 0-5 ppm |
| Examples | Wheat, rice, soybean | Maize, sugarcane, sorghum |
Temperature
Photosynthesis rate rises with temperature up to an optimum (around 25 to 35°C for most plants), then falls as enzymes denature. C4 plants have a higher optimum than C3.
Why the curve is bell-shaped: At low temperature, enzyme-catalysed reactions are slow (low kinetic energy). As temperature rises, reaction rates increase (roughly doubling every 10°C — the Q10 rule). But above the optimum, enzymes begin to denature — their 3D structure unfolds, active sites lose shape, and activity drops sharply.
For photosynthesis, the most temperature-sensitive step is the Calvin cycle (enzymatic dark reactions), not the light reactions (which are largely physical — photon absorption and electron transport).
Water
Water stress closes stomata to conserve water, cutting off CO2 supply. This indirectly reduces photosynthesis even though water itself is not usually the direct limiting substrate.
When a plant is water-stressed:
- Abscisic acid (ABA) is released by roots
- ABA signals guard cells to close stomata
- Closed stomata prevent CO2 entry
- Calvin cycle slows due to CO2 shortage
- Light reactions continue briefly, generating reactive oxygen species
- Photooxidative damage occurs
This cascade explains why drought reduces crop yield even on sunny days — it is the CO2 starvation, not the water shortage directly, that slows photosynthesis.
Blackman’s law
Rate is limited by the single factor in shortest supply. Increasing any other factor has no effect until the limiting factor is raised. A classic experimental set of curves proves this.
“When a process depends on several factors, its rate at any given time is limited by the factor that is present in its minimum.”
Graphically: at low light + normal CO2, increasing light raises rate (light is limiting). At high light + normal CO2, increasing light has no effect (CO2 is now limiting). Increasing CO2 at this point raises rate again. This is the “plateau shift” seen in NEET graph questions.
Chlorophyll content
More chlorophyll means more light can be captured. Nitrogen deficiency causes yellowing (chlorosis) because nitrogen is a component of chlorophyll. Magnesium is at the centre of the chlorophyll molecule — Mg deficiency also causes chlorosis.
Worked Examples
Rate rises linearly at low light, plateaus at saturation. If you then add more CO2, the plateau shifts up — proving CO2 was limiting at high light.
Indoor growers often raise CO2 to 1000 ppm or more, dramatically increasing photosynthesis rate and crop yield. This is direct application of Blackman’s law.
At temperatures above 30°C, RuBisCO’s oxygenase activity increases in C3 plants (photorespiration wastes energy). C4 plants concentrate CO2 around RuBisCO, suppressing photorespiration. They maintain high photosynthetic rates even at 40°C.
A NEET question shows two curves: photosynthesis rate vs light intensity, one at 0.03% CO2 and one at 0.1% CO2. The 0.1% curve plateaus at a higher rate. At low light, both curves overlap (light is limiting for both). At high light, the 0.03% curve plateaus first (CO2 becomes limiting), while the 0.1% curve continues rising (more CO2 available). The difference between the two plateaus represents the effect of CO2.
If a leaf produces 20 mg of O2 per hour during photosynthesis but consumes 5 mg of O2 per hour in respiration, the net (apparent) photosynthesis = 20 - 5 = 15 mg O2/hour. Gross photosynthesis = 20 mg O2/hour. The difference is respiration.
Common Mistakes
Saying water is the most common limiting factor in India. CO2 is the most common limiting factor for photosynthesis, though water limits growth indirectly.
Writing that rate keeps increasing with temperature. It has an optimum.
Confusing light intensity and light quality. Intensity is brightness; quality is wavelength.
Saying C4 plants do not use RuBisCO. They do — but only in bundle sheath cells, where CO2 is concentrated. The initial fixation uses PEP carboxylase.
Thinking that increasing all factors simultaneously always increases rate. Only the current limiting factor matters. If CO2 is limiting, increasing both light and temperature will not help — you must increase CO2.
Exam Weightage and Revision
Factors affecting photosynthesis carry 1-2 NEET questions per year, almost always as graph interpretation. CBSE Class 11 boards ask for Blackman’s law statement + graph + explanation (5 marks). The graph-based question is the single most reliable question from this chapter.
| Question Type | NEET Frequency | Difficulty |
|---|---|---|
| Graph interpretation (light curves) | Every year | Medium |
| Blackman’s law application | Most years | Medium |
| C3 vs C4 comparison | Every year | Easy-Medium |
| Compensation point | Every 2 years | Medium |
| Effect of temperature | Occasional | Easy |
If you can read and interpret a “rate vs light intensity at two CO2 levels” graph, you can answer the most common NEET question from this chapter. Practice reading these graphs until you can explain every feature.
Practice Questions
Q1. A plant is given high light but low CO2. According to Blackman’s law, what is the limiting factor? What happens if you increase light further?
CO2 is the limiting factor (it is in shortest supply relative to demand). Increasing light further has no effect on photosynthesis rate — the rate is already limited by CO2 availability. To increase the rate, you must increase CO2. This is exactly why greenhouses inject CO2.
Q2. Why do C4 plants have a lower CO2 compensation point than C3 plants?
The CO2 compensation point is the CO2 concentration at which photosynthesis equals respiration (net exchange = 0). C4 plants use PEP carboxylase, which has very high affinity for CO2 and can fix it even at very low concentrations (down to 0-5 ppm). C3 plants use only RuBisCO, which has lower CO2 affinity and significant oxygenase activity at low CO2 (causing photorespiration). So C3 plants reach compensation at 50-100 ppm.
Q3. Explain why a plant’s rate of photosynthesis first increases then decreases with rising temperature.
The increase: enzyme-catalysed reactions speed up with temperature (higher kinetic energy, more collisions, more enzyme-substrate complexes formed). The decrease: above the optimum temperature, enzymes (especially RuBisCO, Calvin cycle enzymes) begin to denature — hydrogen bonds and hydrophobic interactions that maintain the 3D structure break, active site shape is lost, activity drops. Additionally, stomata may close at high temperature to prevent water loss, reducing CO2 supply.
Q4. In an experiment, two identical plants are kept at the same light and temperature. One is given normal air (0.04% CO2), the other is given air with 0.1% CO2. Which produces more oxygen? Why?
The plant with 0.1% CO2 produces more oxygen, provided light is not limiting. At 0.04% CO2, the Calvin cycle is limited by CO2 supply. At 0.1% CO2, more CO2 is available for fixation, so more glucose (and O2) is produced. The difference is most pronounced at high light intensity, where CO2 is the limiting factor.
FAQs
Why is CO2 the most common limiting factor, not light?
Atmospheric CO2 is only 0.04% (400 ppm) — a very low concentration. In full sunlight, the light reactions can produce more ATP and NADPH than the Calvin cycle can use at this CO2 level. So the Calvin cycle (which needs CO2) becomes the bottleneck. This is why CO2 enrichment in greenhouses is so effective.
Does photosynthesis happen at night?
The light reactions require light and cannot happen in the dark. The Calvin cycle does not directly require light but depends on ATP and NADPH from light reactions, so it also stops at night. However, CAM plants (crassulacean acid metabolism) fix CO2 at night into organic acids and release it during the day for the Calvin cycle — an adaptation to extremely dry conditions.
What is photorespiration and why is it wasteful?
Photorespiration occurs when RuBisCO binds O2 instead of CO2 (RuBisCO has both carboxylase and oxygenase activity). The product (phosphoglycolate) is toxic and must be converted back through an energy-consuming pathway that releases CO2 — wasting both carbon and energy. C4 plants evolved to minimise this by concentrating CO2 around RuBisCO.
Draw one graph — rate vs CO2 at two light levels. That graph answers most PYQs on this topic.
Factors questions are really about the limiting factor principle. Identify the limit and the rest follows.