Temperature affects photosynthesis mostly through the dark reactions, because these are enzyme-controlled and enzymes are very temperature sensitive. The light reactions are also affected, but much less. Memory hook: "Dark = enzymes = temperature; and C4 loves the heat" — C4 plants respond to higher temperatures with a higher rate of photosynthesis, while C3 plants have a much lower temperature optimum.
Rate of photosynthesis rises with temperature to an optimum, then falls as enzymes are damaged. C4 plants (orange) have a higher temperature optimum than C3 plants (green). The dark reactions are enzymatic and therefore the most temperature sensitive.
Your doubts, answered
Does temperature affect the light reaction or the dark reaction more?
The dark reaction is affected much more. NCERT says the dark reactions are enzymatic, so they are temperature controlled. The light reactions are also temperature sensitive, but they are affected to a much lesser extent. So the correct NEET answer is: temperature acts mainly on the dark (enzyme-driven) reactions.
Why are the dark reactions so sensitive to temperature?
The dark reactions (Calvin cycle) are run by enzymes like RuBisCO. Enzyme activity rises with temperature up to an optimum, then falls as the enzyme is damaged. Because these steps depend on enzymes, changing the temperature changes the reaction rate a lot. The light reactions are driven by pigments absorbing light, so they change less with temperature.
Do C3 or C4 plants have a higher temperature optimum?
C4 plants have the higher temperature optimum. NCERT states clearly: C4 plants respond to higher temperatures and show a higher rate of photosynthesis, while C3 plants have a much lower temperature optimum. This is a very common NEET trap, so remember it in this exact direction: C4 = high temperature, C3 = lower optimum.
Does the best temperature for photosynthesis change with the plant's habitat?
Yes. NCERT says the temperature optimum for photosynthesis depends on the habitat the plant is adapted to. Tropical plants have a higher temperature optimum than plants adapted to temperate (cooler) climates. So there is no single fixed number for all plants.
Is temperature always the limiting factor for photosynthesis?
No. Temperature is one factor, but under Blackman's law of limiting factors, only the factor that is most in short supply limits the rate at a given time. In nature CO2 is usually the major limiting factor. Temperature becomes limiting only when it is far from the plant's optimum.
⚠️ The NEET trap ✗ Light reactions are the most temperature sensitive part of photosynthesis. ✓ The dark (enzymatic) reactions are the most temperature sensitive; light reactions are affected to a much lesser extent. 🧠 Enzymes = temperature. The dark reaction is enzyme-run, so it feels temperature the most. NEET reverses this in options — read carefully.
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 says the reverse: C4 plants respond to higher temperatures with a higher rate of photosynthesis, while C3 plants have a much lower temperature optimum. Option C swaps C3 and C4, so it is wrong and is the answer. Options A, B and D are all stated correctly in NCERT.
NEET 2016 Phase 1
A plant in your garden avoids photorespiratory losses, has improved water use efficiency, shows high rates of photosynthesis at high temperatures and has improved efficiency of nitrogen utilization. In which of the following physiological groups would you assign this plant?
A · C3
B · C4 ✓
C · CAM
D · Nitrogen fixer
Solution: A plant that lacks photorespiration, tolerates high temperatures, and uses water and nitrogen efficiently is a C4 plant. NCERT notes C4 plants tolerate higher temperatures, respond to high light intensities and have greater biomass. The high temperature clue points directly to C4.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Which reactions of photosynthesis are controlled by temperature?
Mainly the dark reactions, because they are enzymatic and enzymes are temperature controlled. Light reactions are temperature sensitive too, but affected to a much lesser extent.
Do C4 plants like high or low temperature?
C4 plants prefer higher temperatures. They respond to higher temperatures with a higher rate of photosynthesis. C3 plants have a much lower temperature optimum.
Is there one fixed optimum temperature for all plants?
No. The optimum depends on the plant's habitat. Tropical plants have a higher temperature optimum than temperate-climate plants.
Why does photosynthesis fall at very high temperature?
At very high temperature the enzymes of the dark reaction (like RuBisCO) get damaged, so the rate drops after the optimum is crossed.
