Warburg effect is a decrease in the rate of photosynthesis at
(a) Low carbon dioxide concentration
(b) High oxygen content
(c) High carbon dioxide concentration
(d) Both A and B

Hint: In photosynthesis, carbon dioxide and water are converted into glucose in the presence of sunlight.

Complete answer:
The Warburg effect signifies the decrease in the rate of photosynthesis due to high oxygen concentrations.
These two mechanisms that are accountable for the Warburg effect in plants are:
- Oxygen is a competitive inhibitor of carbon dioxide fixation by the enzyme RuBisCO which launches photosynthesis. Oxygen competes with carbon dioxide to bind with the enzyme hence photosynthesis can not take place.
- Besides, oxygen encourages photorespiration which reduces photosynthetic yield.
The major limiting factors for photosynthesis include light intensity, temperature, and carbon dioxide levels.
- Since photosynthesis cannot start without light, it is the first limiting factor. The light energy from the sun fires the reactions that convert carbon dioxide and water into glucose and oxygen. Without light, photosynthesis is useless.
- Plants use enzymes, protein molecules, to help in carrying out the reactions necessary to bind and store glucose. The rate at which these enzymes work is influenced by temperature, which makes temperature another major limiting factor. At extremely high temperatures the enzymes may become denatured, which distinctly decreases the rate of photosynthesis.
- Carbon dioxide is required in the fixation of carbon atoms to form glucose. As carbon dioxide concentration rises reaction rates will increase, and more glucose molecules are produced. At a certain concentration of CO₂ photosynthetic rate will become constant, as the enzymes responsible for carbon fixation are completely saturated.
So, the correct answer is “High oxygen content”.

Note: In photosynthesis, solar energy is used as chemical energy to glucose. Oxygen is also released as a byproduct which is used in cellular respiration.