Cambridge O Level Biology · Syllabus 5090 · Plant Nutrition
Limiting Factor
What is Limiting Factor?
The factor in shortest supply that restricts the rate of a process. In photosynthesis the candidates are light intensity, carbon dioxide concentration and temperature: whichever is least available at that moment sets the rate, and increasing any of the others has little or no effect until the limiting one is increased.
This definition is part of the Plant Nutrition chapter in Cambridge O Level Biology.
Limiting Factor in context
Photosynthesis is the process by which plants make carbohydrates from raw materials using energy from light. It happens in chloroplasts, and the green pigment inside them, chlorophyll, absorbs light and transfers that light energy into chemical energy stored in carbohydrate. Two raw materials go in — carbon dioxide and water. Two products come out — glucose and oxygen. Light and chlorophyll are conditions, not reactants: light supplies the energy and chlorophyll transfers it, and neither appears on the reactant side of the equation. The glucose then goes four ways: it is respired, stored as starch, built into cellulose for cell walls, or converted to sucrose for transport in the phloem. How fast the whole thing runs is set by whichever necessary factor is in shortest supply — the limiting factor — and the leaf is built to keep light, carbon dioxide and water in good supply while letting oxygen out.
Questions students ask about Limiting Factor
What does a plateau on a photosynthesis graph mean?
It means the factor on the horizontal axis is no longer the limiting factor. Photosynthesis is still occurring, at a constant and usually high rate; it simply cannot go faster because a different factor — carbon dioxide concentration or temperature — is now the one in shortest supply. A plateau never means photosynthesis has stopped.