Limiting Factors in Photosynthesis

The Concept of Limiting Factors Light intensity Carbon Dioxide Temperatures These are the three factors that determine the of photosynthesis.

Limiting factors If the rate on these factors is changed, the rate of photosynthesis will change. Usually only one of these factors will change at a time, the one changing is the nearest its minimum, hence why it’s called the limiting factor. Change one of the other rates than the limiting factor, will have no effect on the rate, only if you change the limiting factor it will change.

Limiting factors The overall rate of photosynthesis in a plant is determined by the rate of whichever step is proceeding slowest. This is called the rate-limiting step. The three limiting factors affect different rate- limiting steps.

The effect of light intensity At low light intensities, there is a shortage of the products of the light-depending reactions. NADPH (Nicotinamide adenine dinucleotide phosphate) and ATP. The rate-limiting step in the Calvin cycle is the point where glycerate 3-phosphate is reduced. Unless a plant is heavily shaded, or the sun is rising or setting, light intensity is not usually the limiting factor.

The effect of CO2 concentration At low and medium CO2 concentrations, the rate-limiting step in the Calvin cycle is the point where CO2 is fixed to produce glycerate 3- phosphate. At high CO2 concentrations some other factor is limiting. Because the level of carbon dioxide in the atmosphere is never very high, carbon dioxide concentration is often the limiting factor.

The effect of temperature At low temperatures, all of the enzymes that catalyse the reactions of the Calvin cycle work slowly. At intermediate temperatures, some other factor is limiting. At high temperatures, RuBP carboxyalse does not work effectively, so the rate-limiting step in the Calvin cycle is the point where CO2 is fixed. NADPH accumulates.

Cyclic photophosphorylation When light is not the limiting factor, NADPH tends to accumulate in the stroma and there is a shortage of NADP+. The normal flow of electrons in the thylakoid membranes is inhibited because NADP+ is needed as a final acceptor of electrons. An alternative route can be used that allows ATP production when NADP+ is not available.

Cyclic Photophospholyration Photosystem l absorbs light and is photo activated. Excited electrons are passed from Photosystem to a carrier in the chain between Photosystem ll and Photosystem l. The electrons pass along the chain of carriers back to Photosystem l. As the electrons flow along the chain of carriers they cause pumping of protons across the thylakoid