Limiting Factors in Photosynthesis The Concept of Limiting

Limiting Factors in Photosynthesis

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

Limiting factors • If the rate on these factors is changed, the rate of photosynthesis will change. • Usually 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 ratelimiting 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 CO 2 concentration • At low and medium CO 2 concentrations, the rate-limiting step in the Calvin cycle is the point where CO 2 is fixed to produce glycerate 3 - phosphate. • At high CO 2 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, Ru. BP carboxyalse does not work effectively, so the rate-limiting step in the Calvin cycle is the point where CO 2 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