Factors Affecting The Rate Of Photosynthesis

The rate of photosynthesis is affected by 4 main factors light intensity, water availability, temperature and CO2 concentration. Any of these factors can limit the rate of photosynthesis, so the reaction can only proceed at the rate of the limiting factor.

Light intensity:

As light intensity increases, the rate of photosynthesis increases. This is because light is needed for the first stage of photosynthesis the light dependant stage in which photolysis of water occurs and electrons are excited from chlorophyll a. This reaction produces ATP and NADP which are needed for the second stage of photosynthesis the light independent reaction. When there is low light intensity, less NADP and ATP is available for the light independent reaction, so GP can t be converted to TP, and eventually due to a lack of TP, RuBP will not be regenerated.When there is very high light intensity, the chlorophyll may be bleached, causing the rate of photosynthesis to decrease.

Water availability:

This is not usually a limiting factor to the rate of photosynthesis, because by the time the rate of this reaction becomes limited by water availability, the plants cells would already be plasmolysed. If there is not enough water, plant cells can t remain turgid, causing the stomata to shut. As a result, gas exchange can t occur. This will prevent the plant from absorbing CO2 that is needed in the light independent stage of the reaction, so GP will not be converted to TP.

Temperature:

As the temperature increases, the rate of photosynthesis increases until the optimum temperature. After this the rate will decrease. This is because photosynthesis is an enzyme-catalysed reaction, so at high temperatures, the enzymes involved become denatured. There is a change in the tertiary structure of the protein, that prevents the substrate from binding to the enzyme, and as a result, the reaction can t be catalysed. At the optimum, particles in the reaction move faster and collide more, and have increased kinetic energy so are more likely to overcome the activation energy needed for the reaction to occur. Furthermore, at higher temperatures, RuBisCO s active site changes shape, so there is a reduction in how much CO2 can be accepted by RuBP. RuBisCO also promotes the production of H2O2 at higher temperature, and this is toxic to plants, decreasing the rate of photosynthesis as cells begin to die.

CO2 concentration:

This is not usually a limiting factor, due to the high levels of CO2 in the atmosphere.CO2 is required for the light independent stage of photosynthesis, and so without it, GP and subsequently TP can t be made, and so the rate of photosynthesis decreases.