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Photosynthesis: Main points

Explaining the concept of limiting factors

Plants rely on photosynthesis to synthesise carbohydrates and all the other complex molecules they need for growth. If a plant can photosynthesise more quickly, then it can grow more quickly and this is an important consideration in growing crop plants. There are three major factors that affect the rate of photosynthesis and these are the carbon dioxide concentration, the temperature and the light intensity. The effects of carbon dioxide concentration, temperature and light intensity may be plotted on one graph and students may be asked to describe or explain this data.

The effect of light intensity

The rate of photosynthesis is directly proportional to the light intensity in the first part of the curve. The term ‘directly proportional’ is preferred to the less precise ‘photosynthesis increases’. It is said that ‘light intensity is limiting the rate of reaction’ in this part of the curve. A limiting factor is the lone factor that is preventing the reaction from proceeding at a faster rate. However much the levels of the other factors are increased they will not change the rate of reaction. Increasing the light intensity in the first part of the curve does increase the rate of reaction, so at this point it is the limiting factor. The curve flattens out as the light intensity increases, as some other factor such as carbon dioxide concentration becomes the limiting factor.

The effect of carbon dioxide concentration

The carbon dioxide concentration in the atmosphere is very low at around 0.04%. This means that low carbon dioxide concentrations often limit the rate of photosynthesis under normal conditions. Greenhouse growers often enrich the atmosphere with carbon dioxide to enable faster growth of crop plants.

The effect of temperature on the rate of photosynthesis

If the other factors are not limiting, the rate of photosynthesis will approximately double for every 10℃ rise, in the temperature range 0℃ and 25℃, due to the increasing number of collisions between molecules as the kinetic energy increases.

Above this temperature the rate will level off and then start to fall. This points to the fact that enzyme reactions are involved in the process of photosynthesis, and that they are beginning to be denatured above 25℃. It is important to remember that plant enzymes will often have lower optimum temperatures than mammalian enzymes because they have usually evolved to work in lower environmental temperatures. At high temperatures the enzyme called RuBisCo in the Calvin cycle starts to behave oddly and combines with oxygen rather than carbon dioxide. This can seriously affect the rate of photosynthesis when temperature and light intensity are high so some species have adapted the process to prevent this happening.

The light-dependent phase is a photochemical process and is not temperature dependent, whereas the light-independent stage (the Calvin cycle) is catalysed by several enzymes and is therefore temperature dependent. At low light intensities little ATP and reduced NADP can be made, so a rise in temperature may have little effect on the activity of the Calvin cycle enzymes.

Explaining the effect of light on the Calvin cycle

The interpretation and explanation of this type of graph is an important as it can be used to test the basic understanding of the processes involved in photosynthesis.

The Calvin cycle is the light–independent stage of photosynthesis, and the term could be taken to mean that it does not need the energy of light to proceed. Although the light-independent stage does not require light directly it does need the ATP and reduced NADP molecules that are made in the light-dependent reaction. When there are low levels of light, there are low concentrations of ATP and reduced NADP; this means glycerate 3-phosphate (GP) cannot be converted to triose phosphate (TP). If there is no light then GP levels will rise and TP and RuBP levels will fall. The cycle will keep running until most of the TP has been used up and then stop altogether. Temperature changes will not affect the relative concentrations of these compounds.

Explaining the effect of carbon dioxide concentration on the carbon cycle.

Carbon dioxide is needed in the Calvin cycle to make GP from RuBP. If the carbon dioxide supply is limited then less GP can be made, and the concentration of TP will be lower. This also means that less RuBP can be made, since it is made from some of the TP molecules produced, whilst the remainder are made into products such as glucose, amino acids and lipids. The plant may prioritise the regeneration of ribulose bisphosphate (RuBP) but this may be of little use if low carbon dioxide concentrations remain for a long period of time since there is not much carbon dioxide to combine with.