5.6.9 Factors Affecting the Rate of Photosynthesis

• Plants need several factors for photosynthesis to occur:
  • The presence of photosynthetic pigments
  • A supply of carbon dioxide
  • A supply of water
  • Light energy
  • A suitable temperature

• If there is a shortage of any of these factors, photosynthesis cannot occur at its maximum possible rate
• The main external factors that affect the rate of photosynthesis are:
  • Light intensity
  • Carbon dioxide concentration
  • Temperature

• These are known as limiting factors of photosynthesis
• If any one of these factors is below the optimum level for the plant, its rate of photosynthesis will be reduced, even if the other two factors are at the optimum level
• Although a lack of water can reduce the rate of photosynthesis, water shortages usually affect other processes in the plant before affecting photosynthesis and is therefore not one of the main limiting factors

Light intensity

• When temperature and carbon dioxide concentration remain constant, changes in light intensity affect the rate of photosynthesis
• The rate of photosynthesis increases as light intensity increases:
  • The greater the light intensity, the more energy supplied to the plant and therefore the faster the light-dependent stage of photosynthesis can occur
  • This produces more ATP and reduced NADP for the Calvin cycle (light-independent stage), which can then also occur at a greater rate
  • During this stage of the graph below, light intensity is said to be a limiting factor of photosynthesis

• At some point, if light intensity continues to increase, the relationship above will no longer apply and the rate of photosynthesis will reach a plateau
• At this point, light intensity is no longer a limiting factor of photosynthesis – another factor is limiting the rate of photosynthesis
• The factors which could be limiting the rate when the line on the graph is horizontal include temperature being too low or too high, or not enough carbon dioxide

The effect of light intensity on the rate of photosynthesis

Carbon dioxide concentration

• The rate of photosynthesis increases as carbon dioxide concentration increases:
  • Carbon dioxide is one of the raw materials required for photosynthesis
  • It is required for the light-independent stage of photosynthesis, when CO₂ is combined with the five-carbon compound ribulose bisphosphate (RuBP) during carbon fixation
  • This means the more carbon dioxide that is present, the faster this step of the Calvin cycle can occur and the faster the overall rate of photosynthesis

• This trend will continue until some other factor required for photosynthesis prevents the rate from increasing further because it is in short supply
• The natural level of CO₂ in the atmosphere is 0.04%, it is therefore not advisable to increase the CO₂ concentration much higher than this as it can become toxic
• The factors which could be limiting the rate when the line on the graph is horizontal include temperature being too low or too high, or not enough light

The effect of carbon dioxide concentration on the rate of photosynthesis

Temperature

• As temperature increases the rate of photosynthesis increases, as the reaction is controlled by enzymes
• However, as the reaction is controlled by enzymes this trend only continues up to a certain temperature, beyond which the enzymes begin to denature and the rate of reaction decreases
• For most metabolic reactions, temperature has a large effect on reaction rate
• For photosynthesis, temperature does not have a significant effect on the light-dependent reactions, as these are driven by energy from light rather than the kinetic energy of the reacting molecules
• However, the Calvin cycle is affected by temperature, as the light-independent reactions are enzyme-controlled reactions (e.g. rubisco catalyses the reaction between CO₂ and the five-carbon compound ribulose bisphosphate)
• As long as there is enough light energy to produce ATP and NADPH in the light-dependent reaction, increasing temperature up to an optimum temperature (this will vary by species and what its natural habitat is) will increase the rate of the light-independent reactions and therefore the rate of photosynthesis
• Although the rate of enzymatic reactions is the main component affected by temperature, other components of the process can also be affected:
  • Increasing temperature causes stomata on the leaves to close in order to reduce water loss; when the stomata are closed CO₂ cannot enter the leaves and photosynthesis will slow down
  • The light-dependent reaction relies on a proton gradient forming across the thylakoid membrane; membrane permeability can be influenced by extreme temperatures, which may lead to a dissipation of the proton gradient and a slowing down of photosynthesis

The effect of temperature on the rate of photosynthesis

The effect of the light intensity and carbon dioxide concentration on levels of GP, TP and RuBP

• The concentrations of glycerate 3-phosphate (GP), triose phosphate (TP) and ribulose bisphosphate (RuBP) within chloroplasts can be affected by changes to light intensity and carbon dioxide concentration
• A decrease in light intensity causes a decrease in TP and RuBP concentrations but a slight increase in GP concentration
  • When there is less light available the light-dependent stage stops and does not form any more products needed for the light-independent stage (ATP and NADPH)
  • As a consequence, GP builds up as it is not converted to TP
  • A lack of TP means that RuBP will not form
  • Over time the fixation of carbon dioxide will stop and the concentration of GP will plateau

• Very low concentrations of carbon dioxide (less than 0.01%) causes a decrease in the concentration of GP and TP but an increase in RuBP concentration
  • RuBP accepts carbon dioxide so when there is a lack of carbon dioxide molecules it remains unfixed and builds up
  • The lack of carbon dioxide fixation prevents GP and TP molecules from forming

A decrease in light intensity and carbon dioxide concentration has different effects on the concentrations of GP, TP and RuBP.

Agricultural practices balance limiting factors

• An understanding of the effect of limiting factors on the rate of photosynthesis can be used to increase crop yields in protected environments, such as glasshouses
• In the most sophisticated glasshouses, for example, sensors can be used to monitor the light intensity, the humidity of the atmosphere and the carbon dioxide concentration around the crops
  • This means that plants could continue to grow through the night if they are kept lit with artificial lighting
  • Plants can be grown out of their natural season and habitat because the temperature can be kept constant all year round

• All these factors can be managed by a computer and their levels adjusted to ensure the crop can photosynthesis at the highest rate possible
• Water can be supplied by irrigation systems throughout the glasshouse or fields which sometimes contain added fertilisers or growth nutrients such as nitrates to aid plant growth
• Natural pests that may spread disease or eat the crops can be controlled within agricultural settings by pesticides or by separating the plants from the unfiltered outside air
• This maximises the yield of the crop
• Farmers have to find a balance between crop yield and the cost of maintaining 24-hour lighting and year-round heating as well as the environmental implications this has