# Factors Affecting the Rate of Photosynthesis ## What are the Limiting Factors 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

A limiting factor is any variable that, when in short supply, restricts the rate of photosynthesis even if other factors are at their optimum levels

- The main external factors that affect the rate of photosynthesis are: - Light intensity - Carbon dioxide concentration - Temperature - 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 effect of increasing light intensity - The rate of photosynthesis increases as light intensity increases: - The greater the light intensity, the more energy supplied to the plant - 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) - The Calvin cycle can then also occur at a greater rate - During this initial stage, light intensity is said to be a limiting factor of photosynthesis ### The plateau effect - 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 - The factors which could be limiting the rate when the graph line becomes horizontal include: - Temperature being too low or too high - Not enough carbon dioxide

When interpreting graphs: if the rate is increasing (line going up), the x-axis variable is limiting; if the rate plateaus (horizontal line), something else is limiting

### Graph: Light intensity vs rate of photosynthesis The graph below shows the typical relationship between light intensity and photosynthesis rate:

- Notice the curve increases steeply at first (light is limiting) then plateaus (another factor becomes limiting) ## Carbon Dioxide Concentration ### The effect of increasing carbon dioxide - 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 when $\text{CO}_2$ is combined with ribulose bisphosphate (RuBP) during carbon fixation - The more carbon dioxide present, the faster this step of the Calvin cycle can occur - The faster the overall rate of photosynthesis ### Reaching the limit - 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 $\text{CO}_2$ in the atmosphere is 0.04%, so it is not advisable to increase the concentration much higher than this as it can become toxic - The factors which could be limiting the rate when the graph line becomes horizontal include: - Temperature being too low or too high - Not enough light ### Graph: Carbon dioxide concentration vs rate of photosynthesis The graph below shows how increasing $\text{CO}_2$ concentration affects photosynthesis rate:

- Similar to light intensity, the rate increases initially then plateaus when another factor becomes limiting ## Temperature ### Temperature and enzyme activity - 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 this point the enzymes begin to denature - The rate of reaction decreases ### Differential effects on photosynthesis stages - 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: - These reactions are driven by energy from light - Rather than the kinetic energy of the reacting molecules - However, the Calvin cycle is affected by temperature: - The light-independent reactions are enzyme-controlled reactions - For example, rubisco catalyses the reaction between $\text{CO}_2$ and ribulose bisphosphate ### Optimum temperature effects - 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 will: - Increase the rate of the light-independent reactions - Therefore increase the rate of photosynthesis - The optimum temperature will vary by species and what its natural habitat is

Beyond the optimum temperature, enzyme denaturation causes a sharp decrease in photosynthesis rate despite adequate light and $\text{CO}_2$

### Graph: Temperature vs rate of photosynthesis The graph below shows the characteristic bell-shaped curve for temperature effects on photosynthesis:

- The rate increases to an optimum (around 35°C for many temperate plants) then decreases as enzymes denature - This differs from the light and $\text{CO}_2$ graphs which only show a plateau, not a decline ### Other temperature effects - 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 to reduce water loss - When stomata are closed $\text{CO}_2$ cannot enter the leaves - 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 - This may lead to a dissipation of the proton gradient - Causes a slowing down of photosynthesis ## The Effect of Light Intensity and Carbon Dioxide on GP, TP and RuBP Levels - 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 ### Effect of decreased light intensity - A decrease in light intensity causes: - A **decrease** in TP and RuBP concentrations - A **slight increase** in GP concentration **Why this happens:** - 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 ### Effect of very low carbon dioxide concentration - Very low concentrations of carbon dioxide (less than 0.01%) cause: - A **decrease** in GP and TP concentrations - An **increase** in RuBP concentration **Why this happens:** - RuBP accepts carbon dioxide so when there is a lack of $\text{CO}_2$ molecules it remains unfixed and builds up - The lack of carbon dioxide fixation prevents GP and TP molecules from forming

Remember: RuBP + $\text{CO}_2$ → GP → TP. If $\text{CO}_2$ is low, RuBP accumulates; if light is low, GP accumulates

## Agricultural Practices Balance Limiting Factors ### Application in protected environments - 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 sophisticated glasshouses, sensors can be used to monitor: - The light intensity - The humidity of the atmosphere - The carbon dioxide concentration around the crops ### Maximising photosynthesis year-round **Light manipulation:** - Plants could continue to grow through the night if they are kept lit with artificial lighting - Think of commercial tomato growers who use supplementary lighting in winter months to maintain production **Temperature control:** - Plants can be grown out of their natural season and habitat - Temperature can be kept constant all year round - Similar to how tropical orchids can be cultivated in temperate regions using heated greenhouses **Computerised management:** - All these factors can be managed by a computer - Their levels adjusted to ensure the crop can photosynthesise at the highest rate possible ### Additional environmental controls **Water and nutrient supply:** - Water can be supplied by irrigation systems throughout the glasshouse or fields - These sometimes contain added fertilisers or growth nutrients such as nitrates to aid plant growth **Pest control:** - Natural pests that may spread disease or eat the crops can be controlled within agricultural settings - Methods include pesticides or separating the plants from the unfiltered outside air **Result:** - This maximises the yield of the crop ### Economic and environmental balance - Farmers have to find a balance between: - Crop yield - The cost of maintaining 24-hour lighting - Year-round heating costs - The environmental implications this has

Intensive agricultural practices must balance increased yields against energy costs and environmental impact such as carbon emissions

## Summary Table: Limiting Factors | Factor | How it limits photosynthesis | How to increase it (in practice) | |--------|------------------------------|----------------------------------| | Light intensity | Less light = slower light-dependent stage = less ATP and NADPH produced | Move light source closer; use artificial lighting; increase light exposure time | | $\text{CO}_2$ concentration | Less $\text{CO}_2$ = slower carbon fixation in Calvin cycle = less GP formed | Add $\text{CO}_2$ to greenhouse atmosphere; ensure good ventilation outdoors | | Temperature | Too low = slower enzyme activity; too high = enzyme denaturation | Use heating systems; water baths in experiments; greenhouse climate control |