Photosynthesis: Limiting Factors Affecting the Rate of Photosynthesis - (GCSE Biology)

Limiting factors Affecting the Rate of Photosynthesis

Single Factors Affecting Rate of Photosynthesis

• Limiting factors affect the rate of a reaction. A limiting factor is a condition, that when in shortage, slows down the rate of a reaction. Light intensity, carbon dioxide concentration and temperature are limiting factors of photosynthesis. They all affect the rate of the photosynthetic reaction, but in different ways.

Photosynthesis Equation

The equation for photosynthesis is shown below in words. Factors affecting photosynthesis will affect the rate of reaction, will affect the amount of glucose and oxygen produced.

• Carbon dioxide + water + (light energy) → glucose + oxygen gas

Carbon Dioxide

• The higher the CO₂ concentration, the higher the rate until a certain point. As you increase the concentration of carbon dioxide (the reactant), the reaction is driven forwards.
• At high CO₂ concentration, rate levels off. until you reach a point at which the enzymes required are saturated. At this point, the carbon dioxide is no longer a limiting factor.

• You can prove the need for carbon dioxide in photosynthesis by placing a plant in a sealed bell jar with some soda lime. The soda lime will absorb any carbon dioxide present in the jar. After a while you can test the leaves of the plant for the presence of starch:
  
  1. Dip the leaf in boiling water. This stops any further reactions in the leaf.
  2. Place the leaf in a tube with ethanol.
  3. Place the tube in an electric water bath and heat till it’s boiling. This removes any chlorophyll and turns the leaf a white colour.
  4. Rinse the leaf with cold water and add drops of iodine solution.
  5. Observe your results. The leaf won’t turn blue-black which means there is no starch present hence, no photosynthesis took place.

Light Intensity

• The higher the light intensity, the higher the rate until a certain point. If you increase the intensity of the light, more light can be trapped by the chloroplasts to provide energy to drive the photosynthetic reaction.
• At high light intensity, rate levels off. Again, this is up until a certain point, when the maximum amount of light has already been trapped. When you reach this point, light is no longer a limiting factor.

• You can prove the need for light by testing leaves of a plant left in the dark for 48 hours. You can test the leaves for starch in the same way as described above and will find that they won’t turn blue-black as no starch is present. The plant would have used up its starch stores without being able to make more due to the lack of light.

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Temperature

• The higher the temperature, the higher the rate until a certain point. As the temperature increases, the enzymes gain more kinetic energy and so can catalyse the reaction at a greater rate. At the optimum temperature, the rate of reaction is the highest.
• At high temperatures, the rate falls as enzymes denature. When the temperature is greater than the optimum, the enzymes denature (change shape), so the reaction can no longer take place. This means the reaction is no longer feasible and so will not occur. This temperature is often around 45ºC.

Chlorophyll

• Chlorophyll can act as a limiting factor for photosynthesis. As the amount of chlorophyll increases, more light can be trapped by the chloroplasts in order to drive the reaction. However, once there is a certain amount of chlorophyll, it will no longer be a limiting factor and the values of other factors will limit the rate of the reaction. Infection and disease can reduce the amount of chlorophyll in a plant.
• You can prove the need for chlorophyll by testing variegated leaves. Variegated leaves are green and white – only the green areas contain chlorophyll so only they photosynthesise and produce starch. You can test the leaves for starch in the same way as described earlier. You will find that the areas of the leaf that were initially green (with chlorophyll) will turn black due to the presence of starch. The areas that were initially white (without chlorophyll) will remain orange.

Multiple Factors Affecting Rate of Photosynthesis

• Realistically, more than one factor is involved at a period in time. In the wild, plants are not kept under experimental conditions and so will be affected by different temperatures, carbon dioxide concentrations and light intensities.

Graphs can be used to map the effect of different limiting factors on the rate of photosynthesis at the same time:

Light Intensity and Temperature

• In the above graph, CO2 concentration is constant, so the effect of light intensity and temperature on photosynthesis is shown.
• As you increase light intensity, rate increases until we reach a plateau, after which light is no longer the limiting factor.
• As you increase temperature (shift from 20ºC to 30ºC), the rate increases. Also, the light intensity effect plateau’s at a higher level, so light intensity is a limiting factor for longer.

Light Intensity and CO2 concentration

• In the above graph, temperature is constant, so the effect of light intensity and carbon dioxide concentration on photosynthesis is shown.
• As you increase light intensity, rate increases until we reach a plateau, after which light is no longer the limiting factor.
• As you ice increase CO2 (shift from 0.05% to 0.5%), the rate increases. Also, the light intensity effect plateau’s at a higher level, so light intensity is a limiting factor for longer.

Canadian Pondweed Experiment

Setting up the Experiment

This test can specifically measure the effect of light intensity on photosynthesis. The rate of the plant’s oxygen production is proportional to the rate of photosynthesis.

1. Place the plant in a boiling tube. Place the plant underwater in a boiling tube.
2. Connect a gas syringe. Place the boiling tube on a clamp and add a capillary tube and a gas syringe. There should be an air bubble in the capillary tube.
3. Use a ruler to measure movement of the air bubble. You can then use a ruler to see how much the air bubble moves. The amount the bubble moves is proportional to the rate of photosynthesis.
4. Change the variables. You can now vary the light intensity, temperature and CO₂ concentration and observe the effect on bubble displacement (and hence rate of photosynthesis).

Investigating the Effect of Light Intensity

• Vary light intensity. Place a lamp at differing distances from the plant and observe the oxygen production through the movement of the bubble. This will give you an opportunity to see the effect of light intensity on the rate of photosynthesis.
• Control the other variables, and repeat for reliability. Control the temperature of the room and the time taken for the experiment. Make sure to repeat the experiment three times at each distance, then take a mean value in order to attain more reliable results.

Investigating the Effect of Temperature

• Vary the temperature. Place the boiling tube in different temperature water baths and observe the oxygen production through the movement of the bubble. This will give you an opportunity to see the effect of light intensity on the rate of photosynthesis
• Control the other variables, and repeat for reliability. Control the time taken for the experiment and the distance at which the lamp is placed. Make sure to repeat the experiment three times at each distance, then take a mean value in order to attain more reliable results.

Investigating the Effect of CO₂ Concentration

• Vary the CO₂ concentration. Dissolve different amounts of sodium hydrogen-carbonate in the boiling tube. This releases carbon dioxide in water. You can dissolve different amounts into different boiling tubes to see the effect of differing carbon dioxide concentration on the rate of photosynthesis
• Control the other variables, and repeat for reliability. Control the temperature, distance of the lamp and the time taken for the experiment. Make sure to repeat the experiment three times at each carbon dioxide concentration, then take a mean value in order to attain more reliable results.

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