Rate of Photosynthesis

• Not enough light slows down the rate of photosynthesis
• Light provides the energy needed for photosynthesis. As the light level is raised, the rate of photosynthesis increases steadily - but only up to a certain point. Beyond that, it won't make any difference - as light intensity increases, the rate will no longer increase

  
• After a certain point the rate will not increase because it'll either be the temperature or the CO2 level which is now the limiting factor, not light

  
• In the lab you can change the light intensity by moving a lamp closer to or further away from your plant. But if you plot the rate of photosynthesis against ''distance of lamp from the plant'', you get a weird-shaped graph

  
• To get a graph like the one below you either need to measure the light intensity at the plant using a light meter or do some maths with your results

  
• Too little carbon dioxide also slows down the rate of photosynthesis
• CO2 is one of the raw materials needed for photosynthesis. As with light intensity, the amount of CO2 will only increase the rate of photosynthesis up to a point. After this the graph flattens out - as the amount of CO2 increases, the rate no longer increases

  
• As the amount of CO2 increases, the rate ono longer increases. This shows that CO2 is no longer the limiting factor

  
• As long as light and CO2 are in plentiful supply then the factor limiting photosynthesis must be temperature

  
• The temperature has to be just right for photosynthesis
• Usually, if the temperature is the limiting factor it's because it's too low - the enzymes needed for photosynthesis work more slowly at low temperatures

  
• If the plant gets too hot, the enzymes it needs for photosynthesis and its other reactions will be damaged. This happens at about 45°C (which is pretty hot for outdoors, although greenhouses can get that hot if you're not careful)

  
• One graph may show the effect of many limiting factors
• The graph below shows how the rate of photosynthesis is affected by light intensity and temperature. At the start, both of the lines show that as the light intensity increases, the rate of photosynthesis increases steadily

  
• The lines level off when light is no longer the limiting factor. The line at 25°C levels off at a higher point than the one at 10°C, showing that temperature must have been a limiting factor at 10°C

  
• The graph below shows how the rate of photosynthesis is affected by light intensity and CO2 concentration. Both the lines plateau when light is no longer the limiting factor

  
• The line at the higher CO2 concentration of 0.2% plateaus at a higher point than the one at 0.02%. This means CO2 concentration must have been a limiting factor at 0.02% CO2. The limiting factor here isn't temperature because it's the same for both lines (20°C)

  
• The inverse square law links light intensity and distance
• In the experiment that tests the rate of photosynthesis on oxygen production, when the lamp is moved away from the pondweed, the amount of light that reaches the pondweed decreases
• As the distance increases between the lamp and the plant, the light intensity decreases, i.e. distance and light intensity are inversely proportional to each other
• It turns out that light intesity decreases in proportion to the square of the distance. This is called the inverse square law and is written out like:

  light intensity∝1/distance(d)^2
• The inverse square law means that is you halve the distance, the light intensity will be four times greater and if you third the distance, the light intensity will be nine times greater
• If you double the distance, the light intensity will be four times smaller and if you treble the distance, the light intensity will be nine times smaller
• You can use 1/d^2 as a measure of light intensity
• Example

  Use the inverse square law to calculate the light intensity when the lamp is 10 cm from the pondweed

  1)Use the formula 1/d^2

  2) Fill in the values you know - you're given the distance, so put that in

  > light intensity = 1/10^2

  3) Calculate the answer

  > = 0.01 a.u.
• a.u. stands for arbitrary units
• You can artificially create the ideal conditions for farming
• The most common way to artificially create the ideal environment for plants is to grow them in a greenhouse
• Greenhouses help to trap the Sun's heat, and make sure that the temperature doesn't become limiting
• In winter a farmer or gardener might use a heater as well to keep the temperature at the ideal level
• In summer it could get too hot in a greenhouse so a farmer or a gardener might use shades and ventillation to cool things down
• Light is always needed for photosynthesis, so commercial farmers often supply artificial light after the Sun goes down to give their plants more quality photosynthesis time
• Farmers and gardeners can increase the level of carbon dioxide in the greenhouse. E.g. by using a paraffin heater to heat the greenhouse. As the paraffin burns, it makes carbon dioxide as a by-product
• Keeping plants enclosed in a greenhouse also makes it easier to keep them free from pests and diseases. The farmer can add fertilisers to the soil as well, to provide all the minerals needed for health growth
• Sorting out artificial environments for plants costs money - but if the farmer can keep the conditions just right for photosynthesis, the plants will grow much faster and a decent crop can be harvested much more often, which can then be sold
• It's important that a farmer supplies just the right amount of heat, light, etc. - enough to make the plants grow well, but not more than the plants need, as this would just be wasting money