Rate of photosynthesis

• Photosynthesis

  • Light provides the energy needed
  • As light level is raised, rate of photosynthesis increases steadily up to a certain point
  • Beyond that point, light intensity increase won't make a difference as CO2 or temperature becomes the limiting factor
• Measuring light intensity effect on photosynthesis rate

  1. Change light intensity by moving lamp closer or further from plant

  2. Plot rate of photosynthesis against light intensity (not distance of lamp)

  3. Need to measure light intensity at plant using light meter or do calculations
• Effect of CO2 on photosynthesis rate

  • CO2 is a raw material needed for photosynthesis
  • As CO2 increases, rate of photosynthesis increases up to a point
  • After that point, further CO2 increase doesn't increase rate as another factor (temperature) becomes limiting
• Effect of temperature on photosynthesis rate

  • Enzymes needed for photosynthesis work more slowly at low temperatures
  • Enzymes can be damaged at high temperatures (around 45°C)
• If light and CO2 are plentiful, then temperature is the limiting factor for photosynthesis rate
• Malignant tumours contain chemicals that can cause cancer, but benign tumours are not cancerous
• The Rate of Photosynthesis
• One Graph May Show the Effect of Many Limiting Factors
• As light intensity increases


  The rate of photosynthesis increases steadily
• When light is no longer the limiting factor


  The rate of photosynthesis levels off
• At 25°C


  The rate of photosynthesis levels off at a higher point than at 15°C
• As CO2 concentration increases from 0.04% to 0.4%

  The rate of photosynthesis levels off at a higher point
• Oxygen Production Shows the Rate of Photosynthesis
• Measuring the effect of light intensity on the rate of photosynthesis using Canadian pondweed


  1. Place a source of white light at a specific distance from the pondweed

  2. Leave the pondweed to photosynthesise for a set amount of time

  3. Measure the length of the oxygen bubble collected in the capillary tube

  4. Repeat the experiment at different light intensities

  5. Calculate the mean volume of oxygen produced
• The experiment can be altered to measure the effect of temperature or CO2 on photosynthesis
• Artificial lighting is commonly used to grow plants in greenhouses
• Farmers also use paraffin heaters to control temperature in greenhouses
• Keeping the plants at the ideal light, temperature and CO2 levels is important for plant growth
• Sorting and grading the plants is important to ensure the best quality produce
• Temperature


  • 25 °C
  • 15 °C
• Percentage


  • 4%
  • 4%
• The Inverse Square Law


  Links Light Intensity and Distance
• When the lamp is moved away from the pondweed, the amount of light that reaches the pondweed decreases
• As the distance increases

  The light intensity decreases
• Distance and light intensity are inversely proportional to each other
• Inverse square law


  Light intensity is proportional to 1/distance^2
• If you halve the distance, the light intensity will be four times greater
• 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
• If you treble the distance, the light intensity will be nine times smaller
• 1/d^2


  Measure of light intensity
• Calculating light intensity using inverse square law

  1. Use formula

  2. Fill in known distance

  3. Calculate answer
• Arbitrary units (au)


  Unit for light intensity
• Greenhouse


  • Traps Sun's heat
  • Maintains ideal temperature
  • Provides artificial light
  • Increases CO2 levels
  • Protects from pests and diseases
  • Allows addition of fertilizers
• Keeping conditions just right for photosynthesis allows plants to grow much faster and be harvested more often
• Supplying just the right amount of heat, light, etc. is important to avoid wasting money
• The inverse square law should not put you off learning everything on these past three pages
• Experiment to find effect of temperature on rate of photosynthesis


  Control two variables