How Light Intensity Affects the Rate of Photosynthesis – Biology IAA

This article will have all you need to know about the Biology IAA coursework on the investigation into the effect of light intensity on the rate of photosynthesis. Yes, another boring IAA exam which makes it vital to have no slip ups in the method, validity, reliability, evaluation or conclusion.

Firstly, let’s identify what the independent and dependant variables are:

  • Independent Variable – Light Intensity
  • Dependant Variable – Rate of photosynthesis

Independent variable causes a change in Dependent Variable and it isn’t possible that Dependent Variable could cause a change in Independent Variable. So, the light intensity causes a change to the rate of photosynthesis.

 

Controlled Variables

Amount of background light, volume of indicator, carbon dioxide concentration in water, temperature of water, lamp (bulb), colour of indicator at start, length of pondweed, species of pondweed, concentration of indicator, healthiness of pondweed and observer.

 

Equipment List

  • Three test tubes.
  • Healthy Pondweed plant (still alive in water).
  • Glass container for remaining unused pondweed to be in.
  • Hydrogencarbonate indicator.
  • Scissors (to cut pondweed with).
  • Three rubber bungs.
  • A sheet of aluminium.
  • A lamp.
  • 30cm ruler.
  • Stopwatch.
  • A purple spot diagram.

 

Method

  1. Label three tubes 1-3 to help stop contamination.
  2. Cut 6 lengths of pondweed at 10cm long (do this while the pondweed is underwater) and place two in each test tube.
  3. Pour 25ml of hydrogencarbonate indicator into all three test tubes which should be enough to cover the pondweed.
  4. Close each tube with a rubber bung making sure it’s air tight.
  5. Cover tube 1 completely with aluminium foil to exclude light.
  6. Place tube 2 directly next to a light source (such as a lamp) and then tube 3 directly 10cm away from the light source.
  7. Measure the time taken for the indicator to change to purple using the ‘purple spot diagram’ to decide this.
  8. Repeat steps 1-7.
  9. Ignore any anomalous data and repeat to compensate for this.
The graph produced from this experiment isn’t very useful as it will only have 3 points on it making it harder to see a pattern or any type of correlation. However, this is what the table should look like:

Pattern From The Table
From the table, it is clear that light intensity on the pondweed effects the time taken for the hydrogen carbonate indicator to go from red to purple as high light intensity produced an average of 27.8 minutes where low intensity produced an average of 35 minutes. This shows the higher the light intensity, the less time it takes for the indicator to go purple. Where there is no light, there is no photosynthesis so no change.
The method could have been made more valid by (The point/Explanation for the point)
Keeping the temperature the same using a sheet of glass because temperature effects the rate of photosynthesis. 
Use one lamp instead of two as the light source as the two different lamps could have had two different light intensity meaning two different rates of photosynthesis.
Using same amount indicator or use a pH probe as different amounts will produce different shades of colours making it more difficult to establish when the colour has gone purple.
Using same length of pondweed as the different lengths of pondweed (in mm) would have used slightly different amounts of carbon dioxide.

The method was made reliable by..

  • Labelling the tubes to stop contamination.
  • Cutting the pondweed underwater.
  • Close each test tube with a rubber bung.
  • Repeating the experiment.
  • Ignoring any anomalous data and repeat to compensate.

 

Conclusion

(this is where you should use your results and scientific knowledge)
Ultimately, the higher the light intensity the less time it takes for the indicator to turn purple. This is because light effects the rate of photosynthesis. Therefore, the more light, the faster the pondweed will photosynthesis. The change in carbon dioxide (due to pondweed photosynthesising) dissolved in the water effects the acidity of the water: the more carbon dioxide dissolved the more acidic the water will be. As the carbon dioxide in the water has decreased, the acidity of the water decreases causing the indicator to change from red to purple. This proves as light increases, so does rate of photosynthesis which is because the chlorophyll has more light to absorb for photosynthesis.

Update

From the comments below, there seems to be some confusion about what exact indicator is used in this experiment. Hydrogencarbonate indicator (also known as biocarbonate indicator) is used because it is a pH indicator that is sensitive enough to display a colour change when the levels of carbon dioxide changes. When the levels of carbon dioxide decrease, the acidity of the water ever so slightly decreases which the hydrogencarbonate indicator reacts to changing colour from red to purple. Therefore, to sum up:

  • 25ml of hydrogencarbonate indicator is poured into the test tubes with pondweed in them.
  • The pondweed photosynthesises decreasing the level of CO2 in the water. 
  • The decreased level of CO2 in the water causes the acidity of the water to decrease.
  • The hydrogencarbonate changes colour from red to purple due to the change in acidity. 
  • The ‘purple spot diagram’ is a simple diagram with different shades of purple on it. This helps making the test valid because you will be able to stop the clock at the same specific shade of purple for each test tube of pondweed.

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