Investigting the effect of light intensity on photosynthesis Essay

Biology lab: measuring the effect of light intensity on plant growth

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Investigate the effect of light intensity on plant growth

Light intensity will have an effect on plant growth. With an increase in light intensity there will be plant growth at a faster rate in comparison to when the light intensity is low where plant growth is limited and will take place slowly. When there’s too much light intensity, the light intensity will no longer become a limiting factor. A limiting factor is a factor that controls any particular process at the minimum rate. This control the rate of the process which in this case is the light intensity. This can also be called the rate limiting factor as it affects the rate of photosynthesis.

Photosynthesis is an essential process by which plants make their food in order for growth and respiration to happen. Carbon dioxide water and light combine to manufacture food which comes in the form of glucose for use. Light is a very important aspect in plant growth as with light the light dependent reaction in photosynthesis is able to take place. In this process light is used to break down water, and the photon energy is used to move the electron to the reaction centre where it gets excited. The light dependant reaction is essential as products obtained from this process are used in the light independent stage of photosynthesis. The products made in the light independent stage of photosynthesis are NADP+ and ADP.


1 Lamp
6 Green aquatic plant (Elodea)
1 500ml Beaker
1 Thermometer 110°C ±0.5
1 Metre ruler ±0.5
1 Test tube
50g Sodium hydrogen carbonate
1 Funnel
1 Stopwatch
1 water bath

Independent variable
Distance between plant and light bulb affecting the light intensity

Dependant variable
Number of oxygen bubbles of oxygen produced as a bi-product of photosynthesis.

Control variables:
Type of plant
Temperature of the surroundings
Use the same colour light bulb due to effect of different wavelengths Concentration of sodium hydrogen carbonate
Water provided to the plant
Set to optimum temperature, pH and CO2 concentration levels

How to control the control variables.
To control the type of plant we use we have to use the same species of plant of the same length and age during the whole experiment The temperature of the surrounding room will be set at room temperature which will be 24°C. This can be done by using the fans or opening or closing the windows. The concentration of sodium hydroxide must be at the same level at the beginning of each of the different light distances. The water provided to the plant must be at a level as to where the aquatic plant is completely submerged underwater. Optimum temperature must be calculated before plant is submerged underwater. This can be done by heating the water to the specific number of degrees.


1. Ensure all control variables have been administered and controlled 2. Immerse the plant into the 500ml beaker full of water so that the plant is fully covered with water. Water must be at optimum temperature which is 40°C, with a calculated concentration of sodium hydrogen carbonate and a buffer solution to maintain the pH. 3. Place the funnel above the aquatic plant in order to channel the flow of the oxygen bubbles produced. 4. Fill test tube with water till it is completely full as the bubbles produced from photosynthesis will displace the water in the test tube.

5. Place the test tube above the funnel so that the gas bubbles can be collected. This way the number of gas bubbles can be counted and tallied.

6. Place the light source being the lamp 50cm away from the beaker containing the aquatic plant. A protective transparent mirror will not be needed as heat transmitted will be blocked off by the test tube and the beaker.

7. Count the number of bubbles produced over a 10-minute period as the plant needs time to adjust to the new surroundings it has been placed in. Use a stopwatch to count the time.

8. Repeat steps 6 and 7 again this time changing the distance between the light and plant by another 10cm. Do this for 60cm, 70cm and finally 80cm.

9. Regularly check the temperature of the water in order to maintain fairness within the experiment.

10. Repeat the entire experiment two more times for reliable results. When redoing the entire experiment it is essential that a different plant is used at each different trial as the plant from the previous trial has been affected by the adjustments that it has been placed in. a new plant of the same species will better accuracy within the results.


Distance from the plant cm (±0.5)
Number of oxygen bubbles produced (±1)




Diagram illustrating the lab setup for a test on the effect of light intensity on the rate of photosynthesis.

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