❌ 6. Required Practical: Light Intensity And Rate Of Photosynthesis (B8) Flashcards
Aims Of This Practical
In this practical you will plan and carry out an experiment to find out how the rate of photosynthesis changes as you change the light intensity.
Hypothesis for this Practical
Overall, I believe that the higher the light intensity, the more the plant will photosynthesise, meaning there will more a larger number of oxygen bubbles being produced from the stem in a certain time. This means as the light intensity will increase, the rate of photosynthesis will increase.
Health And Safety for this Practical
- Take care with sharp blades and scissors
- Clear up any spilt water to avoid any hazards
- Carefully clear and dispose of any smashed glass
- Be weary of the boiling water as it may scald/burn you
Equipment For This Practical
- pondweed
- scissors
- boiling tube of water
- test tube rack
- large beaker of water
- lamp
- metre rule
- stopwatch
- thermometer
Method for this Practical
- Cut a piece of pondweed 8–10 cm long. Place it in a boiling tube of water/or KHCO3 (Potassium Hydrogen Carbonate Solution). (This solution has all the reactants and things neded to encourage the plant to photosynthesise).
2 Put the pondweed upside down into the solution, and cut the end of the plant at at the top. - Then you use the thermometer to measure the temperature of the water in the boiling tube.
- Place the lamp 15 cm from the boiling tube. Place the large beaker of water between the lamp and the boiling tube.
- Wait until there is a steady flow of bubbles from the cut end of the pondweed. Then count the number of bubbles in 2 minutes. Record this in your results table.
- Measure the temperature of the water in the boiling tube again to make sure it has not changed.
- Repeat steps 1–5 four more times but in step 3 increase the distance between the lamp and boiling tube by 2 cm each time.
- Repeat the whole investigation (steps 1–6) two more times and record the results.
- Calculate the mean number of bubbles for each distance, leaving out any anomalous values from your calculations.
What is the Independant Variable
The Independant Variable in this experiment is the light intensity. This is because it is the variable that this changed and altered to affect the dependant variable in this test.
What is the Dependant Variable
The Dependent Variable in this experiment is the volume of oxygen produced by the plant. This is the dependant variable because it is the variable that is being tested/measured in this experiment.
What are the Control Variables
The Control Variables in this experiment are the temperature, light energy omitted from the light, volume of Potassium Hydrogen Carbonate, length of pondweed. These are control variables because they have to stay the same throughout the experiment to ensure that it is kept as a fair test.
How to lay out the results table
The table can include 4 main rows, which includes the ‘distance the lamp is from the plant’ (m), the ‘distance^2’ (m^2), as well as the ‘Light Intensity’ (worked out by 1/Distance^2), and finally the mean ‘rate of photosynthesis (mm^3 min^-1). (The rate if photosynthesis can have 2 or 3 measurements for the average)
Conclusion of this Practical (Describing the Results + Linking to Hypothesis)
Overall, the results were pretty consistant with the hyposthesis I made at the start. Initially, the lower light intensity leads to a far slower rate of photosynthesis, however, it is quite clear that between 0-50 m^-2, the rate of photosynthesis increases drastically as the light intensity increases, highlighting the fact that the light intensity is needed here. However, after 50 m^-2, the graph plateus, and the rate of Photosynthesis remains as a constant at around 83mm^3 min^-1. This shows that the rate of photosynthesis doesn’t benefit from any more light intensity than around 50 m^-2.
Initially, the Rate of Photosynthesis rapidly increases, because the fact that the light is moving closer increases the light intensity recieved by the plant. At this stage the light intensity is the limiting factor of photosynthesis, which is the reason why there is such an increase from around 0-50 m^-2. However, when the light intensity surpasses 60m^-2 it does not increase the Rate of Photosynthesis by any more, which is because now there are other limiting factors of photosynthesis, like Carbon dioxide and temperature. To increase the rate further, the supply of carbon dioxide would need to be increased to bring up the rate of photosynthesis again, while if the optimum temperature for the enzymes inside the plant, the rate would quicken even more, as these factors directly influence the process of photosynthesis.
Evaluation of this Practical
Generally, the experiment was successful. However, there were a few factors which could have affected the results. These limitations include:
- The volume of each bubble is unknown, so lots of small bubbles could in theory have the same volume as anfew large bubbles. To combat this, instead of counting bubbles, you could instead use a gas syringe to measure the exact volume of gas being emitted from the plant
- Another limitation can be errors in counting the number of bubbles, especially if they are extremely small, or appear when your not concentrating. This can be fixed by using the gas syringe as you won’t need to count bubbles
- The lamp used in the experiment could also be giving off a small amount of heat that could affect the rate of photosynthesis (therefore altering your results). This can be atoned for by using an LED light for example as it gives off less energy as heat, or even use a heat shield to prevent infrared waves from passing through (meaning the plant only revieve the light rather than the extra heat with it).
- The fact that there were more sources of light other than the lamp meant that it wasn’t just the lamp that was helping the plant photosynthesise (this extra light could come through windows for example). In order to ensure the lamp is the only light source, you can ensure that the experiment is carried out in a dark room.
What is the effect of the Light Intensity on the Rate of Photosynthesis
Initially, the lower light intensity leads to a far slower rate of photosynthesis, however, it is quite clear that between 0-50 m^-2, the rate of photosynthesis increases drastically as the light intensity increase. However, after 50 m^-2, the graph plateus, and even slowly begins to drop off showing that the rate of photosynthesis doesn’t benefit from any more light intensity than around 50 m^-2.
Why does the graph level out at the end
The graph begins to level at after the 50 m^-2 mark, because when the plant is expose to a very high intensity of light, the rate of photosynthesis drops quickly as the light starts to damage the plant.
How could the maximum rate obtained be increased
The maximum rate obtained can be increased by giving the plant even better conditions for it to photosynthesise. This includes giving it the optimum light intesity, along with the optimum supply of both Carbon Dioxide and Water, along with ensuring that the plant has the optimum number of chloroplats as well.
How should the graph be laid out?
On the X-Axis, I included the ‘Light Intensity’ (1/D^2) in m^-2.
On the Y-Axis, I includes the ‘Rate of Photosynthesis’ which is measured in mm^3 min^-1)
The graph shows showed a positive trend at the start, meaning the Rate of Photosyntheisis increased as the Light Intensity increased, but then plateus at around 50m^-2 (of Light Intensity).
