To investigate the limiting effect of light intensity on photosynthesis.

Aim: To investigate the limiting effect of light intensity on photosynthesis.

Prediction: I predict that as the light source (desk lamp) is moved closer to the pondweed (Elodea), the rate of photosynthesis will increase therefore more oxygen will be produced creating more bubbles.

I believe this will happen
because when the light source is nearer to the plant more of the plants
surface area is coming in to contact with the light from the desk lamp
therefore more photosynthesis will occur which will mean more oxygen will be produced which will create more bubbles.

Plan:

Method:
I am going to collect my apparatus and set it up as in the diagram, then I am going to fill the beaker with water. I will then set up desk lamp in correct position (eg: 20cm away). I will cut a piece of elodea 5cm long with a Scapular and put it into the beaker of water. Before I turn on the desk lamp I will observe the amount of bubbles the Elodea produces with foil wrapped around the beaker and then again without the foil. I will do this for 1 minute. Then I will turn on the desk lamp and start the stop clock.
I will count the bubbles for the next minute and also make a note of the change in volume.
I will repeat this for a distance of 10cm, 5cm and 2cm away. I will record all my results in a table.

Equipment:

Foil (used to filter the light)
Stop clock
Beaker
Desk lamp
One metre ruler stick
Pondweed (Elodea)
Water
Scapular

To make my experiment safe I will:

Follow lab safety rules (eg no running, bags under desks etc).
Keep the desk lamp away from contact with water.
Be careful when using scapular.

To make sure the experiment is fair, these things must not change:

The same pondweed must be used every time we change the light distance.
Do the experiment twice for each distance to get an accurate
average.
There ...

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Equipment:

Foil (used to filter the light)
Stop clock
Beaker
Desk lamp
One metre ruler stick
Pondweed (Elodea)
Water
Scapular

To make my experiment safe I will:

Follow lab safety rules (eg no running, bags under desks etc).
Keep the desk lamp away from contact with water.
Be careful when using scapular.

To make sure the experiment is fair, these things must not change:

The same pondweed must be used every time we change the light distance.
Do the experiment twice for each distance to get an accurate
average.
There must only be one variable and that is the distance of the light source from the plant.
Keep the experiment at a constant temperature
Take results after one minute for each individual experiment.

Results:

Analysis

Conclusion: Looking at my results a can see a significant increase in the rate of photosynthesis as the distance decreases.

In the experiments, when the distance was 20cm away there was not much photosynthesis taken place only about 20 bubbles of oxygen were produced per minute, but when the desk lamp was 2cm away a lot of photosynthesis was occurring on average around 64 bubbles per minute which was quite a significant increase. I believe this is because when the desk lamp is close to the pondweed, more of the pondweeds surface area has light energy shining upon it, which means more photosynthesis will occur in a shorter time.

Looking at my results it is quite evident that the number of bubbles
produced is much greater when the distance of the pondweed from the desk lamp is less. This proves that my prediction was correct that as the light source (desk lamp) is moved closer to the pondweed (Elodea), the rate of photosynthesis will increase therefore more oxygen will be produced creating more bubbles.

Evaluation:

I think my method was difficult and I don’t think my results are very reliable because counting bubbles is not a very reliable method as some bubbles are bigger than others and it is very difficult to count every bubble. Instead I should have used a gas syringe because this method is more accurate and will make my experiment fairer. I could have also used different light intensity, coloured sheets of plastic over the light to see it that affects the rate of photosynthesis and different plants instead of just using elodea. I could have also repeated my experiment more to make my average results more accurate too because I don’t think I have enough results to support a firm conclusion. I have also found some anomalous results on my graph and I think these have occurred because I haven’t used a very accurate method.

By Kira Rudder