Yr8 Mod7 Elodea Investigation

Photosynthesis

Brief: Investigate the rate of photosynthesis of the water plant Elodea.

Variables:

The Variables that will affect my investigation are:

My Independent variable will be: ______

My Dependent variable will be:______

The Range of my independent variable will be ______to _____

The Values of my independent variable will be;

______, ______, ______, ______, ______

Controlled Variable / Set at Value

It will be difficult to control ______

Because ______

______

Success Criteria:

To be successful I will need to:

So that I can:

To be successful I will need to:

So that I can:

To be successful I will need to:

So that I can:

Evidence

Photosynthesis is a temperature dependant chemical reaction which happens in the leaves of green plants. It is the first step towards making food - not just for plants but ultimately every animal on the planet.

During this reaction, carbon dioxide and water are converted into glucose and oxygen. The reaction requires light energy (photons) which are absorbed by a green substance called chlorophyll.

Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide.

During photosynthesis every molecule of oxygen produced needs one photon of light to hit a chlorophyll molecule.

If you double the distance that a light is from a surface the number of photons hitting the same area of surface will only be a quarter of the number of photons before.

On the surface of the Earth 2 million, million, million (2 x1018) photons from the Sun hit each square cm every second. If the Earth was twice as far from the Sun then only 0.5 million, million, million (5 x 1017) photons would hit every square cm each second.

Diagram

Hypothesis:

  • Explain what you think the relationship will be between your independent and dependent variable.
  • Then explain using scientific knowledge and understanding from the evidence section why you think this relationship will exist.

______

Preliminary work

Lucy tried the experimental setup but found it was impossible to get the lamp as close as 10mm from the syringe. She decided to adjust her range to become 20mm to 60mm.

Mark noticed that the lamp was heating up the syringe. He suggested that heat would make the sodium hydrogen carbonate solution expand forcing the bubble faster along the capillary tube. He decided to place a beaker of cold water between the lamp and the syringe to act as a heatsink. he also suggested to measure the temperature of the sodium hydrogen carbonate solution to make sure that it was controlled.

Sanjeev was measuring how far the meniscus would move in five minutes. After four minutes the meniscus went past the end of the ruler, he decided to reduce the time for each test to three minutes.

Method:

Read the above preliminary work, then using all the information you have been given write a systematic method using the five points for a perfect plan.

Results:

Design a results table to collect sufficient accurate data.

ALN Supplied Method:

1/ Log on to your laptop.

2/ From the programmes menu go to Science and the Focus Educational.

3/ From the list of investigations choose Photosynthesis.

4/ Set the temperature to 25oC.

5/ Set the NaHCO3 concentration to 1.00%

6/ Place the lamp 20mm away from the Elodea.

7/ Click the start icon.

8/ When the oxygen bubbles have been released pull back on the plunger of the syringe to pull the air bubble into the capillary tube.

9/ Set the air bubble in an easy place to measure it’s length and then record it’s length to the nearest mm.

10/ Repeat steps 6-9 using the same settings for temperature and NaHCO3 concentration but increase the distance of the lamp to 30mm, 40mm, 50mm and 60mm consecutively.

ALN Supplied Results table Level 4:

Distance of lamp from Elodea / mm / Group 1
Length of oxygen bubble / mm / Group 2 Length of oxygen bubble / mm / Group 3 Length of oxygen bubble / mm / Average
Length of oxygen bubble / mm
20
30
40
50
60

Graph:

Plot a graph of your data. Make sure you select the correct graph for the data type and try to meet your graph drawing targets.

Conclusion:

The further away the lamp gets from the Elodea the less oxygen produced. It seems from my graph that when I doubled the distance of the light, the volume of oxygen is halved.

This is not what I would expect based on my understanding of the science involved. Four photons are needed for the chlorophyll to produce one molecule of oxygen. The inverse square law tells me that; if I double the distance of the light source the number of photons hitting the same area of leaf should be one quarter of the original value, I would therefore expect the volume of oxygen to be a quarter of the original when the distance of the lamp is doubled.

I predict that if the room was totally dark and the lamp produced no heat, then the inverse square law would be obeyed.

Evaluation:

My results seem very reliable with a very small spread however my results differ significantly from the accepted scientific model. There could be a few explanations for my results differing from the accepted scientific model:

The room in which the experiment was conducted was not completely dark therefore background light intensity was biasing the results.

The lamp causes the temperature of the sodium hydrogen carbonate solution to increase during the experiment. The closer the lamp the higher the temperature rise, the rate of photosynthesis is temperature dependent. I therefore believe my results are invalid and the experiment should be repeated with no background light and a heat sink placed between the lamp and the elodea.