The Rate of Photosynthesis and its Factors
Biology Investigation
The Rate of Photosynthesis and its Factors
Introduction
The aim of our investigation is to find out what factors affect rate of photosynthesis and what happens when you alter one of these factors.
The key factors affecting photosynthesis are availability of Carbon Dioxide, water, light, chlorophyll and heat. Carbon Dioxide is taken in by green plants for use in photosynthesis, and oxygen is released. If the Carbon Dioxide supply is taken away then photosynthesis may only take place until the Carbon Dioxide previously absorbed has been utilised in the reaction.
Water also supplies minerals and nutrients for the plant, which are crucial for photosynthesis.
Light is essential; it is captured by the chlorophyll and its energy used to build up sugar. Photosynthesis, therefore is the system used to obtain energy. Chlorophyll is contained in the chloroplasts of a leaf cell, which is where the light comes into contact with the plant. Chlorophyll is what causes leaves to appear green; it reflects green light while it absorbs others. Heat affects photosynthesis since it speeds up chemical reactions by causing particles to move more quickly, but if the temperature is above 40 degrees the enzymes in the reaction begin to denature.
The key factor we have chosen to test is light. By changing the amount of light available to some elodea (a weed found in fresh water, usually ponds) we should be able to increase or decrease its rate of photosynthesis. I predict that the closer the light, the more rapid the rate of photosynthesis and release of oxygen; the further away the light source the less oxygen is released so the slower the rate of the photosynthesis. This would mean that the distance from the light source is a factor in photosynthesis. If the light is in closer proximity there is less time for light rays to disperse before they reach the Elodea. This means the Elodea can absorb more light energy for use in photosynthesis. The faster the rate of photosynthesis, the more waste gas (oxygen) is produced, pushing up the water level in the capillary tube.
Preliminary Testing
We carried out a preliminary test to ensure our experiment would work, and that we could make it as fair and accurate as possible. We set up the equipment as it would be in the final experiment. We clamped a boiling tube of 1% Sodium Hydrogen Carbonate solution to a stand and inserted a clipping of Elodea. Sodium Hydrogen Carbonate solution is use as it contains far more carbon dioxide than water, so the Elodea will not use up all the carbon dioxide available and the test will be more fair due to the plentiful supply of this vital reactant at all times. With a beaker underneath, we inserted the bung and capillary tube. We adjusted the water level until there was room for it to rise. We then plugged in the bulb and put it 40cm from the Elodea. Timing for 5 minutes, we recorded the rise in water level. We repeated the process with 30cm, 20cm and 10cm distance from the bulb.
Distance from Elodea to light source
Millimetres of rise in water level within capillary tube
0cm
21mm
20cm
1mm
30cm
9mm
40cm
3mm
As you can see, the rise in capillary tube water level rose as the light source was brought towards the Elodea. So it appears the rate of photosynthesis is being affected by the distance it is from the light source, and this indicates that my prediction may be correct. Our results have helped us to decide the best measurements to use for the final experiment. ...
This is a preview of the whole essay
Distance from Elodea to light source
Millimetres of rise in water level within capillary tube
0cm
21mm
20cm
1mm
30cm
9mm
40cm
3mm
As you can see, the rise in capillary tube water level rose as the light source was brought towards the Elodea. So it appears the rate of photosynthesis is being affected by the distance it is from the light source, and this indicates that my prediction may be correct. Our results have helped us to decide the best measurements to use for the final experiment. In respect to our preliminary results, we have decided to test the distance from the light source in units of 5. From 5-25cm in distance; any greater distance and the differences in the amount of oxygen produced are so small they are less worth measuring.
Apparatus
In our experiment we will need to use a clamp and stand, a boiling tube, an elodea clipping 10cm in length, a light source in the form of a bulb, a beaker, a capillary tube with bung and some 1% Sodium Hydrogen carbonate solution.
Method
*Fill the beaker with the 1% Sodium Hydrogen Carbonate solution.
*Clamp the boiling tube about 15cm up the stand. Fill it with solution.
*Underwater, take a clipping 10cm long of Elodea and submerge it immediately into the beaker of solution.
*Take the Elodea from the beaker and insert it into the boiling tube.*
Place the beaker under the boiling tube to catch excess solution.
*Press the bung of the capillary tube as far into the top of the boiling tube as it will go. Some solution should flow over the top of the capillary tube.
*Turn the bung and the water level should fall enough to allow a rise due to oxygen release.
*Plug in the light source and switch it on. Position it near the stand.
*Leave for a while to ensure photosynthesis is already in action.
*Move the light to 5cm from the base of the light to the beaker beneath the boiling tube.
*Mark off the water level in the capillary tube and then leave for 5 minutes.
*Record the rise in water level and repeat with 10cm, 15cm, 20cm and 25cm.
Our range is from 5cm-25cm. This is the only variation in our experiment. To make our experiment as accurate as possible we will use the same piece of Elodea throughout, the same bulb and leave the photosynthesis to take place for exactly 5 minutes. We will also allow some time for the photosynthesising to change pace once we have altered the distance from the bulb. On top of making the test accurate we have to ensure the test is also fair. The fairness of the test also contributes to the accuracy of the experiment and its results. So to keep the test fair we will keep all key factors other than light constant throughout. We will use 1% solution of Sodium Hydrogen Carbonate for the whole experiment, which will provide the Elodea with more than enough Carbon Dioxide for its photosynthesis. I will not alter the heat, the room temperature is unlikely to change much, if at all, while the experiment takes place. Since the same piece of Elodea is being used throughout the amount of chlorophyll present in the reaction will not alter. The solution in which the Elodea is submerged will not be changed at any point in the experiment. It will not be added to or have any removed. These key factors must remain the same so that it is only the differences in light we are testing, and nothing else will be affecting our results.
There are few safety issues with our experiment, since we are not handling any harmful substances. Our only precaution are to handle our equipment with care and to ensure we don't let the hot bulb come into contact with any liquids as this may cause the bulb to explode. We should also be cautious when cutting the Elodea, as it is underwater and our visibility is distorted. To obtain information I have used two textbooks- 'Biology For Life' by M.B.V Roberts and 'The Mind Alive Encyclopedia of Living Things' by Marshall Cavendish. I have used this information to give myself a better understanding of what affects the process of photosynthesis, allowing me to make a more informed prediction.
Rise in Water Level of Capillary Tube According to Distance from Light Source
Distance from Elodea to light source
(cm)
trial 1
trial 2
trial 3
trial 4
average
5cm
9mm
7mm
8mm
8mm
8mm
0cm
5mm
5mm
1mm
1mm
3mm
5cm
3mm
2mm
1mm
1mm
2mm
20cm
8mm
7mm
8mm
9mm
8mm
25cm
5mm
4mm
3mm
4mm
4mm
If you add the four results from the trials together and then divide the answer by four, you will have the average result.
Analysis
My results show that as the Elodea is brought closer to the light, it photosynthesises more rapidly, producing more oxygen. With each 5cm move closer to the light, the average rise in water level increased, showing that more light was being absorbed and proving the rate of photosynthesis had increased. The graph shows that there is a direct relationship between the distance from the light source and the change in the water level of the capillary tube. A best fit straight line can be drawn which rises as it moves from 25cm to 5cm. Had we further extended our experiment into distances above 25cm we could extend this line further and be more certain of our findings. Or if we had used smaller units, 2cm change in distance for example. But since we had to leave the Elodea with time to adjust to its new proximity to the light we didn't have enough time to experiment any further.
A table to show the percentage of oxygen produced by Elodea where the amount produced with the light at a distance of 5cm equals 100%
trial 1
trial 2
trial 3
trial 4
average
5cm
00%
00%
00%
00%
00%
0cm
78.95%
88.24%
61.11%
61.11%
72.35%
5cm
68.42%
70.59%
61.11%
61.11%
65.31%
20cm
42.11%
41.18%
44.44%
50%
44.43%
25cm
26.32%
23.53%
6.67%
22.22%
22.19%
This set of calculations shows that my prediction was correct; that the amount of oxygen produced by the Elodea reduces as the light source is moved further away. This table of percentages makes it easier to see the relative differences in the amount of oxygen produced at different distances from the light source. The only problem there is with our results is that there is quite a difference between the different trials. Particularly at 10cm, where there is a difference of 27% between trials. We'd need to do further experimentation to obtain a more reliable result. But for each 5 cm we move the light away, there is roughly a decrease between averages of 20-25%. But if we moved the light a further 5cm away would the average decrease by another 5% and leave us with no oxygen, meaning no photosynthesis has taken place. We can see from our preliminary testing that this should not be the case. When we moved the light 40cm away, 3mm of oxygen was still produced. We can't use this as a strong argument against the photosynthesis no longer taking place, as the piece of Elodea we had used in our preliminary tests was more successful in its rate of photosynthesis, and released more oxygen.
Our results have been conclusive though, and suggest that the prediction we made was correct. The closer the Elodea was to the light source the more oxygen was released and the more rapid the rate of photosynthesis.
Conclusion
We have been able to show that distance from light source has a direct effect on the rate of photosynthesis. That when distance increases, the rate of photosynthesis slows, producing less oxygen. We would require further experimentation to prove that the drop in percentage of oxygen produced is proportional to the distance from the light source.
Evaluation
We have some anomalous results which show up on our table and our graph. This occurs where the light source is 10cm away from the Elodea. The amount of oxygen produced varied by nearly 30% between trials. This could have been due to us being surrounded by other experiments in progress; there were several light bulbs close by, and natural light from a nearby window. We purposely turned our experiment away from the light of the window, and as the copious amounts of oxygen released by our colleagues' Elodea suggests, this appears to have an effect. Had we either used a larger piece of Elodea or collected the oxygen for a longer period of time we would have had a greater rise in the water level of the capillary tube, which would have been less difficult to measure.
In hindsight we should have allowed a fixed amount of time for the Elodea to acclimatise to each new distance from the light source before we began measuring. We, however, just left it for a while until we thought it had adjusted. Another factor which needs to be considered is the complexity of biological systems, and the numerous factors which affect them. Our results were reliable enough to form the conclusion that light affects photosynthesis, and that as the distance from the light source increases the rate of photosynthesis decreases. However, the results were not good enough to say by how much with any certainty. We would need to do a lot of further experiments to prove the direct relationship between rate of photosynthesis and the distance from the light source. We also should consider that we've only used one plant species to test our prediction. We would need to experiment with other species to develop a more accurate result.
