Temperature affects the rate of photosynthesis because the enzymes are only activated when they reach a certain temperature, and if they reach too high a temperature they are denatured, so they no longer work correctly. If enzymes are denatured their shape and size is altered and they do not join correctly to carbon dioxide and water to convert it into oxygen and glucose. Below is a graph showing the optimum temperature for enzyme activity.
Concentration of carbon dioxide and the amount of water supplied affect the rate of photosynthesis because carbon dioxide and water are both needed for photosynthesis to occur. If the concentration of carbon dioxide and the amount of water supplied are both increased, providing the light intensity is sufficient, the rate of photosynthesis will increase. However, if either the concentration of carbon dioxide or the amount of water supplied are increased in isolation from the other, the rate of photosynthesis will only increase up to a point where the other variable is used up, and then no longer have an effect. In addition, the stomata remaining open, and therefore intake of carbon dioxide, relies on the cell being supplied with enough water. If there is not enough water the stomata will shut, and carbon dioxide will not enter the cell, therefore photosynthesis will not take place.
The surface area of the leaves affects the rate of photosynthesis because the more energy, carbon dioxide and water supplied to the cells, the faster the rate of photosynthesis. If there is a larger surface area there is more space for light energy to be absorbed, and more stomata on the surface for carbon dioxide to diffuse through, so the rate of photosynthesis will increase.
The age of the plant affects the rate of photosynthesis because a young plant is more likely to be healthy, and able to carry out photosynthesis more efficiently, which means the rate of photosynthesis will be faster. An old plant may be dying, and therefore unable to convert carbon dioxide and water at a steady rate. Another possible cause of an unhealthy plant would be, for example, magnesium deficiency, and this may also slow down the rate of photosynthesis.
The amount of chlorophyll in the chloroplasts of the palisade cell affects the rate of photosynthesis because it is the chlorophyll that converts the light energy into chemical energy that the plant can use during photosynthesis. If there is more chlorophyll, more sunlight can be converted into usable chemical energy, so the rate of photosynthesis will increase.
Finally, the colour of the light, or length of wavelength, will affect the rate of photosynthesis because the more light absorbed by the plant, the greater the rate of photosynthesis. Plants reflect green light, which is why they look green, and absorb blue and red light. Therefore, if the only light reaching the plant was blue or red the rate of photosynthesis would be very fast. If the only light reaching the plant was green light, there would be no photosynthesis, as no light energy would be absorbed by the plant.
The law of limiting factors states that each factor on its own will increase photosynthesis up to a point. After that point is reached, continuing to increase this factor will have no affect on the rate of photosynthesis, and to further increase the rate of photosynthesis you must alter one of the other factors.
Law of limiting factors
The factor I am going to investigate is light intensity. I predict that as the light intensity increases the rate of photosynthesis will increase. However, when this reaches a certain point the rate of photosynthesis will no longer continue to increase because either the chlorophyll pigment had been damaged by the very high light intensity, or because, even though the light intensity was enough for a very fast rate of photosynthesis, the concentration of carbon dioxide was not high enough to photosynthesise at this rate. The rate of photosynthesis will increase as light intensity increases, up to this point, because light is needed for photosynthesis, and if you increase the intensity then more energy is being supplied to the cells every second, which is then used to convert carbon dioxide and water into glucose and oxygen. I predict that the inverse square law will apply to my experiment.
When y = number of bubbles released, and x = distance from the plant, the formula for the calculation will be y = 1
x2.
This means that if I double the distance of the lamp from the Elodea, the number of bubbles released per minute will halve. If I then move the elodea further from the lamp, by the same distance as previously, the number of bubbles released per minute will once again halve.
Preliminary work
For my preliminary work I am going to find the maximum and minimum light intensity until photosynthesis is no longer affected. I am going to use the equipment in the diagram, and follow the method below.
Method: -
- Begin with the lamp switched off. Start the stop-clock and for 1 minute count the number of bubbles given off from the Elodea. These are bubbles of oxygen. This is called the control.
- Begin with the lamp switched on, 1 metre away from the elodea. Start the stop-clock. For 1 minute count the number of bubbles given off from the Elodea.
- If the number of bubbles is lower than the control figure, repeat the experiment at a greater distance of the lamp from the Elodea. Continue to do this at 10cm intervals until the number of bubbles released is the same as the control.
- If the number of bubbles is the same as the control, repeat the experiment decreasing the distance of the lamp from the Elodea at 10cm intervals. Continue to do this until the number of bubbles is less than the control. The value 10cm lower than the last place equivalent to the control is the minimum light intensity.
- Place the lamp 20cm from the Elodea. Start the stop-clock and count the number of bubbles released from the Elodea for 1 minute. Decrease the distance of the lamp from the elodea at 2cm intervals, and repeat the experiment at each distance. When the number of bubbles released starts to decrease, the previous value was the maximum light intensity because the chloroplasts in the Elodea have been damaged. I will repeat the experiment 3 times at each distance so I can take an average of the results.
Preliminary results: -
Planning
I will make my experiment a fair test by using the same amount of Elodea at each light intensity, keeping the temperature of the water the same by using a beaker of water as a heat barrier, and by keeping the concentration of carbon dioxide the same. The only factor I will change is the light intensity, by altering the distance of the lamp from the Elodea.
I am going to use the same apparatus as I used in my preliminary work. In my preliminary experiment I discovered that when there was no light no bubbles of oxygen were given off, so there was no photosynthesis. I found that the minimum light intensity was when the lamp was 45cm from the Elodea, and the maximum light intensity was when the lamp was 10cm from the elodea. Using these results I am going to record the number of oxygen bubbles released during photosynthesis at 5cm intervals, from 10cm to 45cm. The distances I will use of the lamp from the elodea are 10cm, 15cm, 20cm, 25cm, 30cm, 35cm, 40cm, and 45cm.
Method: -
- Set up apparatus in a darkened room.
- Place lamp 45cm from the Elodea and switch it on. Start the timer immediately, and count the number of bubbles appearing in the test tube for 1 minute.
- Repeat this experiment until you reach 10cm, decreasing the distance of the lamp from the elodea at 5cm intervals. Move the lamp, not the Elodea.
- After finishing the experiment wash your hands, as many germs may live on the Elodea.
Results: -
Analysis
My results show that the closer the lamp was to the Elodea, the more bubbles were produced, which is what I stated in my prediction. When the graph is furthest away from the Elodea very few bubbles are produced, and the number gradually increases as the lamp is moved closer, so the line of the graph becomes steeper, resulting in a curve. This means that the greater the light intensity, the more oxygen was released, so the faster the rate of photosynthesis. The lower the light intensity, the less oxygen is released, so the rate of photosynthesis is slower.
My graph shows that initially as the distance of the lamp from the Elodea is increased, the graph falls very rapidly. This shows that as the lamp was moved further from the Elodea, light energy was lost on the particles in the air between the lamp and the Elodea, and because as the lamp moved further away some light rays missed the elodea completely. The reason the rate of photosynthesis was faster when the lamp was closer to the Elodea was that when the lamp was closer, more light energy was being focussed on the Elodea. For example, at 10cm from the Elodea 29 bubbles of oxygen were released, but at 20cm only 8 bubbles were released. My second graph of how the inverse square law applies to the rate of photosynthesis shows that the inverse square law does apply to my experiment, as I predicted. This means that if I double the distance of the lamp from the Elodea, the number of bubbles of oxygen released will halve. If I then move the Elodea further from the lamp, by the same distance as previously, the number of bubbles released will again halve. The formula for the number of bubbles released would be y = 1 , where y = number of bubbles and x = distance from Elodea.
x2.
Therefore, theoretically, the number of bubbles of oxygen released at 5cm from the Elodea should have been about 58, and about 116 bubbles released at 0cm from the Elodea. However, at these distances the inverse square law may have no longer applied because of the law of limiting factors, which states that after a certain point is reached, increasing one factor will no longer have any effect, and one of the other factors will need to be increased. For example, increasing the light intensity may have no effect if there is not enough carbon dioxide or water for the reaction to take place at this rate. There was one anomalous result from my experiment, which was not on the line of best fit when I plotted my graph. This was probably due to experimental error, as the result did not concur with my conclusion.
Evaluation
I feel that the procedure I used was satisfactory and my investigation was successful because my results correlated with my prediction, which was based on my research about photosynthesis. My results were reliable because I carried out the experiment 3 times at each distance and took an average, which helped to exclude human error, and I tried to only alter one variable, making it a fair procedure. Also, in my preliminary work I found the range of the results I would have to take.
In my experiment I had one anomalous result, which was at 35cm from the elodea, where 2 bubbles were released. According to my line of best fit, an average of about 0.6 bubbles should have been released. A reason for this could have been human error, because I was required to count the number of bubbles, and it would have been very easy to miss one. Another reason for the anomalous result could have been the equipment used. If I had been able to use more sophisticated equipment, for example, using natural or ultra violet light instead of artificial light, my results may have been more accurate. Controlling the other variables better would also have made my results more accurate. For example, I could have made sure the light intensity was the same each time by measuring it in lux, with a light meter. I could have used Perspex to reduce heat instead of using a beaker of water, as this may have been more effective, and would have also allowed me to continue to move my lamp right up to the beaker. Also, to improve the reliability of my procedure I could collect and measure the volume of gas released in a gas syringe instead of counting bubbles. I could also use a measuring cylinder filled with water and record how much water was displaced by oxygen. This would give me more exact results, as the bubbles may not all be of the same size, therefore allowing me to make a more detailed conclusion. The light from other experiments being carried out in the room could have interfered with my experiment, so an improvement would be to carry out the experiment in a room where the only lighting is from the single lamp, and where there are no other experiments being carried out simultaneously. When doing the experiment I made the assumption that the level of carbon dioxide in the air was always 0.03%. Another improvement I could have made would have been to measure the percentage of carbon dioxide, and make sure it was always the same.
To extend this enquiry I could carry out the experiment at different masses of elodea, to see if the mass of the plant affects the relationship between light intensity and rate of photosynthesis. I could also take all the leaves off the plant, and see how that affects the rate of photosynthesis. To allow me to plot more points on my graph I could take measurements at more regular intervals. Using Perspex instead of a beaker of water would have allowed me a larger range of results, as I would have been able to continue to move the lamp right up to the beaker containing the Elodea.
