A Level Biology Project
I plan to investigate how environmental conditions affect the transpiration of plants.
This is an A-level biology project. It helped me get an A grade for biology many years ago. The whole project is reproduced here for your reference.
The Sun provides the energy to turn the water in the plants into a vapour causing it to evaporate into the leave’s internal air spaces before diffusing out of the stomata into the air. This is known as transpiration. As the water evaporates out of the top of the plant it creates a suction on the column of water below it in the xylem. The upwards force on the column of water created by transpiration and the downwards force due to gravity created a tension in the column of water.
As the upwards pull is greater than the downwards pull the column of water moves up the xylem. Cohesion tension theory tells us that it is the evaporation of water from the leaves which causes the upwards movement of water. The water molecules have a high cohesion as they are polar and so are electrically attracted to each other. They are held together by hydrogen bonds. The column of water does not tend to break as it has a very high tensile strength from the bonds.
The water is able to evaporate out of the leaf as the leaf has a high water potential and the air has a low water potential so the water molecules pass down the concentration gradient from the spongy and palisade mesophyll cells into the leave’s internal air spaces before diffusing out into the air.
Transpiration is needed to keep the cells of the spongy and palisade mesophyll cells moist as this allows carbon dioxide to dissolve before diffusing into the cells for photosynthesis. The stomata open in the day to let carbon dioxide diffuse in, and to let oxygen diffuse out as part of photosynthesis. At night photosynthesis is unable to take place due to the absence of light so the stomata are closed to reduce water loss.
Light causes potassium ions to be pumped into the guard cells which lowers their water potential and so water diffuses into the guard cells causing them to go turgid and so open. At night potassium moves out of the guard cells into the surrounding cells so the water diffuses out of the guard cells causing them to close.
In hot climates the water loss by transpiration can exceed the water uptake from the roots which causes the plants to suffer from water stress. To combat this ABA is produced by the plant which causes the rapid pumping of potassium ions out of the guard cells which closes them and so reduces the water loss by transpiration.
- Privet shoot (used as it has many big leaves)
- Capillary tubing (1.13mm Diameter)
- Bowl of water
For this experiment a simple potometer will be made to measure the rate of water uptake from the plant. The potometer is the easiest way of measuring this and is quite accurate as well. The experiment is limited by the fact that the potometer measures the total water uptake and not just the transpiration rate.
I am planning to use the following method for my experiment.
1. A privet shoot is cut under water in a large bowl about 1 inch up the stem. This should remove any blockages that are within the xylem when the shoot was originally cut. In order to stop the xylem vessels from being crushed when the shoot is cut, it should be cut diagonally with a razor blade. The end of the privet shoot should be left in the bowl of water that it was cut in order to prevent any air bubbles getting into the system.
2. The capillary tube should be completely filled with water by submerging it into the same bowl as the privet shoot.
3. The capillary tube should be attached to the shoot underwater being careful to make sure that there are no air bubbles in the tube.
4. The joins between the capillary tube and the shoot should be sealed with Vaseline to make sure that the system is water tight.
5. Making sure that the open end of the capillary tube remains underwater the plant can be raised out of the water and clamped above the bowl of water in an upright position.
6. The whole system should now be full of water and completely air tight. As the plant transpires it will pull water up through the tubing. This apparatus will measure the total water uptake.
7. Allow the apparatus to equilibrate for about 5 minutes. As this is the control experiment the leaves should be exposed to the maximum possible light intensity by using a bench lamp. There should also be no wind in the area that the experiment is taking place.
8. Introduce an air bubble to the system by holding the apparatus out of the water until a bubble forms.
9. Measure how far the bubble has risen up the capillary tube every thirty seconds for five minutes. When measuring the distances your eye should be at a 90º angle from the bottom of the bubble. This way parallax error can be avoided when looking at the scale on the ruler. You also must measure from the same point of bubble every time in order to ensure consistency in the measurement taking. I am planning to take all my measurements from the bottom of the bubble. The distances travelled should be noted.
Diagram Of The Potometer [not reproduced]
The test should be repeated 5 or 6 more times so that an accurate average of the rate of water uptake can be obtained. After each experiment the tubing can be refilled with water by placing the end of the tube underwater and squeezing it to force the air bubble out of the tube.
10. The air temperature of the room should also be noted. This test will give the rate of transpiration under normal inside conditions. These set of results are a control set of results against which the other results can be compared.
It is very important that the whole apparatus is air tight as if any bubbles of air were to get into the xylem then transpiration would not occur. This is because any air bubble would break the continuous column of water which is usually present in the xylem and so the water molecules below the air bubble would not feel the upwards pull from the above water molecules. The xylem would soon fill with air in a process called cavitation. The plant is cut under water an inch up it’s stem in order to get rid of the bottom part of the xylem which may have an air bubble in it. Doing this makes sure that the shoot which will be used in the experiment has water in it’s xylem.
The experiment can then be repeated but this time changing the following environmental conditions.
To measure how wind affects transpiration a hairdryer or fan that blows cold air can be used to blow the leaves. The fan has to be fairly close to the leaves but not so close that it will buffet the leaves as this may cause the stomata to close.
The shoot will be enclosed in a transparent plastic bag. The humidity will soon increase as the water vapour which has been transpired will not be able to leave the bag and so will stay around the plant thereby increasing the humidity.
The windows will be covered and the lights dimmed to make the conditions darker.
The surface area of the leaves of the plant can be reduced by vasolining the leaves on the top and especially to the bottom side of the leaf. The bottom side of the leaf is where the stomata are so this is where most of the water loss in the plant occurs. Applying Vaseline to the bottom surface of the leaf will block the stomata and so water vapour will be unable to leave the leaf through this leaf. The effect that this has on the rate of transpiration can be measured. The surface area of the leaf that has just been vaselined can be measured by drawing around the leaf on some graph paper. The surface area can be worked out by counting the squares. This vaselining technique can be used on more and more leaves till all the plant’s leaves are vaselined.
Each experiment should be replicated several times in order to obtain more accurate data and to get rid of any anomalies that may occur in a single experiment. I will note the results in a table for each 30 second period in all the experiments.
From these recorded distances I will work out the rate of water uptake for each 30 second period for all the experiments. Using these rates I plan to plot several graphs of the rate of water uptake against the time for various environmental conditions. I shall plot the results for the same environmental factors on the same graph so that the effect that these environmental factors has on the rate can easily be compared.
I shall also work out the volume of water taken up per minute. This can be worked out with the following equation.
Volume Of Water Uptake = p x Radius Of The Capillary Tube² x Distance Travelled
I predict that if the wind is increased the rate of transpiration will increase. In the control test where there will be no wind a band a water vapour will be able to form in the air spaces of the leaf and around it as water transpires out. This will reduce the water potential gradient between inside the leaf and the air so the rate of transpiration will be reduced. If air is blown across the leaves by the wind or in the case of this experiment a hairdryer, this band of water vapour will be blown away and further water vapour will not be able to accumulate. This will lead to an increase in the water potential gradient between the inside of the leaf and the air and so the rate at which water transpires into the air will increase.
I predict that if the surface area of the plant’s leaves available for transpiration is halved then the rate of transpiration will be halved. This is because the plant loses almost all it’s water through it’s leaves so if it loses the ability to transpire through half of it’s leaves then the plant will only lose half as much water through transpiration. The amount of surface area that the plant has should be directly proportional to the rate of transpiration.
Secondary results that were taken in January 1998 under controlled conditions were provided. The environmental conditions were measured with electronic probes. These results showed the distance that the air bubble had travelled up the capillary tube over a period of 4 minutes.
Using these results I worked out the rate of travel up the capillary tube for all the experiments. I also averaged the duplicated experiments to give me an average set of results for experiments 14 and 15, and also an average of experiments 11 and 12. I averaged experiments 1, 2 and 9 to produce control data for the rate of travel up the capillary tube against which the other results can be compared.
The sheet of secondary results is shown on the next page. The results table with my extra calculations for the rates of uptake and the averages of the duplicated experiments are shown on the page after that. [not reproduced]
Graphs of the rate of uptake against time were drawn by me for the surface area of the leaves and the wind speed.
I will analyse the results for the surface area experiments and the wind speed experiments.
The graph shows that as the surface area goes down, the rate of transpiration goes down. This would seem to prove my prediction. When the surface area of the plant’s leaves available for transpiration is reduced, the rate of transpiration is also reduced. When the surface area of the leaves is reduced from 5058mm² to 2773mm² (which is a decrease of 46%), the rate of transpiration decreases 50% which would seem to suggest a directly proportional relationship between the surface area and the rate of transpiration. However, when the surface area is reduced by 76% from (2773mm² to 659mm²) the rate of water uptake only reduces by 33%. This would suggest that perhaps the rate of transpiration is reducing at a directly proportional rate but that the rest of the water is being used for photosynthesis. When the plant had a large surface area only a small percentage of water was lost due to photosynthesis. When the plant’s surface area for transpiration was reduced the effect of photosynthesis on the water loss was a lot more noticeable.
Photosynthesis was able to continue because the leaves were still on the plant and receiving light, they must have had enough carbon dioxide left in the photosynthesising cells for photosynthesis to continue after the stomata had been blocked by the vasoline. If the plant had been left for a longer period of time then the carbon dioxide levels in the photosynthesising cells would have dropped and so the rate of photosynthesis would also have reduced.
The main anomaly with this graph is that it shows the control results, (surface area of 5997mm²) with a slower rate of water uptake than the experiment where the plant had a leaf surface area of 5058mm².
By looking at the provided raw data sheet it can be seen that the first two control experiments with a surface area of 5997mm² (1 and 2) would give an average rate of 0.9cm/minute which would make the rate higher than the experiment where the surface area was 5058mm². However it is the third control experiment (number 9) which drags the average rate down to below that of the experiment with less surface area. I believe the reason the rate for control experiment 9 being significantly lower than experiment 1 and 2 is due to experiment 9 being done straight after experiment 8 where the light level was reduced to 21%. This reduced light level will have slowed down the rate of transpiration to the extent that it had not had time to recover when experiment 9 (the third control experiment) was started. The reduced light level would have caused the stomata to close which would have resulted in less water being able to transpire out of the stomata. Therefore experiment 9 produced a lower average than that of 1 and 2.
Wind moving past the leaves caused a large increase in the rate of water uptake. When there was no wind in the control experiments the average rate of water uptake was 0.85cm/minute. When the wind was added at 0.98m/s, the transpiration rate increased to1.1cm/minute, (an increase of 23%).
As I predicted, increasing the wind increases the water uptake rate. Increasing the wind speed to 1.1m/second further increased the transpiration rate to 1.13cm/second. The rate was much slower in the control test because in the absence of wind, a band of water vapour was able to form in the internal air spaces of the leaf and around the leaf as the water vapour transpired out. This reduced the water potential gradient between the spongy and palisade mesophyll cells inside the leaf and the air so the rate of transpiration was reduced. When air was blown across the leaves by the hairdryer, this band of water vapour was blown away and further water vapour was not be able to accumulate. This led to an increase in the water potential gradient between the spongy and palisade mesophyll cells inside the leaf and the air and so the rate of transpiration into the air increased.
a) The experiment measures the total water uptake which includes water used by the plant as well as the water lost through transpiration. Though most of the water (90 - 99%) is lost through transpiration, a small amount is used by the plant for photosynthesis and so this experiment is not able to tell us the exact rate of transpiration.
b) It is often difficult to change just one environmental condition without changing another. For example the lamps used to give the leaves the maximum possible light intensity in the experiment will also have the effect of slightly heating up the leaves. This could cause the rate of transpiration to go up.
c) These experiments were carried out one after another without leaving enough time for the plant to equilibrate after each experiment. For example, after the light intensity had been reduced to 21% in experiment 8, not enough time was allowed after it for the plants transpiration rate to rise to it’s normal level. Therefore, experiment 9 which was a control experiment produced a significantly lower average rate than the control experiments 1 and 2.
d) There was a general deterioration in the results as the experiments went on. As the experiments were carried out the water would have warmed up to room temperature. This will have caused bubbles to form in the xylem as the air started to dissolve out of it. These air bubbles could have caused a loss of cohesion tension in the leaves, which may have lead to cavitation in the xylem. This cavitation would have reduced the number of working xylem in the stem and could account for the reduced rated of uptake in later experiments.
This experiment could be improved in a number of ways.
1) It could be repeated more times to help get rid of any anomalies. A better overall result would be obtained by repeating the experiment more times because any errors in one experiment should be compensated for by the other experiments.
2) More time should have been allowed between experiments to allow the plant to fully equilibrate before the next experiment was started. This would have reduced the number of anomalies caused by the sudden exposure of the plant to completely different conditions when the previous effects of the last experiment’s conditions had not had time to wear off.
3) The water used in the bowl could have been allowed to warm up to room temperature before the experiment began. This would have stopped the air in the water from dissolving out and leading to cavitation in the xylem vessels.
4) A different method of carrying out the experiment could have been devised that would measure the rate of transpiration instead of the total water uptake.
This is a real A-level school project and as such is intended for educational or research purposes only. Extracts of this project must not be included in any projects that you submit for marking. Doing this could lead to being disqualified from all the subjects that you are taking. You have been warned. If you want more help with doing your biology practicals then have a look at 'Advanced Level Practical Work for Biology' by Sally Morgan. If you want more detailed biology information then I'd recommend the book 'Advanced Biology' by M. Kent.