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Photosynthesis Essay, Research Paper

Photosynthesis is a very important process in nature. It is the production of energy in the form of glucose involving water from the soil, carbon dioxide from the air and light energy. It takes place in all green plants, which use the green chlorophyll, held in chloroplasts in the leaves, to trap light. The main site of photosynthesis is the palisade mesophyll cells in the leaf of a plant. It is these cells that contain the green chloroplasts and are very well adapted to their task. They are near the upper side of the leaf where they can obtain the maximum amount of light, they are packed very closely together and as already mentioned contain green chloroplasts clustered towards the upper side too.

Plants photosynthesise to produce food chemicals that are needed to allow them to grow. The main reaction is to produce oxygen and glucose to be changed into energy during respiration. Glucose is stored in the form of starch which is insoluble and does not affect the osmosis taking pace in the plant. As plants respire both day and night this starch is often used up during the night when photosynthesis cannot take place. The uses of glucose within the plant are for active transpiration, cell division, the production of protein and the production of cellulose. However many other things can also be produced with the addition of special mineral salts.

In photosynthesis the raw materials are carbon dioxide and water. They react to form the products of the reaction-oxygen and starch (glucose that has been stored). The reactions need energy and this comes from light. The green chloroplasts allow light to be used as energy and therefore both of these things are like helpers in the reaction. Glucose is formed firstly then turned into starch to be stored up for when it is needed.

Although photosynthesis is a complicated process it can be summed up in this equation: 6CO2 + 6H3O C6H22O6 + 6C2

carbon dioxide water glucose oxygen It is important to the reaction that certain factors are present when it is occurring. We know that these are carbon dioxide, water, light and chlorophyll. Without these the reaction will not take place at all, but some of them also determine how quickly the reaction takes place. Water, carbon dioxide and light, along with temperature, all have a particular effect on the rate of photosynthesis. In terms of carbon dioxide the levels in the atmosphere do not really alter very much, but if gardeners wish to increase the rate of photosynthesis then sometimes carbon dioxide is pumped into greenhouses. Up to a certain point as temperature goes up so does the rate of reaction. After it reaches a certain point though the enzymes involved in the reaction become denatured and stop working properly. A drop in the amount of water present may cause photosynthesis to occur at only half the normal rate. The reason for this is the stomata are being closed.

The final factor which contributes is light. We decided to investigate how this affects the rate of reaction also. METHOD We need to find out how the of presence light and the intensity of it contributes to the rate of photosynthesis. To be able to measure the rate we need some type of visible sign that photosynthesis is actually taking place. We will use a type of plant that grows in water and produces bubbles when photosynthesising. By counting these bubbles we can tell how fast oxygen is being given off and therefore produced from photosynthesis. We will place the pondweed in a beaker containing water and also a bit of sodium hydrogen carbonate-NaHCO3-(0.5%). This is put in as it acts as carbon dioxide. If it wasn’t there then another limiting factor may be the cause of the rate changing instead of just light.

By placing the beaker next to a lamp we can alter the light intensity. We will move the lamp further away every time and then count the number of bubbles that are produced within one minute. The weed will be given two minutes each time to adjust to the new level of light intensity. To start with the lamp will be 1cm away from the beaker, then the following distances: 2cm



16cm The diagram will help to explain this more clearly.

The rate of reaction will be in number of bubbles per minute (b.p.m).

VARIABLES AND CONSTANTS The factor that will be changed is light intensity. This is the only factor that will be changed. The factors that will be kept constant are the amount of water the weed is put in, carbon dioxide levels, lamp that is used and temperature. This means that out of all the possible factors we have chosen only one to monitor. PREDICTION I predict that as the light intensity is increased the rate of photosynthesis will also increase. However at a certain point the light will reach a certain point where the rate will not increase any more. The chloroplasts will no longer be able to absorb any light so the rate will stay at its optimum level or even decrease. At this point light is no longer limiting.

The graph of results will probably look something like this:

Light is limiting at this point Maximum rate of photosynthesis

light is no longer limiting.








The last set of results is very anomalous and we won’t be using it for our results. And here are the averages of these results.






1614.88 ANALYSIS This is a graph of the averages. The light intensity for the distances used will be shown in the following units: 1cm- 1000 units

2cm- 250 units

4cm- 62.5 units

8cm- 15.6 units

16cm- 3.9 units

As you can see our results have turned out quite similarly to how we expected. In the first table of results there are some slightly different results according to the different experiments that were done. This shows that it can’t have been 100% reliable. It does prove however that as light intensity is increased the rate of photosynthesis is increased also. This is because the more light there is available the more light the chloroplasts can absorb. They use this light in the reaction as energy; therefore the more energy there is available the faster the reaction can take place.


On the graph there wasn’t a point where the rate started to level off. We assumed that this would happen, as the chloroplasts would not be able to absorb any more light energy. However this did not happen so it may be that we did not take the pondweed close enough to the light so it would reach a point where the rate could no longer increase.

There was one set of results in the first table that I decided to leave out. These results were very unusual and anomalous so to include them would have greatly affected the average. I felt it was best to leave them out so they would not give us results that were inaccurate.

These anomalous results, among others, can be explained by many things. First off all our experiment was not completely fair. We did not attempt to regulate the temperature as well as we could have done which as we know is a limiting factor of photosynthesis. We could have put the test tube into a beaker filled with water of a certain temperature. This would have helped to regulate the temperature so we would have been certain that light was the only limiting factor. Also the size of the pieces of pondweed were not all the same so some people may have achieved different results depending on the size of their pondweed and therefore how much surface area was available for photosynthesis to take place in the palisade mesophyll cells. The distance may not have been completely accurately measured and we could also have taken each set of results twice to make doubly sure we were getting an accurate picture.

I think that another way we could have gone about doing this experiment would have been to use a method where the amount of carbon dioxide being produced displaces some water held in a burette. The experiment could be set up as shown:

This would help to give us a greater idea of how much carbon dioxide is being produced and therefore how fast photosynthesis is occuring. As the burette fills with the gas water is displaced and the level drops. By measuring the level every 10 second we would easily be able to work out the rate of the reaction in the pondweed.

I think that overall our evidence is very reliable and that our results show what we thought they would. It could have been more accurate than it is but I think we achieved what we set out to do which was prove that as light intensity is increased photosynthesis speeds up.


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