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How light intensity effects the rate of photosynthesis Essay

In my investigation I am going to establish the effect of changing light intensity on the rate of photosynthesis. What is Photosynthesis? Photosynthesis is how plants get their food. The reason why plants have leaves, roots or being green is all linked to photosynthesis. In photosynthesis, a plant takes carbon dioxide from the air and water from the soil and uses the light from sunlight to turn them into food. Photosynthesis occurs only in the presence of light, and takes place in the chloroplasts of green plant cells. Photosynthesis can be defined as the production of simple sugars (glucose) from carbon dioxide and water causing the release of sugar and oxygen. Oxygen is also produced in photosynthesis, plants don’t really need oxygen so they give it out as a waste product but saves a little bit to use for respiration. The sunlight is absorbed by a green pigment called chlorophyll. Most of the glucose is turned into a substance called starch.

Word equation for photosynthesis:

Light energy

Carbon dioxide + Water Glucose + Oxygen


Word equation for photosynthesis:

Light energy

6CO + 6H O C H O + 6O

2 2 Chlorophyll 12 6 2

My aim:

My aim is to determine the effect of light intensity on the rate of photosynthesis. To accomplish this I will have to control the amount of water, carbon dioxide and light. Also the distance of the light will have to be controlled by undertaking this there should be different levels of oxygen escaping the plant. The plant will be soaked into water so the oxygen can be measured easily. The oxygen will be measured by the amount of bubbles produced from the plant for a certain amount of time.

Investigate the factors of – Independent variable – I am changing the light intensity.

– Dependent variable – I am measuring the rate of photosynthesis.

– Controlled variable – I am going to keep the level of carbon dioxide the same.

Preliminary experiment

Before doing my actual experiment I have decided to do a preliminary investigation; I have done this to overcome errors and mistakes. I have produced a table and graph to show the effect on the rate of photosynthesis when light intensity is increased or decreased.


For this experiment I predict that the further the light is from the leaf the fewer bubbles will be produced. I have come about this answer because for a fact photosynthesis occurs in light only and the rate of reaction is quicker in a warmer condition, so therefore if this condition is colder then the results will be the opposite which is that fewer bubbles (oxygen) will be produced.


Distance Amount of bubbles Average Rate

1 2 3 4

3cm 18 18 19 18 18.25 36.5

6cm 18 20 19 17 18.5 36.5

9cm 17 18 15 17 16.75 33.5

12cn 11 15 9 11 11.5 23

15cm 11 11 12 12 11.5 23

The table above shows my results after I completed the preliminary experiment. I counted the bubbles 4 times as you can see on the tables to be sure and make the experiment fair, the table shows that the higher the light intensity the faster the rate of reaction, this tables proves my prediction for the actual experiment.

Problems occurred

1) Background light – Light from other sources such as room light may have affected the results.

2) Thermometer – I did not have a thermometer therefore I couldn’t measure the temperature.

3) Miscount of bubbles – I may have miscounted the bubbles therefore my result can be inaccurate.

4) Distance of light – The distance of the light from the plant may not have been accurate; this can be due to that no one else verified that the distance was accurate.


To overcome the problems above I will have to make sure to block off all other light entering the room other than the lamps light, to use a thermometer for accurate temperatures, use more pupils to count the bubbles with me and to make sure the distance of the light from the plant is accurate.

The graph above is an interpretation of the tables above the graph. The graph, in a simpler way, shows that the higher the light intensity the faster the rate of photosynthesis. This is clearly shown when, the light is 3cm away from the plant, the rate of the bubbles is 36.5 and when the light is further away at 15cm the rate of bubbles is 23, as you can see there appears to be a less amount of bubbles produced when the light is further away from the plant, this backs up my prediction which is the higher the light intensity the higher the amount of bubbles produced.


Now I have done my preliminary experiment I will do the actual experiment, hoping it to be more accurate and efficient.

Prediction: – I forecast that the increase in the light intensity will increase the rate of photosynthesis by giving out more oxygen via bubbles. I come about this prediction because I know that the higher the temperature the faster the rate of photosynthesis is and the light produces the heat so I think the higher the intensity of the light the faster reaction there is.

Factors effecting photosynthesis

There are a few factors that effect photosynthesis, they are as follows:

Background light – The amount of light in the room, I won’t be able to do a perfect experiment each time but can get the light in the room around the same level each time we do the experiment this can be done by making sure the whole room is dark and the only light source is the lamp we are using for the test.

Heat from other sources – Heat from sunlight which comes through the window into our classroom also can alter our experiment because it gives out heat which speeds up the reaction making our experiment unfair.

Carbon dioxide – The concentration of Carbon dioxide is another factor that can affect our experiment. This can affect the rate of photosynthesis, since if there is too little or more or carbon dioxide, this matter cannot be solved in any particular way.


Table lamp


Ruler (used to measure the distance between the lamp and the leaf)

Stop watch



Here I have drawn out my method; I was not able to do it on computer as I didn’t know how to so I’ve drawn it instead.


Distance from

Lamp in CM Light intensity

Temperature oC Distance, travel

-length in double Rate distance Amount of oxygen

1 2 3 Avg 1 2 3 Avg 1 2 3 Avg 1 2 3 Avg

3cm 5.7 5.7 5.7 5.7 30 30 30.5 30.1 3.5 19 15.9 12.8 1.2 3% 3% 3% 3%

6cm 5.8 5.8 5.8 5.8 30 30 30 30 7.5 9 19.5 12 1.2 3% 3% 3% 3%

9cm 5.7 5.7 5.7 5.7 30.5 30 30 30.1 20.2 19.5 15.5 18.4 1.8 3% 3% 3% 3%

12cm 5.7 5.7 5.7 5.7 30 30 30 30 11 17.8 20 16.2 1.6 3% 3% 3% 3%

15cm 5.6 5.6 5.6 5.6 30 30 30 30 19.9 13.3 17.7 16.8 1.6 3% 3% 3% 3%

This table shows my results after I completed the actual experiment; I filled in the table as I went along and ended up with good results. In this table compared to the preliminary experiment table has more data, also compared to the table for the preliminary experiment this table shows the amount of light intensity, temperature, distance, travel length in double, rate distance and amount of oxygen all of this data also shows an “Avg” sign after the numbers this sign means average, so, this table is more detailed and accurate than the table for the preliminary experiment.


The two graphs below is an interpretation of the table above, this graph is simpler to read. Graph 1.1 shows the rate distance by light intensity, this graph shows the less amount of light intensity the higher the rate distance. Graph 1.2 shows the amount of bubbles (oxygen) by distance (cm), clearly you can see that the oxygen level remains the same as the distance from the lamp is changed; this was not expected to happen therefore this graph maybe incorrect.


After I have carried out my experiment I have discovered that the light intensity has an effect on the rate of photosynthesis. The effect is that the higher the light intensity the increase in the amount of bubbles produced, this bubble produced from the plant is oxygen.

Why is this? This is because light plays a major role in photosynthesis, it helps the leaf to give out oxygen and take in carbon dioxide from the atmosphere which is polluting our world. Also light helps photosynthesis to produce food for the plants and makes the plant produce fruits or vegetables such as apples and tomatoes.

Photosynthesis comes about when trapped light energy turns into carbon dioxide and water into a solution called glucose and oxygen. During this whole process some oxygen is given off.

About my graph

After I have completed my experiment, I produced a graph which shows that when the light was 3cm away from the plant 18 bubbles were produced and when the light was 6cm away from the plant 17 bubbles were produced. This clearly shows that the higher the light intensity the higher the amount of oxygen (bubbles) produced and the lower the light intensity (the further the light is away from the plant) the lower the amount of oxygen (bubbles) produced.


I feel that my experiment was positive overall; however I thought there were many points at which the accuracy was not perfect such as my preliminary experiment which was not accurate enough to justify being used as my main experiment, mostly due to the fact that I was relying on all the bubbles being the same size, which they clearly weren’t, (I did realise during the experiment there was no way round this matter) however many of the smaller inaccuracies also apply to my main experiment.

Firstly, the distance between the light sources and the plant were not measured to a very high degree of accuracy, especially when you note the fact that the distance should have been measured exactly from the filament of the light bulb to the centre of the plant( I couldn’t actually measure from the filament of the light bulb as glass was around it and the light bulb was very hot), and it is possible to find a percentage error. I estimate that the error could have been up to 0.5cm.

Few other problems that occurred

Background light, heat from other sources and carbon dioxide all played a nuisance during my investigation; background light may have a affected the rate of photosynthesis and heat from other sources may have speeded up the reaction, I realised all this factors above during my preliminary experiment which help me overcome other errors as well so when it came down to the actual experiment itself I knew what errors to overcome and made sure background light and other factors effecting my investigation were dealt with so ill have a fair investigation.

What was learnt?

I and my group all learnt that after an intensive investigation into how the rate of photosynthesis is affected by the amount of light intensity that the higher the light intensity the faster the rate of reaction. We also learnt that if the light intensity was too high than the rate of photosynthesis will not work this is because the enzymes denature if the temperature was to be too high, I did not find this out by doing my investigation, this information was found on the internet after some research.

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