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Biology - How Light Intensity Affects the Rate of Photosynthesis

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Biology Coursework ÐŽV Does The Light Intensity

Affect the Rate of Photosynthesis

The Investigation

In this experiment I will investigate the affect in which the light intensity will have on a plants photosynthesis process. This will be done by measuring the bubbles of oxygen and having a bulb for the light intensity variable.


The input variable which will be used in this investigation will be the light intensity (this will be a 100Watt bulb being moved closer and further away from the plant). The outcome variable which will change as a result of this experiment will be the amount of oxygen made in the experiment due to the amount of light intensity increases (causing the plant to photosynthesise more).


I predict that as the input variable, the light intensity increases (the light moved towards the plant) the outcome variable, the amount of oxygen, produced from photosynthesis will be larger.

Plants can absorb and use light energy because they have a green pigment, chlorophyll, contained in the chloroplasts in some of their cells. Chlorophyll allows the energy in sunlight to drive chemical reactions. Chloroplasts act as a energy transducers, converting light energy into chemical energy. So as the plant has more light the chlorophyll inside the chloroplasts can react faster absorbing in more light for food and energy.

The equation for photosynthesis can help to predict the outcome of the investigation.

Light Energy

Carbon Dioxide + Water --------------„Ñ- Glucose + Oxygen


This shows that there could be three variables in this experiment, carbon dioxide, water and light energy. So in our case the variable light energy (light intensity) will be used. The equation also shows that if there is more light energy then more glucose and oxygen will be produced.

I also predict that as the light is moved closer to the plant there will be more bubbles (oxygen) produced due to the increase of photosynthesis speed explained above. So in conclusion I predict that the more light intensity there is on the plant the faster the rate of photosynthesis there will be.

Fair Testing

The fair testing will be carefully checked so the results do not come out to be void. The light intensity will be changed for it is the only variable used in the experiment. Though the temperature, watt of the bulb, amount of water, size of the plant and position of the clamp and stand. The preliminary experiment I did has helped me to decide in which conditions/measurement I will use. The reason in which I will keep the other variables the same is because if increased they would increase the rate of photosynthesis making the experiment pointless and a waste of time. Also if they decrease then it will be likely that the rate of photosynthesis will decrease as well. Me and my partner in this experiment decided that we will set the timer to start after sixty seconds in which the light has been turned on, we chose this because the photosynthesis rate will have been rising and be at a steady rate to take the amount of bubble recording after one minute. We will be doing each light intensity test three times to get an average (for our graphs).


The apparatus I will be using are:

Clamp Stands

Test Tube

100Watt Bulb and Light

Plastic Screen


Automatic Counter


1 Metre Ruler

Light Meter

Preliminary experiment

The preliminary experiment was done to find out what range of values I will use and details of the measuring instruments. I decided that the amount of water will fill the test tube, the light intensity readings will be at 100cm, 75cm, 50cm and 25cm on the metre ruler, the stop watch will be turned on after 60 seconds of the light been on and the results will be taken three times and put into a results table. The results my partner and I ended up with where:

100Cm between light and plant = 6 bubbles

25Cm between light and plant = 12 bubbles

I will be measuring the light intensity with a light meter at each of the distances. When I do this, I will then find the light intensity by dividing the number which came up on the light meter into 1; this will be the light intensity. During the preliminary experiment I did not come up against any noticeable problems so no solutions where needed.


The plan or method I will be using will start of by placing a plant in a test tube filled with water. This test tube will then be held by a clamp stand and around 100 cmÐŽ¦s across from the clamp stand there will be a lamp with a 100 watt bulb in it. The 100 cm ruler will be placed between the clamp stand and lamp so you know how far to move the lamp after you record the bubbles for each light intensity. The light intensity will by using the light meter to record the readings at 100cm75cm, 50cm and 25cms, then dividing the number which comes up on the light meter into one and the way they will be recorded are as follows:

1) Turn on lamp for 1min

2) Start stop watch after 1min of the light being on

3) For 1min record the bubbles starting at the same time as the stopwatch

4) Write down results, then leave lamp off for 1min then repeat the process for each measurement 3 times.

I will intend to plot a graph. Going up on the graph shall be the amount of bubbles and going across on the X axis will be the measurement. The amount of bubbles recorded on the graph will be the averages for each measurement (the reason for doing three results). The graph title will be ÐŽ§Graph to Show Bubble Amounts



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