# How does light intensity affect the rate of photosynthesis?

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• Published: September 10, 2003

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How does light intensity affect the rate of photosynthesis?

Poorly Pondweed

Questions

1) How does light intensity affect the rate of photosynthesis?

2) How does temperature affect the rate of photosynthesis?

sunlight

carbon dioxide + water glucose + oxygen

chlorophyll

sunlight

6CO2 + 6H2O àààààà C6H12O6 + 602

chlorophyll

Prediction

I predict that the more intense the light, the higher the rate of photosynthesis. To photosynthesise, plants need light. It provides the energy for the process to happen. Chlorophyll is an enzyme and it speeds up the reaction. If a plant does not get enough of either of these things, photosynthesis will not happen as quickly, if at all. Therefore, I predict that when the light is not very intense we will not see so many bubbles being produced. This is because the plant will not have so much energy (derived from light) to activate photosynthesis. All reactions require a certain activation energy, and if this is not reached the reaction will occur more slowly.

I think that as we move the lamp away (and therefore reduce the light intensity) from the elodea pondweed the number of bubbles produced will decrease steadily. For instance, say at 10cm distance 50 bubbles are counted, it is likely that at 20cm distance 25 bubbles will be counted, as the lamp is twice the distance away. This means the rate of photosynthesis is halved. I think that if we move the lamp any further away than 50cm no bubbles at all will be produced because there will simply not be enough light for photosynthesis to work.

I predict that for temperature, it will not be a case of an increase in x = an increase in y. I predict that there will be a peak where photosynthesis happens the quickest at around 40-50°C. Chlorophyll is an enzyme, therefore it requires some heat to work, but if it is overheated it stops working. Enzymes work rather like a lock and key. It is important that they are a very specific shape for their purpose (in this case chlorophyll joins carbon dioxide and water together to form glucose). If a key is heated too much, it melts and becomes denatured. It will no longer fit the lock it was designed for. This is why enzymes start to work less well at high temperatures.

I think that at the lower temperatures (i.e. 0-20°C) we will not see many bubbles being produced. This is because enzymes need energy to work, which they get form heat (as well as light). So when the temperature is quite low the chlorophyll does not have much energy, therefore the rate of photosynthesis will be low. As the temperature increases, I predict that the rate of photosynthesis will increase (more bubbles will be produced). This is because more heat energy is being provided to the chlorophyll, meaning that photosynthesis can happen more quickly. However, I think that when the temperature reaches 50°C and above, the rate of photosynthesis will decrease, because the chlorophyll will become denatured, and unable to activate photosynthesis as well. Eventually I predict that no bubbles at all will be produced, say around 100°C. So we should see a pattern where the rate increases dramatically between 0-50°C, but then it should decrease dramatically from 50-100°C.

Plan

Apparatus

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desk lamp

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Elodea pondweed x 2

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boiling tube x 2

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paperclip x 2

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250ml glass beaker x 2

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test tube rack

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thermometer

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sodium hydrogen carbonate solution

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stopwatch

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ice

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tongs

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metre stick

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black A4 paper

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sellotape

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scissors

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1ml pipette

Safety

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Take care of the light bulb, which may get very hot.

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As you will be using electricity (for the light bulb) and water at the same time, special care must be taken.

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As a precaution try not to drop any water onto the beaker in the light intensity investigation as it will be hot and may crack.

Method

Preliminary work...

Take two green,...