Rate of Photosynthesis

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Rate of Photosynthesis

Introduction

I am going to perform an experiment to test the rate of photosynthesis for different light intensities.

Photosynthesis is performed by plants only. It is the process they use to create glucose, the food that all living things consume, from sunlight. It occurs in the green cells of plants in the palisade layer of the leaf. The chemical equation for photosynthesis is

sunlight

6CO2+6H2O                             C6H12O6+6O2     

chlorophyll

CO2 is carbon dioxide, which the plant will take from the air via the leaves. H2O is water, which the plant takes up through the roots. C6H12O6 is glucose, the food the plant will burn when it respires. O2 is oxygen, and although this is used during respiration enough of it is produced for there to be surplus amount, which secondary consumers (i.e. herbivores) can then use to respire (whilst they eat the plants).

        All living things perform respiration and it takes place in every cell of the organism. The chemical equation for respiration is

C6H12O6+6O2                        6CO2+6H2O+Energy

        The rate of respiration for a plant rarely changes, whereas the rate of photosynthesis can be affected by four limiting factors. The amount of light the plant has access to (and the wavelength). The chlorophyll uses light energy to perform photosynthesis and it can only do this as fast as the light is arriving. The amount of carbon dioxide there is in its local atmosphere. There is only 0.03% of carbon dioxide in the air so it isn’t readily available for the plants. The temperature of the plants surroundings is the third limiting factor. Chlorophyll is like an enzyme in this respect and it works best when it is warm, but not too hot.

        The forth-limiting factor is the amount of chlorophyll a plant has. Even if all the other limiting factors are sated then the amount of chloroplasts in the palisade leaf cells will limit the rate of photosynthesis. Most of photosynthesis takes place in the top layer of the leaf, in the palisade cells (refer to leaf diagram). These cells are specially designed to absorb as much sunlight as they can by having a high concentration of chloroplasts. But a leaf can contain only so many chloroplasts and if these are all working to produce glucose then the rate of photosynthesis will balance out and stop rising.          

         

Light intensity can be calculated using inverse square law, which states that if you double the distance you quarter the light intensity. The equation for light intensity is 1/d2 where d=distance.        

We now know that photosynthesis has three main limiting factors and one final factor related to the plant and not the surroundings. These will have to be controlled or varied during my experiment to ensure that my test is fair and that I get some usable results. We also know that the plant produces two gases. The relevance of this will be explained later.

Plan

Method:

To test the rate of photosynthesis I am going to use a 10cm piece of Elodea (pond weed). The Elodea will be immersed in water and a halogen light will be pointed at it (imitating sunlight). The temperature of the water surrounding the Elodea will be affected by the light (which gets very hot) and the temperature of the room. In order to keep temperature change to an absolute minimum I will immerse the boiling tube containing the Elodea in a large beaker of cool water, which will be changed for each distance. The carbon dioxide will be supplied by sodium hydrogencarbonate (NaHCO3), which will be supplied to the boiling tube holding the Elodea once at the beginning. The Elodea will be left for five minutes before any readings are taken so it can come to equilibrium.

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To measure the rate of photosynthesis I will count the bubbles produced by the plant over a five-minute period, recording the amount of bubbles produced every minute. The same piece of Elodea will be used for each distance and it follows that the same water in the boiling tube will also be used. This will effect the concentration of the carbon dioxide, as it will go down the longer the plant is photosynthesising. I will compensate for this by putting a lot of sodium hydrogencarbonate into the water surrounding the Elodea, so that it would be impossible for the plant ...

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