Investigating the effect of Light Intensity on Elodea.

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Tania Lapa 11Q

Investigating the effect of Light Intensity on Elodea

Aim: 

The aim of my experiment is to determine whether or not the intensity of light would affect the rate of photosynthesis in a plant. To do this I will place different pieces of Elodea (pondweed) into a beaker and expose it to varied light intensities, and observe the amount of oxygen given off by the plant.

Introduction:

Photosynthesis is the process of converting light energy into chemical energy and storing it in the bonds of sugar.

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 from carbon dioxide and water causing the release of sugar and oxygen. It is the chemical process, which takes place in every green plant to produce food in the form of glucose.  Plants use the suns energy to join together water and carbon molecules to make the glucose, which is sent around the plant to provide food.  Cells in the root or stem can use the glucose to make energy, if the plant does not need to use all the glucose immediately then it is stored as starch.

It is possible to measure the rate of photosynthesis by counting how many bubbles or the volume of oxygen produced. In this experiment I will collect data to see if it supports my prediction.

The following chemical equation summarises photosynthesis:

Carbon dioxide      +     water                                                   glucose     +     oxygen  

          CO2             +     H2O                                                     C6H12O6    +     O2

From this equation we can see that the photosynthesis reaction requires light. Light is a form of energy, and when it falls on the chloroplasts in the leaf, it is trapped by chlorophyll, which then makes the energy available for chemical reactions in the plant. As the amount of light (energy) falling on the plant increases, the more energy chlorophyll can trap, so more energy is available for chemical reactions, so more photosynthesis can take place in a given time.

If the plant had a plentiful supply of carbon dioxide and water, the only limiting factor would be light. The rate of photosynthesis would increase with an increase in light intensity up to the point that light is no longer the limiting factor.

Light provides energy so that carbon dioxide and water can bond to make glucose. When the glucose is used (the bonds broken), a large amount of energy is released.

If the distance of a lamp from the plant is known, light intensity can be calculated:

Light intensity = 1/d2 (d = distance)

Prediction:

I predict that as light intensity increases, the rate of photosynthesis will increase at a proportional rate until a certain level is reached where an increase in light intensity will have no further effect on the rate of photosynthesis, as there will be another limiting factor.

I predict that the graphs plotted will be non-linear, they will be curves of best fit.

Light intensity is inversely proportional to the distance squared because the light energy spreads out as it travels further from its source. As light energy is released from a point, it is dispersed equally along the circumference. As the distance increases, the same amount of light has to be dispersed along a larger area. I intend to do preliminary work so that I have an idea of how to carry out my final experiment. Preliminary work can give an idea of the results, and whether there is any correlation and any point to continue studying. Preliminary work lets us test our method and whether there are any other factors that need to be taken into account.

Preliminary experiments allow us to decide on a suitable range (distance) to investigate light intensity. To alter the light intensity I placed the lamp at various distances from the plant.

Simplest method to use:

I have decided to use the bubble counting method, as it is simple. It would be much more difficult to measure the volume of oxygen, to get data. Counting bubbles gives numerical data, volume of oxygen would perhaps be more accurate, but very hard to measure accurately. With the equipment available and expertise, it would be easiest to use the bubble counting method.

Preliminary work:

Method used:

Cut pondweed at both ends so that the plant is fresher. The length of the plant should be the same every time (20cm). The longer the plant, the closer to its natural form it is, the more accurate and reliable our results are. The Elodea we received to work with were of about 30cm in length, cutting it down too much would not be a true representative of the plant in its natural state. Fill a beaker with water (500cm3) of water and add a spatula of sodium hydrogen carbonate. Place a funnel and an upturned test tube full of water over the Elodea (as shown below):

Measure a distance with a 1m rule e.g. 50cm from the plant and place the lamp there. Then time with a stop clock how many bubbles are produced within a certain length of time at that distance.

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I took several results for my preliminary experiment and worked out an average.

These are the preliminary results obtained:

From this we can see that there is a definite correlation. As the distance from the plant increases the number of bubbles decreases. For my final experiment I intend to record results from the following distances: 50, 45, 40, 35, 30, 25, 20, 15, 10, 5. I do not intend to record no. of bubbles produced from a distance of 0cm, because the lamp would heat up the experiment too much, making the test unfair. I will not ...

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