An investigation into the effect of light intensity on the rate of photosynthesis of Canadian pondweed (Elodea canadensis)

Authors Avatar

An investigation into the effect of light intensity on the rate of photosynthesis of Canadian pondweed (Elodea canadensis)

Planning

Scientific knowledge and understanding

All green plants need to be able to make their own food. They do this by a process called photosynthesis, which means, “making things with light.” Photosynthesis is a series of organic chemical reactions by which green plants produce glucose and oxygen from carbon dioxide and water. It occurs only in the presence of light, and takes place in the chloroplasts of green plant cells. Land plants get carbon dioxide from the air. Water plants get carbon dioxide from water (carbon dioxide in the air dissolved into water). Photosynthesis creates oxygen and sugar. Oxygen is released into the air and used by all animals, including humans, in order to respire. Sugar is used by the plant to respire or store them inside their body.

The overall chemical equation for photosynthesis can be expressed as:

Plants convert the light energy into stored chemical energy. Photosynthesis is possible because green plants contain an energy-capturing substance called chlorophyll. The plant gets its green colour because chlorophyll is green.

Light energy drives photosynthesis. The chlorophyll captures the light energy and uses it to build carbohydrates from simple raw materials (water, carbon dioxide and minerals). The raw materials that are needed for photosynthesis provide the element to make up carbohydrates – carbon, hydrogen and oxygen. The carbon dioxide in the air is the source of carbon. Hydrogen and oxygen is taken from water by a process called photolysis. These raw materials enter the plant through its roots and leaves. Both water and carbon dioxide enter the pondweed through their leaves.

The rate of photosynthesis is affected by changes in light intensity, temperature and carbon dioxide concentration etc. It may be limited by conditions such as light intensity temperature and carbon dioxide concentration. Light intensity has a more immediate effect on the rate of photosynthesis than any other factor. In the dark, photosynthesis stops completely. Light, temperature and carbon dioxide not only affect the rate of photosynthesis but also limit how fast it goes. They are called limiting factors. For example, as the light intensity increases, so does the rate of photosynthesis. But then it reaches a maximum rate which when the light intensity increases; there is no more affect of the rate of photosynthesis. Now carbon dioxide concentration or temperature may become the limiting factor.

        

(This picture is from: www.marietta.edu/~spilatrs/biol103/photolab/photosyn.html)         

In this investigation, I will investigate the photosynthesis of Canadian pondweed – a water plant. This is because pondweed will have to stay under the water to be alive. Oxygen bubbles will be produced from the cut end of the plant and we can tell from the number of the bubbles that what is the rate of the photosynthesis. If I use a land plant, I will not be able to tell how much oxygen has been produced. So I will not be able to find out the rate of photosynthesis. Also if I use a land plant, water will likely to be a limiting factor as if the soil is not wet enough.

The aim of my investigation has been to determine how the intensity of light affects the rate of photosynthesis in Canadian pondweed. To do this, I placed a piece of pondweed in varying light intensities, and observed the number of oxygen bubbles giving off in a given time. I used pondweed because of its quality of giving off bubbles of gas from a cut end, when placed in water.

Prediction

  1. I predicted that as the light intensity increased, so would the rate of photosynthesis. This is because when the light source is nearer, more light will take part in the reaction therefore more photosynthesis will occur which means more oxygen will be produced.
  2. Furthermore, I predicted that if the light intensity increases, the rate of photosynthesis would increase at a constant rate until a certain level is reached where an increase in light intensity will have no further effect on the rate of photosynthesis. There will be another limiting factor, in this case probably temperature or carbon dioxide.

As one can see that figure 1 supports my prediction.

Key factors

Independent variable        

Light intensity is the independent variable. It is to be varied by increasing and decreasing the distance from the light source to the plant. I am going to put a ruler between the desk lamp and the beaker. Every time I move the desk lamp, I would get a distance. From the distance, I could calculate the light intensity by this equation:

I am going to take 5 distances and measure the oxygen bubble produced in each distance. I am going to take 5 values because it is the minimum value required to draw a smooth curve. It is better to have more values but the time is not allowed.  

Join now!

This is a table to calculate the light intensity from distance

(Table 1)


A table to calculate the distance from light intensity

(Table 2)

The numbers I get from this equation will be too small to make any sense and hard to draw a graph with them. So I decide to use them multiplied by a big number. If I use the light intensity from table 1, it will be quite hard to draw a graph and I will not get a very nice graph. This is because although the distance ...

This is a preview of the whole essay

Here's what a teacher thought of this essay

Avatar

***** An excellent and very detailed account demonstrating a thorough understanding of relevant theory.