An Investigation to find out how Light Intensity affects Photosynthesis in Pondweed.

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Dave Burton 10FD

An Investigation to find out how Light Intensity affects Photosynthesis in Pondweed.

An Investigation to find out how Light Intensity affects Photosynthesis in Pondweed.

The investigation is intended to demonstrate the way in which photosynthesis in pondweed is affected by light intensity.  I have read about photosynthesis in Illustrated Biology by B.S. Becket, as well as Biology, a fictional approach by M.B.V. Roberts, and this is my summery:

Most plants are neither carnivores nor herbivores, they do not feed on complex organic molecules and can instead synthesis organic compounds from inorganic raw materials, this is called Autotrophic Nutrition.  There are two different types of Autotrophic Nutrition: Chemosynthesis and Photosynthesis.  The method used by all green plants is Photosynthesis.  Photosynthesis can be summarised by:

Energy from sunlight is trapped by chlorophyll contained in chloroplasts found mostly in the leaves of plants, and is used to combine carbon dioxide with water.  Carbon dioxide is absorbed through pores called stomata which are usually located on the under surface of the leaves, and water is absorbed from the soil by the roots.  The main product generated by photosynthesis is monosaccharide sugar (glucose).  This can be converted into proteins for growth, broken down into carbon dioxide and water for energy production, or built up into starch for storage.  A waste product of oxygen is also released.

The structure of a leaf


Plan

Oxygen is the only gas released during photosynthesis.  Therefore, to find out how light affects photosynthesis in pondweed, I will measure the volume of gas released from some Canadian pondweed (Elodea).  I will record the volume of gas releases every minute, for six minutes, before changing the intensity of light the pondweed is exposed to, and repeating the procedure.  

The range of equipment available for me to use in school is limited, so this is what I shall be using:

  1. Lamp
  2. Clamp stand
  3. Meter ruler (cm)
  4. Bung with a hole in it
  5. Water containing  Sodium Bicarbonate
  6. Bent delivery tube
  7. Conical flask
  8. Black paper and some sticky tape
  9. Scale (mm)
  10. Elodea (Canadian Pondweed)
  11. Ink bubble
  12. Stopwatch

In order to get the ink bubble into the delivery tube I will use the following method:

  1. Seal the top end of the delivery tube.
  2. Dip the bottom end of the delivery tube into a pool of ink.
  3. Seal the bottom end of the delivery tube.
  4. Remove the seal from the top end of the delivery tube.
  5. Tip the delivery tube up and release seal to position bubble, and then replace seal.
  6. Seal the bottom of the delivery tube to the test tube, and then finally remove the seal from the top of the delivery tube.

To discover accurately the affect of light intensity on pondweed (i.e. maintain a fair experiment), I shall have to supply sufficient amounts of four of the five factors necessary for photosynthesis to take place, these are carbon dioxide, water, chlorophyll, and heat.  This is so that the fifth factor, light, is the only factor limiting any photosynthesis that takes place.  In addition, the levels of carbon dioxide, water, chlorophyll, and heat need to be kept at a constant.  This is so that light intensity is the only variable of which the affect is being measured.  Since the investigation is intended to demonstrate the way in which light intensity affects photosynthesis in pondweed, I will obviously have to vary the light intensity.  If I varied both the light intensity and levels of carbon dioxide, for instance, and kept the other three factors at a constant, my results would show the effect of both light intensity and carbon dioxide on the rate of photosynthesis in pondweed.  It is for this reason that light intensity will have to be the only factor I vary.  I know that the natural amounts of carbon dioxide, water, chlorophyll, and heat are of a sufficient quantity, since pondweed survives in the wild.  For this reason I shall not alter the amount of heat subjected to the pondweed.  Ideally, I would sustain a constant temperature but due to limited equipment, this is not possible.  In addition, I shall not alter the amount of chlorophyll present in the pondweed, or it’s size and surface area.  I shall maintain this by simply using the same piece of pondweed for each set of results.  This may however cause the pondweed to slightly loose it’s efficiency at photosynthesis.  Though this inaccuracy would be lesser than that caused by using different pieces of pondweed for each set of results, since it would be almost impossible to obtain several pieces of pondweed that were identically sized, had the same surface area, and contained an equal amount of chlorophyll.  Ideally I would prefer sustain the pondweed’s natural supply of water and carbon dioxide, but by removing the pondweed from the pond and placing it in a test tube, I have already altered the amount of water and carbon dioxide that the pondweed has to use.  To compensate for this I shall add Sodium Bicarbonate to the conical flask full of pond water I will provide.  Since both water and carbon dioxide are used in photosynthesis, the water will need to be replaced for each set of readings ensuring there will be equal amounts of water and carbon dioxide each time.  The water will be replaced with the same volume of water, which comes from the same source, containing the same amount of sodium bicarbonate.

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I will record the location on the scale, of the beginning of the ink bubble every 30 seconds for 5 minutes.  Then using the formula v=πr2l (where v is volume, r is radius and l is length), I will be able to work out the volume of gas released from the pondweed every 30 seconds over the 5 minutes.  I will repeat this at least three times, replacing the water each time, and moving the lamp 25cm away from it’s previous position, when it’s first position is at 0cm.

Since plants respire, the waste products of carbon dioxide ...

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