Wednesday 18th of Mach 2009

IB Photosynthesis Experiment

Aim of the Experiment: To investigate the effect of light intensity on the rate of photosynthesis. A relation is sought for that would, if existent, allow estimation for untested values of light intensity.

Research Question: How does controlled light intensity affect the rate of increase in the excess of oxygen created by the elodea plant undergoing photosynthesis?

Hypothesis: Many plants use the process of synthesis to convert light energy into chemical energy (carbohydrates). The waste product, useful for other life forms is oxygen, a product of photolysis. Carbon dioxide, water and chlorophyll and light are essential for this reaction. The scarcity of any of these affects the rate of reaction.

By controlling light intensity one should limit the rate of reaction of photosynthesis and hence the excess oxygen produced should have a varying rate linked to the light intensity.

We therefore need to control at least one of the other controlling factors of photosynthesis and in this lab it will be temperature.

For this experiment the variables are the following:

Independent variable: light intensity, changed by varying the light’s distance to the elodea complex. This is the photosynthesis factor which behaviour is investigated.

Dependant variable: the rate of photosynthesis, id est the speed of the bubble which is affected by the excess by the waste product, oxygen. This is the best way to have a good estimation of the rate of photosynthesis, which we are trying to establish a relationship with light intensity.

Controlled variable: Local and ambient temperature (of the room), the experiment was done in the same room with the school air conditioning system keeping the room at a constant temperature. The temperature kept constant by measuring so. Again this is a factor that is likely to cause negative impact on the raw data and disturb the experiment’s results.

                

Method

Material Used:

  • Potometer (With its own 10cm ruler with 0.5 mm uncertainty.)
  • 10 cm Elodea Canadensis
  • 10ml of 0.2M Sodium Carbonate
  • Tap Water
  • A lamp with a 40 W light bulb
  • A large syringe able to contain 60ml of water
  • A 1 meter ruler with 0.5 cm uncertainty
  • A little syringe.
  • A timer
  • A thermometer

Steps:

  1. The sodium carbonate solute was mixed with the water contained in the syringe.
  2. A healthy looking piece of elodea was selected and cut to be 10cm long.
  3. The potometer was set up and the syringe was connected to it.
  4. The elodea plant was placed into the syringe.
  5. The lamp was plugged in but not turned on and its distance from the potometer-elodea complex was controlled using the 1m ruler.
  6. A small amount of air, enough to create a bubble was inserted into the potometer tube by attaching it to the second entry provided for this. The initial location of the bubble in respect to the potometer-supplied ruler was taken in note of.
  7.  The light was turned on and the timer was started. Once the timer went past two minutes (120 seconds) the location of the bubble was written down. This allowed its displacement to be calculated. The light was also turned off.
  8.  Steps 6 and 7 were repeated for values ranging from 10 to 60 cm of elodea-potometer complex and lamp distance. Every two (or three if time was plentiful) steps added a 10cm displacement, that is each different displacement had at least 2 different readings. The thermometer was used to check if the temperature stayed the same throughout the experiment.
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Collected Data.

Table 1.Data collected during the 2 minutes when the plant had access to additional light at a controlled distance. The bubble end and start location were measured by eye and rounded to the nearest 0.05cm.

Processed Data

Light intensity is calculated by this formula:

The intensity of light falling on a given object from a constant source is inversely proportional to the square of the distance between them. 

In mathematical notation:

where I is the light intensity and R the distance in metres.

 The constant in this proportionality is the energy used by the light that ...

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