Required Practical: Photosynthesis 

OXYGEN PRODUCTION by plants during photosynthesis can be measured to determine the RATE of photosynthesis.

• CANADIAN PONDWEED can be used in a lab setup where it is exposed to a source of LIGHT at a fixed distance to photosynthesise and produce oxygen.
• The oxygen accumulates in a CAPILLARY TUBE, and the length of the oxygen bubble is measured with a RULER to estimate the volume of oxygen produced.

  

Steps for the Practical Experiment:

• Set the light source at a SPECIFIC DISTANCE from the pondweed.
• Allow the pondweed to photosynthesise for a set amount of time, collecting oxygen in the capillary tube.
• Use the SYRINGE to draw the oxygen bubble alongside the ruler.
• Measure the length oxygen bubble with the ruler to determine the volume of oxygen produced and hence the rate of photosynthesis.
• Ensure other variables like TEMPERATURE and the DURATION of exposure to light are CONTROLLED.
• Repeat the experiment several times and find a MEAN.
• Repeat the whole process again with the light source at different distances from the pondweed.

 

 

The INVERSE SQUARE LAW and Light Intensity:

• When considering the effect of light on photosynthesis, the INVERSE SQUARE LAW is a fundamental concept. It states that light intensity is INVERSELY PROPORTIONAL to the SQUARE of the distance from the light source.
• Mathematically, this is expressed as

This means if the distance from the light source to the plant DOUBLES, the light intensity at the plant's location is reduced to ONE-FOURTH (quarter).

You can work out LIGHT INTENSITY by using the inverse square law.

 

Example:

When you bring a lamp at 5 cm from the pondweed, it has a light intensity of 16au (arbitary units). Calculate the light intensity if the lamp is moved 10cm from the pondweed.

To go from 5cm to 10cm, you need to multiply the distance by 2. To find the light intensity at 10cm, you need to SQUARE the 2 to give you 4 and then do the INVERSE to give you a QUARTER. So you need to find a quarter of 16.

 

You can also use the inverse square law as an EQUATION to find out the light intensity at any distance:

  

Here are a few examples of light intensities at specific distances: