Scientific calculations

The table shows a set of results obtained by students investigating the effect of light intensity on photosynthesis. They varied the distance of the plant from the light source.

Distance from light source in cmRate of photosynthesis = Number of bubbles produced per minute

If we draw a graph of this data:

A graph showing the number of bubbles produced per minute.

Note that the graph is a curve.


From the graph, suggest the rate of oxygen production after 18 minutes.

40 bubbles per minute.

Using the graph:

A graph showing the number of bubbles produced per minute.

The effect of chlorophyll

A picture of geranium flowers

The effect of the presence or absence of chlorophyll on photosynthesis can be investigated using a variegated plant. Variegated plants have regions of their leaves with, and without, chlorophyll.

Only those areas of the leaf with chlorophyll photosynthesise. They will test positive for starch, which is built up from the glucose produced.

One way of testing for starch on a leaf is to remove the chlorophyll and then stain it to see where the starch had been stored.

  1. The leaf of a variegated Pelargonium is dropped in boiling water to kill and preserve it.
  2. The leaf is left for 10 minutes in hot ethanol in a boiling tube. This removes the chlorophyll.
  3. The leaf is dipped in boiling water to soften it.
  4. The leaf is spread out in a Petri dish and covered with iodine solution.
  5. The areas that had the chlorophyll stain blue-black. The areas that had no chlorophyll remain pale.
An image showing the stages of leaf when dropped in boiling water.


Care must be taken when using boiling ethanol. Make sure that no Bunsen burners are turned on as the ethanol is highly flammable.