Elodea has long been used in experiments to estimate the rate of
photosynthesis. The first published example appears to be by Reinke in 1883. He
counted the bubbles given off by Elodea in spring water with a little
added carbon dioxide when it was exposed to lights of different intensities.
The technique of counting bubbles has been greatly criticised by many
people for being a poor way of measuring photosynthesis. There are several
reasons for this:
the gas released in the
bubbles is not pure oxygen. Can you think why this
should this affect our estimate of the rate of photosynthesis?
oxygen and carbon
dioxide can dissolve in water, rather than form bubbles. Think how this could
affect your estimate of the rate of photosynthesis.
Look very carefully at
the bubbles escaping from a cut stem of Elodea. What do you notice about
their sizes? how could this affect your estimate of the rate of photosynthesis?
Some scientists have preferred to measure the volume of gas produced.
Think about how you might measure gas volume. Does measuring the volume of gas
solve all of the problems we have mentioned above?
Does gas production measure the rate of
photosynthesis?