Question

The experiment is about whether the concentration of sodium bicarbonate will have an effect or not on the rate of photosynthesis in spinach by recording the time taken by the leaf disks to float in the sodium bicarbonate solution with different concentrations (0.7% and 1.5%) as treatment groups and the distilled water as the control group.

Is it possible for leaf disks to float in thecontrol group (Distilled water)?

Why do you think the leaf disks floated at different times in your trial run?

What are the potential sources of error? How will you troubleshoot these when running your experiment?

On what basis will you determine the number of replicates to be used?

Answer 1

If you examine the reaction of photosynthesis, in the light reaction water is split to produce hydrogen ions and oxygen. This gas will make the leaf lighter and make it float.

Being in distilled water the absorption of CO₂ will be retarded; the only little amount of CO₂ dissolved naturally will be available, so no, it is not possible for them to float in distilled water. When you make bicarbonate solutions, you allowed rich carbon dioxide and photosynthesis kicked in. The different time required to float is due to a variety of carbon dioxide levels present in water due to various bicarbonate concentrations.

Sometimes the discs may float instantly because of residual gases in them. They should be gently put to the bottom. If put too harshly they may stick to the surface and rise only at a later time

Some discs might float more rapidly when upside down. This depends on the position of stomata which serves to absorb gases.

We tend to use replicates that gives us the time reading for at least 50 percent of leaves within a reasonable duration. We used the 50% point as it provides a greater degree of reliability and repeatability for this procedure.