What do we Know?

Before going further, it is worth pausing to recall what earlier classes have already established about photosynthesis through a few simple, familiar experiments. Those experiments showed that three things are required for photosynthesis to occur: chlorophyll (the green pigment present in the leaf), light, and carbon dioxide (CO2).

One classic demonstration uses a starch test on two kinds of leaves — typically a variegated leaf (one with both green and non-green patches) or an ordinary leaf that has had part of its surface covered with black paper, then exposed to light for a while. When such a leaf is later tested for starch, the result is telling: starch forms only in the parts of the leaf that were both green and exposed to light — never in the non-green patches, and never in the portion that was kept covered and dark. This confirms that both chlorophyll and light are essential for the process. …