Can chlorophyll be put into humans so we can get energy from sunlight?
Hemoglobin and chlorophyll are rather similar molecules, with a key difference being that at the center of a hemoglobin molecule there is an iron atom and at the center of a chlorophyll molecule there is an atom of magnesium.
Despite their similarities, the two molecules play different roles in living things. Hemoglobin carries oxygen in the blood of many animals, while chlorophyll captures the energy of sunlight to convert carbon dioxide and water into sugars -- a process called photosynthesis. Chlorophyll alone, however, isn't really up to the job, and it has to be incorporated into plant cells to function properly, so if one managed to pull off a trick of the kind you describe you they would need to do something more complicated than just injecting someone with chlorophyll, or putting it under the person's skin.
That said, some sea slugs keep entire chloroplasts (bits of plant cells containing chlorophyll and other material) in them and some corals harbor live algae in a symbiotic relationship, so some animals can perform photosynthesis.
If you want something more human, you might be interested in a special substance you have in your body called cytochrome c, which is very similar to chlorophyll and to a part of the hemoglobin molecule called "heme." It is involved in how your body uses energy and evidence exists that it can be activated by light, somewhat like chlorophyll. This may be involved in the accelerated wound healing that can be produced by low levels of laser light.
Some creatures such as octopi use compounds similar to hemoglobin but with copper in place of iron. This leads to bluish-colored blood. There are also some sea creatures (among them some sea squirts) that have similar compounds again, but now with vanadium in place of iron, giving them green blood.
Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819. 
