can all plants provide us w/ energy? what determines whether we can eat a plant or parts of it for energy?

Question

thanks for the help! could you cite some sources?

Answer 1

The sources of energy to animals (including humans) are carbohydrates, lipids, and proteins. Carbohydrates include sugars, starches, and cellulose.
Humans can digest proteins, lipids, sugars and starches, but not cellulose. So, we can get energy by eating plants (or parts of plants) that contain proteins, lipids, sugars, and/or starches. We can't get energy from sources that only have cellulose. For instance, we don't get any energy from celery or lettuce, though we can get vitamins and minerals, maybe some other nutrients also.
Rabbits, horses, and cattle eat large amounts of plants containing cellulose as the main energy source. They carry microorganisms in their intestines that help them digest the cellulose, breaking it down into sugars. Some microorganisms further process these into proteins. However, humans don't host this kind of microorganism.