Remember that we do not “feed” plants. The major thing that separates plants from animals is that they feed themselves through a process called photosynthesis. This is where they capture the energy from the sun (or other light source) and, in the chlorophyll (green colored) molecule in leaves, they pull together a variety of chemical elements and water to trap that energy. The resulting sugars and carbohydrates are then the basis of almost all living things on earth.
Animals, including we humans, feed on this plant material and convert that energy into proteins and other tissues. We can gain transfer the energy of the sun into our bodies by eating either plant or animal materials.