* And some algae like seaweed, and some bacteria.
Glucose is a molecule with 6 carbon atoms bound together. Humans cannot harness the immense amount of energy needed to get 6 carbon atoms to bind together. Plants, however, can. It's quite a feat actually. They harness the energy from the sun to do this.
Plants take in carbon dioxide, string together 6 carbons to make glucose for their fuel (starch is just a chain of glucoses), and give off the excess oxygen.
Humans take in the oxygen given off from plants and use it to extract the energy from those bonds within the glucose molecule. (One pathway to extract that energy is called glycolysis. I'll return to glycolysis later.) The waste product, if you will, from our energy-extraction process is carbon dioxide. We exhale it. This is the same carbon dioxide that the plant takes in to make glucose - it needs those carbons. Humans and plants have a cyclical relationship.
At night, plants respire just like us. They use oxygen to extract energy from the glucose they made during the day, giving off carbon dioxide in the process.
One last point...
There is a process our bodies evolved to supply glucose in a pinch. It's called gluconeogenesis ... the new making of glucose. Since we can't make glucose from scratch, this process allows us to reassemble preformed 3-carbon and 4-carbon molecules to make the 6-carbon glucose. These precursor molecules' carbons were bound via photosynthesis, via plants' harnessing of energy from the sun. We still can't isolate ourselves from this fuel that plants make - that we eat.
Gluconeogenesis is essentially the glycolysis pathway (recall above) in reverse - not quite though. It uses different enzymes (enzymes make it go), and where glycolysis provides us energy by breaking down glucose, gluconeogenesis uses up energy. From where does the energy come to reassemble those precursors for gluconeogenesis? From plants!
Glucose From Fat?
If we eat too much glucose, we convert some of it to fat for storage. Since we can make fat from glucose, can we make glucose from fat? No.
When we disassemble the fat (fatty acid chain), we're left with a small 2-carbon molecule. Recall that gluconeogenesis requires at least a 3-carbon molecule.