All the energy used to create and maintain the phenomenon we call "life" on this planet starts in the nuclear fusion furnaces of the sun and travels to our planet in the form of electromagnetic radiation.
Visible light is one, small part of this spectrum of radiation, but there is enough energy in the wavelengths of light that reach us to allow living organisms to build complex molecules, cells and other physical structures.
The conversion of "light energy" into "chemical energy" (that stored in the covalent bonds of linked atoms) begins in the cells of plants. Specialized solar receptors trap the light, convert it to "electron energy" and then finally to bond energy as carbon, hydrogen and oxygen atoms are linked together to form complex, organic molecules.
One of the first stable and recognizable category of organic molecule created by plants as they trap solar energy is carbohydrate. This is a very abundant type of organic molecule and carbohydrates are found in one form or another in all living organisms. They perform a variety of roles, but one very important function is to act as an "energy reserve", for fuel, for the cell or creature.
Sunlight is one ingredient needed by plants before they can make carbohydrates, the other two are water and carbon dioxide. An oversimplified summary of how these three ingredients come together is:
nCO2 + nH2O + sunlight --> CnH2nOn + nO2
This summary formula shows two things; oxygen, which is another vital molecule for modern living organisms, is a byproduct of this process, and that all carbohydrates can have the general form;
Written this way, such a molecule could be considered to be "Carbon + Water", or "hydrated carbon", or
- carbohydrate -
which is the most common name for this class of organic molecules.
In theory, a carbohydrate could consist of a molecule that had in its structure one carbon atom, two carbon atoms, or a million carbon atoms - so long as it stayed within the general formula of "carbon + water".