Carbohydrates include the sugars and starches, as well as the indigestible but nevertheless important component of diet, fiber. All three of these classes of carbohydrates are closely related chemically. They're made up of one or more sugar units, called monosaccharides, each of which is itself composed of the elements carbon, hydrogen and oxygen. A hallmark of the carbohydrate sugar unit is that it has a chemical formula in which the quantity of carbon is equal to the quantity of oxygen, and there are twice as many hydrogen atoms as carbon atoms.
Like many organic compounds, meaning molecules of life, carbohydrates have what's often called a "carbon backbone." This means the the overall shape of the molecule is determined by carbon, but while carbon determines the molecule's structure, it doesn't generally participate in a significant proportion of the molecule's chemical reactions. Carbohydrates can have various numbers of carbon atoms, but the most common of the monosaccharides, and the building block of most carbohydrates, is glucose, which has six carbon atoms. Explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry," the glucose molecule consists of a ring of five carbon atoms and one oxygen atom, with a sixth carbon atom attached as a side arm on the ring.
Oxygen is common in organic molecules, and is a component of all three macronutrients -- protein and fat as well as carbohydrate. In carbohydrate, oxygen not only helps determine the overall shape of the sugar rings -- there's an oxygen atom as a member of the main glucose ring as well as of the rings of other common monosaccharides -- but also helps increase water solubility of the sugars. Explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry," the six oxygen atoms in glucose and the many oxygen atoms in other carbohydrates make the molecules capable of dissolving in the bloodstream and other water-based fluids.
By sheer quantity, hydrogen atoms account for approximately half of all atoms in any given carbohydrate. Glucose, for example, is made up of a total of 24 atoms, 12 of which are hydrogen. Unlike carbon, which determines the overall structure and shape of a molecule, hydrogen atoms aren't particularly involved in shape determination. They can, however, participate in chemical reactions. In the case of glucose and other carbohydrates, hydrogen helps increase the water solubility of the molecule where it's bonded to oxygen, and also accounts for the ability of many sugar units to bond to one another to form long chain carbohydrates, like starch, explain Drs. Garrett and Grisham.