Carbohydrates (sugar)

Digestion

The 3 monosaccharides galactose, glucose and fructose are the only sugars that can be absorbed directly into the blood streams, e.g., they don't require any digestion. Hence, they can provide immediate energy. In some countries energy tablets are sold cheaply. They contain glucose in the form of the monosaccharide (not forming glucose or maltose). In Germany, for instance, they are marketed under the trade name "dextro energy" . (dextrose is another name for glucose).

All the other sugars require digestion, so that they can be broken down into monosaccharide units that can be absorbed into the bloodstream. That includes the disaccharides and also polisaccharides. For instance, starch is an important staple food that is broken down using appropriate enzymes (starch is not soluble in water without the enzymes).

Cellulose,however, cannot be broken down in to the constituent glucose units in the human gut, because we lack the enzymes needed for that. That is why we don't eat grass.

Cows can eat grass, not because they have enzymes to digest it, but because they have cellulose digesting bacteria living in their stomach. This is a form of symbiosis.