Food chemistry
Comprising 75% of the biological world and 80% of all food intake for human consumption, the most common known human carbohydrate is Sucrose. The simplest version of a carbohydrate is a monosaccharide which contains carbon, hydrogen, and oxygen in a 1:2:1 ratio under a general formula of CHO where n is a minimum of 3. Glucose is an example of a monosaccharide as is fructose. Combine them in the picture shown to the right and you have sucrose, one of the more common sugar products around.
Sport dog nutrition
Carbohydrates provide a quick energy source, increased palatability as well as fiber in canine diets; if disaccharides and xylitol are avoided. In an ingredient label, starch (a complex carbohydrate) may appear as rice, wheat, barley, corn and/or other varying grains. These ingredients not only supply the dog with rapid and accessible energy while performing, but can also be used after exercise to replenish glycogen stores in organs such as the liver and muscle. Direct carbohydrate levels are not displayed on feed labels due to the inaccuracy of current testing methods and varying methods in which certain sources may effect an animal's glycemic response. 
However, for canines it has been demonstrated that rice has the greatest blood glucose response followed by wheat, barley, corn and sorghum. Diets containing rice or wheat should be blended with some low level carbohydrate sources like (sorghum or barley) to prevent an animal's blood glucose from spiking.