Complex carbohydrates are made up of long chains of sugar as opposed to simple carbohydrates made up of one or two molecules of sugar. Starch and cellulose are two major forms of complex carbohydrate. While starch can be digested by the human body, cellulose is not digestible by the human body. Grains are a source of starchy carbohydrate that is broken down into sugars for energy. Green vegetables and beans contain cellulose or fiber which helps to keep the gastrointestinal tract healthy.