When the simple sugars from the Simple Carbohydrates lesson get linked into chains, things get more "complex." If the chains have at least three but no more than 10 monosaccharides linked together, they are called oligosaccharides. If you have ever considered an onion "sweet," it is due to these oligosaccharides. They aren't easily broken down by the enzymes during the digestive process. Instead, the bacteria in the large intestine breaks it down, with one of the by-products being gas. It goes without saying that beans an other legumes contain oligosaccharides. If the sugars form longer chains, we get polysaccharides. Polysaccharides are starches and fiber. Remember, these chains are formed by monosaccharides getting together.
Another polysaccharide is very important to our bodies, and we wouldn't live without it. It is called glycogen. This is a chain of glucose molecules that has many, many, many branches. The branches make it ideal for breaking. If it is broken up, the glucose molecules are now free and used as energy by our body. We store glycogen in our liver and muscles. Again, the liver and muscles do not make the glycogen, they only store it for when the body needs it for energy.
Starch is glucose molecules linked together in straight and branched ways. Roughly, it is to the plant what glycogen is to an animal. The plant stores it for when it needs energy. A potato plant's growth we see above the ground is fueled by the starch under the ground in its tubers. We agree that this starch is good fuel for our bodies as well, so we dig up the plant, discard the green that was above ground, and eat the tubers. Since plants see starch as fuel, they will also store it in their seeds. Beans and grains contain starch in their seeds.
There is such a thing as resistant starch. This starch is generally resistant to being digested by our digestive tract. It can be resistant because of the way it is protected by its structure. A corn kernel is considered resistant starch. Another reason starches can be resistant to digestion is because of the chemical changes it undergoes being processed or cooked. Bananas that haven't ripened, potatoes that have been cooked but gone cold, beans and other legumes, and rice contain a lot of resistant starch.