Glycogen is a polysaccharide that is made up of about 30,000 glucose molecules! Animals store their excess sugar as glycogen in the mitochondria of liver and muscle cells. The image to the right was taken with an electron microscope. The dark structures show the glycogen granules.

One of the main functions of the liver is to control the levels of glucose in the blood. When the glucose level in the blood is too low, glycogen is broken down into glucose and released into the blood. The glycogen in muscle cells provides an immediate source of glucose for muscle contractions and movement.

Starch is a polysaccharide that is made up of hundreds of glucose molecules. Plants store excess sugar in the form of starch. Plant enzymes break the starch into glucose molecules when a plant needs energy. Most animals also have enzymes that break plant starch into glucose. This is why foods like potatoes, grains, and legumes are a good source of energy for animals. The image below shows a small section of a starch molecule.

Cellulose is another complex carbohydrate found in plants. Cellulose provides structural support for plants as it is the major component of cell walls. Cellulose, like glycogen and starch is made up of many glucose molecules, but in cellulose the glucose monomers join in a slightly different arrangement. In cellulose, every other glucose molecule is upside down. Animal enzymes are not able to break down cellulose. Some animals, such as cows have microbes living in their digestive track that are able to break the bonds in cellulose. In humans and most other animals, cellulose passes through the digestive system without being broken into glucose. This is what is referred to as fiber. The image below shows a small section of cellulose.

Chitin is a polysaccharide that is found in fungi and some animals. It is made of chains of glucose molecules, but each glucose molecule has a nitrogen group attached to it. Chitin is found in the exoskeleton of arthropods. An exoskeleton is a rigid outer covering of some invertebrate animals. Chitin function is similar to cellulose in that it provides structure and support. The image to the right shows a cicada shedding its exoskeleton.