This lesson is on cellulose and glycogen. In this lesson we’ll define and look at characteristics of each molecule. We’ll also specifically look at similarities and differences between cellulose and glycogen.
What Are Cellulose and Glycogen?
Plants and humans are made of a lot of the same things. We both have sugars, proteins and fats that are totally necessary for our survival. One sugar in particular, glucose, is essential for both plants and animals, although we use them in slightly different ways.
Animals use glucose primarily for energy. When we have plenty of glucose from a carbohydrate rich meal, like pasta, our bodies store it away for later in the form of glycogen. Glycogen is a giant polymer of glucose, meaning it is made of many glucose molecules put together. Cellulose is another giant glucose polymer, although this is how plants store glucose.
Shapes of Glycogen and Cellulose
All polymers, like glucose and cellulose, are made of smaller pieces called monomers. Think of monomers like Legos for building a toy car.
Cellulose and glycogen each use the same monomer, glucose. Glucose is a ring structure with six carbon atoms. Individual glucose rings can be connected together at different carbons to create different structures. In addition, some pieces of the ring are flipped, creating two different versions of glucose, called alpha and beta.In glycogen, the glucoses can be connected at the first and fourth carbon, called alpha 1,4-glycosidic linkages or at the first and sixth carbon, alpha 1,6-glycosidic linkages. This allows the glycogen to branch and create a winding pattern. However, cellulose has beta 1,4-glycosidic linkages, making it a firm straight chain.
Let’s look how these shapes convey the function of each molecule next.
Cellulose and Glycogen in Animals
Although glycogen and cellulose are made of nearly the same thing, their shapes allow them to perform different functions inside the cell. Glycogen is a branched structure important in storing energy inside animal cells. When we eat an excess of glucose, or sugar, our intestines send the sugars to the liver. The liver stores the extra sugar as energy in the form of glycogen for later use.
Muscles also store glycogen, which can be used during exercise to provide quick energy. Glycogen is readily broken down and reassembled by animal cells.Cellulose, however, is only found in plant cells. Its beta 1,4-glycosidic linkages can’t be broken down in our bodies. When we eat plants, like vegetables, the cellulose remains mostly intact and undigested. This is what dietitians refer to as fiber.
The fiber helps move food through our digestive system efficiently, keeping the conveyor belt of our digestive system moving smoothly.Some animals can digest cellulose. Herbivores, like cows, deer, antelope, and rabbits have a special structure branching from their small intestine called the cecum. The organ is like a storage bin for breaking down cellulose. Although animals lack the enzyme, or small proteins, needed to break down cellulose, special bacteria live in the cecum that do the job. The bacteria break down the cellulose for the animal in the cecum, and in turn they get some food out of the deal.
Both benefit and live together in symbiosis. Humans have evolved to no longer have a cecum, since we are omnivores, meaning we eat vegetables and meat.
Cellulose in Plants
Plants do not make glycogen. They have a different carbohydrate for storing energy, called starch. However, cellulose is used purely for structure in plants. The long, straight chains are further bound together with bonds, giving the plants nice long rods to support themselves.
This is what makes up the stalks of vegetables and keeps them standing tall. Since the rods of cellulose are bound tightly together, there is little room for chemical interactions with water. Thus, soaking cellulose in water does nothing to dissolve the structure, which is ideal for plants, since water continuously flows through the stalks to the leaves. Cotton and wood are rich in cellulose and are especially useful to humans due to this property.
In summary, cellulose and glycogen are carbohydrates made of the monomer glucose. Animals use glycogen to store extra glucose inside cells, particularly the cells of liver and muscle.
The glucose monomers are linked together using alpha 1,4-glycosidic linkages and alpha 1,6-glycosidic linkages, which allow them to be easily assembled and broken down by the body. Cellulose however, is used for structure in plants and has beta 1,4-glycosidic linkages, which can’t be broken down by our body. Plants do not use glycogen, but do use cellulose for cell structure.