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Carbohydrate Classification

Carbohydrates; structure, function and terms used to discuss them.

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Carbohydrate Classification

The fluffy white fibers on this plant look like cotton balls because that’s what they are. The plant is a cotton plant, and the fibers are used to make cotton fabric. Cotton fibers consist of one of the most common compounds on Earth: cellulose. Cellulose is a biochemical compound found in all plants. It belongs to the class of biochemical compounds called carbohydrates.

What Are Carbohydrates?

Carbohydrates are one of four classes of biochemical compounds. The other three classes are proteins, lipids, and nucleic acids. In addition to cellulose, carbohydrates include sugars and starches. Carbohydrate molecules contain atoms of carbon, hydrogen, and oxygen. Living things use carbohydrates mainly for energy

Q: Which carbohydrates do you use for energy?

A: You may eat a wide variety of carbohydrates—from sugars in fruits to starches in potatoes. However, body cells use only sugars for energy.


Sugars are simple carbohydrates. Molecules of sugars have relatively few carbon atoms. Glucose (C6H12O6) is one of the smallest sugar molecules. Plants and some other organisms make glucose in the process of photosynthesis. Living things that cannot make glucose obtain it by consuming plants or these other organisms.

In the Figure below, you can see structural formulas for glucose and two other sugars, named fructose and sucrose. Fructose is a sugar that is found in fruits. It is an isomer of glucose. Isomers are compounds that have the same atoms but different arrangements of atoms. Do you see how the atoms are arranged differently in fructose than in glucose? You’re probably most familiar with the sugar sucrose, because sucrose is table sugar. It’s the sugar that you spoon onto your cereal or into your iced tea.

Structural formulas for glucose, fructose, and sucrose

Note: Each unlettered point where lines intersect represents a carbon atom.

Q: Compare the structure of sucrose with the structures of glucose and fructose. How is sucrose related to the other two sugars?

A: Sucrose consists of one molecule of glucose and one molecule of fructose bonded together.


Starches are complex carbohydrates. They are polymers of glucose. A polymer is a large molecule that consists of many smaller, repeating molecules, called monomers. The monomers are joined together by covalent bonds. Starches contain hundreds of glucose monomers. Plants make starches to store extra glucose. Consumers get starches by eating plants. Common sources of starches in the human diet are pictured in the Figure below. Our digestive system breaks down starches to sugar, which our cells use for energy.

Examples of starches

Foods that are good sources of starches.


Cellulose is another complex carbohydrate that is a polymer of glucose. However, glucose molecules are bonded together differently in cellulose than they are in starches. Cellulose molecules bundle together to form long, tough fibers, as you can see in the Figure below. Have you ever eaten raw celery? If you have, then you probably noticed that the stalks contain long, stringy fibers. The fibers are mostly cellulose. Cellulose is the most abundant biochemical compound. It makes up the cell walls of plants and gives support to stems and tree trunks. Cellulose also provides needed fiber in the human diet. We can’t digest cellulose, but it helps keep food wastes moving through the digestive tract.

Diagram of cellulose structure


  • Carbohydrates are biochemical compounds that include sugars, starches, and cellulose. They contain carbon, hydrogen, and oxygen, and they are used mainly for energy by living things.
  • Sugars are simple carbohydrates such as glucose, which the cells of living things use for energy.
  • Starches are complex carbohydrates that are polymers of glucose. Starches are used by plants to store energy. Consumers get starches by eating plants. They break down the starches to sugar for energy.
  • Cellulose is another complex carbohydrate that is a polymer of glucose. It is the most abundant biochemical compound. It makes up the cell walls of plants and provides fiber in the human diet.


  1. What are carbohydrates?
  2. Compare and contrast sugars and starches.
  3. Describe the structure of cellulose. What is its function in plants?

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Watch the video about carbohydrates and then answer the questions below.

  1. A single, simple sugar is called a __________.
  2. What makes a molecule a carbohydrate?
  3. When glucose and fructose combine chemically, they form __________ and __________.
  4. Sucrose is a two-sugar carbohydrate, or __________.
  5. What is a polysaccharide? Give an example.


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carbohydrate Biochemical compound such as sugar, starch, or cellulose that contains oxygen in addition to carbon and hydrogen.

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