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Organic compounds composed of carbon, hydrogen and oxygen.

Atoms Practice
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Practice Carbohydrates
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Sugar. Does this look like biological energy?

As a child, you may have been told that sugar is bad for you. Well, that's not exactly true. Essentially, carbohydrates are made of sugar, from a single sugar molecule to thousands of sugar molecules attached together. Why? One reason is to store energy. But that does not mean you should eat it by the spoonful.


Carbohydrates are the most common type of organic compound. A carbohydrate is an organic compound such as sugar or starch, and is used to store energy. Like most organic compounds, carbohydrates are built of small, repeating units that form bonds with each other to make a larger molecule. In the case of carbohydrates, the small repeating units are called monosaccharides. Carbohydrates contain only carbon, hydrogen, and oxygen in a 1:2:1 ratio.

Monosaccharides and Disaccharides

A monosaccharide is a simple sugar such as fructose or glucose. Fructose is found in fruits, whereas glucose generally results from the digestion of other carbohydrates. Glucose (C6H12O6) is used for energy by the cells of most organisms, and is a product of photosynthesis.

Sucrose Molecule. This sucrose molecule is a disaccharide. It is made up of two monosaccharides: glucose on the left and fructose on the right.

If two monosaccharides bond together, they form a carbohydrate called a disaccharide. An example of a disaccharide is sucrose (table sugar), which consists of the monosaccharides glucose and fructose (Figure above). Monosaccharides and disaccharides are also called simple sugars. They provide the major source of energy to living cells


A polysaccharide is a complex carbohydrate that forms when simple sugars bind together in a chain. Polysaccharides may contain just a few simple sugars or thousands of them. Complex carbohydrates have two main functions: storing energy and forming structures of living things. Some examples of complex carbohydrates and their functions are shown in Table below. Which type of complex carbohydrate does your own body use to store energy?

Name Function Example
Starch Used by plants to store energy. A potato stores starch in underground tubers.
Glycogen Used by animals to store energy. A human stores glycogen in liver cells.
Cellulose Used by plants to form rigid walls around cells. Plants use cellulose for their cell walls.
Chitin Used by some animals to form an external skeleton. A housefly uses chitin for its exoskeleton.


  • Carbohydrates are organic compounds used to store energy.
  • A monosaccharide is a simple sugar, such as fructose or glucose.
  • Complex carbohydrates have two main functions: storing energy and forming structures of living things.

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