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Organic compounds composed of carbon, hydrogen and oxygen.

Atoms Practice
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Practice Carbohydrates
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Carbohydrates - Advanced

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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 all attached together. Why? One reason is to store energy. But that does not mean you should eat it by the spoonful.


Carbohydrates are organic compounds that contain only carbon (C), hydrogen (H), and oxygen (O). They are the most common of the four major types of organic compounds. There are thousands of different carbohydrates, but they all consist of one or more smaller units called monosaccharides.

Monosaccharides and Disaccharides

The general formula for a monosaccharide is (CH2O)n, where n can be any number greater than two. For example, if n is 6, then the formula can be written C6H12O6. This is the formula for the monosaccharide glucose. Another monosaccharide, fructose, has the same chemical formula as glucose, but the atoms are arranged differently. Molecules with the same chemical formula but with atoms in a different arrangement are called isomers. Compare the glucose and fructose molecules in Figure below. Can you identify their differences? The only differences are the positions of some of the atoms. These differences affect the properties of the two monosaccharides.

Monosaccharides can be classified by the number of carbon atoms they contain: diose (2), triose (3), tetrose (4), pentose (5), hexose (6), heptose (7), and so on.

In addition to glucose, other common monosaccharides include fructose ("fruit sugar"), galactose, xylose ("wood sugar") and ribose (in RNA) and deoxyribose (in DNA).

If two monosaccharides bond together, they form a carbohydrate called a disaccharide. Two monosaccharides will bond together through a dehydration reaction, in which a water molecule is lost. A dehydration reaction is a condensation reaction, a chemical reaction in which two molecules combine to form one single molecule, losing a small molecule in the process. In the dehydration reaction, this small molecule is water. The bond between two monosaccharides is known as a glycosidic bond.

An example of a disaccharide is sucrose (table sugar), which consists of the monosaccharides glucose and fructose (Figure below). Other common disaccharides include lactose ("milk sugar") and maltose. Monosaccharides and disaccharides are also called simple sugars. They provide the major source of energy to living cells.

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Source: http://commons.wikimedia.org/wiki/File:Saccharose.svg
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Sucrose Molecule. This sucrose molecule is a disaccharide. It is made up of two monosaccharides: glucose on the left and fructose on the right. Sucrose forms through a condensation reaction: glucose (C6H12O6) + fructose (C6H12O6) sucrose (C12H22O11).[Figure2]


An oligosaccharide is a saccharide polymer containing a small number (typically two to ten) of monosaccharides. Oligosaccharides can have many functions; for example, they are commonly found on the plasma membrane of animal cells where they can play a role in cell–cell recognition. In general, they are found attached to compatible amino acid side-chains in proteins or to lipids.

Oligosaccharides are often found as a component of glycoproteins or glycolipids. They are often used as chemical markers on the outside of cells, often for cell recognition. An example is ABO blood type specificity. A and B blood types have two different oligosaccharide glycolipids embedded in the cell membranes of the red blood cells, AB-type blood has both, while O blood type has neither.


Polysaccharides are long carbohydrate molecules of repeated monomer units joined together by glycosidic bonds. A polysaccharide may contain anywhere from a few monosaccharides to several thousand monosaccharides. Polysaccharides are also called complex carbohydrates. Polysaccharides have a general formula of Cx(H2O)y, where x is usually a large number between 200 and 2500. Considering that the repeating units in the polymer backbone are often six-carbon monosaccharides, the general formula can also be represented as (C6H10O5)n, where 40≤n≤3000.

Starches are one of the more common polysaccharides. Starch is made up of a mixture of amylose (15–20%) and amylopectin (80–85%). Amylose consists of a linear chain of several hundred glucose molecules and amylopectin is a branched molecule made of several thousand glucose units. Starches can be digested by hydrolysis reactions, catalyzed by enzymes called amylases, which can break the glycosidic bonds. Humans and other animals have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources of starch in the human diet. The formations of starches are the ways that plants store glucose. Glycogen is sometimes referred to as animal starch. Glycogen is used for long-term energy storage in animal cells. Glycogen is made primarily by the liver and the muscles.

The main functions of polysaccharides are to store energy and form structural tissues. Examples of several other polysaccharides and their roles are listed in the Table below. These complex carbohydrates play important roles in living organisms.

Complex Carbohydrates
Complex Carbohydrate Function Organism
Starch Stores energy


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Amylose Stores energy Plants
Glycogen Stores energy


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Cellulose Forms cell walls


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Chitin Forms an exoskeleton

Some animals

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KQED: Biofuels: From Sugar to Energy

For years there's been buzz – both positive and negative – about generating ethanol fuel from corn. But thanks to recent developments, the Bay Area of California is rapidly becoming a world center for the next generation of green fuel alternatives. The Joint BioEnergy Institute is developing methods to isolate biofeuls from the sugars in cellulose. See Biofuels: Beyond Ethanol at http://www.kqed.org/quest/television/biofuels-beyond-ethanol for further information.


  • amylase: An enzyme that catalyses the breakdown of starch into sugars; present in human saliva.
  • carbohydrate: Organic compound such as sugar or starch; major source of energy to living cells.
  • complex carbohydrate: Carbohydrates with three or more monosaccharides bonded together.
  • condensation reaction: A chemical reaction in which two molecules combine to form one single molecule, together with the loss of a small molecule, often water.
  • disaccharide: A carbohydrate composed of two monosaccharides.
  • glycogen: A carbohydrate used for long-term energy storage in animal cells; human muscle and liver cells store energy in this form.
  • glycolipid: A lipid with a carbohydrate attached; provides energy and serves as markers for cellular recognition.
  • glycoprotein: A protein that contain oligosaccharide chains (glycans) covalently attached to polypeptide side-chains; often important integral membrane proteins, where they play a role in cell–cell interactions.
  • glycosidic bond: A covalent bond that joins a carbohydrate molecule to another group, which may or may not be another carbohydrate.
  • hydrolysis reaction: A chemical process in which a molecule of water is split, resulting in the separation of a large molecule into two smaller molecules.
  • isomers: Molecules with the same chemical formula but with differently arranged atoms.
  • monosaccharide: A simple sugar such as glucose; the building block of carbohydrates.
  • oligosaccharide: A saccharide polymer containing a small number (typically two to ten) of monosaccharides.
  • polysaccharide: A chain of monosaccharides; a complex carbohydrate such as starch or glycogen.
  • starch: A carbohydrate used for long-term energy storage in plant cells.


  • Carbohydrates are organic molecules that consist of carbon, hydrogen, and oxygen. They are made up of repeating units called saccharides. They provide cells with energy, store energy, and form structural tissues.


  1. What are carbohydrates?
  2. State the function of monosaccharides, such as glucose and fructose.
  3. Compare and contrast simple sugars and complex carbohydrates.
  4. What are glycoproteins and glycolipids?
  5. Give examples of polysaccharides.

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