What Is a Carbohydrate or Saccharide?

Mono- Di- and Polysaccharides: Sugar's Sweet Organic Molecules

Carbs, also known as saccharides, are organic molecules that are used as energy sources, structural molecules and as components of other biological molecules.

Inorganic and Organic Molecules

Even the experts don’t agree on how to define the difference between organic and inorganic substances, but a good, broad definition is as follows.

Inorganic molecules are essentially substances that don’t have carbon-hydrogen (C-H) bonds, whereas organic molecules are substances that contain carbon-hydrogen bonds, are found in living things.

The major classes of organic molecule include carbohydrates, proteins, lipids and nucleic acids.

Carbohydrates

The term carbohydrate is actually a descriptor of what these molecules are composed of. They are “carbon hydrates,” in a ratio of one carbon molecule to one water molecule (CH2O)n.

You may recognize carbohydrates as source of energy (starch, glycogen), but they fulfill a wide range of roles, including the structural materials of plants (cellulose in plant cell walls) and of some animals chitin of an insect’s exoskeleton.

Carbohydrates can also be components of other molecules such as DNA, RNA, glycolipids, glycoproteins, and adenosine triphosphate (ATP)

Saccharides

The word saccharide is a synonym for carbohydrate and is generally preceded with a prefix indicating the size of the molecule (mono-, di-, tri- poly-).

Monosaccharides

single sugars (one molecule)
simplest
examples are glucose and fructose

Disaccharides

double sugars
combination of two monosaccharides
sucrose = glucose + fructose
lactose = glucose + galactose

Polysaccharides

polymers composed of several sugars
can be same monomer (many of same monosaccharide) or mixture of monomers
glycogen is the major stored carbohydrate in animals
starch is storage polysaccharide of plants. It is a long chain of glucose molecules.
chitin is a structural carb in some animals
cellulose is the major structural carbohydrate in plants

Building and Breaking Down Sugars

Dehydration and hydrolysis are chemical reactions that make bigger (dehydration) and smaller (hydrolysis).

Dehydration Reactions

Dehydration is when one molecule contributes a hydrogen (H) and the other a hydroxyl group (OH), therefore the removal of a water molecule (H2O) results in the joining of two smaller molecules. With respect to carbohydrates, dehydration reactions make bigger carbohydrate molecules from smaller sugars.

Hydrolysis Reactions

Hydrolysis is the reverse of dehydration and is when the addition of a water molecule breaks (lyses) a larger molecule into two smaller molecules. With respect to carbohydrates hydrolysis, the bonds on the larger carbohydrate are broken through the addition of water. One of the smaller molecules receives a hydrogen (H) and the other received a hydroxyl group (OH)

Grains are one souce of dietary carbohydrates.

Chemical Structureof Carbohydrate Lactose

Chemical Stucture of Carbohydrate Sucrose

Additional Organic Chemistry Resources

To learn more about organic molecules and cell biology, see Science Prof Online and the Organic Chemistry Help page or look to additional Suite101 articles including, What Is a Carbohydrate, Nucleotides and Nucleic Acids, What Is a Lipid, Amino Acids & Proteins and What are Organic Molecules.