Carbohydrates (from 'hydrates of carbon') or saccharides (Greek σάκχαρον, sákcharon, meaning "sugar") are the most abundant of the four major classes of biomolecules. They fill numerous roles in living things, such as the storage and transport of energy (starch, glycogen) and structural components (cellulose in plants, chitin in animals).

Monosaccharides (Simple Sugars)

The basic carbohydrate units are called monosaccharides (single sugars). There are only three important simple sugars, glucose, fructose and galactose. All of the other ‘sugars’ you are likely to encounter in daily life are simply combinations of these three.

For example, the ‘sugar’ you see on the label of a carton of milk is lactose. Lactose is half glucose and half galactose. Maltose, the sugar in beer, is two molecules of glucose joined together in an unusual way.

This food … Contains … Which breaks down to …
Milk and Dairy Foods Lactose Galactose + Glucose
Beer Maltose Glucose + Glucose
Table Sugar, Brown Sugar, Caster Sugar etc. Sucrose Glucose + Fructose

Simple sugars like glucose and fructose can also be present in nature in their uncombined form. Most fruit, for example, contains some sucrose, some fructose and some glucose. To our digestive system however, the sucrose is just a bundle of more fructose and glucose.

Monosaccharides are our major source of fuel. When monosaccharides are not immediately needed by many cells they are often converted to more space efficient forms, often polysaccharides. In many animals, including humans, this storage form is glycogen, especially in liver and muscle cells. In plants, starch is used for the same purpose.

Monosaccharides can be linked together into what are called polysaccharides (or oligosaccharides) in almost limitless ways. Many carbohydrates contain one or more modified monosaccharide units that have had one or more molecules replaced or removed. The names of carbohydrates often end in the suffix -ose.


Two joined monosaccharides are called a disaccharides and these are the simplest polysaccharides. Examples include sucrose and lactose.

Sucrose or what we call 'table sugar' is the most abundant disaccharide, and the main form in which carbohydrates are transported in plants. It is composed of one glucose molecule and one fructose molecule.

Lactose, a disaccharide composed of one galactose molecule and one glucose molecule, occurs naturally in mammalian milk.


Carbohydrates require less water to digest than proteins or fats and are the most common source of energy in living things. Proteins and fat are necessary building components for body tissue and cells and are also a source of energy for most organisms.

Carbohydrates are not essential nutrients in humans: the body can obtain all its energy from protein and fats. However, the brain and neurons need glucose for energy but the body can make some glucose from a few of the amino acids in protein and also from the glycerol backbone in triglycerides.

Carbohydrate contains 3.75 and proteins 4 kilocalories (Calories) per gram, respectively, while fats contain 9 kilocalories (Calories) per gram. In the case of protein, this is a little misleading as only some amino acids are usable for fuel.

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