Carbohydrates composed of carbon atoms bonded by hydrogen atoms and hydroxyl groups (H-C-OH). The simplest building block of a carbohydrate is a monosaccharide. Monosaccharides are assembled into chains and the macromolecule version of sugars known as polysaccharide are formed by dehydration or condensation reactions.

Sugars are used for energy storage in cells and carbon skeletons to form other biological structures.

When there is “ose” on the end of a word, this is a clue that it is about sugar. Some examples of sugars are ribose, glucose, lactose and cellulose.

CH2O, the general formula of carbohydrates shows the proportion of carbon, hydrogen and oxygen in monosaccharides. But, the proportion is different in disaccharides and polysaccharides due to the removal of water molecules (H2O) during a series of condensation reactions.


Glucose (C6H12O6) is the most common monosaccharide. It is produced by green plants during photosynthesis reaction. Glucose is the major fuel source for all type of cells. Cells break down glucose by a series of reactions to release energy and produce water and carbon dioxide.

Different Forms of GlucoseGlucose exists in nature in two forms; ring form and the straight chain. In cells, ring form is more common since it is more stable than the straight form.α-glucose and β-glucose is the two different form of ring glucose. The difference between two is the position of hydroxyl and hydrogen bounded to the carbon 1. Monosaccharides are often in D-isomer form in cells.

Monosaccharides are classified by

  • the location of the carbonyl group (as aldose or ketose)
  • the number of carbons in the carbon skeleton (pentose or hexose)

Ribose and deoxyribose are examples of pentoses. These two sugars take part in the backbone of nucleic acids. One oxygen atom is missing in deoxyribose at carbon 2.

Hexoses (glucose, fructose, galactose and mannose) have the same molecular formula that is structural isomer. The difference between fructose and glucose is whether the double bond is at the edge of the molecule or recessed in one position and so, these two sugars are structural isomers. On the other hand, glucose and galactose are stereoisomer of each other so one of the carbon is flipped.



Glycosidic linkages bond monosaccharides to form disaccharides and polysaccharides. Disaccharides are formed when 2 monosaccharides linked covalently together by dehydration reaction. They are used for sugar transport or energy storage. The examples of disaccharides are sucrose, lactose and maltose.

Lactose (milk sugar) is formed from galactose and glucose. Sucrose contains glucose and fructose. While, maltose contains 2 glucose molecules. Cellobiose also possess 2 glucose molecules. The difference between maltose and cellobiose is glycosidic linkage of former is α-linkage, whereas cellobiose has β-linkage. Maltose and cellobiose are isomers. They have the same molecular formula but different chemical properties.


Oligosaccharides contain 3 to 20 monosaccharides. Many oligosaccharides possess functional groups. They often covalently bound to other biological polymers like lipids and proteins. For example; human blood types (AB0) are determined by oligosaccharides.


Polysaccharides are composed of long chains of monosaccharides linked through dehydration synthesis. Plants and animals use starch and glycogen as energy storage, respectively.

The chemical formula for glucose is C6H12O6. The glycogen polymer composed of 10 glucose molecules and its structural formula is calculated as

10 x C6H12O6 – 9(H2O) = C60H102O51

Storing 1000 glucose as a polysaccharide is crucial for a cell. Since by this way, cell prevents huge osmotic pressure that is going to be formed. This much of osmotic pressure would cause the cell swell due to the water entry to balance osmotic pressure.

Cellulose is another example of polysaccharides. Cellulose is the most abundant organic compound in Earth and found in plant cell walls.

Some carbohydrates are modified by addition of functional groups. For example, when amino group is added to hydroxyl group, amino sugars are formed. Glucosamine and galactosamine are examples of amino sugars. One important derivative of glucosamine is chitin. Chitin is the structural polysaccharide found in skeletons of insects, crabs and lobsters. On the other hand, galactosamine is an important component of cartilage.

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