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Carbohydrates - Part One

Updated on October 7, 2015

What are Carbohydrates?

This hub series serves to explain what carbohydrates are and what different kinds of them there are (including examples). Primarily, this hub will be used for revising the A Level Topics concerning carbohydrates by students who wish to know everything they need to about carbohydrates for their exams.

Any molecule that falls under the molecular formula Cm(H2O)n is classified as a carbohydrate.

To remember this, realise that the very word “carbohydrate” means hydrating (adding water to) carbons. That is, adding H2O to C, or 'CH2O'.

Carbohydrates have a total of 4 categories:

  1. Monosaccharides,
  2. Disaccharides,
  3. Oligosaccharides,
  4. Polysaccharides.

However, sometimes "oligosaccharides" are placed under "polysaccharides" to make things simpler. This is true for school exams such as the AS Edexcel exam where the term "oligosaccharides" will never be used.


Molecular structure of glucose.
Molecular structure of glucose. | Source


Molecular Structure of Galactose
Molecular Structure of Galactose | Source


Monosaccharides are single units of sugar and are the monomers (subunits) of bigger molecules called polymers.

You must know that these indeed are monosaccharides and the facts about them:

  • Glucose - The main sugar used by cells in respiration. Glycogen, starch and cellulose are all polymers of glucose and are broken down into it when energy is needed within an organism.
  • Fructose - Naturally occurs in fruit, honey and various vegetables. It has a sweet scent and is therefore used as part of seed dispersal, attracting animals to eat the fruit and defecate the seeds.
  • Galactose - Combines via condensation reaction to form the disaccharide 'lactose', an abundant sugar in milk.

    Note: The only difference between glucose and galactose is that the -OH on carbon 4 faces a different way. They both have the same molecular formula: C6H12O6.

Table Sugar - "Sucrose"
Table Sugar - "Sucrose" | Source


Disaccharides are compiled of two monosaccharides and are formed out of a condensation reaction (giving off water) between them.

The disaccharides that you must know about include:

  • Sucrose - 'The Table Sugar', a disaccharide consisting of one glucose and one fructose molecule. It is the form in which sugar is transported around the plant.
  • Maltose - A disaccharide of two glucose molecules. It is produced when amylase (found in saliva) breaks down starch, and then is itself broken down into glucose. It is found in germinating seeds which break down their starch reserves in order to access glucose for much needed respiration.
  • Lactose - A disaccharide of glucose and galactose. It is found in milk.

Starch is Everywhere!
Starch is Everywhere!


Polysaccharides are made up of many monosaccharides which join together via glycosidic bonds.

The following three polysaccharides are all polymers of glucose:

  • Starch
  • Glycogen
  • Cellulose

All 3 of these are not very soluble and do not taste sweet.

Carbohydrate Structure Explained by Video


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      victoriaS 4 years ago

      this is very helpful!

    • Philanthropy2012 profile image

      DK 5 years ago from London

      Very glad I could help :)

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      Kate 5 years ago

      This is great! thanks it helped me study!!