Biology 1- Carbohydrates

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Carbohydrates are organic compounds containing the elements carbon, hydrogen and oxygen. 


Monosaccharides are small organic molecules and provide the building blocks for the larger carbodydrates. They have the general formula (CH2O)nand their name is determind by the number of carbon atoms in the molecule. Monosaccharides usually exist as ring structures when dissolved in water. 

Monosaccharides have two main functions:

  • They are used as a source of energy in respiration. Carbon-hydrogen bonds are broken to release energy, which is transferred to make adenosine triphosphate (ATP) from adenosine diphosphate (ADP)


Glucose is a hexose monosaccharide sugar. All hexose sugars share the same general formula C6H12O6but they differ in their molecular structure.

Glucose exists as two isomers, the alpha form and the beta form. These different forms results in considerable biological differences when they form polymers such as starch and cellulose. 



Disaccharides consist of two monosaccharide units linked together with the formaion of a glycosidic bond and the elimination of water. This is called a condensation reaction. 


Disaccharides can also be formed by the joining together of two different monosaccharides:

  • glucose and fructose join to form sucrose
  • glucose and galactose join to form lactose

Disaccharide are used for storage and transport e.g. carbohydrate is transported in the sieve tubes of phloem in the form of sucrose.

Testing for the presence of sugars

Using Benedicts reagent:

1. add an equal volume of Benedict's reagent (blue) to the solution being tested and heat in a boiling water bath

2. if a reducing sugar is present the solution will go from green to yellow to orange and then finally to brick red

Non reducing sugars such as sucrose will provide a negative test. Sucrose can only be detected if it is first broken down to its constituent monosaccharides by heating with hydrochloric acid. Then by neutralising the solutions by adding…


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