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Carbohydrates

TOPICS: Carbohydrates  Lipids  Proteins  Many proteins are enzymes  Nucleic acids  ATP  Water  Inorganic Ions

Carbohydrates:

Monosaccharides are the monomers from which larger carbohydrates are made. These include glucose, fructose and galactose. Only the structure of glucose needs to be known for the whole of a level which is illustrated as follows on either side. Notice the OH group and the hydrogen are on opposite sides on carbon number 1 (there is no particular way of counting the carbons however exam questions will refer to this particular carbon as carbon 1. Number 4 is always the carbon opposite carbon number 4).

Two monosaccharides form a disaccharide in any combination where glucose is always one of the reactants –

Glucose + Glucose = Maltose

Glucose + Fructose = Sucrose

Glucose + Galactose = Lactose

 

Polysaccharides are made by the condensation reactions of many monosaccharides. Glycogen and starch are made from many alpha glucose molecules and cellulose is formed from many beta glucose molecules. As part of AQA, you need to know the characteristics of these three polysaccharides:

Starch:  Is insoluble so it does not draw water in by osmosis. Starch will not diffuse easily out and is stored in a tight place because it is compact due to the coils it has. It can easily be hydrolysed into alpha glucose molecules which can used in respiration. Starch is found in plants.

Glycogen: Is shorter and is more branched than starch. It is more readily hydrolysed than starch.

Cellulose: Is made up of many straight unbranched chains of beta glucose molecules that run parallel to one another. Hydrogen bonds link these chains together which gives cellulose its strength which we are familiar with when talking about cell walls.

Knowing the tests for the sugars and for starch has to be known for the exam:

Reducing sugars (includes glucose, fructose, galactose, maltose and lactose):

  • Add Benedict’s reagent and heat

RESULT

  • Blue to brick red precipitate

Non-reducing sugar (sucrose):

  • Add HCl (hydrochloric acid) and boil
  • Add NaOH (sodium hydroxide) to neutralise
  • Add Benedict’s reagent

RESULT

  • Blue to brick red precipitate

Starch:

  • Add Iodine Potassium solution

RESULT

  • Blue-black colourisation