CIE IGCSE Chemistry

Revision Notes

14.3.3 Natural Polymers

Proteins & Carbohydrates

  • These are two of the main and most important components of food
  • Carbohydrates provide energy which is released during cellular respiration
  • Proteins are the building blocks of cells and are essential for growth and all of the enzyme catalysts in the body are proteins
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Proteins as Polymers

  • Proteins are condensation polymers which are formed from amino acid monomers joined together by peptide bonds, similar to the structure in Nylon
  • The units in proteins are different however, consisting of amino acids
  • Amino acids are small molecules containing NH2 and COOH functional groups
  • Most proteins contain at least 20 different amino acids
  • These are the monomers which polymerise to form the protein

 

Forming-a-Protein, IGCSE & GCSE Chemistry revision notesDiagram showing condensation polymerisation to produce a protein

 

Hydrolysis of proteins

  • Proteins can be hydrolysed by the addition of water in acidic or alkaline conditions
  • Heat and concentrated acid (usually 6 mol/dm3 HCl) are used with a reflux condenser to prevent the acidic vapours from escaping the reaction vessel
  • Aqueous ammonia is added after completion to neutralise the excess acid
  • Enzymes can also be used to hydrolyse some proteins at room temperature, mimicking natural bodily processes

 

Hydrolysis-of-a-Protein, IGCSE & GCSE Chemistry revision notesDiagram showing the rupture of a peptide link by hydrolysis

Exam Tip

When drawing biological polymers it is important that you show the peptide link clearly in your sketch.

 

Peptide-Link, IGCSE & GCSE Chemistry revision notesDiagram showing a peptide link which holds proteins together

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Carbohydrates, Fermentation & Chromatography

Carbohydrates

  • Carbohydrates are compounds of carbon, hydrogen and oxygen with the general formula:
Cx(H2O)y
  • There are simple carbohydrates and complex carbohydrates
  • Simple carbohydrates are called monosaccharides and are sugars such as fructose and glucose
  • Complex carbohydrates are called polysaccharides such as starch and cellulose. These are condensation polymers formed from simple sugar monomers
  • Complex carbohydrates, unlike proteins, are usually made up of the same monomers
  • A H2O molecule is eliminated when simple sugars polymerise. The linkage formed is an -O- linkage called a glycosidic linkage

 

Polysaccharide-with-Glycosidic-linkage, IGCSE & GCSE Chemistry revision notesDiagram of a polysaccharide showing the glycosidic linkages (-O-) binding the monomers together

 

Hydrolysis of carbohydrates

  • The complex carbohydrates also undergo hydrolysis and produce the simple sugar monomers from which they were made
  • This can be done by refluxing with more moderately concentrated HCl

Fermentation of simple sugars

  • Simple sugars can be fermented to produce alcohol
  • They are dissolved in water and yeast is added to be fermented between 15 and 35°C in the absence of oxygen for a few days
  • If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured
  • Yeast contains zymase enzymes (biological catalysts) that break down starch or sugar to glucose
  • The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide:
C6H12O6 + Enzymes → 2CO2 + 2C2H5OH

Chromatography

  • The identification of the products of the hydrolysis of carbohydrates and proteins can be done using chromatography
  • Originally used for separating coloured substances, chromatography can be used to identify colourless compounds using locating agents
  • Both carbohydrate and protein monomers are colourless so locating agents must be used
  • A technique called 2-Dimensional paper chromatography is used as some simple sugars and amino acids have the same Rf value
  • In this technique a run is carried out in one direction, then the paper is rotated by 90º and performed again using a different solvent
  • This further separates sample spots that may not have separated in the first run
  • The resulting chromatogram is dried and sprayed with a locating agent
  • The Rf value of each solvent used is characteristic for each sugar or amino acid

 

Two-dimensional-Paper-Chromatography, IGCSE & GCSE Chemistry revision notesDiagram showing the procedure for performing 2-Dimensional paper chromatography

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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