School chemistry revision 14-16 GCSE level chemistry notes:

Carbohydrates and the structure of natural polymers e.g. starch and DNA

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CARBOHYDRATES - SUGARS - natural polymers - STARCH and DNA

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12. Natural Molecules - carbohydrates - sugars - natural polymers like starch and DNA

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Naturally Occurring Molecules from plants and animals

12. Small Molecules like glucose  ==> Natural Polymers = Macromolecules like starch

Carbohydrates (this page), Proteins and Oils & Fats are the main nutrient constituents of food.

  • Most naturally occurring molecules are based on the elements carbon, oxygen and hydrogen, together with smaller proportions of nitrogen, phosphorus, sulfur (sulphur) and sometimes metal ions like iron (in the haemoglobin molecule) and magnesium (in the chlorophyll molecule).

  • Carbohydrates are important sources of energy for many living organisms and their formation in plants via photosynthesis is the basis of many food chains.

    • Initially this is formed as glucose, but this is converted to glycogen and starch for 'storage'.

    • carbon dioxide + water + sunlight energy ==> glucose + oxygen

    • 6CO2 + 6H2O ==> C6H12O6 + 6O2 

  • Carbohydrates are a whole series naturally occurring molecules based on the elements carbon, hydrogen and oxygen.
  • They are an important source of chemical energy in our diet.
  • e.g. the respiration reaction
    • glucose + oxygen ==> carbon dioxide + water
    • C6H12O6 + 6O2  ==> 6CO2 + 6H2O

  • Carbohydrates like glucose and fructose are used as sweeteners in food as well as sweets themselves.
  • Historically the name 'carbohydrate' comes from the fact that all their formulae seemed to be based on Cx(H2O)y (see key above) BUT this is not the way to think of their formula.
  • They range from relatively small molecules called monosaccharide (means one basic unit), or disaccharide (two basic units combined) to very large natural polymers or macromolecules called polysaccharides (many units combined). A summary of them is shown in the key diagram above along with some familiar names from biology.

Glucose is one of the simpler sugar molecules  (a monosaccharide). The structural formula is shown on the left and you should be able to see that there are 4 bonds to each carbon, 2 to each oxygen and just 1 bond to each hydrogen atom. The right-hand 'shorthand' skeletal formula version uses short straight lines to represent bonds. Most H's and their bonds are not shown, and at AS-A2 level it is assumed you can interpret these structures 'back to' a full structure!, but they are handy for describing large 'biochemical' molecules (see polysaccharide below)


  • Sucrose is a disaccharide made from combining two monosaccharide molecules,  glucose and fructose by the elimination of a water.
    • 2C6H12O6 <==> C12H22O11 + H2O
    • On hydrolysis sucrose reforms the glucose and fructose.
  • The formation of complex carbohydrates:
    • These are made of smaller carbon, hydrogen and oxygen based molecules combining together e.g. the polysaccharides starch and cellulose are formed from glucose, molecular formula C6H12O6.

  • Their formation can be described in terms of a large number of sugar units joined together by condensation polymerisation e.g. starch and cellulose are natural polymers
    • e.g. the 'box' diagram below shows 4 units of a natural carbohydrate polymer being formed
    • Note: Condensation polymerisation means the joining together of many small 'monomer' molecules by eliminating an even smaller molecule between them to form the linkage.
      • e.g. n HO-XX-OH ===> [-XX-O-]n- + nH2O
      • n C6H12O6 ===>  (C5H10O5)n + nH2O
      • (where n is a very large number to form the natural polymer)
    • plus many H2O etc.

This diagram of starch or cellulose is in 'skeletal formula' style and both are polymers of glucose - can you see the connection between each 'unit' and the structure of glucose itself?

A simplified representation - many polymeric carbohydrates have complex cross-linked structures.

  • The resulting natural polymer is called a polysaccharide.


  • Acid hydrolysis of complex carbohydrates (e.g.. starch) gives simple sugars.

    • This can be brought about by e.g. warming starch with hydrochloric acid solution to form glucose.

    • (C5H10O5)n + nH2O ==> n C6H12O6 (where n is a very large number)

    • In your body starch and other complex carbohydrate molecules are broken down by enzymes like amylase.

  • The hydrolysis products from polysaccharides can be analysed with paper chromatography as in the case of amino acids.

  • We can digest long molecules like starch, though they have to be broken down by enzyme action before the smaller molecules like glucose can be used in respiration.

    • Carbohydrates are an important source of energy in our diet e.g. in breakfast cereals, typical nutrition information illustrated below ...

    •  Cereal 1:

    • A breakfast cereal based on oats which contains 60.8% carbohydrate and 3.2% sugar.

    • Cereal 2:

    • A breakfast cereal based on wheat which contains 53.6% carbohydrate and 12.1% sugar. Cereal is less healthy than cereal 1 as regards sugar, but it does have a lot of essential vitamins and minerals added (for children?), see picture below, and the sweet taste is more tempting!

  • However, we cannot digest cellulose because we don't have the enzymes to effect this process, unlike cows etc.!

What happens when potatoes are cooked?

  • Potatoes are a good source of carbohydrates, hence a good source of energy for the body.

  • Raw potato is not easily digested but cooking partially breaks the structure of potato down so that we can digest it AND cooking also improves the texture and taste of the potato to make it more palatable to eat.

  • In the cooking process the plant cell walls of cellulose are softened and this allows hot water to penetrate the cells.

  • Further cooking causes the starch granules to swell and gelatinize as they absorb hot water.

  • This eventually causes the cells to separate producing the soft texture of well-cooked digestible potatoes!

  • Mash with milk, butter and salt to suit taste! Cheap and yummy!

DNA - deoxyribonucleic acid

  • DNA (deoxyribonucleic acid) is a large molecule essential for life and cell replication and is another example of a natural polymer.
  • It is the natural polymer found in all living organisms and viruses too.
  • DNA encodes genetic instructions for the development and functioning of living organisms and viruses e.g. every protein molecule is synthesised by other molecules reading the genetic code and combining the right amino acids in the right order.
  • Most DNA molecules consist of two polymer chains, made from four different monomers called nucleotides, connected together in the form of a double helix.
    • Nucleotides form the building blocks of DNA or RNA and an individual nucleotide consists of three molecular bits combined together - a phosphate group, a base (adenine, cytosine, guanine, and thymine), and a pentose sugar.
    • In RNA the thymine base is replaced by uracil.
  • So a DNA molecule consists of two 'molecular' strands coiled together to form a double helix, but how is this helix held together?
  • The strands are linked by a series of complementary base pairs joined together by weak hydrogen bonds (base-pairing H bonds shown here as ):
  • There are four bases in DNA holding the structure together and the same bases are always paired together.
  • (i) adenine (A) with thymine (T) i.e. AT, and (ii) cytosine (C) with guanine (G) i.e. CG whererepresents the weak (but crucial) intermolecular attractive force between pairs of bases, called the hydrogen bond.
    • It is these cross-links of hydrogen bonds that holds the two DNA strands together as a double helix.
  • The double helix structure is shown in the diagram on the right, illustrating how the DNA is held together by the hydrogen bonds.

Advanced level chemistry notes on amine bases and the hydrogen bonding in DNA and RNA

GCSE biology notes on DNA and RNA structure and Protein Synthesis and an experiment to extract DNA

Multiple Choice Quizzes and Worksheets

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