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 C_x(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)

• 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. 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. HO-XXXXX-OH + HO-XXXXX-O-XXXXX-OH + H₂O etc.
• n C₆H₁₂O₆ ==>  (C₅H₁₀O₅)_n + nH₂O (where n is a very large number to form the natural polymer)
• The XXXXX or the [rectangles] below, represent the rest of the carbon chains in each unit (more detail in the 3rd diagram below).

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?

• 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)

• 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.

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

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!

  • See also ...

  • Oils, fats and margarine notes

  • Aspects of food additives and cooking chemistry