Unit 20: Natural polymers

Unit 20: Natural polymers

1. Name the main constituents of food as carbohydrates, fats and proteins

   • Carbohydrates, fats and proteins are the main constituents of food. Carbohydrates and fats give us energy. Proteins are needed for growth and other essential functions. Proteins and carbohydrates are macromolecules. Fats are large molecules which usually contain three ester linkages.

2. Describe in simple terms the structure of amino acids

   • All amino acids have the amine (-NH2) and carboxylic acid (-COOH) groups in common.

3. Describe in simple terms the structure of proteins, especially the amide linkage

   • When proteins are made, an amide link is formed by the reaction of the amine group of one amino acid with the carboxylic acid group of the next amino acid. This is an example of a condensation reaction. Water is eliminated.

4. Describe the hydrolysis of proteins to amino acids

   • We break down proteins to amino acids by heating them with hydrochloric acid. This is a hydrolysis reaction. In a hydrolysis reaction, a compound reacts with water and breaks down to form two or more products. We often use acid or alkali to help the hydrolysis.
   • Heat the protein with 6 mol/dm3 hydrochloric acid for several hours. The condenser is vertical to stop the loss of hydrochloric acid vapour. At the end of experiment, neutralise excess hydrochloric acid with ammonia.
   • When proteins are hydrolysed, the amide linkages are converted to carboxylic acid and amine functional groups - the reverse of the polymerisation reaction.

5. Describe in simple terms the structure of fats, especially the ester linkage

   • When we react glycerol with fatty acids, an ester linkage is formed. Three ester linkages are formed when a fat or oil molecule is made. This is a condensation reactiom because two molecules have reacted to form a larger molecule, and a small molecule, water, has been eliminated.

6. Describe the hydrolysis of fats and oils to make soap

   • We hyrdolyse fats with concentrated sodium hydroxide solution.
   • Heat a mixture of vegetable oil and concentrated sodium hydroxide in the steam for 15 mins. Add concentrated sodium chloride solution (about 40 cm3). Stir and heat for 5 mis. Let mixture cool. Soap forms on top of the liquid mixture.

7. Describe in simple terms the structure of complex carbohydrates

   • We give the name monosaccharides to simple sugars such as glucose: mono means 'one' and saccharide means sugar. We can join monosaccharide monomers together to make polymers called polysaccharides. This is a condensation polymerisation. The monosaccharide monomers have joined together and water has been eliminated. The linkage in these sugar polymers is called a glycosidic linkage.

8. Describe the hydrolysis of complex carbohydrates to simple sugars

   • When we heat polysaccharides with hydrochloric acid they are hydrolysed to simple sugars. e.g. starch is hydrolysed to glucose: starch + water --hydrochloric acid + heat--> glucose. When starch is hydrolysed the glycosidic linkages break and simple sugars are formed. The hydrochloric acid acts as a catalyst in the reaction.

9. Describe how chromatography is used to identify amino acids and simple sugars

   • Because sugars are colourless, we have to use a locating agent so they show up on the chromatogram.
   • Some compounds have the same or similar Rf values so you cannot always identify a sugar or an amino acid from a chromatogram by simple chromatography. We overcome this by using two-dimensional chromatography. We carry out the chromatography as usual. We allow the paper to dry, then turn it 90 degrees and carry out chromatography using a different solvent. Only then do we spray the chromatogram with a locating agent.

10. Describe enzymes as biological catalysts

    • Enzymes are biological catalysts. They are sensitive to changes in temperature. They work best between 30 - 40 degrees C. If the temperature is too low, they work much more slowly. If the temperature is too high, they denature and don't work.

11. Name fermentation as a method of producing ethanol from simple sugars

    • Fermentation is a method of producing ethanol from simple sugars.
    • Fermentation is the breakdown of organic material with effervescence (bubbles) and release of heat.

12. Describe how simple sugars are fermented to produce ethanol

    • The most commonly used substances for making ethanol are sugars and starch. The bacteria and yeast that cause fermentation are found on the surface of many plants, as well as in the air. When we carry out fermentation to produce ethanol, we use a particular form of yeast so that unwanted reactions do not occur.
    • Add moist yeast to a flask of warm glucose solution. Shake the solution so that the yeast is suspended in the solution. Keep the flask warm for about 30 mins without shaking. 'Pour' the gas from the flask into a boiling tube containing limewater. The limewater turns milky - this shows that carbon dioxide is a product of fermentation.
    • The fermentation of glucose produces ethanol and carbon dioxide. The reaction takes place in the absence of air - it is anaerobic. The ethanol is separated from the reaction mixture by fractional distillation.

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