AQA GCSE Science BIOLOGY 2 Unit B2.5 Proteins – their functions and uses

• Know that proteins have many functions, both inside and outside the cells of living organisms.

• Know that proteins, as enzymes, are now used widely in the home and in industry.

• You should be able to use your skills, knowledge and understanding to evaluate the advantages and disadvantages of using enzymes in the home and in industry.

AQA GCSE Science BIOLOGY Unit B2.5.1 Proteins

• a) Know that protein molecules are made up of long chains of amino acids.

  • Know that these long chains are folded to produce a specific shape that enables other molecules to fit into the protein eg the biological catalysts we call enzymes.

•  Know and understand that proteins act as:

  • structural components of tissues such as muscles

  • hormones

  • antibodies

  • catalysts.

•  b) Know and understand that catalysts increase the rate of chemical reactions.

  • Know that biological catalysts are called enzymes and they enable the vast majority of reactions in cells, hence all the essential chemical reactions of any living organism.

  • Know that enzymes are proteins.

AQA GCSE Science BIOLOGY Unit B2.5.2 Enzymes

• a) Know that the shape of an enzyme is vital for the enzyme’s function.

  • The diagram above illustrates what is known as the 'key and lock mechanism'. The molecule being processed must fit 'snugly' into the right shaped enzyme protein molecule otherwise it won't be changed into the product molecules.

  • Know that high temperatures change the shape - enzymes become denatured and ineffective, the heat interferes with the bonding within the protein molecule.

  • Enzymes work best at their optimum temperature eg ~37^oC for enzymes in the human body.

  • As the temperature increase the rate of catalysis increases (normal effect on the speed of reaction as the average kinetic energy of the molecules increases), but at high temperatures the protein structure of the enzyme is destroyed, so the active site on the enzyme is damaged and won't work correctly.

  • So, a graph of rate versus temperature rises to a maximum (optimum temperature) and then falls away as the enzyme becomes thermally denatured and destroyed and ceases to function at high temperatures (see diagram above on right).

  • 

  • This is illustrated by the graph above, which shows what happens to the speed of the enzyme catalysed process of photosynthesis as the temperature is increased.

• b) Know that different enzymes work best at different pH values.

  • Different enzymes have different optimum pHs (diagrams on right and below).

  • The pH ie how acid or how alkaline the aqueous medium is, affects the protein structure of the enzyme, so like the temperature graph, the graph rises to a maximum of the optimum pH.

  • The optimum is often around pH 7 neutral, which is the most common pH region inside cells.

  • 

  • The graph above shows a variety of optimum pH values for three different enzymes - pepsin, urease and glycine oxidase.

    • Note that pepsin works best in the acid conditions of the stomach.

    • Carbonic hydrase works best in nearly neutral media like blood plasma or cell cytoplasm.

    • Trypsin is most effective in mildly alkaline conditions.

• c) Know that some enzymes work outside the body cells.

  • Know that the digestive enzymes are produced by specialised cells in glands and in the lining of the gut.

  • The enzymes then pass out of the cells into the gut where they come into contact with food molecules.

  • They catalyse the breakdown of large molecules into smaller molecules.

    • eg large carbohydrate molecules and converted to small sugar molecules.

• d) Know that the enzyme amylase is produced in the salivary glands, the pancreas and the small intestine.

  • Know that this enzyme amylase in saliva catalyses the breakdown of starch into sugars in the mouth and small intestine.

  • Starch molecules are too big to pass through the cell membranes of the walls of the digestive system.

• e) Know that protease enzymes are produced by the stomach, the pancreas and the small intestine.

  • Know that protease enzymes catalyse the breakdown of proteins into amino acids in the stomach and the small intestine.

  • Protein molecules are too big to pass through the cell membranes of the walls of the digestive system and must be broken down by protease enzymes into amino acids first.

•  f) Know that lipase enzymes are produced by the pancreas and small intestine.

  • Know that these enzymes catalyse the breakdown of lipids (fats and oils) into fatty acids and glycerol in the small intestine.

  • Again, fat and oil molecules are too big to be absorbed through the intestine walls and must be first broken down into smaller molecules, such as fatty acids and glycerol, by lipase enzymes.

• g) Know that the stomach also produces hydrochloric acid.

  • Know that the enzymes in the stomach work most effectively in these acid conditions.

• h) Know that the liver produces bile, which is stored in the gall bladder before being released into the small intestine.

  • Bile neutralises the acid that was added to food in the stomach, the stomach fluids are too acid for most digestive processes.

  • This provides alkaline conditions in which enzymes in the small intestine work most effectively.

  • Bile also emulsifies fats, breaking them into smaller droplets which are broken down more rapidly because of the increased surface area - typical rates of reaction factor.

• i) Know that some microorganisms produce enzymes that pass out of the cells.

  • Know that these enzymes have many uses in the home and in industry.

    • In the home:

      • biological detergents may contain protein-digesting and fat-digesting enzymes (proteases and lipases)

      • biological detergents are more effective at low temperatures (saving energy) than other types of detergents for breaking down plant and animal substances including blood, food and stains.

    • In the food industry:

      • proteases are used to ‘pre-digest’ the protein in some baby foods to help digestion in immature digestive systems

      • carbohydrases are used to convert starch into sugar syrup to sweeten foods

      • isomerase is used to convert glucose syrup into fructose syrup, which is much sweeter and therefore can be used in smaller quantities in slimming foods and fizzy drinks.

• j) Know and understand that in industry, enzymes are used to bring about reactions at normal temperatures and pressures that would otherwise require.

  • Enzymes are excellent catalysts for speeding up reactions at relatively low temperatures and pressures without the need for increased energy costs of using high temperatures and more costly high pressure engineering in chemical manufacturing plants.

  • They have the advantage of catalysing a specific reaction (no other reaction, no waste chemicals), lower energy costs, they can be recycled over and over again and any waste enzymes are biodegradable.

  • There are a few disadvantages eg they are easily inhibited or denatured by changes in temperature and pH, so reaction conditions must be carefully controlled.

• Your practical work to develop your skills and understanding may have included the following:

  • designing an investigation to find the optimum temperature for biological and non-biological washing powders to remove stains from cotton and other materials

  • investigating the action of enzymes using catalase at different concentrations and measuring the rate at which oxygen is given off from different foods, eg liver, potato, celery and apple

  • planning and carrying out an investigation into enzyme action using the reaction between starch and amylase at different temperatures, pH and concentrations

  • using small pieces of cooked sausage, use 2% pepsin and 0.01M HCl in water baths at different temperatures to estimate the rate of digestion. The concentration of both enzymes can be varied

  • using computer simulations of enzymes to model their action in varying conditions of pH, temperature and concentration.