AQA GCSE Biology B2.3 Enzymes

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B2.3.1 Proteins, Catalysts and Enzymes

  • Proteins:
  • Protein molecules are very important in the body, and are made up of long chains of amino acids. Different arrangements of amiino acids give different proteins.
  • Proteins act as structural components like muscles and tendons, hormones like insulin, antibodies to destroy pathogens and catalysts in the form of enzymes.
  • Catalysts:
  • These are used to speed up chemical reactions. It isn't used up in the reaction and stays unchanged before and after the reaction. It can be used repeatedly.
  • Enzymes:
  • These are biological catalysts and large proteins, made of amino acid chains folded to produce a specific shape.
  • The specific shape allows other molecules; substrates, to fit into the enzyme protein. The part along the enzyme where it fits the substrate is also known as the active site.
  • Enzymes are involved in building large molecules from small ones, changing one molecule into another, and breaking down large molecules into smaller ones (like digestive enzymes).
  • The substrate (reactant) of the reaction fits onto the active site of the enzyme like two jigsaw pieces, and once in place they bind together.
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B2.3.2 Limiting Factors for Enzymes

  • Temperature
  • Enzyme reactions are similar to other reactions when the temperature is increased-the rate is increased.
  • This is however, only true up to around 40ºC. After this the protein structure of the enzyme is affected and the long amino acid chains begin to unravel, which means that the shape of the active site changes and the substrates cannot fit there anymore. Therefore, the enzyme has denatured and it can no longer be a catalyst, so the rate of reaction drops.
  • Most human enzymes work best at around 30ºC.
  • pH
  • Each enzyme also works best at a certain pH. The shape of the active site of an enzyme comes from forces between different parts of the protein molecule, which hold the folded chains in place, but the pH can affect these forces.
  • If the pH changes too much, the active site will be lost and the enzyme will no longer act as a catalyst, and is dentatured.
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B2.3.3 Enzymes in Digestion

  • Food is made up of large, insoluble molecules that your body cannot absorb.
  • Most enzymes work inside cells in your body, but digestive enzymes don't. They are made in specialised cells in glands (like salivary glands and pancreas) and in the lining of your gut.
  • The enzymes pass out of the cells into the gut itself.
  • The gut is a hollow, muscular tube that squeezes you food, helping to break food up into small pieces with a larger surface area for enzymes to work on. It also mixes the food with digestive juices so enzymes come into contact with as much of the food as is possible.
  • The muscles of the gut move your food from one area to the next.
  • Carbohydrates-Enzymes that break down these are carbohydrases. Starch is the most common carbohydrates eaten, and is broken down into sugars in your mouth and small intestine. The enzyme used for this is amylase.
  • Amylase is produced in your salivary glands, pancreas and small intestine. Digestion starts in your mouth but no digestion occurs in the pancreas (all the enzymes from the pancreas flow into the small intestine).
  • Proteins- Enzymes digesting these are protease enzymes. They are made by the stomach, pancreas and small intestine. Proteins are broken down into amino acids, taking place in the stomach and small intestine.
  • Fats-Lipids (fats and oils) are broken down into fatty acids and glycerol in your small intestine, catalysed by lipase. These are made in the pancreas and small intestine. Digestion occurs in the small intestine.
  • Once foof molecules are digested into soluble glucose, amino acids, fatty acids and glycerol, they pass through the wall of your small intestine into the bloodstream to be carried around the body.
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B2.3.4 Speeding up Digestion

  • If the body temperature is kept steadily at 37ºC, your enzymes have an ideal temperature allowing them to work as fast as possible.
  • However, controlling the pH in your gut at ideal levels isn't as easy as different enzymes work best at different pH levels. For example, proteases in the stomach works best in acidic conditions, but the proteases made in the pancreases work best in alkaline condittions.
  • There are about 35 million glands in the lining of the stomach, secreting proteases. They work best in acidis levels, so the stomach produces a concentrated solution of hydrochloric acid from the same glands. In fact, about 3L of hydrochloric acid is produced in the stomach each day.
  • The stomach also produces a thick layer of mucas, coating stomach walls and preventing them from being digested by the acid and enzymes.
  • Amylase and lipase work in the small intestine, where conditions are slightly alkaline. This is where the food goes after a few hours in the stomach.
  • Your liver produces a greenish-yellor alkaline liquid called bile, which is stored in the gall bladder and the squirted into the small intestine, neutralising the stomach acid.
  • The fats you eat don't mix with all the watery liquids in your gut, staying as large globules (like oil in water) making it difficult for lipase to act.
  • The next function of bile is to emulsify fats in food, so it breaks up large drops of fat into smaller droplets, providing a larger surface area for lipases to work.
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B2.3.5 Making Use of Enzymes

  • Biological detergents can be used to remove grass, sweat and food stains from clothes. They contain proteases and lipases, breaking down the proteins and fats in stains, to help give a cleaner wash at lower temperatures. The enzymes denature at too high a temperature so less electricity is used.
  • Proteases can be used to make baby foods, 'predigesting' some of the protein in the food as babies aren't as good at digesting food when they start eating it, so doing this helps babies get the amino acids they need from their food.
  • Carbohydrases are used to convert starch to sugar (glucose) syrup. Huge quanities of sugar syrup is used in food production. Starch is made by plants like corn and is very cheap, so converting it to sugar is a cheap source of sweetness for manufactuerers, and it is also important for the process of making ethanol from plants.
  • Glucose syrup can go through another process with other enzymes-isomerase, which changes it into fructose syrup. Glucose and fructose have the same amount of energy (400kcal per 100g) but fructose is much sweeter than glucose, so less is needed to make food sweet-used in 'slimming foods'.
  • Advantages:
    • Enzyme-based processed are often fairly cheap to run, running at normal pressures and relatively low temperatures.
  • Disadvantages:
    • The temperature has to be carefully contolled below  45ºC, as well as the pH. Controlling these cost money.
    • Many enzymes are expensive to produce-whole microbes are quite cheap, but have to be supplied with food and oxygen and have waste product removed, and use some of the substrate to grow more microbes.
    • Pure enzymes are more efficient, but also more expensive to produce.
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B2.3.6 High-tech Enzymes

  • Biological detergents:
  • Advantages:
    • Enzymes are very effective in biological washing powders and clothes stains are often biological as well.
    • Research (based on 44 different studies), published by the British Journal of Dermatology in 2008 showed that biological detergents don't seem to be a major cause of skin problems.
  • Disadavantages
    • When it was first manufactured, many factory staff developed allergies; reacting to enzyme dust in the air as proteins often trigger allergies. Some of the customers were affected the same way. Therefore, enzymes were put into tiny capsules and most allergy problems stopped. However, this recieved bad publicity and some still remember this.
    • Some worry about enzymes going into rivers and seas from biological detergents.
    • Low temperatures used to wash with biological detergents might not be as good at killing pathogens on clothes.
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