Additional Science-Biology Chapter 4

GCSE AQA Additional Science-Biology Chapter 4

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  • Created by: Mel
  • Created on: 24-05-12 10:57

Enzyme Structure

Enzymes are biological catalysts made from amino acids (builds up to proteins) that speed up reaction. They are large proteins and each has a particular shape (hole or indentation in it) that has an area (active site) where other molecules can fit in (e.g. substrate - reactant).

Too high a temperature will change the enzymes shape, and it will be destroyed or denatured.

Enzymes can catalyse the build up of small molecules into large molecules or the break down of large molecules into small molecules. Enzymes lower the amount of energy necessary for a reaction to take place - 'activation energy'.

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Factors Affecting Enzyme Action

Most chemical reactions take place faster when it is warmer. At the higher temperature the molecules move around more quickly so collide with each other more often and with more energy - e.g. Enzyme substrate particles.

However, if the temperature gets too hot the active site changes shape and the enzyme becomes denatured (stops working). Enzymes work best in certain acidic or alkaline conditions (pH). Enzymes only work well within a narrow pH range.

Chemical reactions which take place in living cells happen at relatively low temperatures. After 40°C, the protein structure of the enzyme is affected. The long amino acid chains begin to unravel causing the active site to change.

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Aerobic Respiration

Aerobic respiration is the release of energy from food when oxygen is available. During aerobic respiration, glucose reacts with oxygen.

Glucose + Oxygen --> Carbon Dioxide + Water [+ energy]

Most reactions take place in the mitochondria, tiny rod-shaped organelles with folded inner membrane which provides a large surface area for the enzymes involved.

The energy released is used to:

  • build larger molecules from smaller ones
  • enable muscle contraction in animals
  • maintain a constant body temperature in mammals and birds
  • build sugars, nitrates and other nutrients in plants into amino acids and then proteins

The muscles store glucose as glycogen which can then be converted back to glucose for use during exercise.

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Anaerobic Respiration

During exercise, not enough oxygen may reach the muscles, so they use anaerobic respiration to obtain energy. It involves the incomplete breakdown of glucose releasing around 5% of the energy released by aerobic respiration, per molecule of glucose. Waste product is lactic acid:

Glucose à Lactic acid (+ little energy)

Muscles become fatigued during long periods of vigorous activity, they stop contracting efficiently. One cause of this is the build-up of lactic acid in the muscles from anaerobic respiration; lactic acid is removed from the muscles by blood flowing through them.

Much less energy is released during anaerobic respiration than during aerobic respiration because the breakdown of glucose is incomplete. Anaerobic respiration produces an oxygen debt – amount of oxygen needed to oxidise lactic acid to carbon dioxide and water.

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Enzymes in Digestion

Most enzymes work inside the cells of the body, digestive enzymes work outside of the cells. Digestion involves the breakdown of large, insoluble molecules into smaller soluble molecules.

  • Amylase (carbohydrase) - produced by salivary glands, pancreas and small intestine; catalyses digestion of starch into sugars in the mouth and small intestine
  • Protease - produced by the stomach, pancreas and small intestine; catalyses the breakdown of proteins into amino acids in the stomach and small intestine
  • Lipase - produced by the pancreas and small intestine; catalyses the breakdown of lipids (fats and oils) to fatty acids and glycerol

The enzymes pass out of the specialised cells (found in glands and in the lining of the gut) into the gut, a hollow muscular tube which squeezes food.

  • It helps to break up food into small pieces with a large surface area for enzymes to work on
  • Mixes food with digestive juices so that enzymes come into contact with as much of the food as possible
  • Uses its muscles to move food along its length from one area to the next
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Speeding Up Digestion

The stomach produces 3 litres of acid a day, which kills most bacteria.

  • Protease enzymes in the stomach work best in acid conditions. Glands in the stomach wall produce hydrochloric acid to create very acidic conditions.
  • Amylase and lipase work in the small intestine. They work best when conditions are slightly alkaline.
  • Liver produces bile (stored in the gall bladder) that is squirted into the small intestine and neutralizes the stomach acid making conditions slightly alkaline.
  • The stomach produces a thick layer of mucus which coats the stomach walls and protects them from being digested by the acids and the enzymes.

Bile also emulsifies fats in the food, physically breaking up large drops of fat into smaller droplets providing bigger surface area for lipase enzymes to act on.

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Making Use of Enzymes

Biological washing powders contain enzymes that digest food stains working at lower temperatures than ordinary powders so can save money.

  • Protease enzymes are also used to pre-digest proteins in some baby foods
  • Isomerases are used to convert glucose into fructose (much sweeter, so less is needed in foods), the foods are therefore not so fattening
  • Carbohydrases are used to convert starch into sugar syrup for use in foods
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