Enzymes

• What are enzymes?

  Biological catalysts that increase the rate of a chemical reaction without being permanently altered themselves.
• What is an advantage of enzymes in the body?

  They enable cellular reactions to take place at lower temperatures.
• What is the active site of an enzyme?

  The region of an enzyme to which a substrate molecule binds and the reaction takes place.
• Why are enzymes described as having a 'high specificity' for their substrate?


  Only substrates with a specific, complementary shape can fit into an enzymes active site.
• Describe the 'lock and key' method.

  1. Substrate collides with the active site of an enzyme
  2. Substrate binds, enzyme-substrate complex forms
  3. Substrate converted to products
  4. Products released from active site, which is now free to bind to another substrate
• What factors affect the rate of an enzyme-controlled reaction?

  • temperature
  • pH
  • Substrate concentration
• Explain how increasing the temperature initially affects the rate of an enzyme-controlled reaction.

  • As temperature increases, molecules have more kinetic energy
  • Movement of molecules increases
  • Probability of a successful collision increases
  • More enzyme-substrate complexes form
  • Rate of reaction increases
• Explain how increasing the temperature above the optimum affects the rate of an enzyme-controlled reaction

  • temperature increases above the optimum
  • Increased vibrations break bonds in enzyme structure
  • Active site changes shape, enzyme is denatured
  • No more enzyme-substrate complexes can form
  • Rate of reaction decreases
• What does this graph show?

  The effect of increasing temperature on the rate of an enzyme-catalysed reaction.

  
• Explain how pH affects the rate of an enzyme controlled reaction

  • Enzymes have an optimum pH
  • pH shifts from the optimum
  • Bonds in the enzyme's structure are altered
  • Active site changes shape, enzyme is denatured
  • Rate of reaction decreases
• Explain how the substrate concentration affects the rate of an enzyme-controlled reaction

  • Substrate concentration increases
  • Number of substrate molecules in the same volume increases
  • More enzyme-substrate complexes form
  • Rate of reaction increases
  • Once all active sites become full, the rate of reaction plateaus
• What does this graph show?

  The effect of increasing substrate concentration on te rate of an enzyme-catalysed reaction.

  
• How can the rate of an enzyme-controlled reaction be calculated when given a value for time?

  rate = 1/time
• What are the units for rate?

  s^-1
• Why must large organic molecules be broken down into smaller, simpler molecules in the body?

  • Larger molecules are too big to be absorbed across the surface of the gut wall
  • So, large molecules are broken down into smaller molecules for absorption into the bloodstream
• Give an example of the breakdown of large molecules into smaller molecules in plants.

  Starch is broken down by enzymes into simpler sugars which are respired to release energy.
• What type of molecules are proteins and carbohydrates?

  Polymers
• What are the monomers of carbohydrates?

  Simpler sugars
• What group of enzymes catalyses the breakdown of carbohydrates?

  Carbohydrase
• Which type of carbohydrase catalyses the breakdown of starch?

  Amylase
• What are the monomers of proteins?

  Amino acids
• Which type of enzyme catalyses the breakdown of proteins?

  Proteases
• What is the function of lipases?

  Enzymes which catalyse the breakdown of lipids into fatty acids and glycerol.
• Why are smaller molecules synthesised into larger organic molecules in the body?

  Large molecules are used for storage (e.g. glycogen) or are used to build structures (e.g. organelles)
• Which enzyme catalyses the formation of glycogen from glucose?

  Glycogen synthase
• How can the amount of energy contained in food be measured?

  Using calorimetry
• What is calorimetry?

  A method of measuring the heat transfer during a chemical reaction.
• Describe the method used to measure the amount of energy in a sample of food.

  1. Add a set volume of water in a boiling tube, record the initial temperature
  2. Record the mass of a small sample of food (e.g. bean)
  3. Stick the sample onto a mounted needle
  4. Using a bunsen burner light the food sample
  5. Hold the burning sample under the boiling water until it burns up
  6. Record the maximum temperature reached by the water
  7. Record the final mass of the food sample