Chapter 4

• Catalysts speed up the rate of chemical reactions
• Catalysts

  Substances that can speed up a chemical reaction without itself being chemically changed at the end of the reaction
• Enzymes

  Catalysts found in living organisms, made of protein
• To catalyse

  To speed up
• Reactions enzymes catalyse

  • Anabolic reactions that build up complex substances
  • Catabolic reactions that break down complex substances
• Enzyme-catalysed chemical reactions in living organisms - Building up complex substances

  • Protein synthesis: amino acids used to build up proteins and form new protoplasm
  • Photosynthesis: glucose is synthesized using carbon dioxide and water

  
• Enzyme-catalysed chemical reactions in living organisms - Breaking down complex substances

  • Digestion in humans: break down large, insoluble food molecules into smaller, soluble food molecules that are soluble in water and small enough to pass through cell membranes
  • Respiration: glucose is broken down to release energy and form carbon dioxide and water
  • Decomposition of hydrogen peroxide: catalase breaks down hydrogen peroxide into water and oxygen

  
• Lock-and-key hypothesis

  Used to describe how an enzyme will only act on a specific substance called the substrate
• Enzyme's active site

  • Has a specific 3D structure that is complementary to the shape of their substrate

  
• Mode of action of an enzyme - Lock and Key hypothesis

  1. Each enzyme has a specific active site with a specific 3D shape complementary to the shape of the substrate

  2. The substrate binds to the active site of the enzyme, they undergo effective collision, forming an enzyme-substrate molecule

  3. The enzyme can catalyse the reaction that will take place at the active site to convert the substrate molecule into a product molecule, which is released from the active site

  4. Leaving the enzyme chemically unchanged and free to catalyse another reaction
• Characteristics of enzymes

  • They speed up chemical reactions
  • They are specific in action
  • They are required in minute amounts and remain chemically unchanged
  • They are affected by temperature
  • They are affected by pH

  
• Enzymes speed up the rate of a chemical reaction

  • By lowering the activation energy of chemical reactions
  • They provide an alternative pathway with a lower activation energy so that more reactant molecules can react to form products
• Enzyme specificity

  • An enzyme molecule has a specific 3D shape that only fits a specific substrate

  
• Enzymes

  Organic catalysts that speed up chemical reactions
• As temperature increases

  • Rate of enzyme activity increases
  • Both enzyme and substrate molecules gain kinetic energy
  • The rate of effective collision between the enzyme and substrate molecule increases, and more enzyme-substrate complexes are formed
• At optimum temperature

  The rate of enzyme activity is the highest
• As temperature increases above optimum

  • The rate of enzyme activity decreases rapidly
  • The active site of the molecule starts to lose its original structure and is no longer complementary to the shape of its substrate
  • The enzyme is said to be denaturing which is the change in the 3D structure of the enzyme
• Denaturation

  Change in the 3D structure of enzymes
• Different enzymes have different optimum pH
• Extreme acidity or alkalinity will cause enzymes to denature
• Enzymes remain chemically unchanged at the end of the reaction
• Enzymes are affected by temperature and pH
• Enzymes are required in small quantities
• Enzymes speed up chemical reactions
• Enzymes are specific in action
• Activation energy means the amount of energy needed to start a chemical reaction
• The higher the activation energy, the slower the rate of reaction
• A catalyst is a substance which speeds up a chemical reaction without being used up or changed itself
• An enzyme is a biological catalyst