AoS 2

• Enzymes

  Biological catalysts that usually consist of multiple polypeptide chains combined to form a functioning protein
• Role of enzymes


  To speed up the rate of specific chemical reactions
• Substrate


  The specific molecule that an enzyme acts on
• Active site


  The location in which a substrate binds to an enzyme, the shape of the active site is key to an enzyme's functioning
• Enzyme substrate models


  • Lock and key
  • Induced fit
• Lock and key model

  • The shape of the active site and the shape of the substrate are specific and complementary to each other
  • The substrate is seen as the key and the active site of the enzyme is the lock
  • Only a matching lock and key work together
• Induced fit model


  • The shape of the active site of the enzyme and the substrate are not perfectly complementary to each other
  • When the substrate comes into contact with the active site of the enzyme, the active site changes shape slightly to accommodate the substrate
• Characteristics of enzymes


  • Specificity - their active site is specific to usually only one substrate
  • Catalytic power - their ability to speed up the rate of reaction
• Activation energy


  The energy input required for a reaction to start
• How enzymes reduce activation energy

  • Changing the orientation and proximity of the reactants
  • Changing the micro-environment that the chemical reaction takes place in to provide more stable conditions
  • Exchanging ions, usually H+, at the active site to facilitate steps in certain reactions
• Biochemical pathway


  A sequence of reactions that take place one after the other to reach an overall end product
• The sum of all the biochemical pathways taking place in an organism is its metabolism
• Photosynthesis


  1. Light-dependent stage

  2. Light-independent stage
• Enzymes involved in photosynthesis

  • Photosystem II - splits water molecules in the light dependent stage
  • ATP synthase - combines ADP and inorganic phosphate to produce ATP for the light independent stage
  • Rubisco - captures carbon dioxide and converts them to 3-carbon molecules that are then assembled to form glucose
• Cellular respiration


  1. Glycolysis

  2. Krebs cycle

  3. Electron transport chain (ETC)
• Enzymes involved in cellular respiration

  • Phosphofructokinase (PFK) - catalyses a step in glycolysis
  • Dehydrogenase enzymes - catalyse the removal of hydrogen ions from substrates
  • Cytochrome c oxidase - responsible for catalysing the transfer of electrons in the ETC
• Cofactor


  A non-protein molecule or ion that is essential for the normal functioning of some enzymes
• Types of cofactors


  • Inorganic cofactors - contain metal ions or clusters of ions
  • Organic cofactors - contain carbon and are produced by organisms
• Coenzyme


  An organic cofactor that acts with an enzyme to alter the rate of chemical reaction
• Role of coenzymes


  • Transfer of atoms or groups of atoms
  • Energy transfers
• Coenzymes involved in cellular respiration

  • NAD+/NADH
  • FAD/FADH
  • ADP + Pi/ATP
  • Coenzyme A (CoA)/Acetyl CoA
• NAD+ and FAD


  Hydrogen and electron acceptors when they are unloaded
• ADP


  The unloaded form of ATP, often shown as ADP + Pi
• Coenzyme A (CoA)


  Required in the step of cellular respiration between glycolysis and the Krebs cycle, the loaded form is acetyl CoA
• Coenzymes involved in photosynthesis

  • NADP+/NADPH
  • ADP + Pi/ATP
• NADP+


  A hydrogen acceptor/donator that is required in the first stage of photosynthesis
• ATP


  Produced in the first stage of photosynthesis and used as the energy input of the pathway that creates glucose
• Energy can neither be created nor destroyed, but it can be transformed from one form to another
• The ultimate source of energy is the sun
• ATP provides the energy for metabolic reactions
• Chloroplast


  The site of photosynthesis in plants, containing thylakoid discs and stroma
• Light-dependent stage of photosynthesis

  1. Light energy is trapped by thylakoid disc membranes

  2. Energy is used to split water into hydrogen ions and oxygen atoms

  3. Energy is used to produce ATP

  4. Hydrogen ions are collected by coenzyme acceptor molecule NADP

  5. Oxygen gas is released into the atmosphere

  6. ATP and NADPH go onto the light independent stage
• Light-independent stage of photosynthesis

  1. ATP and NADPH from the light-dependent stage are used

  2. In a series of enzyme-controlled reactions (the Calvin cycle) hydrogen ions are combined with CO2 to form glucose

  3. Other hydrogen ions bind with oxygen atoms to form water molecules
• Inputs and outputs of photosynthesis

  • Light-dependent stage inputs: Water, ADP + Pi, NADP

  Light-dependent stage outputs: Oxygen, ATP, NADPH

  Light-independent stage inputs: Carbon dioxide, ATP, NADPH

  Light-independent stage outputs: Glucose, Water
• Rubisco


  The enzyme used in the light-independent reactions to fix carbon (from CO2) to make carbohydrates (such as glucose)
• Calvin cycle


  First step utilises rubisco to produce 3-carbon molecules (PGA)

  These C3 molecules are then used to create glucose
• If oxygen is present, rubisco can use oxygen as a substrate instead of carbon dioxide, reducing the efficiency of photosynthesis in C3 plants
• C3 plants


  Less efficient in hot and dry conditions due to photorespiration
• C4 pathway


  Production of the C4 molecule malic occurs in the mesophyll cell

  The C4 compound is then sent to the bundle sheath cells where the malic acid is continuously converted into carbon dioxide, creating a high concentration of CO2 for rubisco to use
• C4 plant leaf anatomy

  • Bundle sheath cells enclose the vascular tissue and contain many chloroplasts

  Mesophyll cells are arranged around the bundle sheath cells