Metabolic Pathways (HL) (HL IB Biology)

Intracellular and extracellular enzyme-catalysed reactions

• Enzymes can be intracellular or extracellular based on whether they are active inside or outside the cell respectively
• Extracellular enzymes are produced inside the cell and then packaged into vesicles before being secreted by the cell
  • These enzymes will catalyse reactions outside the cell
  • Examples of such enzymes are those involved in chemical digestion in the gut
• Most enzymes however are intracellular, meaning that they are produced and function within the cell
  • Glycolysis and the Krebs cycle are two important processes of respiration that are catalysed by intracellular enzymes

Exergonic and endergonic reactions

• During aerobic respiration, glucose is oxidised to carbon dioxide and water and in the process some of the chemical potential energy stored in the bonds of glucose is released
  • This called free energy and can be used to perform different functions
• Reactions such as these, that release free energy, are known as exergonic reactions
• Many metabolic reactions are exergonic, and some of the energy is released as heat
  • This is because the energy transfer in these reactions are not 100% efficient
• Organisms such as birds and mammals rely on the heat released by metabolic reactions to regulate their body temperature
  • They are called endotherms (or 'warm-blooded') and their body temperature remains constant
  • Those that are unable to regulate their body temperature this way are known as ectotherms (or 'cold-blooded' organisms)
• Reactions where energy is absorbed are called endergonic reactions
  • The products formed by these reactions will have more stored energy than the reactants
  • An example of this is the synthesis of proteins from amino acids
• Since endergonic reactions require an energy input, they are often linked to exergonic reactions in metabolism
• Adenosine triphosphate (ATP) acts as the intermediate that links the energy-yielding reactions to the energy-absorbing ones
• ATP therefore plays a very important role in metabolic processes in living organisms

Exergonic and endergonic energy level diagrams

Exergonic reactions release energy when products are formed, while endergonic reactions require the absorption of energy to form product molecules

• Metabolic pathways involve a series of small steps, each step involves a chemical change
• The enzyme-catalysed reactions that make up metabolic pathways usually consist of linear (chain) or cyclical reactions:
  • Linear (or chain) reactions are a linear sequence with a distinct beginning and end
    • Glycolysis, part of respiration, is an example of a linear metabolic pathway
  • Cycles involve the end product starting the next cycle, these are less common than chain reactions
    • The Calvin cycle, part of photosynthesis, is an example of a cyclical metabolic pathway
    • The Krebs cycle, part of aerobic respiration, is another example of a cyclical metabolic pathway

Examples of types of metabolic pathways diagram

A chain metabolic pathway has a distinct start and finish, whereas in a cycle the end product feeds back into the starting reactant

• Chemicals involved in metabolic pathways are called metabolites or intermediates
  • Some form new molecules within cells
  • Others breakdown molecules and involve an energy transfer