DP IB Biology: HL

Revision Notes

Syllabus Edition

First teaching 2014

Last exams 2024

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8.1.1 Metabolic Pathways

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Metabolic Pathways

  • 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 chains or cycles:
    • Chain reactions are a linear sequence with a distinct beginning and end
      • Glycolysis, part of respiration, is an example of a reaction chain 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 cyclic metabolic pathway

chains-and-cycles-metabolic-pathways

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

Enzymes & Activation Energy

  • Metabolic pathways are controlled by enzymes in a biochemical cascade of reactions
    • Virtually every metabolic reaction within living organisms is catalysed by an enzyme
    • Enzymes are therefore essential for life to exist
  • Enzymes are biological catalysts
    • ‘Biological’ because they function in living systems
    • ‘Catalysts’ because they speed up the rate of chemical reactions without being used up or undergoing permanent change

The Enzyme-Substrate Complex

  • The starting point of a metabolic pathway is a substrate which is converted to an end product
  • The enzyme works by binding to the substrate at a special site on the enzyme called the active site
    • The active site of an enzyme has a specific shape to fit a specific substrate
  • Substrates collide with the enzyme's active site and this must happen at the correct orientation and speed in order for a reaction to occur
  • An enzyme-substrate complex is formed, temporarily, when the substrate binds to the active site
    • The substrate is said to be in a transitional state at this moment
  • The product is formed and enzyme is released to take part in another reaction
  • The reaction can be shortened to a simple equation

enzyme-substrate-complex

The simple equation can show how an enzyme reaction proceeds

Enzyme-substrate complex, downloadable AS & A Level Biology revision notes

The formation of the enzyme-substrate complex where the substrate is said to be in a transitional state, before forming the product(s)

Enzymes and the lowering of activation energy

  • All chemical reactions, including metabolic pathways, are associated with energy changes
  • Energy may either be released or absorbed during a reaction
    • If energy is released to the surroundings it is an exergonic reaction
    • If energy is absorbed from the surroundings it is an endergonic reaction
  • For a reaction to proceed there must be enough activation energy
  • Activation energy is the amount of energy needed by the substrate to become unstable enough for a reaction to occur and for new products to be formed
  • Enzymes speed up chemical reactions because they reduce the stability of bonds in the substrate
  • Enzymes lower the activation energy needed to catalyse a reaction
    • The energy released is unchanged but the activation energy required is lowered
    • The rate of reaction is therefore quicker
enzymes--activation-energy
The graph shows how an enzyme lowers the activation energy required for a reaction

Exam Tip

Don't forget that enzymes are proteins, meaning that anything that could denature a protein and make it non-operational (such as extremes of heat, temperature, pH etc.) would also denature an enzyme.

Endergonic and exergonic reactions are defined by the net the intake or output of energy (respectively) this differs from endothermic and exothermic reactions which are defined by the intake or output of thermal energy only.

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Cara Head

Author: Cara Head

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding