Metabolism: Enzymes & Reactions (DP IB Biology)

Increasing Reaction Rates in Cells

Enzymes as catalysts

• Most chemical reactions either do not occur spontaneously or occur very slowly

• In laboratory or industrial settings, some chemical reactions require some sort of catalyst in order to form a sufficient concentration of product molecules
  

  • Other conditions that may speed up the reaction rate include:

    • High temperatures or pressures

    • Extremes of pH

    • High concentrations of the reactants

• Cells are very sensitive to extreme temperatures, pressures and pH-levels, so the chemical reactions occurring in them cannot be sped up by these means

• Enzymes are proteins that act as biological catalysts in cells and allow chemical reactions to occur at a suitable rate in the conditions found in living organisms

  • They are reusable, so only a small number is needed to catalyse reactions

  • They remain unchanged by the reactions that they catalyse

• Without the presence of enzymes, the rate of chemical reactions in organisms would be too low to support life

• To form product molecules, the reactants would need to collide at the correct angle and speed in order for a reaction to occur

  • The chances of this occurring under normal conditions would be so low, that this would be an insignificant event

• Enzymes ensure that molecules (called substrate molecules) are orientated correctly and close enough for a reaction to occur

• The cell has control over the enzymes being produced, which in turn gives the cell control over the chemical reactions occurring in the cytoplasm

Metabolism: Role of Enzymes

What is metabolism?

• Metabolism is a catch-all term used to describe all the chemical reactions that take place within cells and organisms

• Metabolism can be thought of as the chemical reactions of life

  • The molecules involved are metabolites

• Many reactions of metabolism take place in multiple stages

  • Each stage is catalysed by a separate enzyme

• A series of interlinked metabolic reactions is called a metabolic pathway

• Metabolic reactions can be classified broadly as anabolic or catabolic

Role of enzymes in metabolism

• Enzymes are globular proteins

• Critical to the enzyme's function is the active site where the substrate binds

• Enzymes are specific to the substrate

  • The shapes of the enzyme and substrate and their chemical properties are complementary, to allow the substrate to fit into the active site, like two jigsaw pieces fitting together

  • This is called enzyme-substrate specificity

• Due to this specificity, thousands of enzymes are needed throughout an organism, to carry out individual chemical reactions

• This means that control over metabolism can be exerted through these enzymes

Anabolism & Catabolism

Anabolic reactions

• Anabolic reactions are involved with the building of large molecules from smaller ones

• Examples include;

  • Photosynthesis, where CO₂ and water are built up into complex sugars

  • Protein synthesis, where amino acids are joined together in sequence

  • The formation of glycogen by linking glucose molecules together

• Anabolic reactions often include condensation reactions

• Anabolic reactions are endergonic (they require an input of energy to take place)

  • Energy-storing products are the end result

Enzyme-catalysed anabolic reactions diagram

Anabolic reactions involve the linking of more than one substrate molecule to form a more complex product molecule

Catabolic reactions

• Catabolic reactions are involved with breaking down large molecules into smaller, simpler ones

• These reactions are often carried out to release energy for cellular processes and for the excretion of waste

• Examples include:

  • Respiration, where CO₂ and water are produced from the oxidation of sugars

  • Deamination of proteins to release urea

  • Breakdown of macromolecules into monomers during digestion

• Catabolic reactions often include hydrolysis reactions

• Catabolic reactions are exergonic (free energy is released for cellular processes or as excess heat)

Enzyme-catalysed catabolic reactions diagram

Catabolic reactions happens when a single substrate molecule is drawn into the active site and broken apart into two or more product molecules

Comparison of anabolism and catabolism table