9.1.1

Enzymes

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Enzymes

Enzymes are proteins that catalyse reactions. The chemical reactants that enzymes bind to are called substrates.

Biological catalyst

Biological catalyst

  • A substance that speeds up a chemical reaction without being used up itself is a catalyst.
  • Enzymes are proteins that catalyse biochemical reactions.
  • Enzymes can act inside or outside of cells.
Activation energy

Activation energy

  • For a chemical reaction to start, it needs a specific amount of energy. This amount of energy is different for different reactions.
    • This is called the activation energy.
Activation energy and enzymes

Activation energy and enzymes

  • Enzymes lower the activation energies of chemical reactions inside the cell to increase the rate of reactions.
  • Enzymes lower the activation energy by binding to the reactant molecules (substrate) and allowing chemical bond-breaking and bond-forming processes to happen more easily.
The active site

The active site

  • Enzymes catalyse specific reactions.
  • The active site has a specific shape for each enzyme.
  • Substrates with a complementary shape to the active site of an enzyme can bind to form an enzyme-substrate complex.
  • The shape of the active site is determined by the tertiary structure of the polypeptide.

Models of Enzyme Action

Models of enzyme action have changed over time. For many years, it was thought that enzymes worked in a lock and key manner. The induced fit model is now more widely accepted.

The lock and key model

The lock and key model

  • The lock and key model was originally used to explain enzyme action.
  • The model proposes that the enzyme and substrate fit together perfectly.
    • The substrate is a key fitting into a lock (the enzyme).
The induced fit model

The induced fit model

  • The induced fit model suggests there is a more dynamic interaction between enzyme and substrate.
  • The model states that as an enzyme and substrate come together, their interaction causes a small shift in the enzyme’s structure.
  • The shift means that the enzyme and substrate can bind to form an enzyme-substrate complex and catalyse a reaction.
  • This model is now more widely accepted.

Enzyme-Substrate Complexes

Enzymes bind with substrates to form an enzyme-substrate complex in a specific fashion. The specificity of enzymes is determined by two things:

Active site

Active site

  • Every enzyme only catalyses one specific reaction.
  • Every enzyme has a specific active site that is complementary to the specific substrate.
    • This jigsaw puzzle-like match between an enzyme and its substrates is what makes enzymes highly specific.
Environmental factors

Environmental factors

  • The 3D, tertiary structure of the polypetide chain determines the shape of the active site.
  • Environmental changes can change the tertiary structure of the active site and can stop the enzyme from working properly.
    • This is called a denatured enzyme.
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1

Unity & Diversity - Molecules

1.1

Water

1.1.1Water & Hydrogen Bonding1.1.2IB Multiple Choice - Water & Hydrogen Bonding1.1.3Extended Response - Water
1.2

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1.4

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2

Unity & Diversity - Cells

2.1

The Origin of Cells

2.1.1Origins - Endosymbiosis
2.2

Introduction to Cells

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2.3

Ultrastructure of Cells

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2.4

Cell Division

2.4.1The Cell Cycle2.4.2Mitosis2.4.3Regulation of the Cell Cycle2.4.4End of Topic Quiz - The Cell Cycle2.4.5IB Multiple Choice - The Cell Cycle2.4.6Extended Response - Mitosis
2.5

Structure of DNA & RNA

2.5.1DNA & RNA
2.6

DNA Replication, Transcription & Translation

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2.7

Cell Respiration

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2.8

Photosynthesis

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2.9

Viruses

2.9.1Viruses
3

Unity & Diversity - Organisms

3.1

Diversity of Organisms

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3.2

Evidence for Evolution

3.2.1Evidence for Evolution - Fossils & DNA3.2.2Evidence for Evolution - Anatomy & Geography3.2.3IB Multiple Choice - Evidence for Evolution3.2.4Extended Response - DNA & Evolution
3.3

Natural Selection

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4

Unity & Diversity - Ecosystems

4.1

Classification

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4.2

Cladistics

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4.3

Evolution & Speciation

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4.4

Conservation of Biodiversity

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5

Form & Function - Molecules

5.1

Carbohydrates & Lipids

5.1.1Monosaccharides5.1.2Disaccharides & Polysaccharides5.1.3Triglycerides & Phospholipids5.1.4Cellulose5.1.5Starch & Glycogen
5.2

Proteins

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6

Form & Function - Cells

6.1

Membranes & Membrane Transport

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6.2

Organelles & Compartmentalization

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6.3

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7

Form & Function - Organisms

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Gas Exchange

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7.2

Transport

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7.3

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8

Form & Function - Ecosystems

8.1

Species, Communities & Ecosytems

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8.2

Energy Flow

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8.3

Carbon Cycle

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8.4

Climate Change

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9

Interaction & Interdependence - Molecules

9.1

Enzymes

9.1.1Enzymes9.1.2Factors Affecting Enzyme Activity9.1.3Enzyme-Controlled Reactions9.1.4End of Topic Test - Enzymes
9.2

Metabolism

9.2.1Molecules to Metabolism
9.3

Cell Respiration

9.3.1Glycolysis9.3.2Aerobic Respiration
9.4

Photosynthesis

9.4.1Light-Dependent Reaction9.4.2Light-Independent Reaction9.4.3End of Topic Test - Photosynthesis Reactions9.4.4Limiting Factors9.4.5Photosynthesis Experiments9.4.6End of Topic Test - Photosynthesis9.4.7End of Topic Test - Photosynthesis
10

Interaction & Interdependence - Cells

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Chemical Signalling

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Ecological Niches

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11

Interaction & Interdependence - Organisms

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Integration of Body Systems

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Defence Against Disease

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12

Interaction & Interdependence - Ecosystems

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Transfers of Energy & Matter

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13

Continuity & Change - Molecules

13.1

DNA Replication

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13.2

Protein Synthesis

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13.3

Mutation & Gene Editing

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14

Continuity & Change - Cells

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15

Continuity & Change - Organisms

15.1

Inheritance

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15.2

Homeostasis

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16

Continuity & Change - Ecosystems

16.1

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16.2

Stability & Change

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16.3

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Practice questions on Enzymes

Can you answer these? Test yourself with free interactive practice on Seneca — used by over 10 million students.

  1. 1
    What is the name of the molecules that speed up biochemical reactions?Multiple choice
  2. 2
    Which of the following statements regarding enzymes is true?True / false
  3. 3
    Name the two models of enzyme action:Fill in the list
  4. 4
    What is the name of the model of enzyme action shown below?Multiple choice
  5. 5
    Which of the following statements about models of enzyme action are true?True / false
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Maintaining Biodiversity

Factors Affecting Enzyme Activity