Source: https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-1-enzyme-action

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# Enzyme Action

Biology: Cambridge International A Level, topic 3.1.1

## Function of Enzymes

Metabolism is the different processes that take place in living organisms to allow them to survive. Enzymes are the proteins that catalyse (speed up) these reactions, they are biological catalysts.

### Catalysts

![Catalysts](https://image-v2.cdn.app.senecalearning.com/2018-08/543a6cea-45dc-4ae7-9736-505bba78f8cf/baby-eating-child-food.jpg)

- Enzymes are called biological catalysts because they speed up reactions in living organisms.
 - E.g. Rubisco in plant cells catalyses the fixation of CO2 in photosynthesis.
 - E.g. Amylase in your saliva catalyses the break down of starch to sugars when you eat.

### Effect on the organism

![Effect on the organism](https://image-v2.cdn.app.senecalearning.com/2018-09/0e596ba1-dcc7-4886-aba3-3393e3078674/roots-plant-phloem.jpg)

- Enzymes speed up reactions that affect the organism on the cellular and individual level.
 - E.g. Enzymes catalyse photosynthesis that takes place in cells. The products of photosynthesis (fixed carbon) are used around the plant on the organismal level to grow roots and shoots.

### Structure and function

![Structure and function](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/1.2.1%20Chromosomes%20and%20mitosis/bone-316228_640.jpg)

- Enzymes speed up reactions that affect both structure and function.
 - E.g. Enzymes catalyse the production of collagen, a structural protein in bones.
 - E.g. Enzymes catalyse respiration, the reaction that produces ATP.

## 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](https://image-v2.cdn.app.senecalearning.com/2018-09/e385551c-54ca-419e-954a-97c9c57a5a64/enzyme-action-lock-and-key-model-.png)

- 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](https://image-v2.cdn.app.senecalearning.com/2018-09/e54f1c18-f5e1-43d6-9cd2-9d913b4f06c8/induced-fit-model-enzyme-action-.png)

- 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](https://image-v2.cdn.app.senecalearning.com/2018-09/e385551c-54ca-419e-954a-97c9c57a5a64/enzyme-action-lock-and-key-model-.png)

- 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](https://image-v2.cdn.app.senecalearning.com/courseImages/biology/2.2.5%20denatured%20enzyme%20(s).png)

- The 3D, tertiary structure of the polypeptide 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.

## Test yourself on Enzyme Action

- Which of the following statements about the function of enzymes are true?
- What type of biological molecule are enzymes?
- What is the name of the model of enzyme action shown below?
- Which of the following statements about models of enzyme action are true?
- What is the name given to enzymes that have experienced a change in their active site due to environmental factors?

[Practise this topic for free](https://app.senecalearning.com/classroom/course/8d1a05c1-df92-4b13-9a69-b60acc7af79a/section/e0117164-d52b-48e5-a428-068f404fb68d/session)

- Previous topic: [2.4.2 End of Topic Test - Water](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-2-end-of-topic-test-water.md)
- Next topic: [3.1.2 Factors Affecting Enzyme Activity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-2-factors-affecting-enzyme-activity.md)

## Biology: Cambridge International A Level contents

### 1 Cell Structure
- 1.1 Cell Structure
  - [1.1.1 Studying Cells - Microscopes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-1-studying-cells-microscopes.md)
  - [1.1.2 Introduction to Eukaryotic & Prokaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-2-introduction-to-eukaryotic-and-prokaryotic-cells.md)
  - [1.1.3 Ultrastructure of Eukaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-3-ultrastructure-of-eukaryotic-cells.md)
  - [1.1.4 Ultrastructure of Eukaryotic Cells 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-4-ultrastructure-of-eukaryotic-cells-2.md)
  - [1.1.5 Ultrastructure of Eukaryotic Cells 3](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-5-ultrastructure-of-eukaryotic-cells-3.md)
  - [1.1.6 Prokaryotic Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-6-prokaryotic-cells.md)
  - [1.1.7 Viruses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-7-viruses.md)
  - [1.1.8 End of Topic Test - Cell Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-8-end-of-topic-test-cell-structure.md)
  - [1.1.9 Exam-Style Question - Microscopes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-9-exam-style-question-microscopes.md)
  - [1.1.10 A-A* (AO2/3) - Cell Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/1-1-10-a-a-ao23-cell-structure.md)

### 2 Biological Molecules
- 2.1 Testing for Biological Modules
  - [2.1.1 Tests for Sugars & Starch](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-1-1-tests-for-sugars-and-starch.md)
  - [2.1.2 Tests for Lipids & Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-1-2-tests-for-lipids-and-proteins.md)
- 2.2 Carbohydrates & Lipids
  - [2.2.1 Monomers & Polymers](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-1-monomers-and-polymers.md)
  - [2.2.2 Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-2-carbohydrates.md)
  - [2.2.3 Examples of Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-3-examples-of-carbohydrates.md)
  - [2.2.4 Triglycerides & Phospholipids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-4-triglycerides-and-phospholipids.md)
  - [2.2.5 Types of Fatty Acids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-5-types-of-fatty-acids.md)
  - [2.2.6 Exam-Style Question - Carbohydrates](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-6-exam-style-question-carbohydrates.md)
  - [2.2.7 End of Topic Test - Carbohydrates & Lipids](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-2-7-end-of-topic-test-carbohydrates-and-lipids.md)
- 2.3 Proteins
  - [2.3.1 Amino Acids & Overview of Protein Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-1-amino-acids-and-overview-of-protein-structure.md)
  - [2.3.2 Primary & Secondary Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-2-primary-and-secondary-structure.md)
  - [2.3.3 Tertiary & Quaternary Structure](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-3-tertiary-and-quaternary-structure.md)
  - [2.3.4 Globular & Fibrous Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-4-globular-and-fibrous-proteins.md)
  - [2.3.5 Haemoglobin](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-5-haemoglobin.md)
  - [2.3.6 Collagen](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-6-collagen.md)
  - [2.3.7 End of Topic Test  - Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-3-7-end-of-topic-test-proteins.md)
- 2.4 Water
  - [2.4.1 Water Structure & Function](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-1-water-structure-and-function.md)
  - [2.4.2 End of Topic Test - Water](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/2-4-2-end-of-topic-test-water.md)

### 3 Enzymes
- 3.1 Enzymes
  - [3.1.1 Enzyme Action](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-1-enzyme-action.md)
  - [3.1.2 Factors Affecting Enzyme Activity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-2-factors-affecting-enzyme-activity.md)
  - [3.1.3 Factors Affecting Enzyme Activity 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-3-factors-affecting-enzyme-activity-2.md)
  - [3.1.4 A-A* (AO2/3) - Enzymes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/3-1-4-a-a-ao23-enzymes.md)

### 4 Cell Membranes & Transport
- 4.1 Biological Membranes
  - [4.1.1 Fluid Mosaic Model](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-1-fluid-mosaic-model.md)
  - [4.1.2 Cell Signalling](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-2-cell-signalling.md)
  - [4.1.3 End of Topic Test - Biological Membranes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-3-end-of-topic-test-biological-membranes.md)
  - [4.1.4 Exam-Style Question - Membranes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-1-4-exam-style-question-membranes.md)
- 4.2 Movement Into & Out of Cells
  - [4.2.1 Passive Movement Across the Membrane](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-1-passive-movement-across-the-membrane.md)
  - [4.2.2 Active Movement Across the Membrane](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-2-active-movement-across-the-membrane.md)
  - [4.2.3 Osmosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-3-osmosis.md)
  - [4.2.4 Osmosis & Plant Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-4-osmosis-and-plant-cells.md)
  - [4.2.5 Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-5-surface-area-to-volume-ratio.md)
  - [4.2.6 Adaptions to Surface Area to Volume Ratio](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-6-adaptions-to-surface-area-to-volume-ratio.md)
  - [4.2.7 Calculations - Volume](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-7-calculations-volume.md)
  - [4.2.8 Calculations - Surface Area](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/4-2-8-calculations-surface-area.md)

### 5 The Mitotic Cell Cycle
- 5.1 Replication & Division of Nuclei
  - [5.1.1 Chromosomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-1-chromosomes.md)
  - [5.1.2 The Cell Cycle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-2-the-cell-cycle.md)
  - [5.1.3 Mitosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-3-mitosis.md)
  - [5.1.4 Role of Mitosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-4-role-of-mitosis.md)
  - [5.1.5 Stem Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-5-stem-cells.md)
  - [5.1.6 Mitosis & Cancer](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-6-mitosis-and-cancer.md)
  - [5.1.7 End of Topic Test - Replication & Division](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-7-end-of-topic-test-replication-and-division.md)
  - [5.1.8 A-A* (AO2/3) - Cell Cycle & Transport](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/5-1-8-a-a-ao23-cell-cycle-and-transport.md)

### 6 Nucleic Acids & Protein Synthesis
- 6.1 Nucleic Acids
  - [6.1.1 Introduction to DNA & RNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-1-introduction-to-dna-and-rna.md)
  - [6.1.2 Nucleotides](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-2-nucleotides.md)
  - [6.1.3 Polynucleotides](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-3-polynucleotides.md)
  - [6.1.4 ADP & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-4-adp-and-atp.md)
  - [6.1.5 DNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-5-dna.md)
  - [6.1.6 DNA Replication](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-6-dna-replication.md)
  - [6.1.7 RNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-7-rna.md)
  - [6.1.8 End of Topic Test - Nucleic Acids & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-1-8-end-of-topic-test-nucleic-acids-and-atp.md)
- 6.2 Protein Synthesis
  - [6.2.1 The Genetic Code](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-1-the-genetic-code.md)
  - [6.2.2 Polypeptide Synthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-2-polypeptide-synthesis.md)
  - [6.2.3 RNA Processing in Eukaryotes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-3-rna-processing-in-eukaryotes.md)
  - [6.2.4 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-4-mutations.md)
  - [6.2.5 Exam-Style Question - Protein Synthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-5-exam-style-question-protein-synthesis.md)
  - [6.2.6 A-A* (AO2/3) - Coronavirus Translation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/6-2-6-a-a-ao23-coronavirus-translation.md)

### 7 Transport in Plants
- 7.1 Transport in Plants
  - [7.1.1 Plant Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-1-plant-vessels.md)
  - [7.1.2 Xylem](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-2-xylem.md)
  - [7.1.3 Investigating Transpiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-3-investigating-transpiration.md)
  - [7.1.4 Adaptations to Water Availability](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-4-adaptations-to-water-availability.md)
  - [7.1.5 Phloem](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-5-phloem.md)
  - [7.1.6 End of Topic Test - Transport in Animals & Plants](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-6-end-of-topic-test-transport-in-animals-and-plants.md)
  - [7.1.7 A-A* (AO2/3) - Transport in Animals & Plants](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/7-1-7-a-a-ao23-transport-in-animals-and-plants.md)

### 8 Transport in Mammals
- 8.1 Circulatory System
  - [8.1.1 Double Circulation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-1-double-circulation.md)
  - [8.1.2 Blood Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-2-blood-vessels.md)
  - [8.1.3 Important Blood Vessels](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-3-important-blood-vessels.md)
  - [8.1.4 Blood Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-4-blood-cells.md)
  - [8.1.5 Tissue Fluid](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-1-5-tissue-fluid.md)
- 8.2 Transport of Oxygen & Carbon Dioxide
  - [8.2.1 Haemoglobin](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-2-1-haemoglobin.md)
  - [8.2.2 Oxygen Dissociation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-2-2-oxygen-dissociation.md)
- 8.3 The Heart
  - [8.3.1 Structure of the Heart](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-1-structure-of-the-heart.md)
  - [8.3.2 The Cardiac Cycle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-2-the-cardiac-cycle.md)
  - [8.3.3 Coordination of Heart Action](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/8-3-3-coordination-of-heart-action.md)

### 9 Gas Exchange
- 9.1 Gas Exchange System
  - [9.1.1 Mammalian Gas Exchange](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/9-1-1-mammalian-gas-exchange.md)
  - [9.1.2 Smoking](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/9-1-2-smoking.md)

### 10 Infectious Diseases
- 10.1 Infectious Diseases
  - [10.1.1 Transmission of Disease](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-1-transmission-of-disease.md)
  - [10.1.2 Tuberculosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-2-tuberculosis.md)
  - [10.1.3 HIV](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-3-hiv.md)
  - [10.1.4 Malaria](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-4-malaria.md)
  - [10.1.5 Cholera](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-5-cholera.md)
  - [10.1.6 Measles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-1-6-measles.md)
- 10.2 Antibiotics
  - [10.2.1 Antibiotics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/10-2-1-antibiotics.md)

### 11 Immunity
- 11.1 The Immune System
  - [11.1.1 Phagocytes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-1-phagocytes.md)
  - [11.1.2 Antigens](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-2-antigens.md)
  - [11.1.3 Lymphocytes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-3-lymphocytes.md)
  - [11.1.4 Primary & Secondary Response](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-1-4-primary-and-secondary-response.md)
- 11.2 Antibodies & Vaccination
  - [11.2.1 Antibodies](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-1-antibodies.md)
  - [11.2.2 Monoclonal Antibodies](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-2-monoclonal-antibodies.md)
  - [11.2.3 Immunity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-3-immunity.md)
  - [11.2.4 Vaccines](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-4-vaccines.md)
  - [11.2.5 Exam-Style Question - Immune System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-5-exam-style-question-immune-system.md)
  - [11.2.6 A-A* (AO2/3) - Disease & Immune System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/11-2-6-a-a-ao23-disease-and-immune-system.md)

### 12 Energy & Respiration (A2 Only)
- 12.1 Energy
  - [12.1.1 Energy & ATP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-1-energy-and-atp.md)
  - [12.1.2 Respiratory Substrates & Quotient](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-2-respiratory-substrates-and-quotient.md)
  - [12.1.3 Respirometers](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-1-3-respirometers.md)
- 12.2 Respiration
  - [12.2.1 Introduction to Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-1-introduction-to-respiration.md)
  - [12.2.2 Anaerobic Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-2-anaerobic-respiration.md)
  - [12.2.3 Aerobic Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-3-aerobic-respiration.md)
  - [12.2.4 Chemiosmosis & Coenzymes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-4-chemiosmosis-and-coenzymes.md)
  - [12.2.5 Mitochondria](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-5-mitochondria.md)
  - [12.2.6 Respiration Experiments](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-6-respiration-experiments.md)
  - [12.2.7 End of Topic Test - Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-7-end-of-topic-test-respiration.md)
  - [12.2.8 A-A* (AO3/4) - Respiration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/12-2-8-a-a-ao34-respiration.md)

### 13 Photosynthesis (A2 Only)
- 13.1 Photosynthesis
  - [13.1.1 Introduction to Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-1-introduction-to-photosynthesis.md)
  - [13.1.2 Light-Dependent Reactions](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-2-light-dependent-reactions.md)
  - [13.1.3 Light-Independent Reactions](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-3-light-independent-reactions.md)
  - [13.1.4 Limiting Factors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-4-limiting-factors.md)
  - [13.1.5 Investigating Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-5-investigating-photosynthesis.md)
  - [13.1.6 End of Topic Test - Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-6-end-of-topic-test-photosynthesis.md)
  - [13.1.7 A-A* (AO3/4) - Photosynthesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/13-1-7-a-a-ao34-photosynthesis.md)

### 14 Homeostasis (A2 Only)
- 14.1 Homeostasis
  - [14.1.1 Homeostasis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-1-1-homeostasis.md)
  - [14.1.2 Negative Feedback](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-1-2-negative-feedback.md)
- 14.2 The Kidney
  - [14.2.1 Kidneys & Osmoregulation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-2-1-kidneys-and-osmoregulation.md)
- 14.3 Cell Signalling
  - [14.3.1 Messengers & cAMP](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-3-1-messengers-and-camp.md)
- 14.4 Blood Glucose Concentration
  - [14.4.1 Blood Glucose Concentration](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-1-blood-glucose-concentration.md)
  - [14.4.2 Controlling Blood Glucose](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-2-controlling-blood-glucose.md)
  - [14.4.3 Urinalysis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-4-3-urinalysis.md)
- 14.5 Homeostasis in Plants
  - [14.5.1 Stomata & Guard Cells](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/14-5-1-stomata-and-guard-cells.md)

### 15 Control & Coordination (A2 Only)
- 15.1 Control & Coordination in Mammals
  - [15.1.1 Neurones](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-1-neurones.md)
  - [15.1.2 Receptors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-2-receptors.md)
  - [15.1.3 Taste](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-3-taste.md)
  - [15.1.4 Reflexes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-4-reflexes.md)
  - [15.1.5 Action Potentials](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-5-action-potentials.md)
  - [15.1.6 Saltatory Conduction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-6-saltatory-conduction.md)
  - [15.1.7 Synapses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-7-synapses.md)
  - [15.1.8 Cholinergic Synnapses](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-8-cholinergic-synnapses.md)
  - [15.1.9 Neuromuscular Junction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-9-neuromuscular-junction.md)
  - [15.1.10 Skeletal Muscle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-10-skeletal-muscle.md)
  - [15.1.11 Sliding Filament Theory Contraction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-11-sliding-filament-theory-contraction.md)
  - [15.1.12 Sliding Filament Theory Contraction 2](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-12-sliding-filament-theory-contraction-2.md)
  - [15.1.13 Menstruation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-13-menstruation.md)
  - [15.1.14 Contraceptive Pill](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-1-14-contraceptive-pill.md)
- 15.2 Control & Co-Ordination in Plants
  - [15.2.1 Venus Flytrap](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-1-venus-flytrap.md)
  - [15.2.2 Auixns](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-2-auixns.md)
  - [15.2.3 Gibberellins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/15-2-3-gibberellins.md)

### 16 Inherited Change (A2 Only)
- 16.1 Passage of Information to Offspring
  - [16.1.1 Meiosis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-1-meiosis.md)
  - [16.1.2 Gametogenesis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-2-gametogenesis.md)
  - [16.1.3 Meiosis & Genetic Diversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-1-3-meiosis-and-genetic-diversity.md)
- 16.2 Genes & Phenotype
  - [16.2.1 Key Terms in Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-1-key-terms-in-genetics.md)
  - [16.2.2 Inheritance](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-2-inheritance.md)
  - [16.2.3 Linkage](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-3-linkage.md)
  - [16.2.4 Multiple Alleles](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-4-multiple-alleles.md)
  - [16.2.5 Chi-Squared Test](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-5-chi-squared-test.md)
  - [16.2.6 Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-6-mutations.md)
  - [16.2.7 Specific Mutations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-7-specific-mutations.md)
  - [16.2.8 End of Topic Test - Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-8-end-of-topic-test-genetics.md)
  - [16.2.9 A-A* (AO2/3) - Genetics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-2-9-a-a-ao23-genetics.md)
- 16.3 Gene Control
  - [16.3.1 lac Operon](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-1-lac-operon.md)
  - [16.3.2 Transcription Factors](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-2-transcription-factors.md)
  - [16.3.3 Gibberellins & DELLA Proteins](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/16-3-3-gibberellins-and-della-proteins.md)

### 17 Selection & Evolution (A2 Only)
- 17.1 Variation
  - [17.1.1 Phenotypic Variation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-1-phenotypic-variation.md)
  - [17.1.2 Continuous & Discontinuous Variation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-2-continuous-and-discontinuous-variation.md)
  - [17.1.3 t-Tests](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-1-3-t-tests.md)
- 17.2 Natural & Artificial Selection
  - [17.2.1 Natural Selection & Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-1-natural-selection-and-evolution.md)
  - [17.2.2 Types of Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-2-types-of-selection.md)
  - [17.2.3 Types of Selection Summary](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-3-types-of-selection-summary.md)
  - [17.2.4 Genetic Drift & Founder Effect](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-4-genetic-drift-and-founder-effect.md)
  - [17.2.5 Populations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-5-populations.md)
  - [17.2.6 Hardy-Weinberg Principle](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-6-hardy-weinberg-principle.md)
  - [17.2.7 Artificial Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-2-7-artificial-selection.md)
- 17.3 Evolution
  - [17.3.1 Evolution by Natural Selection](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-1-evolution-by-natural-selection.md)
  - [17.3.2 Evidence for Evolution](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-2-evidence-for-evolution.md)
  - [17.3.3 Speciation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-3-speciation.md)
  - [17.3.4 Extinction](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/17-3-4-extinction.md)

### 18 Classification & Conservation (A2 Only)
- 18.1 Biodiversity
  - [18.1.1 Key Terms](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-1-key-terms.md)
  - [18.1.2 Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-2-biodiversity.md)
  - [18.1.3 Sampling Methods](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-3-sampling-methods.md)
  - [18.1.4 Correlation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-4-correlation.md)
  - [18.1.5 Spearman’s Rank](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-5-spearmans-rank.md)
  - [18.1.6 Measuring Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-1-6-measuring-biodiversity.md)
- 18.2 Classification
  - [18.2.1 3-Domain Classification System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-2-1-3-domain-classification-system.md)
  - [18.2.2 Linnaean Classification System](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-2-2-linnaean-classification-system.md)
- 18.3 Conservation
  - [18.3.1 Importance of Biodiversity](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-1-importance-of-biodiversity.md)
  - [18.3.2 Conservation](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-2-conservation.md)
  - [18.3.3 Zoos, Seedbanks, & Other Conservation Methods](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-3-zoos-seedbanks-and-other-conservation-methods.md)
  - [18.3.4 Assisted Reproduction & Contraception](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-4-assisted-reproduction-and-contraception.md)
  - [18.3.5 Non-Governmental Organisations](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/18-3-5-non-governmental-organisations.md)

### 19 Genetic Technology (A2 Only)
- 19.1 Manipulating Genomes
  - [19.1.1 Recombinant DNA & Genetic Engineering](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-1-recombinant-dna-and-genetic-engineering.md)
  - [19.1.2 Restriction Endonucleases](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-2-restriction-endonucleases.md)
  - [19.1.3 Gel Electrophoresis](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-3-gel-electrophoresis.md)
  - [19.1.4 DNA Probes & Microarrays](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-4-dna-probes-and-microarrays.md)
  - [19.1.5 End of Topic Test - Manipulating Genomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-5-end-of-topic-test-manipulating-genomes.md)
  - [19.1.6 A-A* (AO2/3) - Manipulating Genomes](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-1-6-a-a-ao23-manipulating-genomes.md)
- 19.2 Genetic Technology Applied to Medicine
  - [19.2.1 Bioinformatics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-1-bioinformatics.md)
  - [19.2.2 Applications of Recombinant DNA](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-2-applications-of-recombinant-dna.md)
  - [19.2.3 Genetic Screening](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-3-genetic-screening.md)
  - [19.2.4 Gene Therapy](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-4-gene-therapy.md)
  - [19.2.5 Ethics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-5-ethics.md)
  - [19.2.6 DNA Profiling & Forensics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-2-6-dna-profiling-and-forensics.md)
- 19.3 Genetically Modified Organisms in Agriculture
  - [19.3.1 GM Crops & Livestock](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-3-1-gm-crops-and-livestock.md)
  - [19.3.2 Ethics](https://senecalearning.com/en-gb/revision-notes/international-a-level/biology/cambridge-international/19-3-2-ethics.md)