3.1.1Inquiry Process
Metabolism- Enzymes and Reactions — IB Biology HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 101 of 160
Section 1 of 4What Metabolism Is
Every cell runs thousands of chemical reactions at once, and enzymes keep them organised.
Metabolism
- The sum of all the enzyme-catalysed chemical reactions that take place inside a cell or organism, which together keep it alive.
Anabolic reactions
- Reactions that build larger molecules from smaller ones and need an input of energy, such as protein synthesis from amino acids.
Catabolic reactions
- Reactions that break large molecules into smaller ones and usually release energy,
- such as the oxidation of glucose in respiration.
Condensation and hydrolysis
- Condensation joins monomers and releases water,
- whereas hydrolysis uses water to split polymers; both are catalysed by specific enzymes.
| Type | Direction | Energy | Example |
|---|---|---|---|
| Anabolic | Small to large | Takes in energy | Amino acids to polypeptide |
| Catabolic | Large to small | Releases energy | Glucose to carbon dioxide and water |
Section 2 of 4Enzymes as Catalysts
Enzymes as Catalysts
Biological catalyst
- An enzyme speeds up a reaction without being used up, so a small amount of enzyme can convert a very large amount of substrate.
Globular protein
- Enzymes are folded proteins whose precise three-dimensional shape, held by bonds between R groups, creates a specific active site.
Specificity
- Each enzyme normally catalyses one reaction or one type of reaction, because only certain substrates fit its active site.
Lower activation energy
- By stabilising the transition state, an enzyme reduces the energy barrier so that more molecules react at body temperature.
Reusable
- After the products leave, the enzyme returns to its original state and the active site is free to bind another substrate.
Section 3 of 4Where Enzymes Work
1
Most enzymes are intracellular and act inside cells, for example the enzymes of glycolysis that work in the cytoplasm.
2
Some enzymes are extracellular and are secreted to work outside the cell, such as digestive amylase released into the gut.
3
Without enzymes most metabolic reactions would proceed far too slowly at the mild temperatures that cells can tolerate.
4
Because every enzyme is specific, the cell can control which reactions happen by controlling which enzymes are present and active.
5
Metabolic reactions are therefore organised into pathways in which the product of one enzyme becomes the substrate of the next.
Section 4 of 4Exam Checklist & Tips
3.1.1 – Key Points for the Exam
- Metabolism The sum of all the chemical reactions in a cell or organism.
- Enzyme A biological catalyst, usually a globular protein, that speeds up a specific reaction.
- Substrate The molecule on which an enzyme acts.
- Anabolism Reactions that build larger molecules and require energy.
- Catabolism Reactions that break down larger molecules and release energy.
Common Mistakes
- Saying enzymes provide energy for a reaction; they lower the activation energy instead.
- Writing that enzymes are used up in reactions, when they are recycled and remain unchanged.
- Swapping anabolic and catabolic; anabolic means building up.
- Forgetting that enzymes are specific, so a different enzyme is needed for each reaction.
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