Enzymes and metabolism at Higher Level: The same statement as SL. At HL it leads on to how pathways are controlled: inhibitors and feedback inhibition, later in this topic.
Practise this as you read
- Define metabolism.
- Explain why many enzymes are needed and how they control metabolism.
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The big idea: Metabolism is the whole web of chemical reactions inside a living cell. Almost every one is catalysed by an enzyme, which is why metabolism is described as the cell's enzyme-catalysed reactions.
These reactions fall into two types
- Those that build larger molecules: anabolism
- Those that break molecules down: catabolism
- Metabolism
- All of the enzyme-catalysed chemical reactions that take place inside a living organism.
- Anabolism
- The reactions of metabolism that BUILD larger molecules from smaller ones. Anabolism uses (requires) energy.
- Catabolism
- The reactions of metabolism that BREAK larger molecules into smaller ones. Catabolism releases energy.
- Enzyme
- A biological catalyst — a protein that speeds up a metabolic reaction without being used up.
A name to remember it by: Anabolism = building up (think of an athlete building muscle).
Catabolism = breaking down (think of a catastrophe tearing things apart).
Both are part of metabolism, the cell's full set of enzyme-controlled reactions.
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The big idea: Metabolism is a network of linked reactions. Each reaction needs its own enzyme, so a cell needs many different enzymes, and it controls metabolism by controlling them.
The points to remember
- Reactions are linked: the product of one reaction is the substrate of the next.
- Pathways branch and join, so they form a network that depends on every step.
- Enzymes are specific: each catalyses one reaction, so there is a different enzyme for each step.
- The cell controls metabolism by making, activating or inhibiting particular enzymes.
Remember it as: One step, one enzyme; control the enzyme, control the step.
Real example: phenylketonuria. People with phenylketonuria lack a working phenylalanine hydroxylase, the enzyme that changes the amino acid phenylalanine into tyrosine. Phenylalanine builds up in the blood, so they must eat a diet low in it: one missing enzyme disrupts a whole part of metabolism.
Trap: Do not say one enzyme can run a whole pathway. Because of specificity, each step needs its own enzyme.
How it is asked: Expect a definition of metabolism, or an explanation of why so many enzymes are needed and how they control metabolism.
Explain why living organisms need many different enzymes.
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