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The liver performs hundreds of distinct biochemical functions, but three of them are central to the OCR specification: detoxification, deamination of excess amino acids, and the ornithine cycle that converts toxic ammonia to urea. These illustrate perfectly how a single organ can integrate nutrient processing, waste disposal and biosynthesis. This lesson covers these key functions in the depth expected for A-Level, with named enzymes, substrates and products, matching OCR A-Level Biology A specification module 5.1.2(d).
Key Definitions:
- Detoxification — the chemical modification of toxic substances so they can be safely excreted or used.
- Deamination — the removal of the amino group (−NH₂) from an amino acid.
- Ornithine cycle — the sequence of reactions in hepatocytes that combines ammonia with CO₂ to form urea.
- Dehydrogenase — an enzyme that catalyses the removal of hydrogen from a substrate, usually passing it to NAD⁺.
Before focusing on the three OCR functions, it helps to appreciate the breadth of the liver's work:
| Function | Example |
|---|---|
| Nutrient processing | Converts glucose to glycogen for storage (glycogenesis); releases glucose when needed (glycogenolysis); makes glucose from non-carbohydrate sources (gluconeogenesis). |
| Lipid metabolism | Synthesises cholesterol, bile salts and lipoproteins; β-oxidation of fatty acids. |
| Protein synthesis | Produces plasma proteins (albumin, clotting factors, transport proteins). |
| Detoxification | Modifies alcohol, drugs, hormones and toxins for excretion. |
| Deamination | Removes amino groups from surplus amino acids and converts them to urea. |
| Storage | Stores iron, vitamins A, D, B₁₂, and glycogen. |
| Bile production | Produces bile containing bile salts and bile pigments. |
| Immune function | Kupffer cells phagocytose bacteria and old red blood cells. |
OCR focuses on detoxification, deamination and the ornithine cycle — but awareness of this wider context helps you understand why exam questions often integrate liver function with other topics.
Detoxification is the chemical transformation of potentially harmful substances into less harmful (or more easily excreted) compounds. These include:
Most detoxification reactions occur in the smooth endoplasmic reticulum (SER) of hepatocytes, where specific enzymes catalyse:
Alcohol (ethanol) is the classic detoxification example and the one OCR most commonly asks about.
flowchart LR
A[Ethanol<br/>CH3CH2OH] -->|alcohol dehydrogenase<br/>NAD+ → NADH| B[Ethanal<br/>CH3CHO]
B -->|aldehyde dehydrogenase<br/>NAD+ → NADH| C[Ethanoate<br/>CH3COO-]
C -->|+ CoA, ATP| D[Acetyl CoA]
D --> E[Krebs cycle<br/>or fatty acid synthesis]
Chronic alcohol consumption has predictable consequences based on this pathway:
OCR questions often ask you to explain why alcohol causes fatty liver. The answer lies in the disturbed NAD⁺/NADH balance, not merely in "ingesting fat".
Mammals cannot store excess amino acids. Protein eaten in excess of the body's needs must therefore be disposed of. The liver performs this through deamination: the removal of the amino group (−NH₂) from each excess amino acid.
In simplified form:
amino acid+O2→keto acid+NH3
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