B22: Metabolism
Metabolism, liver functions including deamination, detoxification, glycogen storage, lactic acid removal, and enzymes in metabolic reactions for GCSE Biology Higher.
Key Definitions
- Metabolism – the sum of all the chemical reactions that occur in the body to keep an organism alive. These reactions are controlled by enzymes.
- Deamination – the removal of the nitrogen-containing part of amino acids to form ammonia (which is converted to urea), with the remaining carbon skeleton being converted to glucose.
- Detoxification – the breakdown of toxic substances (such as alcohol and drugs) into less harmful products.
- Glycogen – a polymer of glucose that serves as the main carbohydrate storage molecule in animals, stored in the liver and muscles.
What Is Metabolism?
Metabolism includes all the chemical reactions in the body, such as:
- Converting glucose to starch, glycogen, or cellulose
- Combining glucose and nitrates to make amino acids and then proteins
- Respiration – breaking down glucose to release energy
- Deamination – breaking down excess amino acids
- Detoxification – breaking down harmful substances
- Synthesis and breakdown of lipids
- Breakdown of lactic acid
All metabolic reactions are controlled by enzymes. Without enzymes, these reactions would occur too slowly to sustain life.
Functions of the Liver in Metabolism
The liver is a central organ in metabolism and performs several vital functions:
1. Detoxification
The liver breaks down toxic substances into less harmful products that can be excreted from the body.
- Alcohol: The liver breaks down ethanol using enzymes. Ethanol is first oxidised to acetaldehyde (which is toxic), then further oxidised to acetate, which is eventually converted to CO2 and water. Long-term excessive alcohol consumption can damage the liver (cirrhosis).
- Other toxins: The liver also detoxifies drugs and other harmful substances.
Worked Example 1 – Alcohol Detoxification
Question: Explain how the liver detoxifies alcohol and why long-term excessive drinking damages the liver.
Solution: The liver uses enzymes to break down (oxidise) ethanol in two stages. First, ethanol is oxidised to acetaldehyde, which is itself toxic. Then, acetaldehyde is further oxidised to acetate, which is less harmful and can eventually be broken down to CO2 and water. Long-term excessive drinking overworks the liver. The constant production of toxic acetaldehyde damages liver cells, leading to inflammation, fat accumulation (fatty liver), scarring (fibrosis), and eventually cirrhosis, where functional liver tissue is replaced by scar tissue and the liver can no longer carry out its functions properly.
2. Lactic Acid Removal / Conversion to Glucose
After intense exercise, lactic acid produced by anaerobic respiration in the muscles is transported to the liver via the blood. The liver converts lactic acid back into glucose using oxygen (this is part of repaying the oxygen debt).
Worked Example 2 – Lactic Acid and the Liver
Question: After a 200 m sprint, a runner's blood contains elevated lactic acid levels. Describe what happens to this lactic acid in the liver.
Solution: Lactic acid from the muscles is carried in the blood to the liver. In the liver, enzymes convert the lactic acid back into glucose. This process requires oxygen, which is why the runner continues to breathe heavily after the race (repaying the oxygen debt). Some of the glucose produced can be stored as glycogen in the liver for later use.
3. Deamination
The body cannot store excess amino acids. When more protein is consumed than needed, the liver breaks down the excess amino acids through deamination:
- The nitrogen-containing amino group is removed and converted to ammonia, which is then converted to urea
- Urea is transported to the kidneys and excreted in urine
- The remaining carbon-containing part (the carbon skeleton) is converted to glucose or used as a substrate for respiration
Deamination Summary
Amino acid → urea (nitrogen part) + glucose (carbon part)
Worked Example 3 – Deamination
Question: Explain the process of deamination in the liver. Why is it necessary?
Solution: Deamination is necessary because the body cannot store excess amino acids. When a person eats more protein than they need, the surplus amino acids are transported to the liver. In the liver, each amino acid is split: the amino group (containing nitrogen) is removed and converted first to ammonia and then to urea, which is excreted by the kidneys in urine. The remaining carbon skeleton is converted to glucose or used in respiration to release energy. This process ensures toxic ammonia does not build up in the body and that the useful carbon portion of the amino acid is not wasted.
4. Glycogen Storage, Synthesis, and Breakdown
The liver plays a central role in blood glucose regulation by storing and releasing glucose:
- Glycogenesis: When blood glucose is too high, the liver converts excess glucose into glycogen for storage (stimulated by insulin).
- Glycogenolysis: When blood glucose is too low, the liver breaks down glycogen back into glucose and releases it into the blood (stimulated by glucagon).
- Gluconeogenesis: The liver can also make glucose from non-carbohydrate sources, such as amino acids (after deamination) and glycerol from lipids.
Worked Example 4 – Liver and Blood Glucose Regulation
Question: After a large meal, blood glucose rises. Describe the role of the liver in bringing blood glucose back to normal.
Solution: When blood glucose rises above the normal level, the pancreas detects this and secretes the hormone insulin into the blood. Insulin travels to the liver and stimulates liver cells to take up glucose from the blood. The liver then converts this excess glucose into glycogen for storage (glycogenesis). This removes glucose from the blood, causing blood glucose concentration to fall back towards the normal level. This is an example of negative feedback.
Enzymes in Metabolism
Role of Enzymes
All metabolic reactions are catalysed by enzymes. Enzymes are biological catalysts that speed up chemical reactions without being used up. Each enzyme works on a specific substrate and catalyses a specific reaction. Without enzymes, metabolic reactions would be far too slow to sustain life at body temperature.
| Liver Function | Process | Key Details |
|---|---|---|
| Detoxification | Alcohol → acetaldehyde → acetate → CO2 + H2O | Enzyme-catalysed oxidation; protects body from toxins |
| Lactic acid removal | Lactic acid → glucose | Requires oxygen; repays oxygen debt |
| Deamination | Amino acids → urea + glucose | Removes nitrogen as urea; saves carbon as glucose |
| Glycogen storage | Glucose → glycogen (glycogenesis) | Stimulated by insulin; lowers blood glucose |
| Glycogen breakdown | Glycogen → glucose (glycogenolysis) | Stimulated by glucagon; raises blood glucose |
Practice Questions
Higher
Q1. Define metabolism and explain the role of enzymes in metabolic reactions.
Show Answer
Metabolism is the sum of all the chemical reactions that occur in the body. These reactions are all controlled by enzymes, which act as biological catalysts to speed up the reactions. Without enzymes, the reactions would occur too slowly at body temperature to sustain life.
Higher
Q2. Describe the process of deamination, explaining why it is necessary and what happens to each product.
Show Answer
Deamination is necessary because the body cannot store excess amino acids. In the liver, the amino group (containing nitrogen) is removed from each excess amino acid and converted to ammonia, then to urea. Urea is transported to the kidneys and excreted in urine. The remaining carbon skeleton is converted to glucose or used as a substrate for respiration. This prevents toxic ammonia from building up and ensures the carbon portion of amino acids is not wasted.
Higher
Q3. Explain how the liver detoxifies alcohol and why chronic alcohol abuse damages the liver.
Show Answer
The liver uses enzymes to oxidise ethanol in two stages: first to acetaldehyde (which is toxic), then to acetate, which is less harmful and can be further broken down to CO2 and water. Chronic alcohol abuse overworks the liver and continuously produces the toxic intermediate acetaldehyde, which damages liver cells. Over time, this leads to fat accumulation (fatty liver), inflammation, and eventually cirrhosis, where healthy liver tissue is replaced by scar tissue. Cirrhosis prevents the liver from functioning properly and can be fatal.
Higher
Q4. Describe three ways the liver is involved in glucose metabolism.
Show Answer
1. Glycogenesis: The liver converts excess glucose into glycogen for storage when blood glucose is too high (stimulated by insulin).
2. Glycogenolysis: The liver breaks down stored glycogen back into glucose when blood glucose is too low (stimulated by glucagon).
3. Conversion of lactic acid to glucose: After anaerobic exercise, the liver converts lactic acid (produced by muscles) back into glucose, using oxygen.
Higher
Q5. Explain how the liver is involved in repaying the oxygen debt after intense exercise.
Show Answer
During intense exercise, muscles carry out anaerobic respiration, producing lactic acid. After exercise, this lactic acid is transported in the blood to the liver. In the liver, enzymes convert the lactic acid back into glucose. This conversion requires oxygen, which is why breathing remains heavy after exercise – the body is repaying the oxygen debt. The glucose produced can then be stored as glycogen or used in respiration.
Exam Tips
- This is a Higher only topic – it will not appear on Foundation papers.
- Metabolism = ALL chemical reactions in the body, not just one type.
- When writing about deamination, always mention both products: urea (from the nitrogen part) AND glucose (from the carbon part).
- Remember the sequence for alcohol detoxification: ethanol → acetaldehyde → acetate → CO2 + H2O. Acetaldehyde is the toxic intermediate.
- Link liver functions to hormones where relevant: insulin stimulates glycogenesis, glucagon stimulates glycogenolysis.
🔢 Maths Skills
Mathematical Skills
Minimal quantitative skills for this topic. You may need to interpret blood test data showing concentrations of urea, glucose, or liver enzymes, and compare values to normal ranges.
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Metabolism is just another word for respiration Correct: Metabolism refers to ALL the chemical reactions in the body — respiration is just one of many metabolic reactions. Metabolism also includes protein synthesis, deamination, detoxification, and glycogen synthesis/breakdown.
2. Wrong: The liver only removes toxins Correct: The liver also deaminates excess amino acids (converting the nitrogen part to urea), converts lactic acid back to glucose, and stores glucose as glycogen (glycogenesis) or breaks glycogen down (glycogenolysis).
✍️ 6-Mark Question
Extended Answer
6 marks: Explain the role of the liver in metabolism.
The liver plays a central role in metabolism through several key functions. First, it carries out detoxification — breaking down harmful substances such as alcohol (ethanol is oxidised to acetaldehyde then acetate) and drugs into less harmful products. Second, it performs deamination — removing the amino group from excess amino acids and converting it to urea for excretion, while converting the carbon skeleton to glucose. Third, it removes lactic acid produced during anaerobic respiration, converting it back to glucose using oxygen (repaying the oxygen debt). Fourth, it regulates blood glucose by converting excess glucose to glycogen for storage (glycogenesis, stimulated by insulin) and breaking down glycogen back to glucose when blood glucose is low (glycogenolysis, stimulated by glucagon). These interrelated processes make the liver essential for maintaining metabolic balance.
Mark scheme: 1 mark for detoxification with example; 1 mark for deamination (urea + glucose); 1 mark for lactic acid conversion; 1 mark for glycogen storage/breakdown with hormones; 1 mark for linking functions to maintaining metabolic balance; 1 mark for QWC
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
Blood test results for three patients: Patient X (healthy): urea 5.0 mmol/L, glucose 5.5 mmol/L, ALT (liver enzyme) 25 U/L. Patient Y (chronic alcohol abuse): urea 5.2 mmol/L, glucose 4.8 mmol/L, ALT 180 U/L. Patient Z (high-protein diet, no liver disease): urea 9.5 mmol/L, glucose 5.3 mmol/L, ALT 28 U/L. Normal ranges: urea 2.5-7.0 mmol/L; glucose 4.0-6.0 mmol/L; ALT 10-40 U/L.
1. Identify which results are outside the normal range for each patient. 2. Explain why Patient Y has elevated ALT levels. 3. Explain why Patient Z has elevated urea but normal ALT. 4. Evaluate the claim that blood tests for ALT alone are sufficient to diagnose all liver problems.
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