9.4 Metabolism and the Liver

A free GCSE Biology lesson for AQA (8461) from OBA School: what metabolism is, the main metabolic reactions, and how the liver processes lactic acid, alcohol and excess amino acids. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Define metabolism as the sum of all the reactions in a cell or the body
• Explain how energy from respiration is used in enzyme-controlled reactions that build new molecules
• Describe the main metabolic reactions listed in the specification
• Describe the roles of the liver in metabolism, including dealing with lactic acid (Higher tier), alcohol and excess amino acids

Key words

• Metabolism: The sum of all the reactions in a cell or the body.
• Synthesis: Building larger molecules from smaller ones, e.g. proteins from amino acids.
• Lipid: A fat or oil, made from one molecule of glycerol and three molecules of fatty acids.
• Urea: A waste product made in the liver from excess amino acids. It is excreted by the kidneys in urine.
• Detoxify: To break down poisonous substances, such as alcohol, into less harmful ones.

What is metabolism?

Metabolism is the sum of all the reactions in a cell or the body. Most reactions are controlled by enzymes. Some build up (synthesise) large molecules from small ones; others break large molecules down.

The energy transferred by respiration is used by the organism for the continual enzyme-controlled processes of metabolism that synthesise new molecules.

Key fact: Respiration is itself a metabolic reaction. It breaks down glucose and transfers the energy that the building-up reactions need.

The main metabolic reactions

• Glucose → starch, glycogen and cellulose; Notes Starch is a plant store; glycogen is an animal store (liver and muscles); cellulose strengthens plant cell walls.
• Glycerol + three fatty acids → lipid; Notes Lipids are energy stores and form cell membranes.
• Glucose + nitrate ions → amino acids → proteins; Notes Plants absorb nitrate ions from the soil and use them to make amino acids, which are joined to make proteins (e.g. enzymes).
• Respiration; Notes Breaks down glucose to transfer energy.
• Excess proteins (amino acids) → urea; Notes Animals cannot store excess amino acids. The liver breaks them down to form urea, which is excreted.

Metabolism and the liver

The liver carries out many metabolic reactions:

• Lactic acid (Higher tier); What happens Blood carries lactic acid from the muscles to the liver, where it is converted back into glucose. This is part of repaying the oxygen debt.
• Storing glucose; What happens Excess glucose is converted to glycogen and stored. It is turned back into glucose when needed.
• Excess amino acids; What happens Broken down to form urea, which travels in the blood to the kidneys and leaves the body in urine.
• Detoxifying; What happens Poisons such as alcohol (ethanol) are broken down into less harmful substances.
• Old red blood cells; What happens Broken down; the iron is stored.

Worked example: how long to break down alcohol

An adult liver can break down about 8 g of alcohol per hour. Sam drinks two drinks, each containing 20 g of alcohol.
Total alcohol = 2 × 20 = 40 g. Time = 40 ÷ 8 = 5 hours.

Answer: About 5 hours

Key fact: Drinking a lot of alcohol over many years damages liver cells (e.g. cirrhosis). A damaged liver cannot carry out these metabolic jobs properly.

Common mistakes

• Saying metabolism means "how fast you burn food". It is the sum of all the reactions in a cell or body.
• Saying lipids are made of "glycerol and fatty acids" without the numbers: one glycerol and three fatty acids.
• Saying urea is made in the kidneys. It is made in the liver; the kidneys excrete it.
• Saying plants get amino acids from the soil. They make them from glucose and nitrate ions.

Revision questions

Try these, then check your answers below each one.

Question 1. What is metabolism? (Choose from: The rate at which you breathe, The sum of all the reactions in a cell or the body, The breakdown of food in the gut, The amount of energy in food.)

Answer: The sum of all the reactions in a cell or the body

Working: Metabolism is the sum of all the reactions in a cell or the body.

Question 2. True or false? Urea is made in the kidneys.

Answer: False

Working: False. Urea is made in the liver from excess amino acids. The kidneys remove it from the blood and excrete it in urine.

Question 3. A lipid molecule is made from one molecule of glycerol and how many molecules of fatty acids?

Answer: 3

Working: One glycerol joins with three fatty acids to make a lipid.

Related lessons

• Previous lesson: 9.3 Anaerobic Respiration
• Next lesson: 10.1 Principles of Homeostasis
• In this chapter: 9.1 Aerobic Respiration
• In this chapter: 9.2 The Response to Exercise

Continue in the full course

This lesson is section 9.4 (AGCBH0904) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.