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B9.4 Metabolism and the Liver

GCSE Combined Science · AQA · Chapter B9: Respiration

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what metabolism is, building-up and breaking-down reactions, and how the liver deals with lactic acid 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 metabolic reactions involving carbohydrates, lipids and proteins
  • Describe the roles of the liver, including making urea and (Higher tier) converting lactic acid to glucose

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.
  • Breakdown: Splitting large molecules into smaller ones, e.g. respiration, or digestion of starch.
  • 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.

What is metabolism?

Metabolism is the sum of all the reactions in a cell or the body. It includes digestion, respiration, protein synthesis and, in plants, photosynthesis. Most of these reactions are controlled by enzymes.

Some reactions synthesise (build up) large, complex molecules from small ones. These need energy, which is transferred by respiration. Others break down large molecules into smaller ones.

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

  • Carbohydrates; Breakdown reactions Starch is digested to glucose. Glucose is broken down in respiration.; Synthesis reactions Glucose → starch (plant store), glycogen (animal store) and cellulose (plant cell walls).
  • Lipids; Breakdown reactions Each lipid is digested to 1 glycerol and 3 fatty acids.; Synthesis reactions 1 glycerol + 3 fatty acids → lipid.
  • Proteins; Breakdown reactions Proteins are digested to amino acids. Excess amino acids are broken down to form urea.; Synthesis reactions Plants: glucose + nitrate ions → amino acids. All organisms: amino acids → proteins.

The breakdown reactions of digestion happen in the gut and make small, soluble molecules that can be absorbed into the blood. The synthesis reactions happen inside body cells.

Metabolism and the liver

  • 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 Animals cannot store excess amino acids. They are broken down to form urea, which travels in the blood to the kidneys and leaves the body in urine.

Worked example: how a leaf uses its glucose

A plant makes 250 g of glucose in a day. 40% is used in respiration and 20% is made into cellulose. The rest is made into starch or used to make amino acids, in the ratio 3 : 1.
Respiration: 0.40 × 250 = 100 g. Cellulose: 0.20 × 250 = 50 g.
Remaining = 250 − 150 = 100 g. Starch = 100 × 3 ÷ 4 = 75 g; amino acids = 25 g.

Answer: 100 g respired, 50 g cellulose, 75 g starch, 25 g for amino acids

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 digestion of food in the gut, The energy stored 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. Most are controlled by enzymes.

Question 2. True or false? Urea is made in the liver and removed from the body by the kidneys.

Answer: True

Working: True. Excess amino acids cannot be stored. The liver breaks them down to form urea, which the kidneys excrete in urine.

Question 3. When one lipid molecule is broken down completely during digestion, how many fatty acid molecules are released?

Answer: 3 molecules

Working: A lipid is made of one glycerol and three fatty acids, so breaking it down releases 3 fatty acids (and 1 glycerol).

Related lessons

Continue in the full course

This lesson is section B9.4 (AGCSC0904) of our free GCSE Combined Science 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 Combined Science course.

 

 

 

 

 

 

 

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