Metabolism — AQA GCSE Biology
Test yourself on Metabolism with AQA GCSE practice questions.
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Metabolism explained
Metabolism includes both building up and breaking down substances.
Read the full explanation
Sugars, amino acids, fatty acids and glycerol are the small molecules used to synthesise larger storage and structural molecules, and they are also released when those larger molecules are broken down. Glucose molecules join by condensation reactions to form starch, glycogen or cellulose. Amino acids join in a specific sequence to form proteins. Fatty acids and glycerol combine to form lipids such as fats and oils. In breakdown, carbohydrates are hydrolysed to sugars, proteins to amino acids, and lipids to fatty acids and glycerol. These reactions are controlled by enzymes. Understanding this helps explain digestion, growth, repair, energy storage and the movement of substances in living organisms.
Metabolism is the sum of all the reactions in a cell or the body.
Metabolism is not a single process but the total of every chemical reaction occurring in a cell or organism. These reactions fall into two broad groups. Synthesis reactions build larger molecules from smaller ones, such as joining glucose to form glycogen or joining amino acids to form proteins. Breakdown reactions split larger molecules into smaller ones, such as breaking down glucose during respiration or breaking down lipids into fatty acids and glycerol. Many metabolic reactions are controlled by enzymes, and they occur at different rates in different cells depending on the cell's role. The idea of metabolism links respiration, digestion, growth, repair and the synthesis of storage molecules, and it helps explain why organisms need a constant supply of energy and raw materials.
The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise new molecules.
Respiration releases energy from glucose in every living cell. That energy is not wasted: it powers metabolism, the sum of all the enzyme-controlled reactions in an organism. Many of these reactions are synthesis reactions, building large molecules from smaller ones. For example, a liver cell respires glucose to release energy, then uses that energy to join glucose molecules into glycogen for storage. The reactions are continual because cells constantly build and replace molecules. Each step needs a specific enzyme, so temperature and pH affect metabolism. Energy is transferred, not created, so respiration links food to growth, repair and storage.
Metabolism includes: • conversion of glucose to starch, glycogen and cellulose • the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids • the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins • respiration • breakdown of excess proteins to form urea for excretion.
Metabolism is the sum of all chemical reactions in a cell or organism. It includes building large molecules from smaller ones and breaking large molecules down. Glucose can be converted into starch for storage in plants, glycogen for storage in animals and fungi, or cellulose for plant cell walls. Lipids are formed when one glycerol molecule joins with three fatty acid molecules. Amino acids are made from glucose and nitrate ions in plants; amino acids then join to synthesise proteins. Respiration releases energy from glucose and is also part of metabolism. If an animal has excess protein, the amino group is removed and converted to urea in the liver, then excreted by the kidneys.
Your focus
- Identify the small molecules used to synthesise carbohydrates, proteins and lipids.
- Describe the breakdown products of carbohydrates, proteins and lipids.
- Explain why these synthesis and breakdown reactions are important in living organisms.
Show all 12 objectives
- Define metabolism as the sum of all reactions in a cell or body.
- Give examples of synthesis and breakdown reactions.
- Explain how enzymes control metabolic reactions.
- State that respiration transfers energy used for metabolism.
- Describe metabolism as the sum of enzyme-controlled reactions.
- Give an example of a synthesis reaction powered by respiration.
- Describe the conversion of glucose into starch, glycogen and cellulose.
- Explain how lipids, amino acids and proteins are synthesised and how excess protein is broken down to urea.
- Identify respiration as a metabolic process and relate metabolic reactions to their products.
Metabolism exam tips
Marking Points
- Sugars such as glucose are the building blocks for carbohydrates, including starch, glycogen and cellulose.
- Amino acids are the building blocks of proteins, joined in a sequence determined by genes.
- Fatty acids and glycerol combine to form lipids such as fats and oils.
- Breakdown reactions release these small molecules: carbohydrates to sugars, proteins to amino acids, and lipids to fatty acids and glycerol.
- Both synthesis and breakdown are enzyme-controlled metabolic reactions.
- The importance lies in growth, repair, energy storage and the release of usable molecules for respiration and other processes.
- Metabolism is the sum total of all chemical reactions in a cell or the body.
- It includes synthesis reactions that build larger molecules from smaller ones.
- It includes breakdown reactions that split larger molecules into smaller ones.
- Examples include respiration, the formation of glycogen from glucose, and the formation of proteins from amino acids.
- Many metabolic reactions are controlled by enzymes.
- Metabolic activity varies between cells and organisms depending on their functions and energy demands.
- Respiration transfers energy from glucose; this energy is used to drive metabolic reactions rather than being released as heat only.
- Metabolism is the sum of all enzyme-controlled reactions in a cell or organism.
- Many metabolic reactions are synthesis reactions that build larger molecules from smaller ones.
- The reactions are continual because molecules are constantly built, replaced and broken down.
- Enzymes control each metabolic reaction, so metabolism depends on enzyme activity.
- An example is the synthesis of glycogen from glucose in liver and muscle cells using energy from respiration.
- Define metabolism as the sum of all chemical reactions in a cell or organism, including both synthesis and breakdown reactions.
- State that glucose is converted to starch in plants, glycogen in animals and fungi, and cellulose in plant cell walls.
- Describe lipid formation as one glycerol molecule combining with three fatty acid molecules.
- Explain that plants use glucose and nitrate ions to form amino acids, which are then joined to synthesise proteins.
- Recognise respiration as a metabolic process that releases energy from glucose.
- Describe the breakdown of excess proteins to form urea, which is excreted, and identify the liver as the organ where urea is formed.
Examiner Tips
- 💡Use a two-column table for synthesis and breakdown, listing the small molecules and the large molecules for each row.
- 💡When explaining importance, link each molecule to a named process such as growth, repair, storage or respiration.
- 💡Use the terms monomer and polymer only if you can also name the specific examples, such as glucose and starch.
- 💡Use the phrase 'sum of all reactions' in your definition, then give one synthesis and one breakdown example.
- 💡When asked about a specific process, state whether it is synthesis or breakdown and name the molecules involved.
- 💡Link metabolism to enzyme action by noting that enzymes control the rate of metabolic reactions.
- 💡Link respiration to metabolism in one sentence: respiration transfers the energy that enzyme-controlled metabolic reactions use.
- 💡Name a specific synthesis reaction, such as glucose to glycogen, to show understanding rather than repeating the word metabolism.
- 💡Use the phrase enzyme-controlled when describing metabolic reactions, because enzymes are part of the specification statement.
- 💡Use the phrase 'sum of all chemical reactions' when defining metabolism to show the full meaning.
- 💡When asked about protein breakdown, name both the product urea and the organ where it is formed, the liver.
- 💡Link each metabolic reaction to its location or organism, for example cellulose in plant cell walls and glycogen in animal cells.
Common Mistakes
- Saying proteins are made from sugars: correct this by stating that proteins are synthesised from amino acids.
- Confusing the breakdown products of lipids with those of carbohydrates: correct this by stating that lipids break down to fatty acids and glycerol, while carbohydrates break down to sugars.
- Describing synthesis as digestion: correct this by explaining that synthesis builds larger molecules from smaller ones, whereas digestion is a breakdown process that releases smaller molecules.
- Defining metabolism as only breakdown or only synthesis: correct this by stating that it includes both types of reaction.
- Confusing metabolism with respiration: correct this by explaining that respiration is one important metabolic reaction, but metabolism includes many other reactions too.
- Thinking metabolism occurs only in the liver: correct this by stating that all cells carry out metabolic reactions, although the liver has many roles.
- Saying respiration makes energy: energy is transferred from glucose, not created; correct this by writing that respiration transfers energy for metabolism.
- Describing metabolism as only breakdown: metabolism includes both synthesis and breakdown reactions; correct this by stating that synthesis of new molecules is a major part.
- Thinking metabolism happens only in animals: all living organisms carry out metabolism; correct this by applying the idea to plants, animals and microorganisms.
- Thinking metabolism only means breaking down molecules; it also includes synthesis reactions such as building starch, glycogen, cellulose, lipids and proteins.
- Confusing glycogen and starch: glycogen is the storage carbohydrate in animals and fungi, while starch is the storage carbohydrate in plants.
- Stating that lipids are made from glycerol and fatty acids in a one-to-one ratio; one glycerol molecule combines with three fatty acid molecules.