Metabolism — AQA GCSE Combined Science
Test yourself on Metabolism with AQA GCSE practice questions.
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Metabolism explained
Metabolism builds large molecules from small ones and breaks large ones down.
Read the full explanation
Sugars such as glucose are the monomers used to synthesise carbohydrates, including starch and glycogen, by condensation polymerisation. Amino acids are the monomers that join in specific sequences to synthesise proteins. Fatty acids and glycerol are the two components that combine to synthesise lipids. In breakdown, carbohydrates are hydrolysed to sugars, proteins to amino acids, and lipids to fatty acids and glycerol. For example, many glucose units condense to form glycogen in liver and muscle cells, while digestion hydrolyses starch to glucose. These small molecules are also respiratory substrates, so synthesis and breakdown link to energy release and storage.
Metabolism is the sum of all the reactions in a cell or the body.
Metabolism means every chemical reaction occurring in a cell or in the whole body, added together. It includes synthesis reactions that build larger molecules, such as joining glucose to form glycogen or amino acids to form proteins, and breakdown reactions that split larger molecules into smaller ones, such as hydrolysis of starch to glucose. It also includes respiration, which transfers energy from glucose and other substrates. Because metabolism is a sum, it is not one single reaction or one organ; it is the total activity of all cells. For example, a liver cell simultaneously synthesises glycogen, breaks down excess amino acids and respires glucose, and all these reactions together are part of that cell's metabolism.
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, and that energy does not simply escape as heat: it drives the chemical work of the organism. Metabolism is the sum of all the chemical reactions in a cell or body, and many of these reactions build larger molecules from smaller ones, so they are synthesis reactions. Because these reactions would not happen quickly enough on their own, enzymes control them, lowering the activation energy needed. The reactions are continual because cells constantly need new molecules for growth, repair and replacement. For example, amino acids are joined into proteins at ribosomes, and glucose molecules are joined into starch or glycogen. Energy from respiration powers these bonds, so a cell with blocked respiration cannot keep synthesising molecules and will die.
Metabolism includes:
This statement introduces the range of reactions that count as metabolism. Metabolism is not one reaction but the total of all chemical reactions in a cell or body. It includes synthesis reactions that build larger molecules from smaller ones, such as joining glucose molecules to form starch or glycogen, joining amino acids to form proteins, and joining fatty acids and glycerol to form lipids. It also includes breakdown reactions, such as the breakdown of excess protein into urea in the liver, and the breakdown of glucose during respiration to release energy. Some metabolic reactions use energy transferred by respiration, while others release energy. Because all these reactions happen in living cells, they are enzyme controlled and continual. A useful way to remember the range is to think of building up and breaking down, with named examples for each.
conversion of glucose to starch, glycogen and cellulose
Glucose is a soluble sugar made in photosynthesis and used in respiration. Excess glucose is converted into storage or structural polymers by condensation reactions, each glucose unit joining with the loss of a water molecule. Plants convert glucose to starch for storage in leaves, roots and seeds, and to cellulose for cell walls. Animals, including humans, convert glucose to glycogen for storage in the liver and muscles. Starch and glycogen are compact, insoluble and easily hydrolysed back to glucose when needed; cellulose is strong and structural. A useful method is to compare the three products by monomer, solubility, function and location, for example starch in a potato tuber, glycogen in liver cells and cellulose in a plant cell wall.
the formation of lipid molecules from a molecule of glycerol and three molecules of fatty acids
A lipid molecule is formed when one glycerol molecule joins with three fatty acid molecules in a condensation reaction. Glycerol has three hydroxyl groups, and each fatty acid has a carboxyl group, so three ester bonds form and three molecules of water are released. The product is a triglyceride. Fatty acids may be saturated, with only single carbon-carbon bonds, or unsaturated, with at least one carbon-carbon double bond. Lipids are stored in plants and animals as an energy store, provide insulation and protection, and are used to make cell membranes. A useful method is to count the reactants and products: one glycerol plus three fatty acids gives one triglyceride plus three water molecules.
the use of glucose and nitrate ions to form amino acids which in turn are used to synthesise proteins
Plants make glucose by photosynthesis. Glucose provides carbon skeletons and energy. Nitrate ions absorbed by roots supply nitrogen. These combine to form amino acids, the monomers of proteins. For example, glucose from photosynthesis and nitrate ions from soil are used in leaf cells to make amino acids, which are then joined by peptide bonds into proteins. Proteins act as enzymes, hormones, structural components and transport molecules. This is an example of metabolism: the sum of all reactions in a cell or the body. The process requires energy from respiration. If nitrate ions are deficient, plants cannot make enough amino acids, so protein synthesis slows and growth is stunted, with yellowing older leaves. This links mineral nutrition to protein synthesis and plant health.
respiration
Respiration is the sum of enzyme-controlled reactions that release energy from glucose. Aerobic respiration uses oxygen and occurs mainly in mitochondria. It produces carbon dioxide and water and releases energy for metabolism. Anaerobic respiration occurs without oxygen and releases less energy; in animals it produces lactic acid, and in plants and yeast it produces ethanol and carbon dioxide. The energy released is used for muscle contraction, active transport, protein synthesis and maintaining body temperature. For example, muscle cells respire glucose to provide energy for contraction. Respiration is a key part of metabolism because it supplies the energy for all other metabolic reactions. It is not breathing; breathing is ventilation, which brings oxygen to the lungs and removes carbon dioxide.
breakdown of excess proteins to form urea for excretion.
Amino acids absorbed from digested protein cannot be stored by the body, so any surplus is broken down in the liver. The nitrogen-containing amino group is removed (deamination) and converted into urea, while the remaining carbon skeleton is respired to release energy. Urea is toxic, so it passes into the blood and is filtered out by the kidneys, then excreted in urine. For example, a person eating more protein than their cells need will produce extra urea, which is why urine tests can reveal how well the kidneys are working. This links metabolism, the liver, the kidneys and excretion together.
All of these aspects are covered in more detail in the relevant specification section but are linked together here.
This statement is a signpost: the metabolism section brings together ideas that are taught separately elsewhere, such as respiration, protein synthesis, deamination and the roles of the liver and kidneys. Rather than learning these as isolated facts, you should connect them into one system. For example, amino acids from digested protein are used to build new proteins, but any excess is deaminated in the liver to make urea, which the kidneys excrete. Similarly, glucose from digestion is used in respiration or stored as glycogen. Revising with a flow diagram or concept map helps you see how each process supplies or removes substances from the blood.
Your focus
- Identify sugars, amino acids, fatty acids and glycerol as the small molecules used to synthesise carbohydrates, proteins and lipids.
- Describe synthesis as condensation and breakdown as hydrolysis for carbohydrates, proteins and lipids.
- Apply the roles of these molecules to examples such as glycogen formation or protein synthesis.
Show all 30 objectives
- State that metabolism is the sum of all reactions in a cell or the body.
- Give examples of synthesis and breakdown reactions that contribute to metabolism.
- Explain that respiration is part of metabolism because it supplies energy for other reactions.
- State that energy transferred by respiration is used for metabolism.
- Describe metabolism as the sum of enzyme controlled reactions that synthesise new molecules.
- Explain why metabolic synthesis is continual in a living organism.
- Identify examples of synthesis reactions included in metabolism.
- Identify examples of breakdown reactions included in metabolism.
- Describe metabolism as the sum of enzyme controlled reactions in a cell or body.
- State that glucose is converted into starch, glycogen and cellulose.
- Describe each conversion as a condensation reaction that releases water.
- Relate the properties of starch, glycogen and cellulose to their functions in plants and animals.
- State the reactants and products when a lipid is formed from glycerol and fatty acids.
- Describe lipid formation as a condensation reaction that releases water and forms ester bonds.
- Relate the structure of lipids to their functions in organisms.
- State that glucose and nitrate ions are used to form amino acids.
- Describe how amino acids are joined to synthesise proteins.
- Explain the effect of nitrate ion deficiency on protein synthesis and plant growth.
- Describe the process of aerobic respiration and its products.
- Compare aerobic and anaerobic respiration in terms of oxygen requirement and energy release.
- Explain how the energy released by respiration is used in cells.
- State that excess amino acids are broken down in the liver to form urea.
- Describe the route of urea from the liver to excretion in urine.
- Explain why urea must be excreted and how this links to kidney function.
- Identify the separate processes that are linked in the metabolism topic.
- Construct a flow diagram connecting digestion, the liver, the blood and the kidneys.
- Explain how one metabolic process affects another using a named example.
Metabolism exam tips
Marking Points
- Sugars are the small molecules from which carbohydrates are synthesised; many sugar units join by condensation to form polysaccharides such as starch or glycogen.
- Amino acids are the monomers of proteins; they join by condensation in a sequence determined by DNA, forming polypeptides that fold into functional proteins.
- Fatty acids and glycerol are the components of lipids; one glycerol molecule combines with fatty acid molecules by condensation to synthesise a lipid.
- Breakdown is hydrolysis: carbohydrates are broken down to sugars, proteins to amino acids, and lipids to fatty acids and glycerol.
- Synthesis requires energy and removes water, whereas breakdown releases the component molecules and uses water, so the two processes are complementary in metabolism.
- The small molecules produced by breakdown can be respired to release energy or reused to synthesise new biological molecules.
- Metabolism is defined as the sum of all the reactions in a cell or the body, not a single reaction or a single organ.
- It includes both synthesis reactions that build larger molecules and breakdown reactions that split larger molecules.
- Respiration is part of metabolism because it transfers energy from substrates such as glucose for other cellular processes.
- Examples include the synthesis of glycogen from glucose, the synthesis of proteins from amino acids, and the breakdown of excess amino acids.
- Metabolism occurs in all living cells, so the metabolism of the body is the combined metabolism of its cells.
- The idea of a sum means individual reactions are linked; the products of one reaction can be substrates for another.
- Respiration transfers energy from glucose in cells; this energy is not used directly for movement only but also for chemical reactions.
- Metabolism is the sum of all the chemical reactions in a cell or the body, including both synthesis and breakdown reactions.
- Synthesis reactions build larger molecules from smaller ones, for example joining amino acids into proteins or glucose into starch and glycogen.
- The metabolic reactions are enzyme controlled, so each step has a specific enzyme with a complementary active site.
- The processes are continual because new molecules are needed throughout the organism's life for growth, repair and replacement.
- Energy released by respiration is used to drive synthesis, for example in forming bonds when amino acids polymerise into polypeptides.
- Metabolism includes synthesis reactions that build larger molecules from smaller ones, for example glucose to starch or glycogen.
- Metabolism includes the formation of proteins from amino acids and lipids from fatty acids and glycerol.
- Metabolism includes breakdown reactions, such as the breakdown of excess protein to urea in the liver.
- Metabolism includes respiration, in which glucose is broken down to release energy for the cell.
- All metabolic reactions are controlled by enzymes, so temperature and pH affect their rate.
- Metabolism is the sum of all reactions in a cell or body, not a single process or organ.
- Glucose is a monosaccharide; starch, glycogen and cellulose are polysaccharides formed by joining many glucose units together.
- The conversions are condensation reactions: each glucose unit added releases one molecule of water.
- Starch is the storage polysaccharide in plants, found in leaves, roots, tubers and seeds.
- Glycogen is the storage polysaccharide in animals and fungi, stored in the liver and muscles.
- Cellulose is a structural polysaccharide that forms plant cell walls and gives strength.
- Starch and glycogen are insoluble and compact, so they do not affect cell water potential and are readily hydrolysed to glucose for respiration.
- Cellulose molecules form straight chains held by hydrogen bonds, giving strength; it is not used as an energy store in humans.
- All three are polymers of glucose but differ in the arrangement of glucose units and in the bonds between them, so their properties differ.
- One glycerol molecule reacts with three fatty acid molecules to form one lipid (triglyceride) molecule.
- The reaction is condensation: three molecules of water are released, one for each fatty acid joined.
- Three ester bonds form between the glycerol and the fatty acids.
- Fatty acids may be saturated (only single carbon-carbon bonds) or unsaturated (at least one carbon-carbon double bond).
- Lipids are an energy store in plants and animals and also provide insulation and protection.
- Lipids are used to make cell membranes, where they form a bilayer.
- The reaction is the reverse of hydrolysis, which uses water to break a triglyceride into glycerol and fatty acids.
- Glucose is produced by photosynthesis and provides carbon, hydrogen and oxygen for amino acid synthesis.
- Nitrate ions are absorbed from the soil by root hair cells and provide nitrogen for amino acid synthesis.
- Amino acids are the monomers that are joined together to synthesise proteins.
- Protein synthesis is an example of metabolism and requires energy from respiration.
- A deficiency of nitrate ions limits amino acid production, so protein synthesis and plant growth are reduced.
- Respiration is an enzyme-controlled process that releases energy from glucose.
- Aerobic respiration requires oxygen and produces carbon dioxide and water.
- Anaerobic respiration occurs without oxygen and releases less energy than aerobic respiration.
- The energy released by respiration is used for processes such as muscle contraction, active transport and protein synthesis.
- Respiration is a metabolic process and is distinct from breathing or ventilation.
- Excess amino acids from protein digestion are transported to the liver in the blood.
- The liver removes the amino group from excess amino acids, a process called deamination.
- The nitrogen from the removed amino group is converted into urea.
- Urea is toxic and must be excreted rather than stored in the body.
- Urea travels in the blood plasma to the kidneys, where it is filtered out of the blood.
- Urea leaves the body dissolved in urine, which is excreted via the ureters, bladder and urethra.
- The carbon skeleton left after deamination can be respired to release energy or converted to carbohydrate or fat.
- Metabolism is the sum of all the chemical reactions in the body, including both building and breaking-down reactions.
- Ideas such as respiration, protein synthesis and deamination are taught in their own specification sections but are connected in this topic.
- Excess amino acids are deaminated in the liver to form urea, linking protein metabolism to excretion.
- The kidneys filter urea from the blood and excrete it in urine, linking metabolism to homeostasis.
- Glucose metabolism links digestion, respiration, glycogen storage and blood glucose control.
- Understanding these links helps explain how the body handles different nutrients and removes waste products.
Examiner Tips
- 💡Name the monomer or component for each polymer before describing synthesis or breakdown, so each clause of the question is clearly addressed.
- 💡Use the terms condensation and hydrolysis accurately, linking condensation to synthesis and hydrolysis to breakdown.
- 💡Give one concrete example, such as glucose forming glycogen or amino acids forming a protein, to show understanding rather than repeating the statement.
- 💡Use the phrase sum of all reactions when defining metabolism, then give one synthesis and one breakdown example to show the range.
- 💡Link metabolism to respiration by explaining that respiration supplies energy for synthesis reactions.
- 💡Avoid treating metabolism as a single process; refer to reactions in cells or the body collectively.
- 💡Link the statement to a named example, such as amino acids joining to form proteins, so the idea of synthesising new molecules is concrete.
- 💡Use the phrase enzyme controlled when describing metabolic reactions, because this shows the role of enzymes in the process.
- 💡If asked why metabolism is continual, refer to growth, repair and replacement of cells rather than saying it happens all the time without reason.
- 💡Give at least one build-up example and one breakdown example when asked what metabolism includes.
- 💡Name the molecules involved, such as amino acids and proteins, rather than saying big molecules and small molecules.
- 💡Use the term enzyme controlled when describing metabolic reactions to show how they are regulated.
- 💡Link each product to its function and location, for example starch in a potato tuber, glycogen in liver cells, cellulose in a plant cell wall.
- 💡Use the word condensation and state that water is released when glucose units join.
- 💡Compare the three polysaccharides in a table of monomer, solubility, function and where found to cover several marking points concisely.
- 💡Count the reactants and products: one glycerol plus three fatty acids gives one triglyceride plus three water molecules.
- 💡Name the bond formed between glycerol and a fatty acid as an ester bond.
- 💡Link the structure of lipids to their roles, such as energy storage, insulation and cell membranes.
- 💡Link the role of glucose to photosynthesis and the role of nitrate ions to root absorption when explaining amino acid formation.
- 💡Use the terms monomer and polymer correctly: amino acids are monomers, proteins are polymers.
- 💡When describing deficiency symptoms, state the effect on protein synthesis and then on growth, rather than just saying the plant is unhealthy.
- 💡Write the word equation for aerobic respiration accurately: glucose + oxygen → carbon dioxide + water.
- 💡When comparing aerobic and anaerobic respiration, state the presence or absence of oxygen and the relative energy released.
- 💡Link the energy released by respiration to a named cellular process, such as active transport or muscle contraction, to show understanding.
- 💡Use the term deamination accurately and link it to the liver, not the kidney.
- 💡When asked about excretion, name urea as the nitrogenous waste and state that it leaves in urine.
- 💡Sequence the route clearly: excess amino acids → liver → urea → blood → kidneys → urine.
- 💡If asked why urea must be removed, state that it is toxic and would build up if not excreted.
- 💡Practise linking topics by writing short chains such as protein → amino acids → liver → urea → kidney → urine.
- 💡When a question mentions more than one organ, name each organ and its role in the sequence.
- 💡Use connectives such as 'which is then' and 'this links to' to show how processes depend on each other.
- 💡Revise the relevant specification sections so you can recall the detail behind each link.
Common Mistakes
- Stating that proteins are synthesised from sugars; correction: proteins are synthesised from amino acids, while sugars are used for carbohydrates.
- Saying lipids are made from amino acids; correction: lipids are synthesised from fatty acids and glycerol.
- Describing breakdown as condensation; correction: breakdown is hydrolysis, in which water is used to split bonds between monomers.
- Defining metabolism as only breakdown or only respiration; correction: metabolism is the sum of all reactions, including both synthesis and breakdown.
- Thinking metabolism occurs only in the liver or only during exercise; correction: metabolism occurs in all living cells all the time.
- Confusing metabolism with digestion; correction: digestion is only one set of breakdown reactions, whereas metabolism includes all reactions in cells and the body.
- Saying respiration makes energy: correct this by stating that respiration transfers energy from glucose, since energy is not created.
- Confusing metabolism only with digestion or only with breakdown: correct this by defining metabolism as all reactions, including synthesis of new molecules.
- Writing that enzymes provide the energy for synthesis: correct this by explaining that enzymes lower activation energy and speed up reactions, while respiration supplies the energy.
- Listing only synthesis reactions: correct this by also giving a breakdown example such as the formation of urea in the liver.
- Treating metabolism as a single reaction: correct this by defining it as the sum of all chemical reactions in a cell or body.
- Confusing the breakdown of glucose in respiration with digestion: correct this by stating that respiration is a series of enzyme controlled reactions inside cells.
- Saying glucose is converted to glycogen in plants: correction, plants store starch, while animals and fungi store glycogen.
- Describing the conversion as hydrolysis: correction, joining glucose units is condensation, which releases water; hydrolysis uses water to break the polymer down.
- Stating that cellulose is a storage molecule: correction, cellulose is structural, forming plant cell walls, and is not broken down for energy in humans.
- Writing that one fatty acid joins glycerol: correction, three fatty acids join one glycerol, releasing three water molecules.
- Saying water is used up in the reaction: correction, condensation releases water; hydrolysis uses water.
- Confusing glycerol with glucose: correction, glycerol is an alcohol with three hydroxyl groups, while glucose is a sugar.
- Thinking plants absorb nitrogen gas from the air: correction — plants absorb nitrate ions from the soil through root hair cells.
- Confusing amino acids with proteins: correction — amino acids are the monomers; proteins are polymers made from many amino acids.
- Believing glucose alone is enough to make proteins: correction — glucose provides carbon skeletons, but nitrogen from nitrate ions is also needed to form amino acids.
- Confusing respiration with breathing: correction — respiration is a chemical process in cells; breathing is ventilation that moves air in and out of the lungs.
- Thinking anaerobic respiration releases more energy than aerobic respiration: correction — anaerobic respiration releases less energy because glucose is not fully broken down.
- Believing respiration only occurs in animals: correction — respiration occurs in all living cells, including plant cells.
- Thinking that excess protein is stored unchanged in the body: correction — surplus amino acids are broken down in the liver, and the nitrogen is converted to urea.
- Confusing deamination with digestion: correction — digestion breaks protein into amino acids in the gut, whereas deamination breaks down amino acids in the liver.
- Saying that urea is made in the kidneys: correction — urea is made in the liver; the kidneys filter urea out of the blood and excrete it in urine.
- Treating each process as a separate fact rather than linking them: correction — build a concept map showing how digestion, liver, blood and kidneys connect.
- Assuming this statement contains new content to memorise: correction — it is a signpost, so revise the linked sections and practise making connections.
- Confusing metabolism with digestion: correction — digestion is the breakdown of food in the gut, while metabolism covers all chemical reactions in cells and organs.