Practise Metabolism. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Metabolism is the sum of all the reactions in a cell or the body. You need the examples of metabolic reactions in the specification and the small molecules that carbohydrates, proteins and lipids are built from and broken down into. Expect short recall and explain questions, often linked to enzymes, digestion or photosynthesis.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Define metabolismMetabolism is the sum of all the reactions in a cell or the body.
4
Give examples of metabolic reactionsFor example respiration, making starch, glycogen or cellulose from glucose, making lipids and proteins, and making urea.
5
Name the building blocks of large moleculesCarbohydrates are made from sugars, proteins from amino acids, and lipids from glycerol and fatty acids.
5
State how a lipid molecule formsOne molecule of glycerol joins with three molecules of fatty acids.
6
Explain why metabolism needs respiration and enzymesBuilding new molecules uses energy transferred by respiration, and each reaction is controlled by enzymes.
6
Describe what happens to excess proteinExcess proteins are broken down to form urea, which is excreted.
7
Explain the importance of the small building blocksSugars, amino acids, fatty acids and glycerol are what large molecules are built from and broken down into, so they can be reused.
Notes
What metabolism is
Metabolism is the sum of all the reactions in a cell or the body.
Metabolic reactions are controlled by enzymes.
The energy transferred by respiration is used for the metabolic reactions that build (synthesise) new molecules.
Some metabolic reactions build large molecules from small ones; others break large molecules down.
Examples you need to know
Glucose is converted into starch (storage in plants), glycogen (storage in animals) and cellulose (plant cell walls).
A lipid molecule is formed from one molecule of glycerol and three molecules of fatty acids.
Glucose and nitrate ions are used to make amino acids, which are then used to make proteins.
Respiration is itself a metabolic reaction.
Excess proteins are broken down to form urea, which is excreted in urine.
Building blocks: made from and broken down into
Carbohydrates (starch, glycogen, cellulose) are built from sugars such as glucose.
Proteins are built from amino acids.
Lipids are built from fatty acids and glycerol.
Digestive enzymes break large food molecules down into these small units: carbohydrases give sugars, proteases give amino acids, and lipases give fatty acids and glycerol. grade 6+
The small units are absorbed and used to build the body's own carbohydrates, proteins and lipids, and glucose is also used in respiration. So these small molecules link the breakdown of food to the synthesis of new molecules. grade 7+
Cheatsheet
Metabolism = the sum of all the reactions in a cell or the body
Metabolic reactions are controlled by enzymes
Energy for building new molecules comes from respiration
Learn the definition exactly: the sum of all the reactions in a cell or the body.
For examples, choose from the specification list and make sure they fit the organism in the question (starch and cellulose in plants, glycogen in animals).
Example. (a) What is meant by metabolism? (b) Give two examples of metabolic reactions that take place in plant cells.
Show the model answer
(a) The sum of all the reactions in a cell or the body (1) (b) Any two from: respiration; converting glucose into starch; converting glucose into cellulose; making amino acids / proteins; making lipids (1 each)
Complete sentences on building blocks
3 marks5
Match each large molecule to its small units: proteins to amino acids, lipids to glycerol and fatty acids, carbohydrates to sugars.
Remember the numbers for a lipid: one glycerol, three fatty acids.
Example. Complete the sentences. Proteins are made from molecules of .................... . A lipid molecule is made from one molecule of .................... and three molecules of .................... .
Show the model answer
amino acids (1); glycerol (1); fatty acids (1)
Explain why a metabolic reaction needs respiration
2 marks6
Say that building large molecules needs energy.
Say that this energy is transferred by respiration.
Example. A poison stops respiration in a cell. Explain why the cell can no longer make proteins.
Show the model answer
Joining amino acids to make proteins (building large molecules) needs energy (1). This energy is transferred by respiration, so without respiration there is no energy for protein synthesis (1).
Explain how a nutrient is used in metabolism
3 to 4 marks7
Start with breakdown: the large molecule is digested into its small units.
Then synthesis: the small units are used to build the body's own molecules, using enzymes and energy from respiration.
Finish with any excess and what happens to it.
Example. A person eats a meal that contains protein. Explain how the protein is used in the person's metabolism. [4 marks]
Show the model answer
The protein is broken down (digested) into amino acids (1). The amino acids are absorbed and used to build (synthesise) the person's own proteins, for example enzymes (1). These reactions are controlled by enzymes and use energy transferred by respiration (1). Any excess protein is broken down to form urea, which is excreted (1).
Shortcuts and memory tricks
Metabolism = every reaction: all the building up and all the breaking down.
Three carbohydrates from glucose: starch stores (plants), glycogen stores (animals), cellulose supports (cell walls).
Lipid 1 + 3: picture one glycerol backbone with three fatty acid tails, like a capital E.
Pair each large molecule with its units and its enzyme: carbohydrate–sugars–carbohydrase, protein–amino acids–protease, lipid–fatty acids and glycerol–lipase.
Where marks are lost
Defining metabolism as 'digestion' or 'how fast you use food'. It is the sum of all the reactions in a cell or the body.
Saying lipids are made of glucose and fatty acids. They are made of glycerol and fatty acids (one glycerol and three fatty acids).
Saying plants store glycogen or animals store starch. Plants store starch; animals store glycogen.
Saying urea comes from the breakdown of glucose or fat. It comes from the breakdown of excess proteins.
Forgetting that respiration is itself one of the metabolic reactions.
Exam technique
Learn the specification's examples of metabolic reactions; they are often asked as 'give two examples'.
When a question asks about the importance of sugars, amino acids, fatty acids and glycerol, say they are the units large molecules are built from and broken down into.
Use the words 'synthesise' (build) and 'break down' accurately, and mention enzymes and energy from respiration where they fit.
Expect links to other topics: digestive enzymes, photosynthesis (glucose) and blood glucose control (glycogen).
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the waste product formed when excess proteins are broken down.
Urea
One lipid molecule is made from one molecule of glycerol and three molecules of fatty acids. How many fatty acid molecules are needed to make 20 lipid molecules?
60
In which organ do many important metabolic reactions, such as the breakdown of excess amino acids, take place?
Liver
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy5 marks
Metabolism involves many different reactions in cells.
(a) What is meant by metabolism?[1]
(b) Which two of these are part of metabolism? Tick (✓) two boxes.[2]
Breakdown of excess proteins to form urea
Diffusion of oxygen into the blood
Evaporation of sweat from the skin
Formation of lipids from glycerol and fatty acids
Osmosis into root hair cells
(c) Name the waste product formed when excess proteins are broken down.[1]
(d) Name the carbohydrate made from glucose that is stored in animal cells.[1]
Show the answer and mark scheme
(a)
the sum of all the reactions in a cell or the body
(b)Answer: Breakdown of excess proteins to form urea; Formation of lipids from glycerol and fatty acids
(c)Answer: Urea
urea
(d)Answer: Glycogen
glycogen
Question 2Medium7 marks
The energy transferred by respiration is used for the enzyme-controlled reactions of metabolism.
(a) One lipid molecule is made from one molecule of glycerol and three molecules of fatty acids. How many fatty acid molecules are needed to make 20 lipid molecules?[1]
(b) Name two carbohydrates that plant cells make from glucose.[2]
(c) Explain why these metabolic reactions need energy from respiration.[2]
(d) Plant cells make amino acids. Give the two substances plant cells use to make amino acids.[2]
Show the answer and mark scheme
(a)Answer: 60
60
(b)Answer: Starch and cellulose
starch
cellulose
(c)
they build larger molecules from smaller molecules
building larger molecules requires energy (which is transferred by respiration)
(d)Answer: Glucose and nitrate ions
glucose
nitrate ions
Question 3Hard5 marks
An advert for a weight-loss supplement claims: 'Our pill boosts your metabolism by 50%, so you burn more energy without any extra exercise!' The company's own trial gave the supplement to 30 volunteers for 4 weeks. Their mean resting metabolic rate increased from 6800 kJ per day to 7200 kJ per day. There was no control group given a placebo.
(a) Calculate the actual percentage increase in mean resting metabolic rate found in the trial.[2]
(b) Evaluate the company's claim, using the information given.[3]
Show the answer and mark scheme
(a)Answer: 5.9%
(7200 − 6800) ÷ 6800 × 100
5.9 (%)
(b)
the trial found an increase of only about 5.9%, far less than the advertised 50% increase, so the claim is not supported by the company's own data
there was no placebo or control group, so it is not clear whether the small increase was really caused by the supplement, or by another factor, e.g. a change in activity or diet during the trial, or the placebo effect
with only 30 volunteers and no independent replication, the result may not be reliable or repeatable
conclusion, e.g. the evidence given does not support the claim that the supplement boosts metabolism by 50%, and a properly controlled, larger, independently reviewed trial would be needed before the claim could be trusted