⚗ Step 1
Glucose and nitrate ions are used to form amino acids.
Respiration » Metabolism
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.4.2.3
- What metabolism is and why cells need energy for it
- How glucose is turned into starch, glycogen and cellulose
- How lipids and proteins are built from smaller molecules
- What happens to excess protein
🔒 Unlock Full Course Content
Sign up to access the complete lesson and track your progress!
Unlock This CourseWhat is metabolism?
A living cell is a busy chemical factory. Thousands of reactions happen inside it every second. Some build big molecules from small ones. Some break big molecules down. Some release energy.
Metabolism is the sum of all the reactions in a cell or the body. Every reaction in your cells, added together, is your metabolism.
Reactions that build new molecules need energy. Enzymes control these reactions. The energy transferred by respiration in cells is used by the organism for the continual enzyme controlled processes of metabolism that synthesise (build) new molecules.
Key terms:
- Metabolism: the sum of all the reactions in a cell or the body.
- Synthesise: to make a new, larger molecule from smaller molecules.
Making carbohydrates from glucose
Glucose is a sugar. It is the starting point for several larger carbohydrates. Cells join many glucose molecules together into long chains.
🌾 Starch
Plants convert glucose to starch. Starch is a store of energy in the plant.
💪 Glycogen
Animals convert glucose to glycogen. It is a store of glucose that can be broken down again when the body needs energy.
🌿 Cellulose
Plants convert glucose to cellulose. Cellulose is used to build strong cell walls.
These reactions can also run backwards. Breaking starch or glycogen down gives glucose again, which can be used in respiration.
Key terms:
- Glycogen: a carbohydrate made from many glucose molecules, used as a store in animals.
Making lipids
Lipids are fats and oils. A lipid molecule is made from one molecule of glycerol and three molecules of fatty acids. The three fatty acids join to the glycerol.
Remember it like this
Glycerol is like a hook with three places to hang a fatty acid. One glycerol + three fatty acids = one lipid.
Lipids can also be broken down again into glycerol and fatty acids.
Key terms:
- Glycerol: the small molecule that three fatty acids join to when a lipid is made.
- Fatty acid: one of the molecules that joins to glycerol to make a lipid.
Making proteins
Proteins are made from long chains of amino acids. Plants make their own amino acids. They use glucose and nitrate ions to do it. The nitrate ions are taken in from the soil. The glucose is made in photosynthesis.
🔗 Step 2
Many amino acids are joined together to synthesise a protein.
Proteins can be broken down into amino acids too. The amino acids can then be used to build different proteins.
What happens to excess protein?
Excess protein is broken down. The breakdown forms urea, which is a waste product. Urea is removed from the body for excretion. It leaves the body in urine. You will see how this works in the lesson Water Balance and the Kidneys.
Key terms:
- Urea: the waste substance formed when excess protein is broken down, removed from the body in urine.
Putting it together
Metabolism includes all of these processes, plus respiration, which supplies the energy:
- glucose converted to starch, glycogen and cellulose
- one glycerol and three fatty acids forming a lipid
- glucose and nitrate ions forming amino acids, then amino acids forming proteins
- respiration
- breakdown of excess proteins to form urea
Sugars, amino acids, fatty acids and glycerol are the small molecules. They are the building blocks and the breakdown products of carbohydrates, proteins and lipids.
Worked example
A bean seedling is given a sugar solution but no nitrate ions. It has glucose, but it cannot form amino acids, so it cannot synthesise the proteins needed for new cells. This shows how one part of metabolism depends on another.
Common mistakes
Writing that metabolism is just respiration. Respiration is only one part of metabolism. Another mistake is mixing up the small molecules: lipids come from glycerol and fatty acids, proteins come from amino acids, and starch, glycogen and cellulose come from glucose.
Exam-style question
(a) What is meant by metabolism? [1 mark]
(b) Name the two types of small molecule that join to make a lipid. [2 marks]
(c) A plant makes amino acids from glucose and one other substance. Name this substance. [1 mark]
(d) Describe what happens to excess protein in the body. [2 marks]
Model answer
(a) The sum of all the reactions in a cell or the body (1).
(b) Glycerol (1) and fatty acids (1).
(c) Nitrate ions (1).
(d) It is broken down (1) to form urea, which is excreted (1).
Exam tip
In part (b), the question asks for two types of molecule, so name glycerol and fatty acids. Write one name for each mark.