Biological Molecules
SEAB syllabus topic 3: the elements and roles of carbohydrates, fats and proteins, the food tests, how large molecules are built, and how enzymes work, with the why, the traps and the exam checks.
Carbohydrates, fats and proteins all contain carbon, hydrogen and oxygen, and proteins also contain nitrogen.
Carbohydrates give immediate energy, fats insulate and store energy, and proteins build and repair cells.
Iodine, Benedict's solution, biuret solution and ethanol each show whether one food molecule is present.
Glucose, amino acids, and glycerol with fatty acids join by condensation to make large molecules.
An enzyme is a protein that acts as a biological catalyst and works by the lock and key hypothesis.
Enzymes work fastest at their optimum temperature and pH, and are denatured by high temperature or extreme pH.
Elements in food molecules
list the chemical elements which make up:
- carbohydrates
- fats
- proteins
Carbohydrates, fats and proteins are all made of carbon, hydrogen and oxygen, and proteins also contain nitrogen and sometimes sulfur.
- Element
- An element is a substance made of one kind of atom, which cannot be broken down into simpler substances by chemical reactions. supports K325-3(a)
- Carbohydrate
- A carbohydrate is made of carbon, hydrogen and oxygen, with the hydrogen and oxygen atoms in a ratio of about 2 to 1, as in water.
- Fat
- A fat is made of carbon, hydrogen and oxygen, but it has much less oxygen than a carbohydrate.
- Protein
- A protein is made of carbon, hydrogen, oxygen and nitrogen, and some proteins also contain sulfur.
- Lipid
- Lipids are a group of substances that includes fats and oils, and a fat is a lipid that is solid at room temperature. supports K325-3(d)
- Sulfur
- Sulfur is spelt sulphur in older books, and both spellings are accepted in answers. supports K325-3(a)
The elements at a glance
| Food molecule | Elements | Extra detail |
|---|---|---|
| Carbohydrate | Carbohydrates contain carbon, hydrogen and oxygen. | In a simple sugar, the atoms of carbon, hydrogen and oxygen are in the ratio 1 : 2 : 1, so glucose has the formula C6H12O6. |
| Fat | Fats contain carbon, hydrogen and oxygen. | A fat has very little oxygen compared with the amount of carbon and hydrogen. |
| Protein | Proteins contain carbon, hydrogen, oxygen and nitrogen. | Some proteins also contain sulfur, but not all proteins do. |
If a question says that a molecule contains nitrogen, then among carbohydrates, fats and proteins it must be a protein.
Among carbohydrates, fats and proteins, only proteins can contain sulfur, so a food molecule with sulfur is a protein, but not every protein contains sulfur.
For SEAB answers, write the names of the elements, such as carbon, hydrogen and oxygen, and not only the symbols C, H and O. supports K325-3(a)
Depth: why and how
- Why do proteins contain nitrogen? Amino acids, the building blocks of proteins, contain an amino group made of nitrogen and hydrogen, so animals must get nitrogen from the protein in their food.
- Why does a fat release more energy than the same mass of carbohydrate? A fat has a lot of hydrogen and little oxygen, so more of its atoms can be oxidised in respiration.
- Why is the ratio of hydrogen to oxygen in carbohydrates useful to know? A ratio of 2 hydrogen atoms to 1 oxygen atom is the ratio in water, which is where the name carbohydrate, meaning hydrated carbon, comes from.
- Cross-link to Nutrition in humans: the same three groups of food molecules are broken down by amylase, protease and lipase.
Common mistakes
Carbohydrates contain carbon, hydrogen and nitrogen.
Carbohydrates contain carbon, hydrogen and oxygen, and of the three food groups only proteins contain nitrogen.
All proteins contain sulfur.
Some proteins contain sulfur, but not all of them do.
Fats contain the same amount of oxygen as carbohydrates.
Fats contain much less oxygen than carbohydrates.
Fats contain only carbon and hydrogen.
Fats contain carbon, hydrogen and a small amount of oxygen.
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- C Carbon is found in carbohydrates, fats and proteins.
- H Hydrogen is found in carbohydrates, fats and proteins.
- O Oxygen is found in carbohydrates, fats and proteins, but fats have much less of it.
- N Nitrogen is found in proteins but not in carbohydrates or fats.
- S Sulfur is found in some proteins.
Carbohydrates and fats are made of carbon, hydrogen and oxygen, and proteins also contain nitrogen and sometimes sulfur.
Exam check
Q1 Which elements are found in a carbohydrate?
- A Carbon and hydrogen
- B Carbon, hydrogen and nitrogen
- C Carbon, hydrogen and oxygen
- D Carbon, hydrogen, oxygen and nitrogen
Show answer
MCQ answer key C (Carbon, hydrogen and oxygen): a carbohydrate is made of carbon, hydrogen and oxygen, and it has no nitrogen.
Q2 A molecule contains carbon, hydrogen, oxygen and nitrogen. Which food molecule is it most likely to be?
- A A protein
- B A carbohydrate
- C A fat
- D A carbohydrate or a fat
Show answer
MCQ answer key A (A protein): of the three food groups, only proteins contain nitrogen.
Q3 Which statement about the elements in food molecules is correct?
- A All proteins contain sulfur.
- B Fats contain more oxygen than carbohydrates.
- C Carbohydrates contain nitrogen.
- D Fats contain less oxygen than carbohydrates.
Show answer
MCQ answer key D (Fats contain less oxygen than carbohydrates.): fats have much less oxygen than carbohydrates, some proteins but not all contain sulfur, and carbohydrates contain no nitrogen.
Structured Carbohydrates, fats and proteins are the three main groups of food molecules.
- Name the elements found in a carbohydrate. [1]
- Name the element that is found in proteins but not in carbohydrates or fats. [1]
- A student says that fats and carbohydrates contain the same elements in the same proportions. State whether the student is correct, and explain. [2]
Total: 4 marks
Show mark scheme
(a) 1 mark
- Mark scheme: carbon, hydrogen and oxygen
(b) 1 mark
- Mark scheme: nitrogen
(c) 2 marks
- Mark scheme: the student is not correct
- Mark scheme: fats contain much less oxygen than carbohydrates, even though both contain carbon, hydrogen and oxygen
Roles of carbohydrates, fats and proteins
state the main roles of carbohydrates, fats and proteins in living organisms:
- carbohydrates as an immediate source of energy
- fats for insulation and long-term storage of energy
- proteins for growth and repair of cells
Carbohydrates are the immediate source of energy, fats give insulation and long-term energy storage, and proteins are needed for the growth and repair of cells.
- Immediate source of energy
- Glucose is broken down in aerobic respiration to release energy that the cells can use straight away.
- Energy store
- Carbohydrates are also stored as energy reserves, as starch in plants and as glycogen in animals. supports K325-3(b)
- Insulation
- Insulation slows the loss of heat from the body, and a layer of fat under the skin acts as a thermal insulator.
- Growth and repair
- Proteins are the building material for new cells and for repairing damaged cells.
- Antibody
- An antibody is a protein made by white blood cells that helps to destroy disease-causing organisms. supports K325-3(b)
- Hormone
- A hormone is a chemical messenger, and some hormones, such as insulin, are proteins. supports K325-3(b)
Roles of each food molecule
| Food molecule | Main role for SEAB | Other roles |
|---|---|---|
| Carbohydrate | The main role of carbohydrates in the syllabus is to be the immediate source of energy for cells. | Carbohydrates store energy, as starch in plants and as glycogen in animals. Cellulose is a carbohydrate that gives structural support in plant cell walls. |
| Fat | Fats form a layer under the skin that acts as thermal insulation, and fats are also the long-term store of energy; SEAB expects both roles. | A fat releases more energy per gram than a carbohydrate, which makes it a compact long-term energy store. A layer of fat around vital organs, such as the kidneys, protects them from damage. Fats act as a solvent for fat-soluble vitamins, so that these vitamins can be absorbed from the food. Oil secreted by glands in the skin makes the skin waterproof and reduces water loss. |
| Protein | The main role of proteins in the syllabus is the growth and repair of cells. | Proteins are used to make enzymes, antibodies and some hormones. Proteins make the new muscle fibres when a muscle grows bigger. If there is not enough carbohydrate or fat, a protein can be used as a source of energy. supports K325-3(b) |
Lipids such as phospholipids form part of cell membranes, but a phospholipid is not a fat.
Insulation and long-term energy storage point to fats, immediate energy points to carbohydrates, and growth and repair points to proteins.
Depth: why and how
- Why is glucose used as the immediate energy source rather than fat? Glucose is small and soluble, so it is carried quickly in the blood and used in respiration, while a fat must be broken down first.
- Why is a layer of fat a good insulator? Fat conducts heat poorly, so a layer of fat under the skin slows the loss of heat from the body.
- Why does a growing child need more protein? A growing child is making more new cells and muscle fibres, and proteins are the building material for these.
- Cross-link to Nutrition in humans: the liver converts excess glucose into glycogen and stores it, and muscle cells also store glycogen.
Common mistakes
Fats are the immediate source of energy.
Carbohydrates, mainly glucose, are the immediate source of energy, and fats are the long-term energy store.
Proteins are the main long-term store of energy.
Fats are the long-term store of energy, and proteins are mainly used for growth and repair of cells.
Fat in the body is only used for insulation.
Fat also stores energy, protects vital organs and dissolves fat-soluble vitamins, and lipids such as phospholipids form parts of cell membranes.
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- E Carbohydrates are the immediate source of energy.
- I Fats provide insulation under the skin, which slows the loss of heat.
- S Fats are the long-term storage form of energy.
- G Proteins are needed for the growth and repair of cells.
Carbohydrates give immediate energy, fats give insulation and long-term energy storage, and proteins provide for the growth and repair of cells.
Exam check
Q1 Which food molecule is the immediate source of energy for the cells?
- A Fat
- B Carbohydrate
- C Protein
- D Vitamin
Show answer
MCQ answer key B (Carbohydrate): glucose from carbohydrates is broken down in respiration to release energy that the cells can use straight away.
Q2 A child who is growing quickly needs more of which food molecule for the growth and repair of cells?
- A Carbohydrate
- B Fat
- C Starch
- D Protein
Show answer
MCQ answer key D (Protein): proteins are the building material for new cells and for repairing damaged cells.
Q3 Which row matches a food molecule with a correct role?
- A Fat: insulation under the skin
- B Protein: immediate source of energy
- C Carbohydrate: solvent for fat-soluble vitamins
- D Fat: making new muscle fibres
Show answer
MCQ answer key A (Fat: insulation under the skin): a layer of fat under the skin acts as thermal insulation, whereas immediate energy comes from carbohydrates, vitamins dissolve in fat, and muscle fibres are made of protein.
Structured Seals that live in cold water have a thick layer of fat under the skin.
- State the role of the layer of fat under the skin. [1]
- Name the food molecule that is the immediate source of energy for cells. [1]
- State two roles of proteins in the body. [2]
Total: 4 marks
Show mark scheme
(a) 1 mark
- Mark scheme: it acts as thermal insulation, which reduces the loss of heat from the body
(b) 1 mark
- Mark scheme: carbohydrate, or glucose
(c) 2 marks
- Mark scheme: one mark for any one of: growth and repair of cells (making new cells), making enzymes, making antibodies, or making some hormones
- Mark scheme: one more mark for a different role from the same list
Food tests
describe and carry out tests for:
- starch (using iodine in potassium iodide solution)
- reducing sugars (using Benedict’s solution)
- protein (using biuret solution)
- fats (using ethanol)
Each food test uses a reagent that changes colour or appearance when one food molecule is present: iodine solution for starch, Benedict's solution for reducing sugars, biuret solution for protein and ethanol for fats.
- Reagent
- A reagent is a chemical that is added to the food being tested, to show whether a particular substance is present. supports K325-3(c)
- Reducing sugar
- A reducing sugar is a sugar that reacts with Benedict's solution, when heated, to give a coloured precipitate.
- Precipitate
- A precipitate is a solid that forms in a liquid during a chemical reaction. supports K325-3(c)
- Non-reducing sugar
- Sucrose is a double sugar that is non-reducing, so it does not give a precipitate with Benedict's solution unless it is first boiled with dilute acid, while glucose, fructose and maltose are reducing sugars.
- Emulsion
- An emulsion is a mixture of tiny droplets of one liquid spread through another liquid, such as fat droplets in water. supports K325-3(c)
- Control test
- A control test uses water in place of the food, to show what a negative result looks like. supports K325-3(c)
The four tests at a glance
| Food molecule | Reagent | Negative result | Positive result |
|---|---|---|---|
| Starch | Starch is tested with iodine solution, which is iodine in potassium iodide solution. | With no starch, the iodine solution stays yellow-brown. | With starch, the iodine solution turns blue-black. |
| Reducing sugar | Reducing sugars are tested with Benedict's solution, which is blue, and the mixture is heated. | With no reducing sugar, the mixture stays blue. | With a reducing sugar, the mixture forms a green, yellow, orange or brick-red precipitate. |
| Protein | Protein is tested with biuret solution, which is blue. | With no protein, the mixture stays blue. | With protein, the mixture turns violet or purple. |
| Fat | Fats are tested with ethanol, followed by water. | With no fat, the mixture stays clear. | With fat, the mixture turns into a cloudy white emulsion. |
Benedict's test for reducing sugars
Liquid food
- 1Put equal volumes of Benedict's solution and the liquid food into a test tube.
- 2Heat water in a beaker until it boils vigorously.
- 3Place the test tube into the boiling water, and make sure that the water level in the beaker is higher than the level of the mixture in the tube.
- 4Watch the mixture for a colour change, and compare the colour with the scale of results.
Solid food and safety
- For a solid food, cut it into very small pieces, add 2 cm3 of distilled water and shake the tube.
- Add 2 cm3 of Benedict's solution to the tube and then continue in the same way as for a liquid food.
- Use a test tube holder, wear safety goggles, and point the mouth of the tube away from everyone.
Reading the result
- The colour changes from blue, through green, yellow and orange, to brick-red as the amount of reducing sugar increases.
- A mixture that stays blue shows that there is no reducing sugar, or too little to detect.
- Glucose, fructose and maltose are reducing sugars, so they give a positive result with Benedict's solution.
- Sucrose is a non-reducing sugar, so it gives a negative result and the mixture stays blue.
- Reducing sugars change the blue copper(II) ions in Benedict's solution into copper(I) oxide, which is the coloured precipitate. supports K325-3(c)
Sucrose gives a positive result with Benedict's solution once it has been broken into glucose and fructose, which are reducing sugars, for example by boiling it with dilute acid. supports K325-3(c)
Benedict's solution works in alkaline conditions, so the acid must be neutralised with an alkali, such as sodium hydrogencarbonate, before the Benedict's solution is added. supports K325-3(c)
Iodine test for starch
- 1For a liquid food, put a few drops of the liquid on a white tile and then add a drop of iodine solution.
- 2For a solid food, add drops of iodine solution straight onto the food.
- 3Look at the colour: the iodine solution is yellow-brown, and it turns blue-black if starch is present.
- 4The iodine test does not need heating.
Biuret test for protein
Steps
- 1Add an equal volume of biuret solution to the liquid food and shake the tube.
- 2Leave the tube to stand for 5 minutes.
- 3Look at the colour: the mixture stays blue if there is no protein, and turns violet or purple if protein is present.
Solid food and safety
- For a solid food, cut it finely, add 2 cm3 of biuret solution and shake the tube, and then continue in the same way as for a liquid food.
- Biuret solution contains sodium hydroxide, which is corrosive, so wear safety goggles.
Ethanol emulsion test for fats
Steps
- 1Add an equal volume of ethanol to the liquid food and shake the tube well.
- 2Add 2 cm3 of water to the tube and shake it again.
- 3Look at the mixture: it stays clear if there is no fat, and it turns into a cloudy white emulsion if fat is present.
Solid food and safety
- For a solid food, cut it finely, add 2 cm3 of ethanol, shake the tube and leave the solid to settle.
- Pour the ethanol into a tube that has 2 cm3 of water in it, and shake the tube.
- Fat dissolves in ethanol but not in water, so when water is added the fat comes out of the solution as tiny droplets that make the mixture look cloudy.
- Ethanol is flammable, so keep it away from flames.
Depth: why and how
- Why is the mixture heated in the Benedict's test? The reaction between a reducing sugar and Benedict's solution needs heat to be fast enough to see a colour change.
- Why does the colour scale matter in the Benedict's test? The colour depends on the amount of reducing sugar, so the test gives a rough idea of how much reducing sugar there is, and not only whether it is present.
- Why is a beaker of boiling water used instead of heating the tube directly over a flame? The water bath heats the mixture evenly and stops the liquid from spitting out of the tube.
- Why is water added in the ethanol emulsion test? Fat dissolves in ethanol but not in water, so adding water makes the fat separate as tiny droplets, which is the cloudy emulsion.
- Cross-link to Nutrition and transport in flowering plants: the iodine test on a leaf that has had its colour removed shows whether photosynthesis has made starch.
Common mistakes
Benedict's solution turns brick-red at room temperature without heating.
Benedict's test needs heating in boiling water, and without heating the reaction is too slow to see.
Sucrose gives a brick-red precipitate with Benedict's solution straight away.
Sucrose is a non-reducing sugar, so it stays blue unless it has first been boiled with dilute acid.
Iodine solution turns from blue to black when starch is present.
Iodine solution turns from yellow-brown to blue-black when starch is present.
Biuret solution is used to test for sugars.
Biuret solution is used to test for protein, and it turns violet.
A positive fat test is when the mixture stays clear.
A positive fat test gives a cloudy white emulsion, and a negative test stays clear.
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- I Iodine solution turns blue-black when starch is present.
- B Benedict's solution is heated in boiling water and forms a green to brick-red precipitate when a reducing sugar is present.
- B Biuret solution turns violet when protein is present.
- E Ethanol is mixed with the food and then water is added, and a cloudy white emulsion shows that fat is present.
Iodine turns blue-black with starch, Benedict's solution gives a green to brick-red precipitate when heated with a reducing sugar, biuret solution turns violet with protein, and ethanol gives a cloudy white emulsion with fat.
Exam check
Q1 A food is heated with Benedict's solution and the mixture stays blue. What does this show?
- A There is no reducing sugar, or too little to detect
- B There is starch in the food
- C There is a large amount of glucose
- D There is protein in the food
Show answer
MCQ answer key A (There is no reducing sugar, or too little to detect): a mixture that stays blue after heating means that no reducing sugar is present, or that too little is present to detect.
Q2 Which row shows the correct positive result for a food test?
- A Starch with iodine solution: violet
- B Reducing sugar with biuret solution: brick-red precipitate
- C Protein with biuret solution: violet
- D Fat with iodine solution: cloudy white emulsion
Show answer
MCQ answer key C (Protein with biuret solution: violet): protein turns biuret solution violet, whereas starch gives blue-black with iodine, reducing sugars give a precipitate with Benedict's solution, and fat gives a cloudy emulsion with ethanol.
Q3 A student boils sucrose solution with dilute acid, neutralises it, adds Benedict's solution and heats it. What is the result, and why?
- A The mixture stays blue, because sucrose is a non-reducing sugar
- B A brick-red precipitate forms, because the acid has broken sucrose into glucose and fructose
- C The mixture turns blue-black, because starch forms
- D The mixture turns cloudy white, because fat forms
Show answer
MCQ answer key B (A brick-red precipitate forms, because the acid has broken sucrose into glucose and fructose): the acid breaks sucrose down into glucose and fructose, which are reducing sugars, so Benedict's solution gives a precipitate.
Structured A student tests a liquid food. The food turns blue-black with iodine solution and turns violet with biuret solution.
- Name the two food molecules that are present in the food. [2]
- Describe how the student would test the food for a reducing sugar. [3]
- The food stays blue in the test for reducing sugars. Suggest a further test to find out whether sucrose is present. [2]
Total: 7 marks
Show mark scheme
(a) 2 marks
- Mark scheme: starch
- Mark scheme: protein
(b) 3 marks
- Mark scheme: add an equal volume of Benedict's solution to the food
- Mark scheme: heat the mixture in a beaker of boiling water for a few minutes
- Mark scheme: a green, yellow, orange or brick-red precipitate shows that a reducing sugar is present
(c) 2 marks
- Mark scheme: boil the food with dilute acid, and neutralise it
- Mark scheme: then heat with Benedict's solution, and a precipitate shows that sucrose was present
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- Building large molecules
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Notes version 1.0 · Last updated 30 Sept 2026 · Topic 3 of 14