What you need to know
- 1.2(i–ii)Monosaccharides and disaccharides, and their roles in supplying energy.
- 1.4Named sugars, glycosidic bonds, condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. and hydrolysisA reaction that breaks a covalent bond using a molecule of water..
By the end, you should be able to…
- Distinguish a monosaccharideA single sugar unit, such as glucose, fructose or galactose. Monosaccharides are the monomers of carbohydrates. from a disaccharideA sugar made of two monosaccharides joined by a glycosidic bond, such as maltose, sucrose or lactose..
- Identify the components of maltoseA disaccharide of two α-glucose molecules joined by a 1,4 glycosidic bond., sucroseA disaccharide of glucose and fructose. It is the sugar transported in phloem. and lactoseA disaccharide of glucose and galactose, found in milk..
- Explain condensation and hydrolysis using water and glycosidic bonds.
Build your understanding
Single sugar units
A monosaccharideA single sugar unit, such as glucose, fructose or galactose. Monosaccharides are the monomers of carbohydrates. is a single sugar unit. glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate., fructoseA hexose monosaccharide found in fruit. Glucose and fructose join to form sucrose. and galactoseA hexose monosaccharide. Glucose and galactose join to form lactose. are monosaccharides. All three have the molecular formula C₆H₁₂O₆, but different arrangements of atoms give them different structures.
Glucose can be used as a respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids.: its breakdown in respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. transfers energy to ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi.. Soluble sugars can be transported in solution to the cells that need them. Avoid saying that sugars “make energy”; energy is transferred, not created.
Joining two units
A disaccharideA sugar made of two monosaccharides joined by a glycosidic bond, such as maltose, sucrose or lactose. consists of two monosaccharide residues joined by a glycosidic bondThe covalent bond that joins two monosaccharides, formed by a condensation reaction.. A condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. reaction forms that bond and releases one molecule of water. For two hexose sugars: C₆H₁₂O₆ + C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O.
maltoseA disaccharide of two α-glucose molecules joined by a 1,4 glycosidic bond. is formed from two glucose molecules. sucroseA disaccharide of glucose and fructose. It is the sugar transported in phloem. contains glucose and fructose. lactoseA disaccharide of glucose and galactose, found in milk. contains glucose and galactose. Learning the paired components is more useful than memorising the formula alone: all three disaccharides share the formula C₁₂H₂₂O₁₁.
Splitting the bond
hydrolysisA reaction that breaks a covalent bond using a molecule of water. breaks a glycosidic bond using water. Hydrolysis of one disaccharide produces its two monosaccharides. For example, lactose + water → glucose + galactose.
Digestion of disaccharides supplies monosaccharides that can be absorbed and used in metabolism. The enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. involved are specific to particular reactions; detailed enzyme action is developed later in Topic 2.
Follow the process
Condensation: two monosaccharide hydroxyl groups meet → a glycosidic bond forms → one water molecule is released → a disaccharide remains. Hydrolysis is the reverse: water is used → the glycosidic bond breaks → the two monosaccharides separate. The biological order is join, then later split; an answer may start with the product if the question asks what is formed.
Maltose joins carbon 1 of one glucose to carbon 4 of the other. Sucrose joins carbon 1 of glucose to carbon 2 of fructose. Lactose joins glucose to galactose by a glycosidic bond. Those carbon numbers name the bond. They do not change which sugars each disaccharide contains. α- and β-glucoseThe form of glucose with the –OH on carbon 1 above the ring. It is the monomer of cellulose., and celluloseA structural polysaccharide of β-glucose. Straight chains are linked by hydrogen bonds into microfibrils that strengthen plant cell walls., are developed with plant walls in Unit 2, not as extra Topic 1 recall.
NOT: two C₆H₁₂O₆ do not make C12H24O12. One water leaves.
- Maltose = glucose + glucose
- Sucrose = glucose + fructose
- Lactose = glucose + galactose
- Condensation releases water; hydrolysis uses water
Two traps this diagram blocks
- Do not forget the water in the formula. C12H24O12 is the wrong product.
- Do not swap the direction: joining releases water; splitting uses water.
From the diagram alone
Question
Describe how two glucose molecules form maltose, and name the reaction that reverses it. [3]
Show a model answer
Condensation joins the two glucose molecules by a glycosidic bond and releases one water molecule, forming maltose, C₁₂H₂₂O₁₁. Hydrolysis uses water to break that glycosidic bond and release the two glucose molecules.
| Disaccharide | Monosaccharide components | Bond |
|---|---|---|
| Maltose | Glucose + glucose | 1,4 glycosidic |
| Sucrose | Glucose + fructose | 1,2 glycosidic |
| Lactose | Glucose + galactose | Glycosidic |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: maltoseA disaccharide of two α-glucose molecules joined by a 1,4 glycosidic bond. forms. Y: condensationA reaction that joins two molecules with a covalent bond and releases a molecule of water. joins two glucoseA hexose (six-carbon) monosaccharide, C₆H₁₂O₆. It is the main respiratory substrate. molecules with a glycosidic bondThe covalent bond that joins two monosaccharides, formed by a condensation reaction. and releases water. Z: the product is one disaccharideA sugar made of two monosaccharides joined by a glycosidic bond, such as maltose, sucrose or lactose., formula C₁₂H₂₂O₁₁, not two free sugars. Incomplete: “The sugars stick together.” Improved: “Two glucose molecules join by a glycosidic bond in a condensation reaction, releasing water, so maltose forms.” The improvement names the bond, the reaction and the water.
Read the direction of the reaction
Joining units releases water; splitting the bond uses water. Check which direction the question asks you to describe before writing.
Avoid this misconception
The claim: Two sugars join to make C₁₂H₂₄O₁₂.
The correction: One water molecule is released during condensation, leaving C₁₂H₂₂O₁₁ in the disaccharide.
How do two sugars join, and how are they split?
Two monosaccharides join in a condensation reaction that releases water. Hydrolysis uses water to split them again.
What the model shows
Simplified sugar rings joining and splitting, the water involved, the glycosidic bond and the atom count.
What to look for
Where the water comes from, which carbons are joined, and why every disaccharide has the same formula.
What it tells us
Condensation releases one water per glycosidic bond; hydrolysis uses one. The three disaccharides differ in their monosaccharides.
Be careful what you claim
Simplified rings; α and β forms are not shown.
Try it in the model: press Condense, then Hydrolyse.
Joining and splitting sugars
Choose two monosaccharides and join them by condensation, then split them by hydrolysis. Follow the water and count the atoms.
What you see
- Two monosaccharides as simplified rings, the water molecule formed or used, the glycosidic bond and an atom count for each side.
- Bars for a batch of reactions: monosaccharides used, disaccharides made and water released.
How to use it
- Choose two monosaccharides, then press Condense or Hydrolyse. Drag Number of reactions for a batch.
What it shows
- Condensation joins two monosaccharides by a glycosidic bond and releases water; hydrolysis uses water to break it (1.4).
- Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose. All are C₁₂H₂₂O₁₁.
Limits of the model
- Rings are simplified: only the carbons and OH groups that react are labelled. α and β forms are not shown (Unit 2).
- Other pairs of monosaccharides are not named disaccharides in this course. Enzymes are not shown.
Guided tour · step 1 of 8
- In Sugars, just below these steps, keep First sugar and Second sugar on glucose.
- Press Condense.
An OH group on carbon 1 of one glucose and an OH on carbon 4 of the other give one water molecule. The two rings are joined by a 1,4 glycosidic bond: maltose, C₁₂H₂₂O₁₁.
Sugars
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Name the products of sucrose hydrolysis and the bond broken.
Show suggested answer
Glucose and fructose are produced. A glycosidic bond is broken using water.
Practice question
Retrieval and explanation · suggested answer
Ten separate maltose molecules are synthesised from glucose. How many glucose and water molecules are involved?
Show suggested answer
Twenty glucose molecules are used and ten water molecules are released. Each maltose requires two glucose molecules and the formation of one glycosidic bond.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A sample contains maltose and lactose. After complete hydrolysis, it contains 15 glucose molecules and 5 galactose molecules. How many molecules of each disaccharide were originally present?
Show a hint
Each lactose contributes one galactose and one glucose.
Explore the reasoning
Five lactose molecules account for the five galactose and five glucose molecules. The remaining ten glucose molecules came from five maltose molecules. The original sample contained five molecules of each disaccharide.
Recall this lesson
Lesson recall questions
Question 1 of 5: Explain, in causal order, why a branched polysaccharide is hydrolysed faster than an unbranched one of the same mass.
Saccharides and energy stores
Practice question
Retrieval and explanation · suggested answer
Explain, in causal order, why a branched polysaccharide is hydrolysed faster than an unbranched one of the same mass.
Lesson: 1A 3: Carbohydrates 2: Polysaccharides
Build the sequence from memory first. Then reveal the model order.
Tap the steps in the order you recall. Tap a placed step to send it back.
- 01Next step
- 02Waiting
- 03Waiting
Tap to place
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The chemistry behind lactose-free milk
1A · Carbohydrates 1: Monosaccharides and DisaccharidesLactase breaks down lactose. Low lactase activity can lead to incomplete lactose digestion and, in some people, symptoms of lactose intolerance. This gives food scientists a practical reason…Specification 1.4Read & use the scienceClose article
Why this matters
Lactase breaks down lactose. Low lactase activity can lead to incomplete lactose digestion and, in some people, symptoms of lactose intolerance. This gives food scientists a practical reason to investigate sugar hydrolysis. For a food scientist, changing a disaccharide into two monosaccharides changes the product without removing its carbohydrate. Water is used to hydrolyse the glycosidic bond; glucose and galactose remain. A label saying lactose-free therefore answers a different question from a label about total sugars. Product testing must distinguish the molecules present, rather than assuming a successful reaction has made them disappear. The NIDDK explanation provides the digestive context; applying that chemistry to a milk label is the biological reasoning.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.4 know how monosaccharides (glucose, fructose and galactose) join together to form disaccharides (maltose, sucrose and lactose) and polysaccharides (glycogen, amylose and amylopectin) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions
Use the science
If lactose is hydrolysed in milk, does that mean all the sugar has been removed?
Explore a possible response
No. Hydrolysis converts lactose into glucose and galactose. Changing the identity of a sugar is different from removing sugar.
Explore the sources
Can a blue test tube prove a drink has no sugar?
1A · Carbohydrates 1: Monosaccharides and DisaccharidesA food analyst and a drinks advertisement may use the word sugar in different ways. The FDA distinguishes total sugars from sugars added during manufacture: a drink can…Specification 1.2(i) · 1.3 · 1.4Read & use the scienceClose article
Why this matters
A food analyst and a drinks advertisement may use the word sugar in different ways. The FDA distinguishes total sugars from sugars added during manufacture: a drink can contain naturally occurring sugars without having sugar added. Your Benedict’s test asks another question, about reducing sugars. It is not a universal detector for every carbohydrate, nor can it tell whether a glucose molecule came from fruit or a factory. Comparing a chemical test with a label requires checking exactly what each measures, using appropriate standards and allowing for the colour of the drink itself.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 1.2(i) know the difference between monosaccharides, disaccharides and polysaccharides, including glycogen and starch (amylose and amylopectin)
- 1.3 CORE PRACTICAL 1 Use a semi-quantitative method with Benedict’s reagent to estimate the concentrations of reducing sugars and with iodine solution to estimate the concentrations of starch, using colour standards.
- 1.4 know how monosaccharides (glucose, fructose and galactose) join together to form disaccharides (maltose, sucrose and lactose) and polysaccharides (glycogen, amylose and amylopectin) through condensation reactions forming glycosidic bonds, and how these can be split through hydrolysis reactions
Use the science
A sucrose-containing drink gives a negative Benedict’s test before hydrolysis. Explain why the claim “laboratory-proven sugar-free” does not follow.
Explore a possible response
Sucrose is non-reducing under the test conditions. Hydrolysis produces reducing monosaccharides. A negative reducing-sugar result cannot establish absence of all sugars; appropriate controls and sensitivity also matter.