Q.Classify the following into monosaccharides and disaccharides.
Ribose, 2-deoxyribose, maltose, galactose, fructose and lactose.
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Lactose Hydrolysis Products – From Intuition to Precision
Imagine you have a glass of milk. That slightly sweet taste comes from a sugar called lactose. But lactose is a disaccharide – it's actually two smaller sugar units joined together. If you could "unstick" those two units, you'd get two simpler sugars. That unsticking process is hydrolysis (water + breaking), and the two simpler sugars you get are the hydrolysis products.
The Intuition: Breaking a Sugar Chain
Think of lactose as a train with exactly two carriages. The coupling between them is a chemical bond. When you add water and the right conditions (like an enzyme called lactase, or an acid), that bond snaps. The train splits into two separate carriages. Each carriage is now a free, smaller sugar molecule.
So the hydrolysis products are simply the two individual sugar units that were originally linked to form lactose.
The Precise Statement
Lactose (C12H22O11) is a disaccharide composed of one molecule of D-galactose and one molecule of D-glucose linked by a β(1→4) glycosidic bond. Upon hydrolysis (reaction with water), this bond is cleaved, yielding the two monosaccharides:
Lactose+H2Olactase or acidD-Galactose+D-Glucose
The two hydrolysis products are:
- D-Galactose – a monosaccharide (aldohexose, C6H12O6)
- D-Glucose – a monosaccharide (aldohexose, C6H12O6)
Both are reducing sugars, and both have the same molecular formula (C6H12O6) but differ in the arrangement of the hydroxyl group on carbon 4 (they are C-4 epimers).
In the body, the enzyme lactase (present in the small intestine) performs this hydrolysis so that the resulting glucose and galactose can be absorbed into the bloodstream. Lactose intolerance occurs when lactase activity is low, leaving lactose undigested.
Why This Matters for Exams
- Always name both products: galactose and glucose. Never just "sugars" or "monosaccharides."
- Know the bond: β(1→4) glycosidic linkage. Hydrolysis breaks this specific bond. …
Why this formula?
Lactose Hydrolysis Products — Understanding the Why
Lactose is a disaccharide composed of two monosaccharides linked by a glycosidic bond. When it undergoes hydrolysis, the bond is broken, yielding specific products. Let's build the reasoning step by step.
1. What is lactose chemically?
- Lactose = galactose β(1→4) glucose
- The bond is between:
- Carbon-1 of galactose (in β configuration)
- Carbon-4 of glucose
So the structural formula is:
Galactose−O−Glucose
2. What does hydrolysis do?
Hydrolysis means "splitting with water." The reaction is:
Lactose+H2Olactase or acidGalactose+Glucose
The water molecule adds across the glycosidic bond:
- The H from water attaches to the oxygen of the galactose (forming a free –OH on galactose)
- The OH from water attaches to the carbon-1 of glucose (forming a free –OH on glucose)
3. Why are the products exactly galactose and glucose?
Because the glycosidic bond is between specific carbons:
- Galactose contributes its anomeric carbon (C1)
- Glucose contributes its C4
When the bond breaks, each sugar regains its free anomeric carbon (in the case of galactose) or free hydroxyl at C4 (in the case of glucose). No rearrangement occurs — the monosaccharides are released as they were originally linked.
4. Key formula — the hydrolysis equation
The balanced chemical equation:
CX12HX22OX11+HX2OCX6HX12OX6+CX6HX12OX6
- Lactose: CX12HX22OX11
- Water: HX2O
- Products: two molecules of CX6HX12OX6 (one galactose, one glucose)
Why the same molecular formula?
Both galactose and glucose are aldohexoses — they have the same molecular formula CX6HX12OX6 but differ in the arrangement of –OH groups (epimers at C4).
5. The "why" behind the formula
- Mass conservation: The total number of C, H, O atoms before and after must match. …
The key idea is that monosaccharides are single sugar units, while disaccharides consist of two monosaccharide units joined by a glycosidic bond.
- Monosaccharides: Ribose, 2-deoxyribose, galactose, and fructose are each single polyhydroxy aldehydes or ketones. …
Monosaccharides are single sugar units; disaccharides are two monosaccharides linked by a glycosidic bond. Ribose, 2-deoxyribose, galactose, and fructose are monosaccharides; maltose and lactose are disaccharides.
Why This Classification Matters
The distinction between monosaccharides and disaccharides is fundamental in carbohydrate chemistry. A monosaccharide is the simplest sugar — it cannot be hydrolysed into smaller carbohydrates. A disaccharide, on the other hand, consists of two monosaccharide units joined by a glycosidic linkage, and it can be hydrolysed (by acid or enzymes) into its constituent monosaccharides.
The question gives you six common sugars. Your job is to decide, for each one, whether it is a single unit or a dimer.
Step-by-Step Reasoning
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Ribose — This is a pentose (5-carbon) sugar, formula C5H10O5. It is the sugar found in RNA. It cannot be broken down into simpler sugars by hydrolysis. Therefore, it is a monosaccharide.
-
2-Deoxyribose — This is a modified pentose where the 2′-hydroxyl group is replaced by hydrogen. Its formula is C5H10O4. Like ribose, it is a single sugar unit and a component of DNA. It is also a monosaccharide.
-
Maltose — Maltose is a disaccharide composed of two glucose units linked by an α(1→4) glycosidic bond. On hydrolysis (e.g., by the enzyme maltase), it yields two molecules of D-glucose. Hence, it is a disaccharide.
-
Galactose — This is a hexose (6-carbon) sugar, an aldohexose with the formula C6H12O6. It is a monosaccharide that occurs naturally in milk and is a component of lactose. It cannot be hydrolysed further. So it is a monosaccharide.
-
Fructose — A ketohexose, also C6H12O6, commonly found in fruits and honey. It is a single sugar unit and is a monosaccharide. …
Method: Structural Classification of Carbohydrates
This method uses monomer counting — the core concept that monosaccharides are single sugar units, while disaccharides are two monosaccharides linked by a glycosidic bond.
Steps
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Identify the basic unit count
- A monosaccharide cannot be hydrolysed into smaller sugars.
- A disaccharide yields two monosaccharides upon hydrolysis.
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Apply to each sugar
Sugar Unit Count Classification Ribose Single pentose Monosaccharide 2-deoxyribose Single deoxy sugar Monosaccharide Maltose Two glucose units (hydrolyses to glucose + glucose) Disaccharide Galactose Single hexose Monosaccharide
Here are the common mistakes students make when classifying these six sugars into monosaccharides and disaccharides, along with how to avoid each.
Mistake 1: Misclassifying Lactose
- The Error: Students often forget that lactose is a disaccharide made of glucose + galactose. When asked to classify "lactose," they might incorrectly list it as a monosaccharide or mix up its components.
- How to Avoid: Memorize the composition of key disaccharides:
- Lactose = Glucose + Galactose (milk sugar)
- Sucrose = Glucose + Fructose
- Maltose = Glucose + Glucose
- Cellobiose = Glucose + Glucose (but β-1,4 linkage)
- Exam Tip: If a sugar name ends in -ose and is not one of the common monosaccharides (glucose, fructose, galactose, ribose, deoxyribose), check if it's a disaccharide. Lactose, maltose, and sucrose are the three main disaccharides for NEET/JEE.
Mistake 2: Forgetting that Galactose is a Monosaccharide
- The Error: Because galactose is a component of lactose, some students mistakenly classify galactose itself as a disaccharide.
- How to Avoid: Remember that galactose is an aldohexose (a monosaccharide with 6 carbons and an aldehyde group). It is a single sugar unit, just like glucose and fructose.
- Key Fact: All monosaccharides are the simplest form of carbohydrates that cannot be hydrolyzed into smaller sugars. Galactose cannot be broken down further by hydrolysis.
Mistake 3: Misclassifying Ribose and 2-Deoxyribose
- The Error: Students sometimes think "ribose" and "2-deoxyribose" are disaccharides because they are found in RNA and DNA (which are large molecules).
- How to Avoid: Focus on the definition: a monosaccharide is a single sugar unit. Ribose (C₅H₁₀O₅) and 2-deoxyribose (C₅H₁₀O₄) are pentoses (5-carbon monosaccharides). They are the building blocks of nucleotides, not disaccharides.
- Memory Aid: "Ribose and deoxyribose are monosaccharides that make up nucleic acids."
Mistake 4: Forgetting that Fructose is a Monosaccharide
- The Error: Since fructose is very sweet and often paired with glucose (as in sucrose), some students misclassify it as a disaccharide.
- How to Avoid: Fructose is a ketohexose (a monosaccharide with a ketone group). It is a single sugar unit. It is not a disaccharide unless it is chemically bonded to another monosaccharide (e.g., in sucrose).
Mistake 5: Using the General Formula to Decide the Classification
- The Error: Students test whether a formula fits Cx(H2O)y and then wrongly reject 2-deoxyribose (C5H10O4) as "not a proper sugar" because it does not fit the hydrate pattern.
- Why it happens: The old "hydrate of carbon" formula is memorised as if it were the definition, but it is neither necessary nor sufficient — acetic acid (CH3COOH) fits the formula yet is not a carbohydrate, while rhamnose (C6H12O5) is a carbohydrate that does not fit it. …
Showing the 12 most recent of 16 on this concept.
- CBSE 2026Set 56/3/11 markMCQQ.On hydrolysis, which of the following carbohydrates gives only glucose ? (A) Sucrose (B) Galactose (C) Lactose (D) Maltose
›Reveal solutionSolution
Hydrolysis breaks glycosidic bonds in disaccharides to release monosaccharides. Only maltose yields glucose alone because it is composed of two glucose units linked together. The answer is (D).
Understanding Carbohydrate Hydrolysis
When a disaccharide undergoes hydrolysis, water molecules break the glycosidic bond connecting two monosaccharide units. The products depend entirely on which monosaccharides were originally joined to form that disaccharide. To answer this question, we need to know the composition of each carbohydrate listed.
The key insight: if a disaccharide is made from two identical glucose molecules, hydrolysis will give us only glucose. If it contains any other monosaccharide (fructose, galactose), those will appear in the products too.
Analyzing Each Option
- Sucrose (Option A) Sucrose is a disaccharide formed from glucose and fructose linked by an α(1→2) glycosidic bond. When hydrolyzed:
Sucrose+H2O⟶Glucose+Fructose
This gives two different monosaccharides, so sucrose does not yield only glucose.
-
Galactose (Option B)
Galactose is already a monosaccharide (a simple sugar). It cannot be hydrolyzed further because there are no glycosidic bonds to break. This option is a distractor—hydrolysis doesn't apply here.
-
Lactose (Option C)
Lactose, the sugar found in milk, is composed of glucose and galactose joined by a β(1→4) glycosidic bond. Hydrolysis produces:
Lactose+H2O⟶Glucose+Galactose
Again, two different monosaccharides result, not just glucose.
- Maltose (Option D) …
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: Sucrose on hydrolysis gives glucose and ________.
›Reveal solutionSolution
Sucrose is a disaccharide of glucose and fructose; hydrolysis breaks the glycosidic bond releasing both monosaccharides.
Sucrose + H2O --(acid or enzyme invertase)--> Glucose + Fructose
…
- CBSE 2025Set BOTANY1 markMCQQ.Fill in the blank selecting the appropriate one: The enzyme beta-galactosidase breaks lactose into ____ and glucose.(a) galactose(b) sucrose(c) glycerol(d) erythrose
›Reveal solutionSolution
beta-galactosidase splits the disaccharide lactose into its two monosaccharide units, galactose and glucose.
Lactose is a disaccharide made of one galactose unit joined to one glucose unit by a beta-1,4 glycosidic bond. The enzyme beta-galactosidase (encoded by the z gene of the lac operon) hydrolyses this bond, releasing galactose and glucose, which the cell can then metabolise. Since the questio …
- CBSE 2024Set ANNUAL1 markQ.Name the milk sugar present in milk.
›Reveal solutionSolution
Lactose ("milk sugar") is the main sugar found in milk, a disaccharide made of glucose and galactose joined by a beta-1,4-glycosidic linkage.
Milk contains lactose as its principal carbohydrate, typically making up around 4-5% of milk by mass. Lactose is a disaccharide formed from one molecule of beta-D-galactose and one molecule of beta-D-glucose linked by a beta-1,4-glycosidic bond. On hydrolysis (e.g. b …
- CBSE 2023Set F1 markMCQQ.Which of the following disaccharides is present in milk?(a) Sucrose(b) Lactose(c) Maltose(d) None of these
›Reveal solutionSolution
Lactose (glucose + galactose) is the disaccharide naturally present in milk.
Lactose, commonly called milk sugar, is made of one D-glucose and one D-galactose unit joined by a beta-1,4-glycosidic bond. It occurs in the milk of mammals. Sucrose (cane sugar) is gluco …
- CBSE 2023Set A1 markQ.Match the following. Column A item: 'Milk sugar'. Choose its correct match from Column B:(a) Ether(b) Primary amine(c) Lactose(d) C12H22O11(e) Glucose(f) Negative ions(g) C6H5SO2Cl(h) +7
›Reveal solutionSolution
'Milk sugar' is the common name for lactose, the disaccharide found in milk.
Lactose is a disaccharide composed of β-D-galactose and β-D-glucose units joined by a glycosidic linkage; it is naturally pr …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following disaccharide is present in milk?(i) Sucrose(ii) Glucose(iii) Lactose(iv) Cellulose
›Reveal solutionSolution
Lactose ('milk sugar') is the characteristic disaccharide of milk.
Sucrose is cane/beet sugar; glucose is a monosaccharide, not a disaccharide; cellulose is a polysaccharide (plant cell walls). Lactose is a disaccharide made of one glucose and one galactose unit joined by a β-1,4 glycosidic linkage, an …
- CBSE 2022Set ANNUAL1 markQ.What are the expected products of hydrolysis of lactose?
›Reveal solutionSolution
Lactose (milk sugar) is a disaccharide of glucose + galactose; on hydrolysis it yields one molecule of glucose and one of galactose.
Lactose, or milk sugar, is a reducing disaccharide.
It is composed of one β-D-galactose unit and one β-D-glucose unit linked by a β(1→4) glycosidic bond.
On hydrolysis (acid- or enzyme-catalysed by lactase), this glycosidic linkage is cleaved:
…
- CBSE 2021Set ANNUAL1 markQ.Name the sugar present in the milk.
›Reveal solutionSolution
Milk sugar = lactose, a disaccharide of glucose + galactose.
Milk contains the disaccharide lactose ("milk sugar"), made of one molecule of β-D-galactose and one molecule of β-D-glucose joined by a β-1,4-glycosidic linkage.
…
- CBSE 2020Set 56/3/11 markQ.Name the disaccharide which on hydrolysis gives two molecules of glucose.
›Reveal solutionSolution
The disaccharide that yields two molecules of glucose upon hydrolysis is maltose, because it is composed of two α-D-glucose units linked by an α(1→4) glycosidic bond.
The key to this question lies in understanding what a disaccharide is and how hydrolysis works. A disaccharide is a carbohydrate formed when two monosaccharides join together through a glycosidic bond, with the elimination of a water molecule. Hydrolysis is the reverse process — adding water breaks that bond, releasing the two original monosaccharides.
So the question is simply: which common disaccharide is built from two glucose units? Let’s recall the major disaccharides and their monosaccharide components.
- Sucrose (table sugar) is made of one glucose and one fructose. Hydrolysis gives glucose + fructose — not two glucoses.
- Lactose (milk sugar) is made of one glucose and one galactose. Hydrolysis gives glucose + galactose — again, not two glucoses.
- Maltose (malt sugar) is made of two glucose units. Hydrolysis of maltose yields exactly two molecules of glucose.
- Cellobiose is also made of two glucose units, but it is a product of cellulose breakdown and less commonly encountered in basic exam contexts. However, it too gives two glucoses on hydrolysis. …
- CBSE 2020Set ANNUAL1 markQ.Which disaccharide on hydrolysis in presence of the catalyst invertase produces glucose and fructose?
›Reveal solutionSolution
Sucrose is the disaccharide that, on enzymatic hydrolysis by invertase, breaks the glycosidic bond to give one molecule each of glucose and fructose.
Sucrose is a non-reducing disaccharide formed by an alpha-1,2-glycosidic linkage between the anomeric carbons of alpha-D-glucose and beta-D-fructose (which is why, unlike maltose or lactose, it shows no free reducing end). On hydrolysis - either by dilute acid or by the enzyme invertase (sucrase) - this glycosidic bond is cleaved:
Sucrose + H2O --invertase--> Glucose + Fructose
…
- CBSE 2020Set ANNUAL1 markMCQQ.The disaccharides present in milk is(a) sucrose(b) maltose(c) lactose(d) cellulose
›Reveal solutionSolution
Milk's characteristic sugar is lactose, a disaccharide of galactose and glucose — option (c).
Lactose is the principal carbohydrate found in milk. It is a reducing disaccharide made of one unit of β-D-galactose and one unit of β-D-glucose, joined by a β-1→4 glycosidic linkage. Sucrose (cane sugar) comes from sug …
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