Name the sugar present in milk. How many monosaccharide units are present in it? What are such oligosaccharide

NCERT Exemplar · Q20

Q. Name the sugar present in milk. How many monosaccharide units are present in it? What are such oligosaccharides called?

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Concept understanding — Lactose Hydrolysis Products

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 ( $\text{C}_{12}\text{H}_{22}\text{O}_{11}$ ) is a disaccharide composed of one molecule of D-galactose and one molecule of D-glucose linked by a $\beta(1 \rightarrow 4)$ glycosidic bond. Upon hydrolysis (reaction with water), this bond is cleaved, yielding the two monosaccharides:

\text{Lactose} + \text{H}_2\text{O} \xrightarrow{\text{lactase or acid}} \text{D-Galactose} + \text{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 ( $\text{C}_6\text{H}_{12}\text{O}_6$ ) but differ in the arrangement of the hydroxyl group on carbon 4 (they are C-4 epimers).

Note

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: $\beta(1 \rightarrow 4)$ 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 $\beta(1 \to 4)$ glucose
The bond is between:
- Carbon-1 of galactose (in $\beta$ configuration)
- Carbon-4 of glucose

So the structural formula is:

\text{Galactose} - O - \text{Glucose}

2. What does hydrolysis do?

Hydrolysis means "splitting with water." The reaction is:

\text{Lactose} + \text{H}_2\text{O} \xrightarrow{\text{lactase or acid}} \text{Galactose} + \text{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:

\ce{C12H22O11 + H2O -> C6H12O6 + C6H12O6}

Lactose: $\ce{C12H22O11}$
Water: $\ce{H2O}$
Products: two molecules of $\ce{C6H12O6}$ (one galactose, one glucose)

Why the same molecular formula?

Both galactose and glucose are aldohexoses — they have the same molecular formula $\ce{C6H12O6}$ 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. …

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The sugar in milk is lactose, a disaccharide made of two monosaccharide units (glucose + galactose). Oligosaccharides with exactly two monosaccharide units are called disaccharides.

Why This Question Matters

This is a foundational biochemistry question that tests your understanding of carbohydrate classification. Milk is often the first food we consume, and knowing its chemical composition helps connect everyday biology to molecular structure. The question has three distinct parts, each testing a different concept: identification, counting, and nomenclature.

Step-by-Step Reasoning

1. Identify the sugar in milk

The principal carbohydrate in mammalian milk is lactose. It accounts for about 4.5–5% of cow's milk and roughly 7% of human milk. Lactose is less sweet than table sugar (sucrose) and is the reason milk has a mild, slightly sweet taste.

Watch out

A common mistake is to say "glucose" or "sucrose." Glucose is present in many foods but is not the dominant sugar in milk. Sucrose is common in plants, not milk.

2. Determine how many monosaccharide units make up lactose

Lactose is a disaccharide. This means it is formed by a glycosidic bond between exactly two monosaccharide units:

One molecule of D-glucose
One molecule of D-galactose

The bond is a $\beta(1 \to 4)$ glycosidic linkage between the anomeric carbon of galactose (C1) and the C4 hydroxyl of glucose.

\beta(1 \to 4)

So the answer to "how many monosaccharide units" is two.

3. Classify such oligosaccharides …

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Here is a breakdown of the common mistakes students make on this specific question, along with the correct reasoning to avoid them.

The Core Concept: Milk Sugar and Oligosaccharides

The sugar in milk is lactose. It is a disaccharide composed of two monosaccharide units: glucose and galactose. Oligosaccharides containing two monosaccharide units are called disaccharides.

Common Mistake #1: Naming the wrong sugar

The Mistake: Students often write sucrose (cane sugar) or maltose (malt sugar) instead of lactose. This is a classic memory slip.
Why it happens: Sucrose is the most common "table sugar," so it becomes the default answer. Students fail to link the specific source (milk) to the specific sugar.
How to Avoid: Create a "Source-Sugar" mental map. Memorize these three key pairs:
- Milk → Lactose
- Sugar cane / Beet → Sucrose
- Germinating grains / Malt → Maltose
- Tip: The word "Lactose" shares the root "lact-" with "lactation" (milk production). This is your memory anchor.

Common Mistake #2: Getting the number of monosaccharide units wrong

The Mistake: Writing "1" (thinking it's a monosaccharide) or "3" (confusing it with a trisaccharide like raffinose).
Why it happens: Students confuse the classification of carbohydrates. They might remember that lactose is a "sugar" but forget its specific size (di- vs. mono- vs. oligo-).
How to Avoid: Always break down the name. The suffix "-ose" means sugar. The prefix tells you the size:
- Mono- = 1 unit (e.g., glucose, fructose)
- Di- = 2 units (e.g., lactose, sucrose, maltose)
- Oligo- = 2 to 10 units (e.g., lactose is a specific type of oligosaccharide called a disaccharide)
- Poly- = many units (e.g., starch, cellulose)
- Action: For lactose, explicitly say to yourself: "Lactose is a disaccharide, so it has 2 monosaccharide units."

Common Mistake #3: Mislabeling the class of oligosaccharide

The Mistake: Writing "oligosaccharide" as the answer to "What are such oligosaccharides called?" The question asks for the specific name for an oligosaccharide with two units. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

COMEDK 2025Set 2025-M1 markMCQ
$\alpha-D-$
›Reveal solutionSolution
Maltose is a reducing disaccharide from starch hydrolysis, but its hydrolysis yields two glucose units, not glucose and fructose; thus Assertion is correct, Reason is wrong, so option (C) is correct.

Concept & Intuition

Maltose is a disaccharide composed of two glucose molecules linked by an α(1→4) glycosidic bond. Because one of the glucose units retains a free anomeric carbon (the hemiacetal group), maltose can reduce Cu²⁺ ions (e.g., in Benedict’s test) — that’s what makes it a reducing sugar. The assertion correctly states that maltose is obtained by partial hydrolysis of starch using the enzyme diastase. However, the reason claims that hydrolysis of maltose gives one glucose and one fructose — that’s actually the hydrolysis product of sucrose, not maltose. The classic pitfall here is confusing the hydrolysis products of common disaccharides.

Step-by-step reasoning

Check the Assertion (A)

Maltose is indeed a reducing sugar because its structure has a free anomeric carbon on the non-reducing end? Actually, careful: In maltose, the glycosidic bond involves C1 of one glucose and C4 of the other. The glucose unit that provides C1 has its anomeric carbon tied up in the bond, so it cannot open to a free aldehyde. But the other glucose unit (the one with the free C1) retains a hemiacetal group, which can open to an aldehyde form. Thus maltose reduces Tollens’ or Benedict’s reagent.

Partial hydrolysis of starch (a polymer of glucose) with the enzyme diastase (an amylase) does yield maltose as a major product.

Therefore, Assertion (A) is correct.

Check the Reason (R)

Hydrolysis of one mole of maltose (C₁₂H₂₂O₁₁) with water yields two moles of D-glucose.

The reaction: $\text{Maltose} + \text{H}_2\text{O} \xrightarrow{\text{acid or enzyme}} 2\, \text{D-glucose}$ …
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KCET 2024Set B-41 markMCQ
$Q. Stanley Miller simulated the conditions of pre-biotic earth using spark-discharge apparatus. Which organic compounds were observed by him on analysing the end product of his experiment? (A) Pigments (B) Fats (C) Nitrogen bases (D) Amino acids$
›Reveal solutionSolution
Miller's spark-discharge experiment recovered simple amino acids, demonstrating that organic monomers can form abiotically from a reducing atmosphere.

Step 1 — What the experiment set out to test.

Oparin (Russia) and Haldane (England) had proposed that the first form of life arose from pre-existing non-living organic molecules (e.g. RNA, proteins) — chemical evolution preceding biological evolution. The claim needed an experimental test: can organic molecules form from purely inorganic precursors under primitive-Earth conditions?

Step 2 — The apparatus and the conditions simulated.

Stanley L. Miller, a student of Harold Urey, built a closed flask (1953) and created conditions like those of the primitive, reducing (oxygen-free) atmosphere:

Gases: methane ( $\mathrm{CH_4}$ ), ammonia ( $\mathrm{NH_3}$ ) and hydrogen ( $\mathrm{H_2}$ ), in a $2:1:2$ ratio.

Water vapour supplied by boiling water in a connected flask.

Temperature about $800^\circ\text{C}$ .

Electric discharge between electrodes, simulating lightning — the energy source.

The vapour was condensed and collected in a U-trap, and the accumulated liquid analysed after a week.

Step 3 — The result. …
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KCET 2023Set D-21 markMCQ
$Q. Sucrose is dextrorotatory but after hydrolysis the mixture show laevorotation, this is because of (A) Laevorotation of glucose is more than dextrorotation of fructose. (B) Sucrose is a non-reducing sugar. (C) Racemic mixture is formed. (D) Laevorotation of fructose is more than dextrorotation of glucose.$
›Reveal solutionSolution

Add the specific rotations of the two hydrolysis products: fructose's large negative rotation outweighs glucose's smaller positive one, so the sign of the mixture inverts.

Step 1 — The reaction
$\underset{\text{sucrose}}{\mathrm{C_{12}H_{22}O_{11}}} + \mathrm{H_2O} \xrightarrow{\;\mathrm{H^+/invertase}\;} \underset{\text{glucose}}{\mathrm{C_6H_{12}O_6}} + \underset{\text{fructose}}{\mathrm{C_6H_{12}O_6}}$
Sucrose is a 1→2 glycosidic disaccharide of $\alpha$ -D-glucose and $\beta$ -D-fructose. Hydrolysis cleaves it into equimolar glucose and fructose.

Step 2 — The specific rotations

Species Specific rotation $[\alpha]_D$
Sucrose $+66.5^\circ$ (dextrorotatory)
D-(+)-Glucose $+52.5^\circ$
D-(−)-Fructose $-92.4^\circ$

Step 3 — Net rotation of the product mixture

Optical rotations of components in a mixture are additive. For a 1:1 mixture the net rotation goes as
$[\alpha]_{\text{mix}} \propto \tfrac12\big(+52.5^\circ\big) + \tfrac12\big(-92.4^\circ\big) = \tfrac12\big(-39.9^\circ\big) < 0 .$
The magnitude of fructose's laevorotation ( $92.4^\circ$ ) exceeds glucose's dextrorotation ( $52.5^\circ$ ), so the sum is negative — the mixture rotates plane-polarised light to the left.

Step 4 — Why this is called "inversion" …
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COMEDK 2022Set 20221 markMCQ
$\alpha$
›Reveal solutionSolution

So the monosaccharide units of maltose are alpha-D-glucose and alpha-D-glucose.

Concept: structure of common disaccharides.

Maltose = two units of alpha-D-GLUCOSE joined by an alpha(1->4) glycosidic linkage (C1 of the first glucose to C4 of the second). It is a reducing sugar since the second glucose retains a free anomeric -OH. …
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COMEDK 2021Set 20211 markMCQ
$\alpha$
›Reveal solutionSolution
Hence the correct pair is alpha-D-glucopyranose and beta-D-fructofuranose.

Concept: Structure of the disaccharide sucrose.

Sucrose is a non-reducing disaccharide in which the anomeric C1 of glucose and the anomeric C2 of fructose are joined head-to-head through a glycosidic linkage, so no free anomeric -OH (hemiacetal) remains - that is why it is non-reducing.

The two units are:

alpha-D-glucopyranose (six-membered pyranose ring, alpha at C1)

beta-D-fructofuranose (five-membered furanose ring, beta at C2) …
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Species	Specific rotation $[\alpha]_D$
Sucrose	$+66.5^\circ$ (dextrorotatory)
D-(+)-Glucose	$+52.5^\circ$
D-(−)-Fructose	$-92.4^\circ$

←Previous questionQ19 — Three structures, labelled (A), (B) and (C), each show two glucose uni…→Next questionQ21 — How do you explain the presence of all the six carbon atoms in glucose…

Q.Name the sugar present in milk. How many monosaccharide units are present in it? What are such oligosaccharides called?

Concept understanding — Lactose Hydrolysis Products

Lactose Hydrolysis Products – From Intuition to Precision

The Intuition: Breaking a Sugar Chain

The Precise Statement

Why This Matters for Exams

Why this formula?

Lactose Hydrolysis Products — Understanding the Why

1. What is lactose chemically?

2. What does hydrolysis do?

3. Why are the products exactly galactose and glucose?

4. Key formula — the hydrolysis equation

5. The "why" behind the formula

Why This Question Matters

Step-by-Step Reasoning

Concept: Carbohydrates – Disaccharides and Oligosaccharides

Steps:

The Core Concept: Milk Sugar and Oligosaccharides

Common Mistake #1: Naming the wrong sugar

Common Mistake #2: Getting the number of monosaccharide units wrong

Common Mistake #3: Mislabeling the class of oligosaccharide

Step 1 — The reaction

Step 2 — The specific rotations

Step 3 — Net rotation of the product mixture

Step 4 — Why this is called "inversion" …

Q.Name the sugar present in milk. How many monosaccharide units are present in it? What are such oligosaccharides called?

Why This Question Matters

Step-by-Step Reasoning

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