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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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

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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