Aldopentoses named ribose and 2-deoxyribose are found in nucleic acids. What is their relative configuration?

NCERT Exemplar · Q27

Q. Aldopentoses named ribose and 2-deoxyribose are found in nucleic acids. What is their relative configuration?

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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 relative configuration of ribose and 2-deoxyribose is identical at all chiral centres except C-2, where ribose has an OH group and 2-deoxyribose has only an H atom — they share the same D-configuration at C-4 (the reference carbon), so both belong to the D-series.

The question asks about the relative configuration of two aldopentoses: ribose and 2-deoxyribose. This is a classic concept from carbohydrate chemistry in the context of nucleic acids.

Let’s first understand what “relative configuration” means here. In sugars, the configuration is defined by the chiral centre farthest from the carbonyl group — for aldopentoses, that’s carbon-4 (C-4). If the OH on C-4 points to the right in a Fischer projection, the sugar is D; if to the left, it’s L. This is the reference for the entire family.

Now, ribose and 2-deoxyribose differ only at carbon-2: ribose has an OH group at C-2, while 2-deoxyribose has just a hydrogen (hence “deoxy”). All other chiral centres — C-3 and C-4 — are identical in configuration. Both sugars are naturally occurring in the D-form.

Here’s the step-by-step reasoning:

Identify the chiral centres. Both ribose and 2-deoxyribose are aldopentoses: a five-carbon sugar with an aldehyde group at C-1. The chiral carbons are C-2, C-3, and C-4. (C-1 is not chiral because it’s an aldehyde; C-5 is a primary alcohol, not chiral.)
Compare the structures. In ribose, C-2 has an OH group. In 2-deoxyribose, C-2 has only H — that’s the only difference. The configurations at C-3 and C-4 are exactly the same in both sugars.
Determine the reference carbon. For aldoses, the configuration is assigned based on the highest-numbered chiral carbon — here, C-4. In both ribose and 2-deoxyribose found in nucleic acids, the OH at C-4 is on the right in the Fischer projection, making them D-sugars. …

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Here are the common mistakes students make when determining the relative configuration of ribose and 2-deoxyribose, along with how to avoid each.

Mistake 1: Confusing "Relative" with "Absolute" Configuration

The Error: Students often try to memorize the specific D/L designation (e.g., "ribose is D-ribose") but fail to understand what "relative configuration" means in this context. They might also confuse it with the anomeric configuration ( $\alpha$ vs $\beta$ ) found in nucleosides.
Why It Happens: The question asks for the relative configuration between the two sugars, not the absolute configuration of one sugar in isolation. Students jump to naming the sugar (D-ribose) instead of comparing the two.
How to Avoid: Focus on the word relative. The question is asking: "How does the stereochemistry of 2-deoxyribose compare to that of ribose?" The answer is that they share the same relative configuration at all chiral centers except for the missing -OH group. Specifically, both have the same configuration at C-2, C-3, and C-4 (in the open-chain form). The only difference is that 2-deoxyribose lacks the -OH at C-2.

Mistake 2: Forgetting the "Deoxy" Implication

The Error: Students treat 2-deoxyribose as a completely different sugar with a different stereochemical family, or they incorrectly assume the missing -OH changes the configuration of the remaining chiral centers.
Why It Happens: The prefix "2-deoxy" means the -OH group at carbon 2 is replaced by a hydrogen atom. This removal does not invert or alter the stereochemistry at C-3 or C-4. Students sometimes think the loss of a chiral center (C-2 becomes achiral) somehow flips the other centers.
How to Avoid: Draw the Fischer projections of both sugars side-by-side.
- Ribose: Has chiral centers at C-2, C-3, and C-4 (all with specific -OH placements).
- 2-Deoxyribose: Has chiral centers only at C-3 and C-4. The -OH at C-2 is gone (replaced by H).
- Key observation: The -OH groups at C-3 and C-4 are in exactly the same positions in both sugars. Therefore, their relative configuration is identical at those positions.

Mistake 3: Mixing Up the Numbering or Ring Forms

The Error: Students try to compare the sugars in their cyclic (furanose) forms and get confused by the anomeric carbon (C-1) or the ring oxygen.
Why It Happens: In nucleic acids, ribose and deoxyribose exist as five-membered rings (furanose). The ring closure creates a new chiral center (the anomeric carbon), which is not relevant to the relative configuration of the parent sugars.
How to Avoid: Always compare the open-chain (linear) forms when discussing relative configuration. The question is about the aldopentose backbone, not the cyclic hemiacetal. In the open chain: …

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

Showing the 12 most recent of 16 on this concept.

CBSE 2026Set 56/3/11 markMCQ
$Q. On hydrolysis, which of the following carbohydrates gives only glucose ? (A) Sucrose (B) Galactose (C) Lactose (D) Maltose$
›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 $\alpha(1 \to 2)$ glycosidic bond. When hydrolyzed:

$\text{Sucrose} + \text{H}_2\text{O} \longrightarrow \text{Glucose} + \text{Fructose}$
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 $\beta(1 \to 4)$ glycosidic bond. Hydrolysis produces:

$\text{Lactose} + \text{H}_2\text{O} \longrightarrow \text{Glucose} + \text{Galactose}$
Again, two different monosaccharides result, not just glucose.

Maltose (Option D) …
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CBSE 2025Set ANNUAL1 mark
$Q. Fill in the blank: Sucrose on hydrolysis gives glucose and ________.$
›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

…

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CBSE 2025Set BOTANY1 markMCQ
$Q. Fill in the blank selecting the appropriate one: The enzyme beta-galactosidase breaks lactose into ____ and glucose.$
(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 …

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CBSE 2024Set ANNUAL1 mark
$Q. Name the milk sugar present in milk.$
›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 …

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CBSE 2023Set F1 markMCQ
$Q. Which of the following disaccharides is present in milk?$
(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 …

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CBSE 2023Set A1 mark
$Q. Match the following. Column A item: 'Milk sugar'. Choose its correct match from Column B:$
(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 …

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CBSE 2023Set ANNUAL1 markMCQ
$Q. Which of the following disaccharide is present in milk?$
(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 $\beta$ -1,4 glycosidic linkage, an …

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CBSE 2022Set ANNUAL1 mark
$Q. What are the expected products of hydrolysis of lactose?$
›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 $\beta$ -D-galactose unit and one $\beta$ -D-glucose unit linked by a $\beta(1\rightarrow4)$ glycosidic bond.

On hydrolysis (acid- or enzyme-catalysed by lactase), this glycosidic linkage is cleaved:

…

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CBSE 2021Set ANNUAL1 mark
$Q. Name the sugar present in the milk.$
›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.

…

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CBSE 2020Set 56/3/11 mark
$Q. Name the disaccharide which on hydrolysis gives two molecules of glucose.$
›Reveal solutionSolution
The disaccharide that yields two molecules of glucose upon hydrolysis is maltose, because it is composed of two $\alpha$ -D-glucose units linked by an $\alpha(1 \to 4)$ 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. …
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CBSE 2020Set ANNUAL1 mark
$Q. Which disaccharide on hydrolysis in presence of the catalyst invertase produces glucose and fructose?$
›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

…

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CBSE 2020Set ANNUAL1 markMCQ
$Q. The disaccharides present in milk is$
(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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←Previous questionQ26 — The prefixes 'D' and 'L' before a sugar's name give the configuration …→Next questionQ28 — Which sugar is called invert sugar? Why is it called so?

Q.Aldopentoses named ribose and 2-deoxyribose are found in nucleic acids. What is their relative configuration?

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

Concept: Relative Configuration of Aldopentoses in Nucleic Acids

Method: Fischer Projection Analysis

Mistake 1: Confusing "Relative" with "Absolute" Configuration

Mistake 2: Forgetting the "Deoxy" Implication

Mistake 3: Mixing Up the Numbering or Ring Forms

Understanding Carbohydrate Hydrolysis

Analyzing Each Option

Q.Aldopentoses named ribose and 2-deoxyribose are found in nucleic acids. What is their relative configuration?

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