Define carbohydrates. On what structural basis are they classified as aldoses or ketoses?

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Q. Define carbohydrates. On what structural basis are they classified as aldoses or ketoses?

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✓ Free question

Carbohydrates are defined as polyhydroxy aldehydes or polyhydroxy ketones, or larger molecules (such as disaccharides and polysaccharides) that yield such compounds when hydrolysed. Many, though not all, fit the general formula $\text{C}_x(\text{H}_2\text{O})_y$ .

Within this definition, a monosaccharide is classified purely by the type of carbonyl group it carries: if it has an aldehyde group ( $-\text{CHO}$ ), it is called an aldose; if it has a ketone group ( $\text{C}{=}\text{O}$ ), it is called a ketose. Glucose (aldehyde at C1) is therefore an aldose, and fructose (ketone at C2) is a ketose.

✓Final answer

Carbohydrates are polyhydroxy aldehydes/ketones (or their hydrolysis products); the aldose/ketose split depends only on whether the carbonyl group present is an aldehyde or a ketone.

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 23 on this concept.

CBSE 2026Set ANNUAL1 markMCQ
$Q. Polysaccharide is$
(a) Ribose
(b) Lactose
(c) Sucrose
(d) Glycogen
›Reveal solutionSolution
Polysaccharides are polymers of many monosaccharide units joined by glycosidic bonds.

Ribose: a 5-carbon monosaccharide (aldopentose).

Lactose: a disaccharide of glucose + galactose.

Sucrose: a disaccharide of glucose + fructose.

Glycogen: a highly branched polymer of many glucose units - a true polysaccharide, the animal storage form of carbohydrate.

✓Final answer
$(d) Glycogen.$
CBSE 2026Set ANNUAL1 markMCQ
$Q. Which of the following is an Aldohexose?$
(a) Cellulose
(b) Sucrose
(c) Raffinose
(d) Glucose
›Reveal solutionSolution
An aldohexose is a simple six-carbon sugar (monosaccharide) that carries an aldehyde (-CHO) functional group; glucose is the most common example.

Glucose has the molecular formula C6H12O6, with a straight-chain structure carrying an aldehyde group at C1 and hydroxyl groups on the remaining carbons - so it is classified as an aldohexose ('aldo' for the aldehyde group, 'hexose' for six carbons).

Cellulose is a polysaccharide (polymer of many glucose units), not a monosaccharide.

Sucrose is a disaccharide (glucose + fructose), not a monosaccharide.

Raffinose is a trisaccharide (galactose + glucose + fructose).

Only glucose among the options is itself a simple aldohexose.

✓Final answer
$(d) Glucose.$
CBSE 2025Set JZ1 markMCQ
$Q. Sweetest sugar is :$
(a) Glucose
(b) Lactose
(c) Sucrose
(d) Fructose
›Reveal solutionSolution
Fructose is the sweetest naturally occurring sugar — option (d).

Concept. Sweetness is compared on a relative scale taking table sugar (sucrose) as the reference, 100.

Relative sweetness of the options:

Glucose ≈ 74

Lactose ≈ 16 (least sweet)

Sucrose = 100 (reference)

Fructose ≈ 173 (the sweetest)

Fructose, the ketohexose found abundantly in fruits and honey, tops this list.

✓Final answer
$(d) Fructose is the sweetest sugar.$
CBSE 2025Set D1 markMCQ
$Q. Which of the following is a disaccharide?$
(a) Sucrose
(b) Glucose
(c) Fructose
(d) Starch
›Reveal solutionSolution
Disaccharides give two monosaccharide units on hydrolysis; sucrose (glucose + fructose) is the disaccharide here.

Carbohydrates are classified by the number of monosaccharide units produced on hydrolysis:

Monosaccharides: cannot be hydrolysed further, e.g. glucose, fructose.

Disaccharides: give two monosaccharide units on hydrolysis, e.g. sucrose, maltose, lactose.

Polysaccharides: give many units, e.g. starch, cellulose.

Sucrose hydrolyses to one molecule of glucose and one of fructose, so it is a disaccharide. Glucose and fructose are monosaccharides; starch is a polysaccharide.

✓Final answer
$(A) Sucrose is a disaccharide.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. Disaccharide is -$
(a) Glucose
(b) Fructose
(c) Sucrose
(d) Starch
›Reveal solutionSolution
A disaccharide yields exactly two monosaccharide units on hydrolysis; among the options only sucrose fits.

Glucose and Fructose are themselves monosaccharides (single sugar units), not disaccharides.

Starch is a polysaccharide (many glucose units joined together).

Sucrose is a disaccharide: on hydrolysis (acid or the enzyme invertase) it splits into one molecule of glucose and one molecule of fructose:

Sucrose + H2O -> Glucose + Fructose

✓Final answer
$(c) Sucrose.$
CBSE 2025Set ANNUAL1 markMCQ
$Q. Which of the following monosaccharides is a ketohexose?$
(a) Glucose
(b) Galactose
(c) Fructose
(d) Mannose
›Reveal solutionSolution

Monosaccharides are classified by their carbonyl group as either aldoses (aldehyde) or ketoses (ketone); among the options, fructose is the ketohexose.

Glucose, galactose and mannose all carry an aldehyde group (-CHO) at C1, making them aldohexoses. Fructose instead carries a ketone group (C=O) at C2, making it the ketohexose among the options - it is often written with the structural formula CH2OH-(CHOH)3-CO-CH2OH.

✓Final answer
$(c) Fructose.$
CBSE 2024Set FZ1 markMCQ
$not$
(a) Sucrose
(b) Cellulose
(c) Lactose
(d) Maltose
›Reveal solutionSolution
Cellulose is a polysaccharide, not a disaccharide → option (b).

Concept. Carbohydrates are classified by how many monosaccharide units they release on hydrolysis: a disaccharide gives two, a polysaccharide gives many.

Checking each option.

Sucrose → glucose + fructose (disaccharide)

Lactose → glucose + galactose (disaccharide)

Maltose → glucose + glucose (disaccharide)

Cellulose → a long chain of many $\beta$ -D-glucose units (polysaccharide)

✓Final answer
$(b) Cellulose is not a disaccharide (it is a polysaccharide).$
CBSE 2024Set D1 markMCQ
$Q. Which of the following is a ketohexose?$
(a) Glucose
(b) Fructose
(c) Sucrose
(d) Starch
›Reveal solutionSolution
Fructose = 6 carbons + keto group = ketohexose.

A ketohexose has six carbon atoms (hexose) and a ketone functional group.

Glucose is an aldohexose (aldehyde group, 6 C).

Fructose is a ketohexose (keto group at C-2, 6 C).

Sucrose is a disaccharide (glucose + fructose) and starch is a polysaccharide.

Hence fructose is the ketohexose.

✓Final answer
$(b) Fructose — a ketohexose.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Glucose is -$
(a) An aldopentose
(b) A ketopentose
(c) An aldohexose
(d) A ketohexose
›Reveal solutionSolution

Glucose has the molecular formula C6H12O6, with an aldehyde group at C-1, making it an aldohexose.

Sugars are classified by (i) the number of carbon atoms (tri-, tetr-, pent-, hex- ose, etc.) and (ii) the type of carbonyl group present (aldo- for -CHO, keto- for C=O within the chain).

Glucose has six carbon atoms (hexose) and its open-chain form has an aldehyde group at carbon 1 (CHO-CHOH-CHOH-CHOH-CHOH-CH2OH), so it is classified as an aldohexose.

Fructose, by contrast, is the classic ketohexose (carbonyl at C-2).

✓Final answer
$(c) An aldohexose.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Milk sugar is -$
(a) Sucrose
(b) Lactose
(c) Maltose
(d) Galactose
›Reveal solutionSolution

Lactose, often called 'milk sugar', is the principal sugar naturally present in mammalian milk.

Lactose is a disaccharide formed by a glycosidic (beta-1,4) linkage between a molecule of galactose and a molecule of glucose; it occurs naturally in milk (about 4-5% in cow's milk).

Sucrose ('cane/table sugar') is glucose + fructose, obtained from sugarcane/sugar beet, not milk.

Maltose ('malt sugar') is glucose + glucose, obtained from starch hydrolysis.

Galactose alone is a monosaccharide, a component of lactose, not the disaccharide itself.

✓Final answer
$(b) Lactose.$
CBSE 2024Set ANNUAL1 markMCQ
$Q. Glucose and fructose are$
(a) Isomers of each other
(b) Homologus of each other
(c) Anomers of each other
(d) Enantioners of each other
›Reveal solutionSolution

Glucose and fructose both have the molecular formula C6H12O6 but differ in the position of the carbonyl group (glucose = aldehyde at C1, fructose = ketone at C2), making them structural (functional) isomers, not anomers or enantiomers of one another.

Glucose is an aldohexose (has a -CHO group at C1). Fructose is a ketohexose (has a C=O group at C2). Both share the same molecular formula C6H12O6, but their functional group and connectivity differ - this is the definition of (structural) isomerism.

They are not anomers of each other (anomers are stereoisomers differing only in configuration at the anomeric carbon of the SAME sugar, e.g. alpha-D-glucose vs beta-D-glucose). They are not enantiomers either (enantiomers are non-superimposable mirror images of the same compound; glucose and fructose are different compounds entirely, not mirror images).

✓Final answer
$(a) Isomers of each other.$
CBSE 2024Set ANNUAL1 mark
$Q. Complete the following. Amylopectin is a polymer of ______.$
›Reveal solutionSolution
Amylopectin, a component of starch, is a branched polysaccharide built entirely from alpha-D-glucose monomers.

Starch is a polysaccharide made of two components: amylose (linear, alpha-1,4 glycosidic linkages) and amylopectin (branched).

Amylopectin is a branched-chain polymer of alpha-D-glucose in which glucose units are joined by:

alpha-1,4-glycosidic linkages in the linear chain, and

alpha-1,6-glycosidic linkages at branch points (roughly every 25-30 glucose units).

✓Final answer
$Amylopectin is a polymer of alpha-D-glucose .$

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Q.Define carbohydrates. On what structural basis are they classified as aldoses or ketoses?

Concept understanding — Classification of Carbohydrates

Classification of Carbohydrates

Q.Define carbohydrates. On what structural basis are they classified as aldoses or ketoses?

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