Key Fact
Carbohydrate classification
Monosaccharides (simplest): glucose, fructose, galactose. Disaccharides: sucrose (glucose+fructose), lactose (glucose+galactose), maltose (2×glucose). Polysaccharides: starch, cellulose, glycogen.
-- NCERT, p. 2Carbohydrates Classification
8 MCQs1 revision card9-step worked example
Source: NCERT Unit 19PYQ coverage: NEET 2021, 2022, 2023, 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
The trap that catches repeaters on carbohydrate classification: confusing the basis of classification. You know glucose and fructose share the molecular formula C₆H₁₂O₆ — but one is an aldohexose and the other a ketohexose. NEET distractors exploit exactly this overlap.
What carbohydrates are. NCERT Class 12 Chemistry Chapter 14 (Biomolecules) defines carbohydrates as polyhydroxy aldehydes or ketones, or compounds that yield these on hydrolysis. The general formula for monosaccharides is Cₙ(H₂O)ₙ.
Classification hierarchy:
-
By hydrolysis products:
- Monosaccharides — cannot be hydrolysed further (glucose, fructose, ribose)
- Oligosaccharides — yield 2–10 monosaccharide units (sucrose → glucose + fructose; maltose → glucose + glucose; lactose → glucose + galactose)
- Polysaccharides — yield many monosaccharide units (starch, cellulose, glycogen)
-
By carbonyl group:
- Aldoses — aldehyde at C-1 (glucose, galactose, ribose)
- Ketoses — ketone at C-2 (fructose, ribulose)
-
By carbon count:
- Trioses (n=3), tetroses (n=4), pentoses (n=5: ribose, deoxyribose), hexoses (n=6: glucose, fructose, galactose)
Reducing vs non-reducing sugars: A sugar is reducing if it has a free anomeric carbon (hemiacetal/hemiketal) that can open to expose the carbonyl. All monosaccharides are reducing. Sucrose is non-reducing (both anomeric carbons are locked in the glycosidic bond). Maltose and lactose are reducing (one free anomeric carbon remains).
Watch-out for NEET: When a stem says "a ketohexose that is laevorotatory," the answer is fructose — but the distractor will offer glucose (an aldohexose) because both are C₆H₁₂O₆. Classify by functional group first, carbon count second.
Practice MCQs
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which of the following is a non-reducing sugar?
MCQ 2Easy RecallPractice
Fructose is classified as a:
MCQ 3Easy RecallPractice
Hydrolysis of sucrose yields:
MCQ 4Direct ApplicationPractice
A monosaccharide has the molecular formula C₅H₁₀O₅. It contains an aldehyde group. This sugar is best classified as:
MCQ 5Direct ApplicationPractice
Which of the following disaccharides will give a positive Tollens' test?
MCQ 6Direct ApplicationPractice
Cellulose and starch are both polysaccharides of glucose. The key structural difference is:
MCQ 7Concept TrapPractice
Glucose and fructose have the same molecular formula (C₆H₁₂O₆) but differ in their functional group position. They are best described as:
MCQ 8CalculationPractice
A polysaccharide on complete hydrolysis yields only D-glucose. A sample is found to dissolve in hot water, giving a blue colour with iodine. The polysaccharide and its structural component are:
Quick recall before you leave
Worked Example
- 1
Given
- Sugar: Ribose - Need to determine: carbon-count class, functional group type, molecular formula
- 2
Required
Complete classification of ribose by (a) carbon number, (b) carbonyl type, and (c) molecular formula using Cₙ(H₂O)ₙ.
- 3
Concept
Monosaccharide classification: first identify the number of carbons (triose/tetrose/pentose/hexose), then identify the functional group (aldehyde → aldose; ketone → ketose). Apply the general formula Cₙ(H₂O)ₙ.
- 4
Formula
Cₙ(H₂O)ₙ with n = number of carbons
- 5
Substitution
Ribose has 5 carbons → n = 5 C₅(H₂O)₅ = C₅H₁₀O₅
- 6
Calculation
Molecular formula: C₅H₁₀O₅ Ribose has an aldehyde group at C-1 → aldose Five carbons → pentose Note: n = 5 is a counting integer (exact); it does not affect significant-figure considerations.
- 7
Final answer
Ribose is an **aldopentose** with molecular formula **C₅H₁₀O₅**.
- 8
Common trap
Confusing ribose (aldopentose, C₅H₁₀O₅) with deoxyribose (also a pentose but with formula C₅H₁₀O₄ — one fewer oxygen). The prefix "deoxy" means one –OH is replaced by –H. NEET distractors may offer C₅H₁₀O₄ for ribose.
- 9
Similar NEET-style question
"Ribulose is a monosaccharide with 5 carbons and a ketone group. Classify it and write its molecular formula." (Answer: ketopentose, C₅H₁₀O₅.)
Before solving, remember these
Formulas
DNA hydrogen bonds per base pair
Used to compute total H-bonds in a duplex of given GC%/AT% composition.
| Symbol | Quantity | SI Unit |
|---|---|---|
| %GC | GC content | - |
Valid when
- Watson–Crick double helix
General formula of monosaccharides
Empirical formula of simple monosaccharides; glucose/fructose are C6H12O6.
| Symbol | Quantity | SI Unit |
|---|---|---|
| n | carbon count | - |
Valid when
- Open-chain or cyclic forms of aldoses/ketoses
Peptide bonds in a polypeptide of N residues
Number of amide (peptide) bonds in a linear polypeptide of N amino acids.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N | residue count | - |
Valid when
- Linear (non-cyclic) polypeptide
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Student writes A=U for DNA or A=T for RNA. DNA: A=T, G≡C. RNA: A=U (no T), G≡C.
When it triggers
Question on base pairing or sugar identity.
How to avoid
DNA: deoxyribose, A-T-G-C bases. RNA: ribose, A-U-G-C bases (uracil instead of thymine). H-bond pairs: A=T (DNA) or A=U (RNA), G≡C (3 H-bonds, both).
Category: Similar Terms
Student claims denaturation breaks peptide bonds. Denaturation only breaks H-bonds, ionic, hydrophobic interactions; primary structure (peptide bonds) intact.
When it triggers
Question about protein denaturation effects.
How to avoid
Denaturation: heat/pH/organic solvents disrupt secondary, tertiary, quaternary structure. Primary structure (covalent peptide bonds) requires hydrolysis to break.
Root cause: concept gap
Correction
Denaturation breaks H-bonds, ionic, hydrophobic; preserves peptide bonds. Hydrolysis (acid/base/enzyme) breaks primary structure.
Root cause: concept gap
Correction
DNA: deoxyribose, A=T, G≡C. RNA: ribose, A=U (no T), G≡C.
Past Year Questions
6 questions from NEET 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1A-IV, B-III, C-II, D-I
2A-I, B-III, C-II, D-IV
3A-IV, B-III, C-I, D-II
4A-II, B-III, C-I, D-IV
NTA Answer: Option 3(final)
NEET 2025
1Statement I is incorrect but Statement II is correct
2Both Statement I and Statement II are correct
3Both Statement I and Statement II are incorrect
4Statement I is correct but Statement II is incorrect
NTA Answer: Option 2(final)
NEET 2024Revised key
1A-IV, B-I, C-III, D-II
2A-III, B-I, C-II, D-IV
3A-IV, B-I, C-II, D-III
4A-I, B-IV, C-II, D-III
NTA Answer: Option 3(revised_final)
NEET 2023
1Both A and R are true and R is NOT the correct explanation of A
2A is true but R is false
3A is false but R is true
4Both A and R are true and R is the correct explanation of A
NTA Answer: Option 3(final)
NEET 2022
The incorrect statement regarding enzymes is
1Enzymes are very specific for a particular reaction and substrate.
2Enzymes are biocatalysts.
3Like chemical catalysts enzymes reduce the activation energy of bio processes.
4Enzymes are polysaccharides.
NTA Answer: Option 4(final)
NEET 2021
1CH
2CH 4
3CH
4CH 2 3
NTA Answer: Option 4(final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Biomolecules — recall and classify carbohydrates, proteins, nucleic acids, vitamins; identify base pairs and peptide bonds.
RecallMedium
Common distractors
swapped classes
Tempts surface-level recall.
Test yourself on this topic with real past-paper questions:
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