What Are Carbohydrates? Meaning, General Formula and Saccharides
Chemistry · Biomolecules · NEET
Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis. Their general formula is Cx(H2O)y, which looks like "carbon plus water," so they were once called hydrates of carbon. They are also called saccharides, and NEET asks about their meaning, formula, and the three types. Memory hook: "Sweet CHO water" — Carbon + H2O + a CHO or C=O group.
Carbohydrates are polyhydroxy aldehydes/ketones (formula Cx(H2O)y) classified by hydrolysis into monosaccharides, oligosaccharides, and polysaccharides.
Your doubts, answered
What is the exact NCERT definition of a carbohydrate?
NCERT defines carbohydrates as optically active polyhydroxy aldehydes or ketones, or the compounds which produce such units on hydrolysis. Break it down: 'polyhydroxy' means many -OH groups, 'aldehyde or ketone' means it has a -CHO or a C=O carbonyl group, and 'optically active' means it can rotate plane-polarised light. Glucose fits directly (it is a polyhydroxy aldehyde). Starch fits because it breaks down into glucose on hydrolysis. NEET often tests this exact wording.
Why is the general formula Cx(H2O)y and why 'hydrates of carbon'?
Many carbohydrates have the formula Cx(H2O)y. For example, glucose is C6H12O6, which can be written as C6(H2O)6 — 6 carbons and 6 waters. Because the hydrogen and oxygen are in the same 2:1 ratio as in water, early chemists thought these were carbon combined with water, so they named them 'hydrates of carbon' (carbo-hydrates). This is only a naming reason; there is no real water molecule inside them.
Does every carbohydrate follow the Cx(H2O)y formula?
No. The formula is a general guide, not a strict rule. Some carbohydrates like rhamnose (C6H12O5) and deoxyribose do NOT fit Cx(H2O)y, yet they are still carbohydrates because they are polyhydroxy aldehydes/ketones. Also, some compounds like acetic acid (C2H4O2) fit the formula but are NOT carbohydrates. So NEET wants you to rely on the polyhydroxy aldehyde/ketone definition, not just the formula.
What are saccharides, and are all of them sweet?
Saccharide is just another word for carbohydrate (from Greek 'sakcharon' = sugar). But not all are sweet. Sweet carbohydrates like glucose, fructose, and sucrose are called sugars. Non-sweet ones like starch, cellulose, and glycogen are polysaccharides, also called non-sugars. So every sugar is a carbohydrate, but not every carbohydrate is a sugar.
How are carbohydrates classified based on hydrolysis?
They are split into three groups by what they give on hydrolysis. Monosaccharides (like glucose, fructose, ribose) cannot be hydrolysed to simpler sugars. Oligosaccharides give 2 to 10 monosaccharide units (disaccharides like sucrose and maltose are the common ones). Polysaccharides (like starch, cellulose, glycogen) give many monosaccharide units. This 3-way split is the next concept and a frequent NEET question.
Is glucose a carbohydrate, and what type is it?
Yes, glucose (C6H12O6) is a carbohydrate. It is a monosaccharide because it cannot be hydrolysed to anything simpler. It is a polyhydroxy aldehyde (aldose) with a -CHO group and several -OH groups, matching the definition perfectly. Fructose is also a monosaccharide but is a polyhydroxy ketone (a keto sugar).
⚠️ The NEET trap ✗ Every carbohydrate must have the formula Cx(H2O)y and must be sweet. ✓ The real definition is 'optically active polyhydroxy aldehyde or ketone (or gives such units on hydrolysis).' Cx(H2O)y is only a general formula with exceptions, and non-sugars like starch and cellulose are not sweet. 🧠 Definition beats formula: if NEET gives you rhamnose or acetic acid, the Cx(H2O)y trick fails — always check for polyhydroxy + carbonyl.
Real NEET questions
NEET 2016 Phase 1
Which one given below is a non-reducing sugar?
A · Maltose
B · Lactose
C · Glucose
D · Sucrose ✓
Solution: All four are carbohydrates. Glucose is a monosaccharide with a free -CHO, and maltose and lactose each keep one free anomeric carbon, so all three are reducing sugars. In sucrose, the anomeric carbons of glucose (C-1) and fructose (C-2) are locked together in the glycosidic linkage, so no free -CHO or C=O group is left. Hence sucrose is the non-reducing sugar. This tests that you know carbohydrates carry aldehyde/ketone groups unless they are blocked.
NEET 2020
Sucrose on hydrolysis gives:
A · α-D-Glucose + β-D-Fructose ✓
B · α-D-Fructose + β-D-Fructose
C · β-D-Glucose + α-D-Fructose
D · α-D-Glucose + β-D-Glucose
Solution: Sucrose is a disaccharide (an oligosaccharide). By the classification of carbohydrates, it hydrolyses into its two monosaccharide units. The glycosidic linkage between α-D-glucose and β-D-fructose is cleaved by water to give an equimolar mixture of α-D-glucose and β-D-fructose. This directly tests the meaning of hydrolysis in the definition 'or the compounds which produce such units on hydrolysis.'
NEET 2025
Sugar 'X': A. is found in honey, B. is a keto sugar, C. exists in α and β anomeric forms, D. is laevorotatory. 'X' is
A · Maltose
B · Sucrose
C · D-Glucose
D · D-Fructose ✓
Solution: A 'keto sugar' means a polyhydroxy ketone carbohydrate. Fructose (C6H12O6) is a ketohexose found in honey, exists in α and β anomeric forms, and rotates plane-polarised light to the left (laevorotatory). All four clues point to D-Fructose. This reinforces that carbohydrates can be polyhydroxy ketones, not only aldehydes.
Solved Biomolecules NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Carbohydrates are optically active polyhydroxy aldehydes or ketones, or compounds that give such units on hydrolysis, with general formula Cx(H2O)y.
Why are carbohydrates called saccharides?
'Saccharide' comes from the Greek word 'sakcharon' meaning sugar. It is simply another name for carbohydrates, whether they are sweet sugars or non-sweet polysaccharides.
Are all carbohydrates optically active?
Yes. Because carbohydrates contain chiral (asymmetric) carbon atoms, they rotate plane-polarised light, so 'optically active' is part of their definition.
What are the three main classes of carbohydrates?
Monosaccharides (cannot be hydrolysed, e.g. glucose, fructose), oligosaccharides (give 2 to 10 units, e.g. sucrose, maltose), and polysaccharides (give many units, e.g. starch, cellulose, glycogen).
Which carbohydrates are called non-sugars?
Polysaccharides such as starch, cellulose, and glycogen are not sweet in taste, so they are called non-sugars. Sweet carbohydrates like glucose and sucrose are called sugars.
