Biology · Biomolecules · NEET
| Number of sugar units | Monosaccharide: one single sugar unit | Disaccharide: two sugar units joined by one glycosidic bond |
| Examples | Glucose, fructose, ribose, deoxyribose | Sucrose, maltose, lactose |
| Can it be hydrolysed into a smaller sugar? | No, it is the simplest sugar | Yes, hydrolysis breaks the glycosidic bond into monosaccharides |
| Water change on forming | Building block, no bond to form | Loses one water molecule (dehydration) per glycosidic bond |
A glycosidic bond is the chemical link that joins two sugar units together. It forms when the -OH group of one monosaccharide reacts with the -OH group of another, and in the process one water molecule is removed. This same bond joins all the sugar units in disaccharides (like sucrose, maltose) and in polysaccharides (like starch, cellulose, glycogen, inulin).
A monosaccharide is one single sugar unit (glucose, fructose, ribose). A disaccharide is two sugar units joined by one glycosidic bond (sucrose, maltose, lactose). A polysaccharide is a long chain of many sugar units, all joined by glycosidic bonds (cellulose, starch, glycogen, inulin). So the number of sugar units is the difference: one, two, or many.
A glycosidic bond joins two glucose molecules. For example, when two glucose units link, they form the disaccharide maltose. NCERT states this bond forms by a dehydration reaction, meaning one water molecule is lost as the two sugars connect.
A glycosidic bond is FORMED by dehydration (also called condensation), because one molecule of water is removed when the two sugars join. The reverse process, breaking the bond by adding water, is hydrolysis. In NEET, enzymes like amylase break glycosidic bonds by hydrolysis.
Sugars carry a carbonyl group and hydroxyl groups. A sugar is a polyhydroxy aldehyde or a polyhydroxy ketone, so it has one carbonyl (C=O, the aldehyde or ketone) plus many hydroxyl (-OH) groups. This was directly asked in NEET 2018.
Two glucose molecules are each C6H12O6, so together they give C12H24O12. But when the glycosidic bond forms, one water molecule (H2O) is removed by dehydration. Subtracting H2O from C12H24O12 gives C12H22O11, the correct formula of maltose. This loss of one water on linking is the key idea NEET 2022 tested.
A dehydration reaction links two glucose molecules to produce maltose. If the formula for glucose is C6H12O6 then what is the formula for maltose?
The two functional groups characteristic of sugars are
Identify the substances having glycosidic bond and peptide bond, respectively in their structure:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Common monosaccharides are glucose and fructose (both hexoses, 6 carbons) and ribose and deoxyribose (both pentoses, 5 carbons, found in nucleic acids). NCERT lists ribose as a monosaccharide pentose and deoxyribose (2'-deoxyribose) as the sugar in DNA.
Both starch and cellulose are polymers of glucose joined by glycosidic bonds, but the exact type of glycosidic linkage differs. This is why starch forms helices and gives a blue colour with iodine, while cellulose does not form those helices and gives no blue colour.
Amylase breaks the glycosidic bonds of polysaccharides (like starch) by hydrolysis. In NCERT, hydrolases catalyse hydrolysis of ester, peptide, glycosidic and other bonds. This was tested in NEET 2024 (amylase matched to glycosidic bond).
Sugars like glucose are primary metabolites, because they have direct roles in normal physiology and metabolism. NEET 2021 asked this: amino acids and glucose are primary metabolites, not secondary ones like alkaloids or rubber.
