Chemistry · Biomolecules · NEET
When two sugar units come close, the -OH (hydroxyl) group of one sugar reacts with the -OH group of the other. One water molecule (H2O) is removed. The two sugars are now joined through a single oxygen atom, like a bridge: C-O-C. This oxygen bridge is called a glycosidic linkage. NCERT calls it an 'oxide linkage formed by the loss of a water molecule.' It is the bond that turns single sugars into disaccharides (2 units) and polysaccharides (many units).
Step 1: Take the anomeric carbon (C-1) of the first sugar, which carries an -OH group. Step 2: Bring the -OH group of the second sugar next to it. Step 3: One -OH loses an H, the other -OH loses an -OH, so together they lose one water molecule (this is a condensation reaction). Step 4: The remaining oxygen now links both carbons: C-O-C. Because water is lost, the reverse reaction (adding water back) is called hydrolysis, and it breaks the linkage.
Oxygen. A glycosidic linkage is a C-O-C bond, so the connecting atom is always an oxygen atom. This is why NCERT also calls it an 'oxide linkage.' Do not confuse it with a peptide bond (which has -CO-NH-, containing nitrogen) or a disulphide bond (which has sulphur). Only oxygen bridges two sugar carbons.
The numbers tell you WHICH carbons are joined. In alpha-(1->4), carbon 1 of one glucose links to carbon 4 of the next glucose (this makes long straight chains, like amylose and maltose). In alpha-(1->6), carbon 1 links to carbon 6, which creates a branch point (this is why amylopectin and glycogen are branched). 'Alpha' or 'beta' just tells the direction the anomeric -OH points. Cellulose uses beta-(1->4) links, which is why humans cannot digest it.
A sugar is reducing only if it still has a FREE anomeric carbon (free -OH on C-1 or C-2 that can open into an aldehyde/ketone). If the glycosidic linkage uses up the anomeric carbon of ONLY ONE sugar, the other anomeric carbon is still free, so the sugar is reducing (maltose, lactose). But in sucrose, the linkage joins the anomeric carbon of glucose (C-1) to the anomeric carbon of fructose (C-2). Both reducing groups are locked, so no free anomeric carbon is left, and sucrose is non-reducing.
No. A glycosidic bond joins SUGARS through oxygen (C-O-C) and is broken by hydrolysis. A peptide bond joins AMINO ACIDS through -CO-NH- (an amide bond, contains nitrogen) and forms proteins. An ester bond joins acids and alcohols (-COO-). NEET often mixes these in one option list, so remember: sugars = glycosidic (oxygen), amino acids = peptide (nitrogen).
In a protein molecule various amino acids are linked together by:
Which one given below is a non-reducing sugar?
Sucrose on hydrolysis gives:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the C-O-C oxygen bridge that joins two monosaccharide units, formed by the loss of one water molecule (a condensation reaction).
Yes. Adding water (hydrolysis), using dilute acid or an enzyme, breaks the oxygen bridge and gives back the separate monosaccharides.
An oxygen atom. The linkage is C-O-C, which is why NCERT also names it an oxide linkage.
Carbon 1 of one sugar is joined to carbon 4 of the next sugar, with the anomeric -OH in the alpha position. This makes straight chains like maltose and amylose.
In sucrose both anomeric carbons (glucose C-1 and fructose C-2) are used in the linkage, leaving no free reducing group. In maltose only one anomeric carbon is used, so the other stays free and reducing.
A glycosidic bond joins sugars through oxygen (C-O-C). A peptide bond joins amino acids through -CO-NH- (contains nitrogen). NEET often lists both as options to confuse you.
