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MHT-CET Chemistry · Biomolecules

Glycosidic Linkages in Di- and Polysaccharides

A glycosidic linkage is the C–O–C bridge formed when the anomeric OH of one sugar condenses with an OH of another: alpha-1,4 in maltose, amylose and the chains of amylopectin and glycogen, alpha-1,6 at their branch points, beta-1,4 in lactose and cellulose, and alpha-1,beta-2 between C-1 of glucose and C-2 of fructose in sucrose.

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Why this matters

19 PYQs, one HARD. Thirteen are the disaccharides and oligosaccharides — the linkage in maltose (asked four times) and lactose (three), which carbons of glucose and fructose join in sucrose (three), invert sugar, how many moles of glucose each gives on hydrolysis, and which sugars contain galactose; six are the polysaccharides — the linkages in cellulose, amylose and amylopectin, what animals store, and the false statement about cellulose in animal cells. Two cards.

Concept 1 of 2

Sucrose, Maltose, Lactose, Raffinose, Stachyose: the Linkage and the Hydrolysis Count

Intuition

Name the two monosaccharides, then the two carbons. Maltose is glucose–glucose through C-1 of one and C-4 of the other, alpha at the anomeric carbon: alpha-1,4. Lactose is galactose–glucose, beta-1,4. Sucrose is the odd one: C-1 of alpha-glucose to C-2 of beta-fructose, so BOTH anomeric carbons are used and no free hemiacetal is left — non-reducing, and no mutarotation. Hydrolysis counts follow from the units: maltose gives 2 glucose per mole, sucrose and lactose 1 each.

Definition

  • Maltose: 2 alpha-D-glucose, alpha-1,4; reducing. Hydrolysis: 2 mol glucose per mole — double what sucrose gives.
  • Lactose (milk): beta-D-galactose + D-glucose, beta-1,4; reducing. Hydrolysis: 1 glucose + 1 galactose.
  • Sucrose: C-1 of alpha-D-glucose to C-2 of beta-D-fructose (alpha-1,beta-2); non-reducing. Hydrolysis by dilute acid or invertase gives equimolar glucose + fructose = invert sugar (dextro +66.5° → laevo, because fructose's −92.4° outweighs glucose's +52.7°).
  • Raffinose (trisaccharide): galactose + glucose + fructose. Stachyose (tetrasaccharide): 2 galactose + glucose + fructose. Both contain galactose; sucrose and maltose do not.
  • Equal glucose per mole on hydrolysis: sucrose and lactose (1 each); raffinose and stachyose also give 1 glucose each.
SugarUnitsLinkageReducing?Glucose per mole
MaltoseGlucose + glucosealpha-1,4Yes2
LactoseGalactose + glucosebeta-1,4Yes1
SucroseGlucose + fructoseC-1 (alpha-Glc) to C-2 (beta-Fru)No1
Both anomeric carbons are in the bond — hence non-reducing and invert sugar on hydrolysis.
RaffinoseGal + Glc + FruTrisaccharideNo1
Stachyose2 Gal + Glc + FruTetrasaccharideNo1
Maltose is the only common disaccharide that gives two glucose per mole.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 1BiomoleculesHARD
Which from the following n mole molecules of carbohydrate contains 2n2n mole molecules of galactose, nn moles of glucose and nn moles of fructose in it?

[Q61 · 2nd May Shift 2 · 2023]

Lactose as the double-glucose sugar

Lactose gives ONE glucose and one galactose. Only maltose gives two glucose per mole; the 'double quantity compared with sucrose' answer is maltose.
Drill 12 more on sucrose, maltose, lactose, raffinose, stachyose: the linkage and the hydrolysis count

Concept 2 of 2

Starch, Glycogen and Cellulose

Intuition

Three polymers of glucose, told apart by the anomer and the branching. Starch is plant storage: amylose, an unbranched alpha-1,4 chain, plus amylopectin, alpha-1,4 chains with alpha-1,6 branches. Glycogen is the same pattern, more branched, and it is how ANIMALS store excess glucose. Cellulose is beta-1,4, straight, hydrogen-bonded into fibres — the plant cell wall, and indigestible to us because we lack the enzyme for the beta link.

Definition

  • Amylose (15–20% of starch): linear, alpha-1,4 only.
  • Amylopectin (80–85%): alpha-1,4 chain, alpha-1,6 at branch points.
  • Glycogen (animal storage; liver, muscle): alpha-1,4 chain + alpha-1,6 branches, more branched than amylopectin. 'Excess glucose stored in animals' → alpha-1,4 and alpha-1,6.
  • Cellulose: linear, beta-1,4, beta-D-glucose; plant cell wall — NOT animal cells (animals have no cell wall).
  • Starch is the food-grain constituent; lactose is the milk sugar; glycogen is the animal reserve.

Three glucose polymers

amylose: α-1,4;amylopectin / glycogen: α-1,4+α-1,6;cellulose: β-1,4\text{amylose: } \alpha\text{-1,4};\quad \text{amylopectin / glycogen: } \alpha\text{-1,4} + \alpha\text{-1,6};\quad \text{cellulose: } \beta\text{-1,4}

Worked example

A polysaccharide is hydrolysed only by an enzyme that cleaves beta-1,4 links, and it gives glucose alone. Identify it and say where it occurs.
Practice this conceptself-check · 4 quick reps

From the bank · past-year question

Example 2BiomoleculesMODERATE
What type of glycosidic linkages are developed when excess glucose is to be stored for future use in animals?

[Q70 · 16th May Shift 2 · 2023]

Amylose with a 1,6 branch

Amylose is the UNBRANCHED half of starch — alpha-1,4 only. The option 'alpha-1,4 and alpha-1,6' describes amylopectin and glycogen.
Drill 5 more on starch, glycogen and cellulose

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

  • Starch, Glycogen and Cellulose

    Three glucose polymers

    amylose: α-1,4;amylopectin / glycogen: α-1,4+α-1,6;cellulose: β-1,4\text{amylose: } \alpha\text{-1,4};\quad \text{amylopectin / glycogen: } \alpha\text{-1,4} + \alpha\text{-1,6};\quad \text{cellulose: } \beta\text{-1,4}

Reference tables (1)

Sucrose, Maltose, Lactose, Raffinose, Stachyose: the Linkage and the Hydrolysis Count5 rows
SugarUnitsLinkageReducing?Glucose per mole
MaltoseGlucose + glucosealpha-1,4Yes2
LactoseGalactose + glucosebeta-1,4Yes1
SucroseGlucose + fructoseC-1 (alpha-Glc) to C-2 (beta-Fru)No1
Both anomeric carbons are in the bond — hence non-reducing and invert sugar on hydrolysis.
RaffinoseGal + Glc + FruTrisaccharideNo1
Stachyose2 Gal + Glc + FruTetrasaccharideNo1
Maltose is the only common disaccharide that gives two glucose per mole.

Watch out for (2)

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