Grade 12

Glucose, fructose, sucrose, starch, cellulose

Carbohydrates range from single sugars to giant polymers: the same glucose unit gives energy-storing starch or structural cellulose depending on how it is linked.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: the same brick, two different buildings

Rice, bread and cotton are all built from glucose units. We digest starch for energy but cannot digest cellulose, although both are chains of glucose. The difference lies in how one unit is joined to the next, and that changes the shape of the whole chain.

A nearly straight chain with alternating residues, like cellulose’s β(1→4) repeat; drag to rotate. It is a schematic, not an atomic structure.

SchoolSchool level: monosaccharides, disaccharides, polysaccharides

Main carbohydrates in the curriculum
Carbohydrate	Formula	Type and key feature
Glucose	C₆H₁₂O₆	Aldohexose; reducing sugar
Fructose	C₆H₁₂O₆	Ketohexose; isomer of glucose
Sucrose	C₁₂H₂₂O₁₁	Disaccharide of glucose + fructose; non-reducing
Starch	(C₆H₁₀O₅)ₙ	α-Glucose polymer: amylose (helical) and amylopectin (branched)
Cellulose	(C₆H₁₀O₅)ₙ	β-Glucose polymer, linear, structural

In solution glucose exists mostly as a six-membered ring (glucopyranose) in equilibrium with a small amount of open-chain aldehyde. The ring closes when the C-5 hydroxyl adds to the C-1 aldehyde, giving two anomers, α and β, that differ in the orientation of the new OH at C-1.

\ce{C12H22O11 + H2O ->[H+] C6H12O6 + C6H12O6}\quad(\text{glucose} + \text{fructose})

(\mathrm{C_6H_{10}O_5})_n + n\,\ce{H2O} \xrightarrow{\ \mathrm{H^+}\ \text{or amylase}\ } n\,\ce{C6H12O6}

Example: Mass of glucose from starch

Complete hydrolysis of 81 g of starch. What mass of glucose forms? (M of the C₆H₁₀O₅ unit = 162 g/mol; glucose 180 g/mol.)

Solution

81/162 = 0.500 mol of units gives 0.500 mol glucose, so m = 0.500 × 180 = 90 g. The extra mass is the water added in hydrolysis.

UndergraduateUniversity: glycosidic bonds decide the shape

Starch (amylose) has α(1→4) linkages, which make the chain curl into a helix; amylopectin adds α(1→6) branches about every 24–30 residues. Cellulose has β(1→4) linkages, so each glucose is flipped relative to its neighbour and the chain is straight; parallel chains pack by hydrogen bonds into strong microfibrils. Human amylases cleave α-links but we have no enzyme for β(1→4).

Definition:

A sugar with a free hemiacetal (anomeric) OH that can open to an aldehyde or α-hydroxy ketone and reduce Tollens’ or Benedict’s reagent. Glucose and fructose reduce; sucrose does not, because both anomeric carbons are tied up in the glycosidic bond.

References

Lehninger Principles of Biochemistry (8th ed.) · David L. Nelson, Michael M. Cox, Aaron A. Hoskins, 2021