Carbohydrate Structures: Sugars, Starch, Cellulose and Glycogen

Revision notes

Monosaccharides

Carbohydrates contain carbon, hydrogen and oxygen. Monosaccharides are single sugar units. Glucose, galactose and fructose are hexoses with six carbon atoms and formula C₆H₁₂O₆, but their atom arrangements differ. Ribose is a pentose, C₅H₁₀O₅; deoxyribose is C₅H₁₀O₄ and has one fewer oxygen atom. Ribose occurs in RNA nucleotides, while deoxyribose occurs in DNA nucleotides.

In conventional glucose ring drawings, α-glucose has the carbon-1 hydroxyl below the ring plane, opposite the CH₂OH group; β-glucose has it above, on the same side. This small structural difference has major consequences when glucose units form polymers. Ring orientation matters, so identify the anomeric carbon rather than memorising an unlabelled shape.

Disaccharides and bonds

Two monosaccharides join by condensation, forming a glycosidic bond and releasing water. Hydrolysis uses water to split the bond. Maltose contains two glucose units; lactose contains glucose and galactose; sucrose contains glucose and fructose. They are all disaccharides, but their bonds and chemical test responses differ. Sucrose is non-reducing under the standard Benedict’s test conditions.

Polysaccharides

Starch is a plant storage polysaccharide made from α-glucose. Amylose is mainly unbranched, with α-1,4 links producing a coiled chain. Amylopectin has α-1,4 chains with α-1,6 branch points. Glycogen is an animal and fungal storage polymer with similar link types but more frequent branching. Branch ends allow rapid addition and removal of glucose units.

Cellulose contains β-glucose joined by β-1,4 links. Alternate glucose units are inverted, producing straight chains. Many hydrogen bonds between adjacent chains form strong microfibrils in plant walls. Humans lack cellulase and cannot digest cellulose into glucose using their own enzymes.

Compare structure, not just names

Starch and cellulose are both glucose polymers, yet their different monomers’ orientations and bonds produce different shapes and functions. Storage polysaccharides are relatively insoluble and compact, reducing their osmotic effect compared with many free glucose molecules. Cellulose is suited to tensile strength rather than rapid glucose storage mobilisation.

Worked application and data skills

A chain of 100 glucose units linked into one unbranched molecule has 99 joining bonds and releases 99 water molecules during its assembly by condensation. Joining just two glucose molecules gives maltose and one water molecule: C₆H₁₂O₆ + C₆H₁₂O₆ → C₁₂H₂₂O₁₁ + H₂O. This counting example assumes each connection is made by one condensation.

Common misconceptions

Starch contains α-glucose; cellulose contains β-glucose. Sucrose is not a reducing sugar in the standard direct test. A glycosidic bond is not a peptide bond.

Examination tips

Compare monomer, linkage, branching and function. Label the carbon-1 hydroxyl correctly in glucose diagrams.

Check your understanding

Five original questions. Use Tab to move between controls, arrow keys within a radio group, and Enter or Space on buttons. Feedback appears after Check Answer. Try Again clears the question. Attempts are not saved.

Sources and scope

Scope: Unit 1 B2.1. Original BTECRevision notes, schematics and questions. Independent resource; not endorsed by Pearson.

Official Pearson specification. Scientific review references are listed in the accompanying QA report.