describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms

Published by Patrick Mutisya · 14 days ago

Cambridge A-Level Biology – Carbohydrates and Lipids: Polysaccharides

Carbohydrates and Lipids – Polysaccharides

Learning Objective

Describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms.

Key Concepts

  • Monosaccharide building blocks – glucose (\$\mathrm{C6H{12}O_6}\$)

  • Glycosidic linkages (α‑1,4 and α‑1,6)

  • Degree of polymerisation and branching

  • Storage vs. structural roles

Starch – The Plant Storage Polysaccharide

Starch is a mixture of two glucose polymers:

Amylose

• Linear chain of D‑glucose units linked by α‑1,4‑glycosidic bonds.

• Typically 200–2 000 glucose residues long.

• Forms a helical conformation (≈6 glucose residues per turn).

• Limited solubility in water; can form a gel when heated with water (gelatinisation).

Amylopectin

• Branched polymer of D‑glucose.

• Main chain consists of α‑1,4‑glycosidic bonds; branch points are α‑1,6‑glycosidic bonds.

• Branches occur every 24–30 glucose residues on average.

• Highly soluble; creates a more open, fluffy structure compared with amylose.

Suggested diagram: Helical structure of amylose and branched structure of amylopectin.

Glycogen – The Animal Storage Polysaccharide

Glycogen is structurally similar to amylopectin but more highly branched.

  • Core of glucose residues linked by α‑1,4 bonds.

  • Branch points are α‑1,6 bonds occurring roughly every 8–12 glucose residues.

  • Average molecular weight ≈ 10⁸ Da; contains \overline{10} 000 glucose units.

  • Highly compact and water‑soluble, allowing rapid mobilisation.

Suggested diagram: Highly branched glycogen molecule with α‑1,4 and α‑1,6 linkages.

Structure–Function Relationships

The differences in branching and chain length dictate how each polysaccharide functions in the cell.

PolysaccharideStructural FeaturesFunctional Implications
AmyloseLinear, α‑1,4 linkages; helical; few branch pointsForms compact granules; slower enzymatic breakdown; contributes to starch rigidity and gelatinisation behaviour.
AmylopectinBranched, α‑1,4 backbone with α‑1,6 branches every 24–30 residuesMore accessible to amylase; rapid release of glucose; gives starch a fluffy texture.
GlycogenHighly branched, α‑1,4 backbone with α‑1,6 branches every 8–12 residuesMaximises surface area; allows simultaneous action of multiple enzymes; rapid mobilisation of glucose during high‑energy demand.

Biological Context

  1. Plants: Starch is stored in chloroplasts (amyloplasts) as granules. Amylose provides structural stability, while amylopectin ensures quick energy release during germination.

  2. Animals: Glycogen is stored mainly in liver and skeletal muscle. Liver glycogen maintains blood glucose levels; muscle glycogen supplies immediate ATP for contraction.

  3. Enzymatic degradation:

    • α‑amylase cleaves internal α‑1,4 bonds (acts on both amylose and amylopectin).

    • Debranching enzyme (α‑1,6‑glucosidase) removes branch points, essential for glycogenolysis.

Key Equations (LaTeX)

General formula for a glucose polymer:

\$\text{(C}6\text{H}{10}\text{O}5\text{)}n + n\ \text{H}2\text{O} \rightarrow \text{C}6\text{H}{12}\text{O}6\ (n\ \text{glucose})\$

Rate of glycogen synthesis (simplified):

\$\frac{d[\text{Glycogen}]}{dt}=V{\text{GS}}-\;V{\text{GP}}\$

where \$V{\text{GS}}\$ is the activity of glycogen synthase and \$V{\text{GP}}\$ the activity of glycogen phosphorylase.

Summary

Starch (amylose + amylopectin) and glycogen are glucose polymers whose degree of branching determines their physical properties and biological roles. Linear amylose provides structural rigidity, moderately branched amylopectin allows rapid enzymatic access, and highly branched glycogen maximises surface area for swift glucose release, matching the metabolic demands of plants and animals respectively.