Lesson 2 of 6 / Biological molecules
Starch, glycogen and cellulose
Why do storage and structural carbohydrates have different shapes?
In this lesson: Relate polysaccharide bonds and branching to their roles.
About 7 min
The key ideaAlpha-glucose polymers store glucose compactly; beta-1,4 cellulose chains form strong microfibrils.
Explore the idea
Change the linkage, branching and role
alpha-1,4. One unbranched chain coils compactly. Glycosidic bonds form its backbone.
Original chain schematic. It compares organisation rather than exact bond angles or relative molecular size.
Explanation
Starch is a plant storage polysaccharide containing amylose and amylopectin. Amylose has alpha-1,4 glycosidic bonds and a coiled chain. Amylopectin has alpha-1,4 chains with alpha-1,6 branch points. Both are relatively insoluble, so stored carbohydrate has less osmotic effect than many free glucose molecules.
Glycogen is an animal and fungal storage polysaccharide with alpha-1,4 chains and frequent alpha-1,6 branches. Branching gives many terminal sites for enzymes to add or remove glucose, allowing rapid mobilisation. It is incorrect to say each branch itself produces energy; released glucose must be metabolised.
Cellulose consists of beta-glucose linked by beta-1,4 bonds. Successive glucose residues are inverted, producing straight, unbranched chains. Numerous hydrogen bonds between parallel chains bundle them into microfibrils with high tensile strength, resisting stretching of plant cell walls.
The identity of the monomer is therefore not enough to predict function. Compare the stereochemistry of the bond, branching and interactions between chains. Humans lack enzymes that hydrolyse cellulose beta-1,4 bonds efficiently, even though cellulose is made from glucose.
Step by step
- 1
Specify the backbone
Name alpha-1,4 or beta-1,4 bonds.
- 2
Add branching or bundling
Distinguish covalent branch points from interchain hydrogen bonds.
- 3
Explain the consequence
Connect many ends to mobilisation or microfibrils to tensile strength.
Worked example
Work through the evidence
Two insoluble glucose polymers have equal mass. A has many branch ends; B has straight parallel hydrogen-bonded chains. Predict their roles.
One way to explain it
A is suited to storage and rapid glucose mobilisation, as in glycogen. B is suited to mechanical support, as in cellulose.
Why this answer works
- Structure suggests how enzymes access terminal units.
- Interchain bonding gives collective strength.
Is this true? "Cellulose is strong because it contains alpha-1,6 branches."
Cellulose is unbranched; its beta-1,4 chains are strengthened by numerous interchain hydrogen bonds.