Lesson 2 of 7 / Biological molecules
Starch, glycogen and cellulose
Why do storage and structural carbohydrates have different shapes?
I can relate polysaccharide bonds and branching to their roles.
I can relate starch, cellulose, glycogen, triglyceride and phospholipid structure to properties and roles.Syllabus
Syllabus 9477, Core 1(i). Relate starch, cellulose, glycogen, triglyceride and phospholipid structure to properties and roles.
About 7 min
Make a guess. You are not marked.
Starch and cellulose are both chains of glucose. Why can you digest rice starch but not the cellulose in vegetables?
Show the answer
Your enzymes cannot hydrolyse beta-1,4 bonds.
Cellulose links beta-glucose by beta-1,4 bonds. Human amylase acts on alpha bonds only.
The key idea
Alpha-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.
Watch out for this
A student says: "Cellulose is strong because it contains alpha-1,6 branches." What is wrong with this?
Show the answer
Cellulose is unbranched; its beta-1,4 chains are strengthened by numerous interchain hydrogen bonds.
Check your understanding
Which feature helps glycogen release glucose rapidly?
Show the answer
Many chain ends from frequent branching
More terminal sites can be acted on by appropriate enzymes.
Exam-style questions
Check your understanding
A polysaccharide releases glucose very quickly when hydrolysed by an enzyme that acts only at chain ends. It is also insoluble. Which conclusion is best supported?
Show the answer
It is highly branched, giving many chain ends.
An end-acting enzyme works fastest when there are many ends, which branching supplies. Glycogen fits this.
Quick recall
Why do storage and structural carbohydrates have different shapes?