Compare starch and glycogen storage polymers
Starch and glycogen are α-glucose polymers whose branching and compact forms suit carbohydrate storage.
Section 2.2 · Carbohydrates and lipids
AS Level; Cambridge International Biology 9700, 2028–2030. Original explanatory diagrams and examples support syllabus learning; written answers are self-checked and teacher review remains pending.
Before you begin
- Recognise that biological molecules are built from smaller molecules and that structure is related to function.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which storage polysaccharide is highly branched?
What you will learn
- Describe amylose and amylopectin structure
- Compare starch and glycogen structure and relate their features to storage
Two polysaccharides form starch
Amylose consists of long, unbranched chains of α-glucose joined by glycosidic bonds. The chain coils into a compact shape. Amylopectin also contains α-glucose but has branches, created by additional glycosidic links. Starch contains both amylose and amylopectin.
Starch is a storage carbohydrate in plants. Its large polymer molecules are relatively insoluble, so storing many glucose units as starch has less effect on water potential than storing the same amount as free glucose.
Glycogen stores glucose in animals and fungi
Glycogen is a highly branched polymer of α-glucose. Its compact structure and many ends allow enzymes to access multiple chain ends when glucose units are needed. Like starch, it is a storage polysaccharide and is relatively insoluble.
Glycogen is generally more highly branched than amylopectin. State the comparison precisely; do not describe starch as one single unbranched molecule because starch includes amylopectin as well as amylose.
Pause and explain
What is one benefit of many glycogen branches?
Worked example
A storage polymer has many branches and many chain ends. Identify the likely molecule and explain one advantage.
Show the worked solution
- Glycogen is highly branched.
- Many ends allow rapid enzyme access.
Answer The molecule is consistent with glycogen. Its many branches create many chain ends that enzymes can access to release glucose units when needed.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare amylose and amylopectin by chain shape.
Give me a hint
State the shared monomer and difference in branching.
Compare my reasoning
- Both are polymers of α-glucose.
- Amylose is unbranched; amylopectin is branched.
Both are starch polysaccharides made from α-glucose. Amylose has unbranched chains that coil, while amylopectin has branched chains.
Look for these in your work
- Shared monomer
- Correct structural difference
Explain why glycogen is suitable for storage in cells.
Give me a hint
Connect the polymer’s properties to storage and release.
Compare my reasoning
- It stores many glucose units in a relatively insoluble polymer.
- Its many branches give enzymes access to many ends.
Glycogen stores many α-glucose units in a relatively insoluble, compact polymer, limiting the effect on water potential. Its many branches provide multiple ends for enzymes to release glucose units when needed.
Look for these in your work
- Storage property
- Branching linked to mobilisation
A student says amylose has branches because starch is branched. Correct the comparison.
Give me a hint
Starch includes two glucose polymers.
Compare my reasoning
- Amylose is unbranched and coils.
- Amylopectin is the branched component of starch.
Starch contains both amylose and amylopectin. Amylose is unbranched and coiled; amylopectin is branched. The branched structure of amylopectin should not be assigned to amylose.
Look for these in your work
- Starch composition
- Individual structures distinguished
Common mistakes
- Calling all starch unbranched
- Confusing glycogen with cellulose
- Saying branching changes the glucose monomer from α to β
What can you explain now?
Which storage polysaccharide is highly branched?
Compare with the explanation
Glycogen
Glycogen has many branches.
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Make it stick
How do amylose, amylopectin and glycogen differ in branching and storage roles?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.