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The key difference
All three are polysaccharides made from glucose, but they serve different functions: starch stores energy in plants, glycogen stores energy in animals, and cellulose provides structural support in plant cell walls.
Starch, glycogen, and cellulose are all polymers of glucose, yet they have very different properties and functions. Understanding why glucose is assembled into different polysaccharides for different purposes is a key concept in biological molecules.
Feature
Starch
Glycogen
Cellulose
Found in
Plants
Animals (mainly liver and muscle cells)
Plants
Function
Energy storage
Energy storage
Structural: forms plant cell walls
Monomer
α-glucose
α-glucose
β-glucose
Structure
Coiled / helical chains (amylose) and branched chains (amylopectin)
Highly branched chains
Straight, unbranched chains that form strong fibres
Solubility
Insoluble: does not affect water potential in cells
Insoluble
Insoluble: forms rigid fibres
Branching
Some branching (amylopectin)
Very highly branched: allows rapid release of glucose
No branching: straight chains bond together into fibres
Digestible by humans?
Yes: amylase breaks starch into maltose
Yes: broken down when blood glucose is low
No: humans lack the enzyme (cellulase) to digest cellulose; it acts as dietary fibre
Why Storage Molecules Are Insoluble
Both starch and glycogen are insoluble in water. This is essential for their storage function. If they were soluble, they would dissolve in the cytoplasm and affect the water potential of the cell, drawing in water by osmosis and potentially causing the cell to burst. Being insoluble means large amounts of energy can be stored without disrupting osmotic balance.
Why Glycogen Is Highly Branched
Glycogen is more highly branched than starch. This means it has many free ends where glucose molecules can be rapidly removed. Animals need to mobilise glucose quickly (e.g. During exercise), so the branched structure allows many glucose molecules to be released simultaneously from different branch points. This is faster than removing glucose from a single chain.
Cellulose and Dietary Fibre
Humans cannot digest cellulose because we do not produce the enzyme cellulase. However, cellulose is important in the diet as dietary fibre (roughage). It adds bulk to food, stimulates peristalsis in the intestine, and helps prevent constipation. Herbivores like cows have bacteria in their gut that produce cellulase, allowing them to break down cellulose and extract energy from grass.
Try a comparison question (3 marks)
Explain why starch is a suitable storage molecule in plant cells. [3]
Show the answer and marking guidance
Starch is insoluble, so it does not affect the water potential of the cell / does not cause water to enter by osmosis
Starch is compact / can be stored in large quantities in a small space (e.g. In starch grains)
Starch can be easily hydrolysed / broken down by amylase to release glucose when energy is needed
Original Cambridge-style practice written for this site.
Common comparison mistakes
When answering a compare question, write matched pairs rather than two separate descriptions.
Saying cellulose stores energy: it provides structural support in cell walls, not energy storage
Confusing starch and glycogen: starch is in plants, glycogen is in animals
Claiming humans can digest cellulose: humans lack cellulase; cellulose passes through as fibre
Forgetting to explain WHY insolubility is important (osmotic effect): just saying "insoluble" without linking to function loses marks