Cellulose at Higher Level: The same statement as SL. HL papers use cellulose inside longer answers comparing alpha- and beta-glucose polymers, so be ready to set its structure against starch and glycogen point by point.
Practise this as you read
- Identify cellulose and its monomer from a structure diagram.
- Explain how its structure gives the cell wall strength.
- Compare it with starch: monomer, shape, function.
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The big idea: Cellulose is made of beta-glucose. Each unit is turned over compared with its neighbour, so the chain is straight.
The points to remember
- Cellulose is a polymer of beta-glucose, joined by condensation (1,4 links).
- Each beta-glucose is turned over compared with the one before: the units alternate.
- So the chain stays straight, with no branches and no coiling.
Remember it as: Beta: flip, flip, flip, so the chain stays straight.
Real example: cattle and grass: Grass is mostly cellulose. Cattle have no enzyme to digest it either: microbes in their rumen do it for them, which is why cattle can live on grass.
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The big idea: Straight chains can lie close together and hydrogen-bond into bundles. That makes cellulose strong enough to build cell walls.
The points to remember
- Straight chains lie side by side and pack closely.
- Hydrogen bonds form between neighbouring chains, cross-linking them.
- Many chains make a bundle (a microfibril): a fibre with high tensile strength.
- In the cell wall this strength stops a plant cell bursting when water enters by osmosis.
- Humans have no enzyme to break its links, so cellulose passes through us as fibre.
Starch (a store)
- Alpha-glucose
- Coiled and branched
- Compact, glucose released easily
Cellulose (a structure)
- Beta-glucose, alternate units turned over
- Straight, unbranched chains
- Bundles held by hydrogen bonds: strong
Remember it as: Straight, side by side, hydrogen-bonded: strong.
One trap when you explain its function:
Say what the strength does: "It has a structural function" on its own is not enough. Say it gives the cell wall strength and stops the cell bursting.
How it is asked: Multiple-choice items on the structure or the function of cellulose, and a structure diagram with two parts: identify the monomer, then explain how the structure suits the function.
The structure of part of a cellulose molecule is shown. Identify the monomer from which cellulose is composed.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Then the three-mark part:
Explain how the structure of cellulose is related to its function.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.