International A Level Biology / Plant structure and function

Cellulose and the plant cell wall

Edexcel International A Level Biology (YBI11), chapter 10: Plant structure and function, specification point 4.3. Free lesson.

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Cellulose is a structural polysaccharide made from β-glucose. Its straight chains bond into strong microfibrils, which help plant cell walls resist bursting while remaining permeable to water and ions.

How α-glucose differs from β-glucose

Both molecules contain the same atoms, but they are arranged slightly differently. In α-glucose, the OH group on carbon 1 points below the ring. In β-glucose, it points above the ring. This small difference affects how the glucose units join and the shape of the resulting polysaccharide. Starch and glycogen are made from α-glucose. Cellulose is made from β-glucose.

How β-glucose forms straight cellulose chains

Thousands of β-glucose units join by condensation to make each cellulose molecule. The units are joined from carbon 1 to carbon 4 by 1,4 glycosidic bonds. Because the OH group on carbon 1 points up while the one on carbon 4 points down, every other glucose unit is flipped. This arrangement keeps the chain straight. Cellulose has no 1,6 bonds, so its chains do not branch. Flipping alternate units places OH groups on both sides of the chain. These groups form hydrogen bonds with neighbouring cellulose chains. Each bond is weak, but thousands of bonds hold the chains firmly alongside…

How cellulose builds up into a cell wall

Cellulose chains lie parallel because they are straight and do not coil. Many chains held together by hydrogen bonds form a microfibril, a bundle with high tensile strength. Microfibrils bundle into larger fibres. In the cell wall, layers of fibres lie in criss-cross directions. A matrix of hemicelluloses and pectins glues the fibres together, helping the wall resist the pressure of water entering the cell. The wall is fully permeable. Water and dissolved ions pass through gaps between the microfibrils.

How cellulose differs from starch

Both starch and cellulose are polysaccharides of glucose, but their different glucose forms give them different structures and roles. Amylose chains coil into a helix, while amylopectin has branches, so starch is compact. Cellulose stays straight and unbranched, with chains bonded into microfibrils. Starch uses α-glucose and has 1,4 bonds, plus 1,6 bonds at branches in amylopectin. Cellulose uses β-glucose and has 1,4 bonds only, with alternate units flipped. Starch molecules remain separate and pack into grains. Cellulose chains bond into microfibrils. Starch is an energy store. It can…

How to explain cellulose's strength

For a four-mark explanation, build the link from glucose units to the role of the wall. Each structural level adds strength, so the cell wall can resist the pressure caused by water entering the cell.

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