Plant cells and cell walls — Edexcel International AS Level Biology
Plant cell ultrastructure compared with animal cells, starch and cellulose, and how wall structure gives xylem and fibres their properties.
Revision notes
3 short notes, in the order of the specification. Each one in short:
Plant cells are eukaryotic cells with a cellulose cell wall, chloroplasts, amyloplasts and a large permanent vacuole surrounded by a tonoplast. Neighbouring cells are cemented together by the middle lamella and linked through pits and plasmodesmata. These features separate plant cells from animal cells and can be recognised in electron micrographs.
Starch is a storage polysaccharide made of α-glucose, and cellulose is a structural polysaccharide made of β-glucose. Hydrogen bonds between many parallel cellulose chains form strong microfibrils. Starch is compact and insoluble, so it stores glucose without affecting water potential, while cellulose resists stretching.
Cellulose microfibrils in the cell wall are laid down in layers at different angles, giving strength in all directions. Secondary thickening adds extra cellulose and lignin inside the wall of xylem vessels and sclerenchyma fibres. This makes them rigid, waterproof and strong, properties that humans exploit in fibres and timber.
Exam questions
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Quick check questions
Name the membrane that surrounds the plant vacuole.
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The tonoplast.
State the function of plasmodesmata.
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They are channels through cell walls, lined by the cell surface membrane, that link the cytoplasm of adjacent cells and allow substances and signals to pass between them.
Which type of glucose is cellulose made from?
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β-glucose, with alternate units rotated through 180°, joined by β-1,4 glycosidic bonds.
Why does amylopectin release glucose faster than amylose?
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It is branched, so it has many free ends where enzymes can hydrolyse glycosidic bonds at the same time.
Name the polymer in secondary walls that makes them waterproof and rigid.
Show answer
Lignin.
Frequently asked questions
What is the difference between a plant cell and an animal cell?
Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole surrounded by a tonoplast. They store starch rather than glycogen and have a fixed shape. Animal cells have none of these and have centrioles. Both have a nucleus, mitochondria, ribosomes, endoplasmic reticulum and a cell surface membrane.
What do plasmodesmata and pits do?
Plasmodesmata are narrow channels through the cell wall, lined by the cell surface membrane, that join the cytoplasm of neighbouring cells so substances and signals can pass directly between them. Pits are thin areas of wall without secondary thickening, which let water and dissolved substances move between adjacent cells.
Why is cellulose so strong?
Cellulose is made of straight, unbranched chains of β-glucose, with every other unit rotated through 180°. Many hydrogen bonds form between parallel chains. Each bond is weak, but together they hold chains in microfibrils with very high tensile strength, which are layered in the cell wall.
What is the difference between starch and cellulose?
Starch is an energy store made of α-glucose, coiled (amylose) or branched (amylopectin), and it is insoluble and compact. Cellulose is a structural polysaccharide made of β-glucose in straight chains, with hydrogen bonds between chains forming microfibrils that strengthen the cell wall.
What is secondary thickening in plants?
Secondary thickening is the extra layers of cellulose, usually with lignin, laid down inside the primary wall of cells such as xylem vessels and sclerenchyma fibres. It makes the wall rigid, waterproof and very strong, and the cell contents die, leaving a hollow, supportive cell.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026