Plant cells, cellulose and storage organelles

What you’ll learn

• Relate plant-cell structures to their functions and compare them with animal cells.
• Explain how cellulose differs from starch.
• Recognise cell-wall, vacuolar and plastid features in micrographs.

Understand the idea

Plant cells share a nucleus, mitochondria, ribosomes, ER and Golgi with animal cells, but typically also have a cellulose wall and a large central vacuole. The vacuole contains cell sap and is enclosed by the tonoplast; water uptake contributes to turgor against the wall. The middle lamella, rich in pectin, helps bind adjacent cells. Plasmodesmata are cytoplasmic connections across walls, while pits are regions where secondary wall thickening is absent or reduced, allowing exchange between neighbouring cells.

Chloroplasts contain an envelope, stroma and internal membranes forming thylakoids, often stacked as grana; their pigments and enzymes support photosynthesis. Amyloplasts are non-photosynthetic plastids that store starch, for example in storage tissues. Not every plant cell contains chloroplasts: a root storage cell may contain amyloplasts instead. An animal cell lacks a cellulose wall and chloroplasts, though it can contain small vesicles and vacuoles.

Cellulose is a polymer of $\beta$-glucose joined by $\beta$-1,4 glycosidic bonds, with alternate glucose units inverted. Straight chains lie alongside one another, and hydrogen bonding between chains forms strong microfibrils. Starch uses $\alpha$-glucose and includes coiled amylose and branched amylopectin, making it suitable for storage. The glycosidic bonds along a cellulose chain and hydrogen bonds between chains play different roles. Microfibril orientation and additional wall materials affect the direction and strength of resistance to stretching.

Worked example

An electron micrograph of a root storage cell shows a cell wall, a large vacuole and plastids filled with dense starch grains, but no stacks of photosynthetic membranes. Identify the plastids and explain the evidence.

1. The cell wall and large vacuole support identification as a plant cell.
2. The plastids contain stored starch rather than visible thylakoid stacks.
3. In a non-photosynthetic storage tissue, these features are consistent with amyloplasts.

Answer: Amyloplasts, specialised for starch storage.