Cell Structure: Question 10
Syllabus 1.2
A researcher places a sample of identical human red blood cells and a sample of identical onion epidermal cells into separate beakers of pure water. After several hours, the red blood cells have burst, but the onion cells remain intact and firm.
(a) Name the structure present in the onion cell but absent from the red blood cell that prevents the onion cell from bursting. [1]
(b) Describe two properties of this structure that allow it to prevent the onion cell from bursting while still permitting water to enter the cell. [2]
(c) State two functions of this structure other than preventing the cell from bursting. [2]
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Worked solution
Part (a): Identifying the structure
The structure present in the onion epidermal cell but absent from the red blood cell is the cell wall. Both cells have a cell surface membrane, but only the plant cell has an additional rigid layer, made of cellulose, surrounding this membrane.
Part (b): How the cell wall prevents bursting
The cell wall has two properties that together explain why the onion cell survives in water while the red blood cell bursts:
- It is freely permeable: it does not stop water, or dissolved solutes, passing through it. Water can still reach the cell surface membrane and enter the cell by osmosis, exactly as it does in the red blood cell.
- It is built from cellulose microfibrils, arranged to give the wall considerable mechanical (tensile) strength. As water enters and the cytoplasm and cell surface membrane push outward against the wall, the wall resists this outward pressure instead of stretching indefinitely.
Because the wall resists further expansion, an internal pressure called turgor pressure builds up inside the cell. Once turgor pressure rises enough to balance the tendency of water to keep entering, net water movement stops and the cell becomes firm (turgid) rather than continuing to swell and burst. The red blood cell has no such wall, so its cell surface membrane alone cannot resist the pressure of the entering water, and it bursts.
Part (c): Other functions of the cell wall
Beyond preventing the cell from bursting, the cell wall also:
- Provides mechanical support, both to the individual cell and, when many turgid, walled cells are packed together, to the whole plant. This is a major reason herbaceous (non-woody) plant tissues and organs stay upright and rigid.
- Maintains a fixed cell shape, giving the onion cell a definite, regular outline, unlike an animal cell such as a red blood cell, whose shape depends only on its flexible cell surface membrane and cytoplasm.
(A further valid function is allowing water and dissolved substances to move between adjacent cell walls, contributing to movement of water through plant tissue.)
Final answers
- (a) The cell wall.
- (b) It is freely permeable to water, so it does not block water entry; its cellulose microfibrils give it high mechanical strength, so it resists outward pressure and allows turgor pressure to build up until the cell becomes turgid rather than bursting.
- (c) It provides mechanical/structural support to the cell and the whole plant, and it maintains a fixed cell shape.