Although it is not easy to see, a plant cell also has a cell surface membrane just like an animal cell. The cell membrane is partially permeable. A plant cell in pure water will take in water by osmosis through its partially permeable cell membrane in the same way as an animal cell. As the water goes in, the cytoplasm and vacuole will swell. However, the plant cell has a very strong cell wall around it. The cell wall is much stronger than the cell membrane and it stops the plant cell from bursting. The cytoplasm presses out against the cell wall, but the wall resists and presses back on the contents. A plant cell in this state is rather like a blown-up S tyre – tight and firm. It is said to be turgid. The turgidity of its cells helps a plant that has no wood in it to stay upright, and keeps the leaves firm. Plant cells are usually turgid. Figure 3.8 and Figure 3.9 illustrate a plant cell in a concentrated solution. Like the animal cell in Figure 3.6, it will lose water by osmosis. The cytoplasm shrinks, and stops pushing outwards on the cell wall. Like a tyre when some of the air has leaked out, the cell becomes floppy. It is said to be flaccid. If the cells in a plant become flaccid, the plant loses its firmness and begins to wilt.
Figure 3.8 These onion cells have been placed in a concentrated solution. The cytoplasm has shrunk inwards, leaving big gaps between itself and the cell walls (Ă— 300).
Activity 3.4
Investigate and describe the effects on plant tissue of immersing them in different solutions
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Cambridge IGCSE Biology
space X cell wall
cell membrane
concentrated solution outside the cell less concentrated solution inside the cell
Osmosis takes place. Water diffuses out of the cytoplasm and vacuole through the partially permeable cell membrane. First, the cell shrinks slightly and becomes flaccid. Then the cell membrane pulls away from the cell wall, and the cell is plasmolysed. Figure 3.9 Plant cells become flaccid and may plasmolyse in a concentrated solution.
If the solution is very concentrated, then a lot of water will diffuse out of the cell. The cytoplasm and vacuole go on shrinking. The cell wall, though, is too stiff to be able to shrink much. As the cytoplasm shrinks further and further into the centre of the cell, the cell wall gets left behind. The cell membrane, surrounding the cytoplasm, tears away from the cell wall. A cell like this is said to be plasmolysed. This does not normally happen because plant cells are not usually surrounded by very concentrated solutions. However, you can make cells become plasmolysed if you do Activity 3.4. Plasmolysis usually kills a plant cell because the cell membrane is damaged as it tears away from the cell wall.
Questions 3.8 3.9 3.10
What happens to an animal cell in pure water? Explain why this does not happen to a plant cell in pure water. Which part of a plant cell is: a fully permeable? b partially permeable?
Original material Š Cambridge University Press 2014
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