Question

A plant cell placed in pure water will
A. Expands until the osmotic potential or solutes potential reaches that of water
B. Becomes more turgid until the pressure potential of a cell reaches its osmotic potential
C. Becomes more turgid until the osmotic pressure reaches that of pure water
D. Becomes less turgid until the osmotic pressure reaches that of pure water

Answer 1

Hint: The potential of water molecules to move from hypotonic solution to hypertonic solution across a semipermeable membrane is called osmotic potential. It is negative in plant cells and zeroes in distilled water. The solutes reduce water potential by consuming some of the potential energy available in the water.

Complete answer: Plant cells are enclosed by a rigid cell wall. When the plant cell is placed in a hypotonic solution, it absorbs water by the process of osmosis and begins to swell, however, the cell wall prevents the cell from bursting. As the water gets filled inside, the plant cell then becomes turgid, i.e. swollen and hard. The pressure inside the cell increases until the internal pressure is equal to the pressure outside. This liquid or hydrostatic pressure which holds the cell is called the turgor pressure. This pressure prevents further intake of water. When a plant cell is placed in pure water, it takes up water molecules by osmosis because the water potential of the cell is lower than that of the water surrounding it. However, it does not absorb water until it bursts because it has a strong cellulose cell wall. Once the cell has absorbed as much water as it can, it then becomes turgid. The water inside the present inside the cell starts to exert pressure on the cell wall, called pressure potential. In a plant cell, the cytoplasm contains salts and sugars (solutes) that reduce its water potential. This is because when the solutes are present the free water molecules in cytoplasm decreases. Therefore, all plant cells always have negative water potential.
Hence, the correct answer is option C.

Note: There are two types of cell pressures; turgor pressure and wall pressure. The cell pressure is defined as the pressure exerted by the cell wall on the contents of a cell, whereas the turgor pressure is defined as the pressure exerted by the cell contents on the cell wall. Both the turgor pressure and wall pressure are opposite to each other.