Question

A cell will swell up if
A) The concentration of water molecules in the cell is higher than the concentration of water molecules in the surrounding medium.
B) The concentration of water molecules in the surrounding medium is higher than water molecules concentration in the cell.
C) The concentration of water molecules is the same in the cell and in the surrounding medium.
D) Concentration of water molecules does not matter.

Answer 1

Pure water has maximum number of free and active water molecules, that means it has maximum water potential. Addition of any soluble substance reduces the number of free and active water molecules, thus, decreases its potential. That is why a weak solution or hypotonic solution has more water potential as compared to a strong solution or hypertonic solution.

Complete Answer:
A) When two solutions are separated by means of a semipermeable membrane the molecules of water (or solvent) move from the region of higher potential (pure solvent or comparatively weak solution) to the region of lower potential (more concentrated solution). This movement of water (or solvent) is referred to as osmosis.

B) The plant cell has a cell wall which is permeable both to solvent and to solute molecules. On the other hand, the plasma membrane of plant cells as well as animal cells is a selectively permeable membrane. It means it allows the passage of only selected molecules like water and gases through it and does not allow the passage of ions and macromolecules.

C) When a cell is placed in a fluid, there are two regions – one inside the cell and one outside the cell i.e. the surrounding region, and these two regions are divided by plasma membrane.
If the cell is placed in a hypotonic solution, the water potential of the surrounding medium will be higher as compared to the water potential inside the cell. Thus, water will enter into the cell through osmosis and the cell will become swollen. This is referred to as endosmosis.
If the cell is placed in a hypertonic solution the direction of movement of water is reverse i.e., from the cell to the surrounding medium. In this case the cell will shrink. This osmosis is termed as exosmosis.

D) If the cell is placed in an isotonic solution (water potential inside the cell and of the surrounding medium will be the same), water will diffuse in and out of the cell and there will be no change in the shape of the cell.

Thus, the correct answer is B, i.e., the cell will swell up when the water potential or the concentration of water is higher in the surrounding medium than the water potential or the concentration of water in the cell.

Note: The plant cells possess a rigid cell wall surrounding the plasma membrane, thus, due to endosmosis they become swollen and turgid. Whereas, the animal cells as a result of endosmosis first swell up and finally burst and this is called osmotic lysis.