Exercise 2.2
Two similar animal cells are placed in two different solutions: y Cell X is placed in pure water. y Cell Y is placed in a concentrated salt solution.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
NCERT’s answer
(iii)
(iii) Water moved into Cell X and moved out of Cell Y through the cell membrane.
Cell X is in pure water. Outside there is more water and less solute than inside, so the surroundings are hypotonic; water enters by osmosis and the cell swells.
Cell Y is in concentrated salt solution. Outside there is less water and more solute, so the surroundings are hypertonic; water leaves by osmosis and the cell shrinks.
One fact explains both: the cell membrane is selectively permeable — water crosses it, salt does not.
(i) is wrong — salt molecules do not move into Cell Y. What leaves is water, and that is why it shrinks.
(ii) is wrong for the same reason: it assumes salt solution entered the cell, so there is nothing for the outgoing water to be "more than".
(iv) is wrong — osmosis is the diffusion of water across a selectively permeable membrane. Solute movement is not what caused it here, and the solute did not move at all.