A red blood cell is placed in a solution with a very high salt concentration. What happens to the cell?
AIt swells and may burst (lyse) as water rushes in
BIt shrinks (crenates) as water moves out of the cell
CNothing changes — the cell membrane blocks salt from affecting it
DIt divides rapidly to restore osmotic balance
A high-salt solution is hypertonic relative to the cell interior. The solute concentration outside exceeds that inside, so water moves by osmosis out of the cell toward the higher external solute concentration. This causes the cell to shrink and wrinkle (crenate). Lysis (option A) is the opposite scenario — it occurs in hypotonic solutions when water rushes into the cell.
Question 2 True / False
Two solutions with identical total solute concentrations usually have identical tonicity effects on a cell.
TTrue
FFalse
Answer: False
Tonicity depends not only on solute concentration but also on membrane permeability to each solute. A solute that freely crosses the membrane (like urea) equilibrates across it and therefore does not sustain an osmotic gradient — it contributes nothing to effective tonicity. A solute that cannot cross (like NaCl in most contexts) maintains the gradient and drives net water movement. Two solutions with equal total solute concentrations can differ dramatically in tonicity if their solutes have different permeabilities.
Question 3 Short Answer
A cell has an internal solute concentration of 0.3 M. It is placed in a 0.1 M external solution. In which direction will water move, and what term describes the external solution relative to the cell?
Think about your answer, then reveal below.
Model answer: Water moves into the cell. The external solution is hypotonic relative to the cell — it has a lower solute concentration, so the cell has a lower water potential, drawing water inward by osmosis.
Osmosis always moves water toward higher solute concentration (lower water potential). Here the cell interior has more solutes, so it 'pulls' water inward. The external solution is hypotonic, meaning it has less solute than the cell. This can cause the cell to swell and, if unchecked, lyse.