A red blood cell is placed in a solution with a much higher solute concentration than the cell's interior. What happens, and why?
AWater rushes into the cell because solutes in the external solution attract water molecules
BWater leaves the cell, causing it to shrivel, because the external solution has lower water concentration
CSolutes move into the cell until concentrations equalize on both sides
DNothing happens because cell membranes block both water and solutes equally
The external solution is hypertonic (more solutes, less free water). Because the semipermeable cell membrane allows water through but blocks most solutes, water moves passively from where water is more concentrated (inside the cell) to where water is less concentrated (outside). The cell loses water and shrivels — crenation. Option A reverses the direction: water moves from inside (higher water concentration) to outside (lower). Option C is wrong because solutes cannot cross the semipermeable membrane in this scenario. The key insight: water follows its own concentration gradient, not the solute gradient.
Question 2 Multiple Choice
What does 'osmotic pressure' actually measure?
AThe pressure generated by water molecules pushing outward as they rush into a cell
BThe minimum pressure that must be applied to a solution to prevent osmotic water inflow
CThe speed at which water crosses a semipermeable membrane
DThe force exerted by solute molecules colliding with the membrane
Osmotic pressure is not a pushing force — it is a resisting force. Specifically, it is the pressure you would need to apply to the more-concentrated side to prevent water from flowing in by osmosis. A high-solute solution has high osmotic pressure, meaning a large counter-pressure is needed to stop osmosis. This explains turgor pressure in plant cells: the rigid cell wall exerts a counter-pressure equal to the osmotic pressure, preventing further water inflow. Option A describes the common misconception — osmosis is not water 'pushing,' it is a passive flow that can be countered by pressure.
Question 3 True / False
Water moves into a cell placed in a hypotonic solution because the solutes inside the cell attract water molecules.
TTrue
FFalse
Answer: False
Solutes do not attract water in a directional sense that drives osmosis. The correct explanation is that adding solutes to a solution lowers the concentration of water molecules in that solution — there are fewer free water molecules per unit volume. Water, like any substance, moves passively from where it is more concentrated to where it is less concentrated. A cell in a hypotonic solution (fewer external solutes, higher external water concentration) experiences net water inflow because the cell's interior has lower water concentration. The movement follows water's own gradient, not attraction by solutes.
Question 4 True / False
Osmotic pressure depends on the total concentration of solute particles in a solution, not on what type of solutes are present.
TTrue
FFalse
Answer: True
Osmotic pressure is a colligative property — it depends on the number of solute particles per unit volume (molarity), not on their chemical identity. One mole of glucose and one mole of NaCl (which dissociates into two ions) have different osmotic effects: NaCl contributes two particles per formula unit. This is why medical saline solutions are formulated to match the osmolarity of blood — it doesn't matter whether the solute is sodium chloride or glucose, only the total particle count matters. The same principle applies to all colligative properties: boiling point elevation, freezing point depression, vapor pressure lowering.
Question 5 Short Answer
Why does water appear to move 'toward' solutes during osmosis, even though water is a passive transport molecule following its own concentration gradient?
Think about your answer, then reveal below.
Model answer: Solutes displace water molecules — adding solutes to a solution reduces the fraction of the volume occupied by free water. So the side with more solutes has a lower concentration of water, not a higher one. Water following its concentration gradient (from high water concentration to low water concentration) therefore flows toward the side with more solutes. The motion appears counterintuitive because we're used to thinking about solute gradients, but osmosis is simply water doing what every diffusing substance does: moving down its own concentration gradient.
The key reframe is: instead of asking 'where do the solutes pull water?' ask 'where is water more concentrated?' Adding solutes to one side creates a region of lower water concentration, so water flows there. This framing resolves the apparent paradox and connects osmosis to the general principle of passive transport you already understand. Water potential (Ψ) formalizes this: Ψ is always lower where solute concentration is higher, and water flows from higher Ψ to lower Ψ.