AP BiologyUnit 2: Cells2.7
AP Biology · Unit 2 · Topic 2.7

Tonicity and Osmoregulation

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Water moves toward more solute, from higher Ψ to lower Ψ. Isotonic means equal traffic both ways, not zero traffic.

Tonicity is always a comparison between two solutions. It tells you which way water will move: toward the side with more dissolved solute. Osmoregulation is the work living things do, and pay for, to manage that flow.

Key terms

  • Osmosis — diffusion of water across a selectively permeable membrane, toward the side with more solute.
  • Hypertonic — has more solute than the solution you're comparing it with. A cell in a hypertonic bath loses water.
  • Hypotonic — has less solute. A cell in a hypotonic bath gains water.
  • Isotonic — equal solute on both sides. Water still crosses, equally in both directions.
  • Water potential (Ψ) — water's tendency to move. Water flows from higher Ψ to lower Ψ, and adding solute lowers Ψ.
  • Osmoregulation — how an organism holds its water and solute balance steady, usually at an energy cost.

How it works

  1. Compare the solute concentration inside the cell with the solution outside. Water moves by osmosis toward the side with more solute.
  2. An animal cell has no wall. In a hypotonic bath it swells and can burst; in a hypertonic bath it shrivels.
  3. A plant cell has a wall that pushes back. In hypotonic water the cell turns firm with turgor pressure. In hypertonic water the membrane pulls away from the wall: plasmolysis.
  4. Water potential puts numbers on it: Ψ = Ψs + Ψp (solute plus pressure), and Ψs = −iCRT. More dissolved particles make Ψs more negative.
  5. A freshwater protist is hypertonic to its pond, so water leaks in nonstop. A contractile vacuole collects the water and pumps it out, spending ATP.
  6. Desert rodents make urine several times more concentrated than their blood. That removes the same waste in much less water, and it costs ATP.

Why it matters

On the exam, this topic arrives as lab data. With mass-change tables, like potato cores soaked in sucrose, find where the change crosses zero. That bath is isotonic to the cells, so it estimates their solute concentration. With volume curves, read direction, not size: shrinking means the outside is hypertonic, swelling means it's hypotonic, and a flat line means the two match.

Exam traps

TrapActually
In an isotonic solution, water stops moving.Water keeps crossing both ways at equal rates. The net change is zero, so the cell holds its size.
The bath with the biggest mass change matches the cells best.The matching bath is the one with zero change. Look for where the numbers cross from gain to loss.
Tissue got lighter in strong sugar because solute leaked out.Mass change tracks water. The tissue lost water to the more concentrated bath.
0.1 M NaCl and 0.1 M sucrose have the same solute potential.NaCl splits into two ions, so i = 2. Its Ψs is twice as negative.
Worked example

A plant cell with a water potential of −6.0 bars is placed in an open beaker of 0.2 M sucrose at 22 °C. Does the cell gain or lose water?

  1. Sucrose doesn't split apart in water, so i = 1. Convert the temperature to kelvin: 22 + 273 = 295 K.
  2. Solute potential: Ψs = −iCRT = −(1)(0.2 mol/L)(0.0831 L·bar/mol·K)(295 K) ≈ −4.9 bars.
  3. The beaker is open, so Ψp = 0 and the solution's Ψ is −4.9 bars.
  4. Compare: −4.9 bars is higher than −6.0 bars, and water flows from higher Ψ to lower Ψ.

The cell gains water: water moves from the solution (−4.9 bars) into the cell (−6.0 bars).

Water always heads toward the more negative water potential. The near-miss is reading 6.0 as the bigger number and sending water out of the cell. With negative values, the one closer to zero is higher. As water enters, turgor pressure builds and the cell's Ψ rises until the two sides match.

Now try it on real exam-style questions

Ten questions on Tonicity and Osmoregulation, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 2.7 Tonicity and Osmoregulation

What is the term for a plant cell's plasma membrane pulling away from its cell wall as the cell loses water?

  1. A Lysis
  2. B Plasmolysis
  3. C Turgidity
  4. D Hydrolysis
Show the answer

B — Plasmolysis happens when a walled cell loses so much water that the shrinking protoplast separates from the rigid wall. Lysis is the opposite outcome, bursting from too much water entering, and it happens to wall-less cells such as red blood cells.

Question 2 · 2.7 Tonicity and Osmoregulation

In osmosis, net water movement is toward the region that has which property?

  1. A Higher water potential
  2. B Higher pressure potential
  3. C Lower water potential
  4. D Lower solute concentration
Show the answer

C — Water moves from higher to lower water potential, which is why it flows toward the side with more dissolved solute. Choosing lower solute concentration reverses this: solutes pull water toward them, so water leaves the dilute side.

Question 3 · 2.7 Tonicity and Osmoregulation

A cell placed in a hypertonic solution is surrounded by fluid that has what, compared with its cytosol?

  1. A A higher solute concentration
  2. B A lower solute concentration
  3. C A higher concentration of water
  4. D A higher pressure potential
Show the answer

A — Hypertonic means the surrounding fluid has more dissolved solute than the cytosol, so water tends to leave the cell. Reading "hyper" as "more water" is the common slip; a solution with more solute actually has less free water.