AP Biology · Unit 2: Cell Structure & Function

2.8 Tonicity & Osmoregulation

Tonicity: which way water moves

Water moves by osmosis toward the side with more solute. Hypertonic = more solute outside → cell loses water and shrinks. Hypotonic = less solute outside → cell gains water and swells (an animal cell may burst). Isotonic = equal solute → no net water movement.

Osmoregulation

Osmoregulation is how organisms control their water and solute balance. Plant cells have a rigid cell wall, so in a hypotonic solution they become firm (turgid) instead of bursting. Some single-celled organisms use contractile vacuoles to pump out excess water.

Key terms

Hypertonic: More solute outside the cell; water leaves and the cell shrinks.
Hypotonic: Less solute outside the cell; water enters and the cell swells (may burst).
Isotonic: Equal solute concentrations; no net movement of water.
Osmoregulation: The control of water and solute balance in an organism.
Turgor pressure: The firmness of a plant cell when water fills it against the cell wall.

Try an AP-style question

Data

Potato cores were soaked in sucrose solutions for 24 hours. Percent change in mass — 0.0 M +12%, 0.2 M +4%, 0.4 M −3%, 0.6 M −9%.

The potato cells' solute concentration is closest to…

0.3 M
0.0 M
0.6 M
0.5 M

Show the answer

A. No net mass change occurs where the solution is isotonic to the cells. The mass change crosses zero between 0.2 M (+4%) and 0.4 M (−3%), near 0.3 M.

Practice 8 AP-style questions with instant explanations, flashcards and spaced review.

Study 2.8 free in APrep