2.7Cells

Tonicity and Osmoregulation — AP BIOLOGY Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 12 1 quiz

0%read

Lesson summary

2.7 Tonicity and Osmoregulation — Notes — AP Biology notes: Explain how concentration gradients affect the movement of molecules across membranes and how organisms regulate water balance.Relative — the tonicity of a cell's surroundings is always described relative to the cell con…

Water potential
Into the cell
Out of the cell
Solute potential (Ψs)
Pressure potential (Ψp)
Calculating Ψs

Section 1 of 8Tonicity

Relative

Relative the tonicity of a cell's surroundings is always described relative to the cell contents

Types

Hypotonic solution — a lower solute concentration than the cytoplasm.
Hypertonic solution — a higher solute concentration than the cytoplasm.
Isotonic solution — an equal solute concentration, so there is no net water movement.

Water potential

Water potential the tendency of water molecules to move from one place to another, and units are bars or MPa

Section 2 of 8Osmosis

Osmosis

Osmosis the movement of water molecules from a region of high water potential (a hypotonic solution) to a region of low water potential (a hypertonic solution)

Into the cell

Into the cell water enters a cell in a hypotonic solution, which has a higher water potential and a lower solute concentration

Out of the cell

Out of the cell water leaves a cell in a hypertonic solution, which has a lower water potential and a higher solute concentration

Solution	Animal cell	Plant cell	Water movement
Hypotonic	Swells and may lyse	Turgid (normal, healthy)	Into the cell
Isotonic	Normal	Flaccid	No net movement
Hypertonic	Shrivels (crenation)	Plasmolysed, as the membrane pulls away from the wall	Out of the cell

Section 3 of 8Water potential and solute potential

Equation

Ψ = Ψp + Ψs

Solute potential (Ψs)

Solute potential (Ψs) the effect of solutes on water potential
Pure water has Ψs = 0.
Adding solute makes Ψs more negative, and with constant Ψp a lower Ψs lowers Ψ.
Solute molecules bind water by hydrogen bonds as they dissolve.

Pressure potential (Ψp)

Pressure potential (Ψp) the hydrostatic pressure on the water
Most biological systems are at atmospheric pressure, so Ψp = 0 and Ψ is set by Ψs alone.
Inside plant cells Ψp is usually positive because the cytoplasm presses on the wall.
Ψp can be negative in xylem vessels where water is under tension.

Calculating Ψs

Ψs = −iCRT
i = the number of ions a solute forms (1 for sucrose and glucose, 2 for NaCl).
C = molar concentration in mol/L.
R = 0.0831 L·bar/(mol·K).
T = temperature in kelvin (°C + 273).

Direction

Direction water moves to the lower water potential

Section 4 of 8Osmoregulation

Osmoregulation

Osmoregulation control of internal water and solute balance, an example of homeostasis, for example between red blood cells and plasma

Harm from failure

Red blood cells take in too much water and burst if the water potential of the plasma rises.
Plant cells lose water in drought, Ψp falls and the plant wilts and may die.

Paramecium

Paramecium freshwater protists take in water by osmosis because their surroundings are hypotonic, and a contractile vacuole pumps the excess out

Plant central vacuole

Plant central vacuole it stores water to maintain pressure potential and structural support,
- and ion channels in its membrane regulate cytoplasmic ion concentration

Animals

Animals freshwater fish excrete dilute urine and actively take up salts, and the kidneys adjust water reabsorption

Section 5 of 8Finding the water potential of tissue

Cut equal potato cores and record their initial masses.

Place the cores in sucrose solutions from 0.0 to 1.0 M for 24 hours.

Blot dry and record the final masses, then calculate the percentage change.

Plot percentage change in mass against molarity.

Read the x-intercept, where there is zero change, because that concentration is isotonic with the tissue.

Calculate Ψs = −iCRT for that concentration, which equals the water potential of the tissue.

Section 6 of 8More from the specification

Hydrophilic

Water-loving; polar or charged and able to dissolve in water.

Facilitated diffusion

Passive movement through transport proteins.

Passive transport

Movement down a gradient without energy input.

Carrier protein

A membrane protein that binds specific molecules or ions and changes shape to move them across.

Phospholipid bilayer

Two layers of phospholipids with hydrophobic tails inwards

Protein

A long chain of amino acids containing carbon, hydrogen, oxygen and nitrogen.

Phospholipid

Glycerol, two fatty acids and a phosphate group, which is amphipathic.

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient,
- as a result of random movement.

Exocytosis

Release of materials from vesicles at the cell surface.

Endocytosis

Bulk transport into a cell using a membrane vesicle.

Hydrophobic

Water-hating; non-polar and unable to dissolve in water.

Permeability

How easily substances cross a membrane.

Section 7 of 8Key terms

Term	Meaning
Tonicity	The ability of a solution to change the water content of a cell.
Hypertonic	Higher solute concentration outside; the cell loses water.
Hypotonic	Lower solute concentration outside; the cell gains water.
Isotonic	Equal solute concentration; no net movement.
Water potential (Ψ)	Ψ = Ψp + Ψs; water moves to lower Ψ.
Solute potential	Ψs = −iCRT.
Osmoregulation	Control of water and solute balance.
Contractile vacuole	Organelle that expels excess water in freshwater protists.
Pressure potential (Ψp)	Hydrostatic pressure on the water.
Osmosis	Movement of water to the lower water potential.

Section 8 of 8Exam technique

Saying water moves towards the lower solute concentration — it moves to the lower water potential (higher solute concentration).

Forgetting that Ψs is negative.

Confusing plasmolysis (plant cell in hypertonic solution) with lysis (animal cell in hypotonic solution).

Assuming isotonic means no water movement at all — there is no net movement.

Using °C instead of kelvin in Ψs = −iCRT.

Last updated: 18 October 2026