Tonicity and Osmoregulation: Study Guide (Biology 2.7)

Why water moves into and out of cells

In Topics 2.4 to 2.6 you saw that the plasma membrane is selectively permeable. Water crosses it, slowly through the lipid bilayer and quickly through aquaporins. Many solutes, such as ions and sugars, cannot cross freely. So a membrane can separate two solutions that hold different amounts of solute. This gives a concentration gradient.

When solutes cannot cross but water can, water moves instead. The net movement of water across a selectively permeable membrane is osmosis. It is passive: no ATP is used. Water molecules cross in both directions all the time. Osmosis is the net flow, from the side where water is more free to move to the side where it is less free.

Why does adding solute make water “less free”? Dissolved particles attract water molecules into hydration shells (Topic 1.1). Water held around solutes is less able to move across the membrane. So the side with more dissolved particles gains water overall.

Tonicity: always a comparison

Tonicity describes a solution compared with another solution, usually the inside of a cell. It depends on solutes that cannot cross the membrane.

Term	Meaning (solution compared with the cell)	Net water movement
Hypotonic	fewer dissolved solute particles than the cell	into the cell
Isotonic	the same concentration of solute particles	no net movement (equal flow both ways)
Hypertonic	more dissolved solute particles than the cell	out of the cell

A short rule: water moves from hypotonic to hypertonic. In other words, it moves from the region of low solute concentration (low osmolarity, the total concentration of dissolved particles) to the region of high solute concentration.

The words only make sense as a pair. “The solution is hypertonic” means nothing until you say “hypertonic to the cell”. If the solution is hypertonic to the cell, the cell is hypotonic to the solution.

What happens to cells

Animal cells have no wall. In a hypotonic solution they gain water, swell and may burst (lysis). In a hypertonic solution they lose water and shrivel. So animal cells need an isotonic surrounding fluid. This is why fluid given into a patient’s vein is made isotonic with blood plasma (for example, 0.9% sodium chloride).

Plant cells have a cell wall (Topic 2.4). In a hypotonic solution, water enters the large central vacuole. The cell swells until the membrane presses on the wall, and the wall pushes back. The cell becomes turgid. It does not burst, because the wall resists further swelling. Turgid cells hold soft parts of the plant upright. In an isotonic solution the cell is flaccid, and the plant wilts. In a hypertonic solution the cell loses water from its vacuole and the membrane pulls away from the wall. This is plasmolysis, and it usually kills the cell if it lasts.

So the “normal” state differs: isotonic for an animal cell, hypotonic surroundings for a plant cell.

Water potential: one number that predicts the direction

Tonicity words compare solute concentrations. But plant cells also have pressure from the wall. To include both effects, biologists use water potential, symbol Ψ (the Greek letter psi), measured in bars (1 bar is close to atmospheric pressure).

Water moves from higher water potential to lower water potential.

Ψ = Ψp + Ψs

• Ψp, pressure potential. Physical pressure on the water. In a turgid plant cell, the wall pushes back on the contents, so Ψp is positive. For a solution in an open container, Ψp = 0. In xylem under tension, Ψp can be negative.
• Ψs, solute potential (also called osmotic potential). The effect of dissolved solutes. Pure water has Ψs = 0. Adding solute always makes Ψs negative. More solute means more negative Ψs.

Pure water in an open beaker has Ψ = 0 + 0 = 0. Any solution in an open container has Ψ = Ψs, which is below zero. So water moves from pure water into any solution, as you would expect.

Calculating solute potential

Ψs = −iCRT

Symbol	Meaning	Units or value
i	ionization constant: particles formed per formula unit	1 for sucrose and glucose; about 2 for NaCl; about 3 for CaCl₂
C	molar concentration	mol L⁻¹ (M)
R	pressure constant	0.0831 L bar mol⁻¹ K⁻¹
T	temperature	kelvin: °C + 273

The units work out: (mol L⁻¹) × (L bar mol⁻¹ K⁻¹) × K = bar. The minus sign is part of the equation, so Ψs comes out negative. The values of i for salts assume the salt separates completely, which is close enough for dilute solutions.

Worked example 1: will this cell gain or lose water?

Question. A plant cell has Ψs = −7.5 bar and Ψp = +2.0 bar. It is placed in an open beaker at 22 °C containing either (a) 0.20 M sucrose or (b) 0.20 M sodium chloride. Predict the direction of net water movement in each case.

1. Water potential of the cell: Ψ = Ψp + Ψs = +2.0 + (−7.5) = −5.5 bar.
2. Temperature in kelvin: T = 22 + 273 = 295 K.
3. (a) Sucrose (i = 1): Ψs = −(1)(0.20 mol L⁻¹)(0.0831 L bar mol⁻¹ K⁻¹)(295 K) = −4.90 bar. The beaker is open, so Ψp = 0 and Ψ = −4.90 bar.
4. Compare: −4.90 bar is higher (less negative) than −5.5 bar. Water moves into the cell, from higher to lower Ψ. The solution is hypotonic to the cell.
5. (b) Sodium chloride (i = 2): Ψs = −(2)(0.20)(0.0831)(295) = −9.81 bar, so Ψ = −9.81 bar.
6. Compare: −9.81 bar is lower than −5.5 bar. Water moves out of the cell. The solution is hypertonic to the cell.

Check. Same molar concentration, opposite results. The only difference is i: NaCl gives two particles per unit, so it lowers Ψ twice as much.

Interpretation. In (a), water entering the cell pushes the membrane harder against the wall. Ψp rises, so the cell’s Ψ rises, until it equals −4.90 bar. Then net movement stops. The pressure term is what lets a plant cell reach equilibrium without bursting. In (b), the cell loses water, Ψp falls toward zero, and if water loss continues the cell plasmolyses.

Osmoregulation: keeping water balance within limits

Cells need a fairly steady volume and internal solute composition. Enzymes work within limits of concentration, membranes tear if stretched too far, and growth needs water to enter in a controlled way. Growth and homeostasis therefore depend on molecules moving across membranes all the time. Osmoregulation is the control of water balance and internal solute concentration, and so of the organism’s water potential.

• Contractile vacuoles in freshwater protists. A single-celled organism such as Paramecium lives in pond water that is hypotonic to its cytoplasm. Water enters by osmosis all the time. A contractile vacuole collects the extra water and expels it through the membrane. This uses energy. Without it, the cell would swell and burst, because it has no wall.
• The central vacuole in plant cells. The vacuole stores water and solutes. By keeping its Ψs low, it draws water in, keeps the cell turgid and supports the plant. Cell growth also depends on this: water entering the vacuole provides the push that enlarges a young cell.
• Fish (an extension example). A freshwater fish has body fluids hypertonic to the water around it. It gains water across its gills, so it produces large amounts of dilute urine and takes up salts actively. A marine bony fish is hypotonic to seawater, so it loses water, drinks seawater, gets rid of excess salt through its gills and makes little urine. In each case the organism spends energy to keep its internal fluid different from its surroundings.

Worked example 2: graphing osmosis data to find a tissue’s water potential

Question. A student cuts identical cylinders from one carrot root and places three cylinders in each of six sucrose solutions in open dishes at 20 °C. After 24 hours she records the percentage change in mass. All data are fictional.

Sucrose / M	Trial 1 / %	Trial 2 / %	Trial 3 / %
0.0	+17.2	+18.9	+17.9
0.1	+11.0	+12.3	+11.2
0.2	+4.1	+5.6	+5.3
0.3	−1.9	−0.6	−1.1
0.4	−8.4	−7.0	−7.4
0.5	−13.1	−14.6	−14.0

(a) Construct a suitable graph. (b) Estimate the sucrose concentration that is isotonic to the carrot cells. (c) Estimate the water potential of the carrot tissue.

(a) Constructing the graph.

1. Calculate the means: +18.0, +11.5, +5.0, −1.2, −7.6 and −13.9%.
2. Choose a line graph (a scatter plot of means), because both variables are continuous.
3. Independent variable on the x-axis: sucrose concentration / M. Dependent variable on the y-axis: mean percentage change in mass / %. Include the negative values, so the y-axis must cross zero.
4. Use even scales that fill the grid. Plot the means. Show the spread with error bars; here they show the range (lowest to highest trial).
5. Draw a line through the points (a straight best-fit line suits these data).

(b) Isotonic point. The tissue neither gains nor loses mass where the line crosses 0%. Between 0.2 M (+5.0%) and 0.3 M (−1.2%), the change falls by 6.2 percentage points. Zero is 5.0 ÷ 6.2 of the way along: 0.2 + (5.0 ÷ 6.2) × 0.1 = 0.28 M (a best-fit line through all six points gives the same value to 2 s.f.).

(c) Water potential. At the isotonic concentration, the tissue’s Ψ equals the solution’s Ψ. The dish is open, so Ψ = Ψs = −iCRT = −(1)(0.28)(0.0831)(293) = −6.8 bar.

Interpretation. Using percentage change, not mass change, allows for cylinders that start at slightly different masses. The error bars are short and do not overlap, so the trend is clear. The estimate assumes sucrose does not enter the cells, and that 24 hours was long enough to reach equilibrium.

Common misconceptions

• “Water moves to where the concentration is higher.” Only if you mean solute concentration. Water moves from high water potential to low water potential, which is toward the higher solute concentration.
• “Hypertonic describes the cell.” Tonicity is a comparison. Always say “hypertonic to” something.
• “In an isotonic solution water stops moving.” Water keeps crossing both ways at equal rates. There is no net movement.
• “Osmosis moves solutes.” In osmosis, water moves; the solutes that set up the gradient cannot cross.
• “Plant cells burst in pure water.” The wall resists expansion, so the cell becomes turgid. Animal cells and wall-less protists can burst.
• “A turgid plant cell is unhealthy, like a swollen red blood cell.” Turgor is the normal, healthy state for most plant cells. Flaccid and plasmolysed cells are the stressed ones.
• “Solute potential can be positive.” Ψs is zero for pure water and negative for any solution.
• “i does not matter.” Forgetting i = 2 for NaCl halves the solute potential and can reverse your prediction.
• “Use °C in −iCRT.” T must be in kelvin.

Where this leads

Topic 2.8, Mechanisms of Transport, explains the active transport that cells use to build the solute gradients you met here. Water potential returns whenever you study water movement in plants, and osmoregulation returns as an example of homeostasis in animals. Now try the practice questions, then use the revision notes and the checklist to consolidate.