01 / 21

Tonicity &
Osmoregulation

How cells manage water balance across selectively permeable membranes: from the physics of osmotic pressure to the biology of survival.

AP Biology

What Is Osmosis?

Osmosis is the net movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration. Water moves down its own concentration gradient: it travels toward the side where water is relatively less abundant.

Understanding Water Potential \( \Psi \)

Water potential (\( \Psi \)) predicts the direction of water flow. It has two components:

\( \Psi_s \) : Solute Potential
Always \( \leq 0 \) in biological systems. More solute means a more negative value. Calculated as \( \Psi_s = -iCRT \).
\( \Psi_p \) : Pressure Potential
Physical pressure on the solution. Positive in turgid plant cells; zero in an open beaker.

The Solute Potential Equation

The solute potential is calculated using the equation:

i
Ionization constant: number of particles the solute dissociates into (e.g., NaCl \( \rightarrow i = 2 \); sucrose \( \rightarrow i = 1 \)).
C
Molar concentration: molarity of the solute in \( \text{mol/L} \).
R
Pressure constant: \( R = 0.0831 \; \text{L} \cdot \text{bar} / \text{mol} \cdot \text{K} \).
T
Temperature: in Kelvin (\( K = \;{}^\circ\text{C} + 273 \)).

Defining Tonicity

Tonicity describes the relative solute concentration of an external solution compared to the cell's internal environment. It determines the direction and extent of water movement across the cell membrane.

Hypotonic
External solution has less solute than the cell. Water enters the cell.
Isotonic
Equal solute concentration. No net water movement.
Hypertonic
External solution has more solute than the cell. Water exits the cell.

Tonicity in Animal Cells

Hypotonic Swells / may lyse
\( \text{H}_2\text{O} \) enters
 Isotonic Normal No net change Hypertonic Shrinks / crenates
\( \text{H}_2\text{O} \) exits

Tonicity in Plant Cells

Hypotonic Turgid (ideal)
\( \text{H}_2\text{O} \) enters
\( \Psi_p \) increases
 Isotonic Flaccid No net change Hypertonic Plasmolyzed
\( \text{H}_2\text{O} \) exits
 Membrane pulls away Cell wall Cell membrane

A plant cell presses against a wall while an animal cell changes shape

Plant wall is thick and distinct from inner membrane; large central vacuole and peripheral cytoplasm visible. Animal cell has flexible boundary and no wall. Separate exact panel: Paired plant states retain the same wall while the inner membrane is pressed outward or retracted; animal membrane has no surrounding wall.For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.

Wall support changes the response

Wall support changes the response. For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.For nonpenetrating solutes, a plant wall limits expansion as water enters. Water loss retracts the protoplast from the wall during plasmolysis; animal cells lack this rigid wall.

Osmolarity vs Tonicity

Osmolarity Tonicity
Measures Total solute concentration Effect on cell volume
Considers All solutes (penetrating + non-penetrating) Only non-penetrating solutes
Predicts Initial direction of water flow Final steady-state cell volume
Example Urea crosses membranes: iso-osmolar but hypotonic NaCl stays outside: iso-osmolar and isotonic
Exact ideal-volume comparison.
Exact ideal-volume comparison.
Equal initial total osmolarity does not imply equal late volume. The late cell in mixed bath B has volume 1.5 V0 and radius cube root of 1.5 times the initial radius, under an ideal model.
The ideal initial cell contains 0.3 Osm nonpenetrating solute. Bath A has 0.3 Osm nonpenetrant; bath B has 0.2 Osm nonpenetrant plus 0.1 Osm freely penetrating solute. In a large bath, neglecting pressure and volume regulation, penetrant equilibration gives a late volume of 1.5 V0 in B while A remains V0. The model assumes survival of this expansion.

What Is Osmoregulation?

Osmoregulation is the active regulation of osmotic pressure in an organism's body fluids to maintain homeostasis of water content. Organisms must balance water gain and loss to keep their cells functioning properly.

Osmoconformers
Match their internal osmolarity to the environment. Most marine invertebrates. Low energy cost but limited habitat range.
Osmoregulators
Maintain internal osmolarity regardless of surroundings. Most vertebrates, freshwater fish, terrestrial organisms. Higher energy cost.

Strategies for Water Balance

Freshwater Fish Hypertonic body
\( \text{H}_2\text{O} \) in
 Strategies: 1. Dilute urine (lots of it) 2. Active uptake of salts via gills 3. Rarely drinks water Saltwater Fish Hypotonic body
\( \text{H}_2\text{O} \) out
 Strategies: 1. Drinks seawater constantly 2. Excretes salt via gill chloride cells 3. Concentrated, small urine volume

A fish balances water and ions through different routes

Distinct freshwater and marine fish have gill-only windows; an authored route comparison separates water movement,gill ions,drinking and urine.Representative freshwater bony fish gain water, actively take up salts and produce large volumes of dilute urine. Marine bony fish lose water, drink seawater and secrete excess salts at their gills.

Two environments, different net challenges

Two environments, different net challenges. Representative freshwater bony fish gain water, actively take up salts and produce large volumes of dilute urine. Marine bony fish lose water, drink seawater and secrete excess salts at their gills.Representative freshwater bony fish gain water, actively take up salts and produce large volumes of dilute urine. Marine bony fish lose water, drink seawater and secrete excess salts at their gills.

Solving Water Potential Problems

1
Identify the solute: Determine \( i \) (ionization constant). Sucrose: \( i = 1 \). NaCl: \( i = 2 \). \( \text{CaCl}_2 \): \( i = 3 \).
2
Calculate \( \Psi_s \): Use \( \Psi_s = -iCRT \). Convert temperature to Kelvin first.
3
Determine \( \Psi_p \): In an open container, \( \Psi_p = 0 \). In a plant cell at equilibrium, \( \Psi_p \) may be positive.
4
Sum for \( \Psi \): \( \Psi = \Psi_s + \Psi_p \).
5
Predict water flow: Water moves from higher \( \Psi \) to lower \( \Psi \).

Explain why zero mass change estimates a tissue water-potential condition.

Use comparable potato cores, the same 24-hour immersion at 22 °C, and at least three replicates per sucrose concentration: 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 M. Blot consistently before final weighing. Percent mass change is 100(final−initial)/initial. An interpolated zero mass change estimates tissue water potential under these conditions; it does not prove every cell is isotonic. No experimental results are supplied.

Generated illustrative potato-core balance and immersion stages accompanies a four-step protocol: weigh initially, immerse, blot consistently, and weigh again. Percent mass change divides the mass difference by initial mass.
Use comparable potato cores, the same 24-hour immersion at 22 °C, and at least three replicates per sucrose concentration: 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 M. Blot consistently before final weighing. Percent mass change is 100(final−initial)/initial. An interpolated zero mass change estimates tissue water potential under these conditions; it does not prove every cell is isotonic. No experimental results are supplied.
Conditions and interpretation.
Conditions and interpretation.

Example 1: Open Beaker

A 0.5 M sucrose solution sits in an open beaker at \( 25^\circ\text{C} \). What is the water potential?

Example 2: Plant Cell at Equilibrium

A plant cell is placed in a 0.3 M NaCl solution at \( 22^\circ\text{C} \). At equilibrium, the cell's \( \Psi_p = 3.5 \; \text{bars} \). Find the cell's \( \Psi \).

Example 3: Predicting Water Flow

Side A has \( \Psi = -5 \; \text{bars} \). Side B has \( \Psi = -12 \; \text{bars} \). A selectively permeable membrane separates them. Which way does water flow?

Side A
\( \Psi = -5 \) bars
Higher \( \Psi \)
 Membrane Side B
\( \Psi = -12 \) bars
Lower \( \Psi \)
\( \text{H}_2\text{O} \)

Common Mistakes

✕ Confusing osmolarity with tonicity
A penetrating solute (like urea) contributes to osmolarity but not to tonicity. Always ask: can the solute cross the membrane?
✕ Forgetting the ionization constant
NaCl yields 2 particles, not 1. Neglecting \( i \) halves your calculated \( \Psi_s \), giving wrong predictions.
✕ Saying water flows toward lower concentration
Water flows toward lower water potential (more negative \( \Psi \)), which is the side with higher solute concentration. Don't reverse the direction.

Summary Table

Hypotonic Isotonic Hypertonic
Solute outside Less than cell Equal to cell More than cell
Water moves Into cell No net flow Out of cell
Animal cell Lysis Normal Crenation
Plant cell Turgid Flaccid Plasmolyzed
\( \Psi \) comparison \( \Psi_{\text{out}} > \Psi_{\text{cell}} \) \( \Psi_{\text{out}} = \Psi_{\text{cell}} \) \( \Psi_{\text{out}} < \Psi_{\text{cell}} \)

Question 1 of 5

A red blood cell is placed in a hypertonic solution. What will happen to the cell?

Question 2 of 5

What is the solute potential of a 0.2 M NaCl solution at \( 20^\circ\text{C} \)?
(\( R = 0.0831 \; \text{L} \cdot \text{bar/mol} \cdot \text{K} \))

Question 3 of 5

A plant cell with \( \Psi_s = -8 \; \text{bars} \) and \( \Psi_p = 3 \; \text{bars} \) is placed in pure water (\( \Psi = 0 \)). What will happen?

Question 4 of 5

A freshwater fish constantly faces the problem of water entering its body by osmosis. Which adaptation helps it maintain water balance?

Question 5 of 5

Two solutions are separated by a selectively permeable membrane. Solution X contains 0.4 M urea (which freely crosses membranes). Solution Y contains 0.4 M sucrose (which cannot cross). Which statement is correct?