Tonicity & Osmoregulation

Osmosis Review

Osmosis is a fundamental process in biology, describing the movement of water across a selectively permeable membrane. It is driven by differences in solute concentration on either side of the membrane.

• Osmosis: The diffusion of water from an area of lower solute concentration to an area of higher solute concentration through a semipermeable membrane.

• Example: In the provided diagram, water moves from the left side (lower solute concentration) to the right side (higher solute concentration), equalizing the concentrations.

Water Balance

Cells must maintain water balance to survive. The ability of an extracellular solution to cause a cell to gain or lose water is called tonicity. This depends on the concentration of solutes that cannot pass through the cell membrane.

• Tonicity: The effect of a solution on cell volume due to osmosis.

• Cells can be in three types of solutions:

  • Isotonic

  • Hypertonic

  • Hypotonic

• Osmoregulation: The process by which cells regulate solute concentrations and maintain water balance.

• Animal cells react differently than cells with cell walls (plants, fungi, some protists).

Tonicity: Isotonic Solutions

In an isotonic solution, the concentration of nonpenetrating solutes is equal inside and outside the cell. There is no net movement of water.

• Isotonic Solution: No net water movement; water diffuses into and out of the cell at equal rates.

• Animal Cells: Remain normal in shape.

• Plant Cells: No net change in turgor pressure; cells are flaccid.

Tonicity: Hypertonic Solutions

Cells in a hypertonic solution lose water to their surroundings because the concentration of nonpenetrating solutes is higher outside the cell.

• Hypertonic Solution: Water moves out of the cell.

• Animal Cells: Shrivel and may die due to dehydration.

• Plant Cells: Undergo plasmolysis—the vacuole shrinks and the plasma membrane pulls away from the cell wall.

Tonicity: Hypotonic Solutions

Cells in a hypotonic solution gain water because the concentration of nonpenetrating solutes is lower outside the cell.

• Hypotonic Solution: Water moves into the cell.

• Animal Cells: Swell and may lyse (burst).

• Plant Cells: Become turgid (firm), which is optimal for structural support due to turgor pressure.

Osmoregulation in Aquatic Animals: Saltwater Fish & Hypertonic Environments

Saltwater fish live in hypertonic environments (seawater), where water tends to leave their bodies by osmosis. They have specialized adaptations to maintain water and salt balance.

• Drinking seawater: Saltwater fish drink seawater to replace water lost by osmosis.

• Excreting excess salt: Specialized cells in their gills actively pump out excess salt taken in from seawater.

• Producing concentrated urine: Their kidneys excrete small amounts of concentrated urine to conserve water while removing salts.

• Relatively impermeable skin: Limits water loss directly through the body surface.

Water Potential

Water potential (Ψ) is a physical property that predicts the direction water will flow. It includes the effects of solute concentration and physical pressure.

• Water Potential (Ψ): Water moves from areas of high water potential to areas of low water potential.

• Solute Potential (Ψs): Adding solutes lowers water potential (expressed as a negative value).

• Pressure Potential (Ψp): Physical pressure increases water potential (can be positive or negative).

• Formula:

• Measured in pressure units: megapascals (MPa) or bars.

• Pure water in open air: MPa.

Calculating Solute Potential

Solute potential can be calculated using the following formula:

• i: Ionization constant (number of particles formed in solution; e.g., NaCl = 2, sucrose = 1)

• C: Molar concentration (mol/L)

• R: Pressure constant ( liter·bar/mol·K)

• T: Temperature in Kelvin (K = °C + 273)

Practice Problems

• Example 1: If the concentration of NaCl inside a plant cell is 0.15M and the cell is placed in a 0.3M NaCl solution, water will leave the cell (moves from low to high solute concentration).

• Example 2: Calculate the solute potential of a 0.3M sugar solution at 27°C:

  • (sucrose does not ionize)

  • mol/L

  • liter·bar/mol·K

  • K (27 + 273)

  • bars

• Example 3: Calculate the solute potential of a 0.2M NaCl solution at 30°C:

  • (NaCl dissociates into Na+ and Cl-)

  • mol/L

  • liter·bar/mol·K

  • K (30 + 273)

  • bars

• Example 4: If root tissue has bars and is placed in a 0.2M sucrose solution at 20°C ( bars), water will move into the solution (from higher to lower water potential).

Summary Table: Types of Solutions and Cell Responses

Additional info: Academic context and definitions have been expanded for clarity and completeness. Practice problems have been solved using standard water potential equations.