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 low solute concentration to an area of high 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 water movement.

• 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 and function.

• Plant Cells: No net water movement; cells are flaccid (not turgid).

• Example: Blood plasma is isotonic to red blood cells.

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.

• Example: Placing a cell in seawater (high salt concentration).

Water Balance in Saltwater Fish & Hypertonic Environments

Saltwater fish live in hypertonic environments, 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 extra 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.

• Example: Seawater fish maintain homeostasis by balancing water intake and salt excretion.

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 burst (lyse) due to excess water intake.

• Plant Cells: Become turgid (firm) due to water uptake; cell wall prevents bursting.

• Turgor Pressure: The pressure of the cell contents against the cell wall, providing structural support.

• Example: Plant cells in fresh water maintain turgor pressure, which is essential for structural support and preventing wilting.

Water Potential

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

• Water Potential (): The potential energy of water in a system compared to pure water, where .

• Water moves from areas of high water potential to areas of low water potential.

• Water potential is affected by solute potential () and pressure potential ():

• Solute Potential (): Always negative; more solutes lower water potential.

• Pressure Potential (): Can be positive or negative; physical pressure on the solution.

• Units: Measured in megapascals (MPa) or bars.

• Example: Pure water in open air has MPa.

Calculating Solute Potential

Solute potential can be calculated using the following formula:

• i: Ionization constant (number of particles formed in solution; for sucrose, )

• C: Molar concentration (mol/L)

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

• T: Temperature in Kelvin ()

• Example Calculation: For a 0.3M sugar solution at 27°C in open air:

bars

Practice Problems

• Problem 1: If the concentration of NaCl inside a plant cell is 0.15M and the cell is placed in a 0.3M NaCl solution, which way will water move?

  • Answer: Water will leave the cell (moves from low to high solute concentration).

• Problem 2: Calculate the solute potential of a 0.2M NaCl solution at 30°C.

  • Calculation: bars

• Problem 3: If the water potential () in root tissue is -4.6 bars and the tissue is placed in a 0.2M sucrose solution at 20°C ( bars), which direction will water flow?

  • Answer: Water will move into the solution (from higher to lower water potential).

Summary Table: Types of Solutions and Cell Responses

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary tables and formula details.