Membrane Structure, Transport, Tonicity, Osmosis, and Water Potential

Membrane Structure and Function

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a dynamic arrangement of proteins and phospholipids. The membrane is a 'mosaic' of proteins floating in or on the fluid lipid bilayer, which is constantly moving and changing.

• Phospholipid bilayer: Composed of hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Proteins: Embedded within the bilayer, serving as channels, carriers, or receptors.

• Selective permeability: Hydrophobic tails in the center restrict passage of polar and charged molecules, allowing only certain substances to cross.

Example: Oxygen (O2) and carbon dioxide (CO2) can diffuse freely, while ions and large polar molecules require transport proteins.

Kinds of Transport

Transport across membranes can be classified as passive or active, and may involve proteins or vesicles.

• Passive Transport: No energy required; includes diffusion, osmosis, and facilitated diffusion via channels or carriers.

• Active Transport: Requires energy (usually ATP); moves substances against their concentration gradient via pumps.

• Vesicular Transport: Includes endocytosis (phagocytosis, pinocytosis) and exocytosis for bulk movement of materials.

Tonicity and Osmoregulation

Definitions and Concepts

Tonicity refers to the ability of a solution to cause a cell to gain or lose water, depending on the relative concentrations of solutes inside and outside the cell.

• Isotonic solution: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypertonic solution: Higher solute concentration outside the cell; cell loses water and shrinks.

• Hypotonic solution: Lower solute concentration outside the cell; cell gains water and may swell or burst.

Example: Animal cells in distilled water (hypotonic) undergo cytolysis (bursting); in concentrated salt solution (hypertonic), they undergo crenation (shriveling).

Osmoregulation

Osmoregulation is the process by which organisms maintain water balance and control internal solute composition.

• Essential for cell survival and homeostasis.

• Specialized structures (e.g., contractile vacuole in Paramecium) help regulate water uptake and loss.

• Fish in saltwater actively pump ions out and urinate less; freshwater fish pump ions in and urinate more.

Osmosis and Water Movement

Osmosis

Osmosis is the passive diffusion of water across a semi-permeable membrane from an area of high water concentration (low solute) to low water concentration (high solute).

• Does not require energy.

• Water moves to balance solute concentrations on both sides of the membrane.

Example: Grocery stores spray water on vegetables to keep them turgid (full of water).

Aquaporins

Aquaporins are specialized transport proteins that facilitate rapid water movement into and out of cells.

• Discovered in the early 1990s.

• Critical for maintaining water balance in cells.

Effects of Tonicity on Cells

Animal Cells

• Hypotonic: Cells swell and burst (cytolysis).

• Hypertonic: Cells shrink and shrivel (crenation).

• Isotonic: Cells remain the same size; water enters and leaves at equal rates.

Plant Cells

• Hypotonic: Cells become turgid (firm); cell wall prevents bursting.

• Hypertonic: Cells undergo plasmolysis; cytoplasm shrinks away from cell wall.

• Isotonic: Cells become flaccid (limp).

Water Potential

Definition and Equation

Water potential (Ψ) is the measure of the potential energy of water in a system, determining the direction of water movement. It is measured in bars or megapascals (MPa).

• Water moves from regions of higher water potential to lower water potential.

• Pure water at atmospheric pressure has Ψ = 0.

Equation: $\Psi = \Psi_s + \Psi_p$ Where:

• $\Psi_s$ = solute potential (osmotic potential)

• $\Psi_p$ = pressure potential

Solute Potential

• Adding solute decreases water potential (makes it more negative).

• Calculated as: $\Psi_s = -iCRT$

• Where:

  • i = ionization constant (number of particles formed in solution)

  • C = molar concentration

  • R = pressure constant (0.0831 liter bars/mole K)

  • T = temperature in Kelvin (273 + °C)

Pressure Potential

• Results from physical pressure on a solution.

• Can be positive (turgor pressure in plant cells) or negative (when cell loses water).

Water Potential in Practice

• Water flows toward areas with lower (more negative) water potential.

• Plant cells in distilled water become turgid due to positive pressure potential from the cell wall.

• In open systems (e.g., beaker), pressure potential is zero.

Sample Calculations

Solute Potential Calculation

For a 0.15 M solution of sucrose at 25°C:

• i = 1 (sucrose does not dissociate)

• C = 0.15 M

• R = 0.0831 liter bars/mole K

• T = 298 K (273 + 25)

Equation: $\Psi_s = -iCRT$ $\Psi_s = -(1)(0.15)(0.0831)(298)$ $\Psi_s = -3.7$ bars

For NaCl (i = 2):

$\Psi_s = -(2)(0.15)(0.0831)(298)$ $\Psi_s = -7.4$ bars

Summary Table: Effects of Tonicity on Cells

Solution Type	Animal Cell Effect	Plant Cell Effect
Hypotonic	Swells, bursts (cytolysis)	Turgid (firm)
Isotonic	No change	Flaccid (limp)
Hypertonic	Shrinks, shrivels (crenation)	Plasmolysis (cytoplasm shrinks)

Properties of Water

Key Properties

• Cohesion and Adhesion: Water molecules stick to each other and to other surfaces (capillary action).

• Surface Tension: Caused by hydrogen bonding between water molecules.

• High Specific Heat: Water resists temperature changes due to strong hydrogen bonds.

• Less Dense as Solid: Ice floats, insulating water below and protecting aquatic life.

• Universal Solvent: Water dissolves many substances due to its polarity.

Example: Water's high specific heat helps moderate Earth's climate.

Additional info:

• Water potential is a central concept in plant physiology, predicting water movement in and out of cells and tissues.

• Understanding osmoregulation is essential for studying how organisms adapt to different environments.