cell membrane

Membrane Structure and Function

  • General Characteristics

    • All cell membranes, including the plasma membrane, share similar structural properties.

    • Composed of two layers with hydrophobic tails facing each other.

    • Cholesterol is embedded within the membrane, influencing its fluidity and functionality.

  • Hydrophilic and Hydrophobic Regions

    • Memebrane has hydrophilic (water-loving) and hydrophobic (water-fearing) parts.

    • Being amphipathic (having both hydrophilic and hydrophobic regions) is essential for membrane structure.

    • The hydrophobic parts control permeability and what substances can cross the membrane, making membranes selectively permeable.

Fluid Mosaic Model

  • Fluidity of the Membrane

    • The membrane is fluid, meaning that its components can move freely within the layer.

    • Proteins and other molecules embedded in the membrane are also capable of lateral movement.

  • Components of the Membrane

    • Proteins: Integral and transmembrane proteins facilitate various functions, including transport and signal transduction.

    • Glycoproteins and Glycolipids: Sugars attached to proteins or lipids act as identifiers for cell recognition and signaling.

Movement Across Membranes

  • Diffusion

    • Molecules move from areas of high concentration to areas of low concentration, referred to as diffusion.

    • Small and hydrophobic (nonpolar) molecules easily pass through the membrane via simple diffusion.

  • Osmosis

    • Specifically refers to the diffusion of water across a membrane, typically requiring the use of aquaporin proteins for transport due to water’s polar nature.

  • Passive Transport

    • Occurs without the input of cellular energy, moving substances along their concentration gradient.

  • Active Transport

    • Involves energy consumption to move substances against their concentration gradient (from low to high concentration).

Specific Transport Mechanisms

  • Sodium-Potassium Pump

    • Regulates ionic balance by exchanging sodium (Na+) for potassium (K+) across the membrane, maintaining a negative charge inside the cell.

    • Essential for various cellular functions including nerve impulse transmission.

  • Coupled Transport

    • Example: Glucose transport often involves coupling it with sodium ions to facilitate the crossing of the membrane.

Tonicity and Solutions

  • Isotonic Solution

    • Has equal solute concentrations inside and outside of the cell, resulting in no net movement of water.

  • Hypertonic Solution

    • Has higher solute concentration outside the cell leading to water leaving the cell, causing it to shrivel.

  • Hypotonic Solution

    • Has lower solute concentration outside the cell leading to water entering the cell, causing it to swell.

  • Movement of Water

    • Water moves towards areas of higher solute concentration to establish equilibrium.