Cellular Membranes: Structure, Function, and Transport Mechanisms

Concept 7.1: Cellular Membranes are Fluid Mosaics of Lipids and Proteins

Selective Permeability

Cellular membranes regulate the passage of substances, allowing some molecules to cross more easily than others. This property is known as selective permeability.

• Definition: Selective permeability refers to the ability of the cell membrane to allow certain molecules or ions to pass through it by means of active or passive transport.

Fluid Mosaic Model

The fluid mosaic model describes the structure of cell membranes as a mosaic of components—including phospholipids, cholesterol, proteins, and carbohydrates—that gives the membrane a fluid character.

• Phospholipid bilayer: Composed of amphipathic molecules with hydrophilic heads and hydrophobic tails.

• Membrane fluidity:

  • Influenced by the presence of saturated and unsaturated fatty acids.

  • Cholesterol modulates fluidity by preventing tight packing of phospholipids.

• Membrane proteins:

  • Peripheral proteins: Loosely bound to the membrane surface.

  • Integral (transmembrane) proteins: Span the membrane and are involved in transport and signaling.

  • Functions: Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix.

• Glycoproteins/glycolipids: Carbohydrate chains attached to proteins or lipids on the extracellular side, important for cell recognition.

Example: The red blood cell membrane contains specific glycoproteins that determine blood type.

Concept 7.2: Membrane Structure Results in Selective Permeability

Transport Proteins

Transport proteins facilitate the movement of substances across the membrane that cannot diffuse freely.

• Channel proteins: Form hydrophilic channels that allow specific molecules or ions to pass (e.g., aquaporins for water transport).

• Carrier proteins: Bind to molecules and change shape to shuttle them across the membrane.

Example: Aquaporins increase the rate of water transport in kidney cells.

Concept 7.3: Passive Transport is Diffusion of a Substance Across a Membrane with No Energy Investment

Diffusion (Passive)

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, down their concentration gradient.

• Concentration gradient: The difference in concentration of a substance across a space.

Diffusion Across a Membrane (Passive Transport)

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Tonicity in cells/osmoregulation:

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

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

  • Hypotonic: Lower solute concentration outside the cell; cell gains water.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across the membrane via transport proteins.

• Carrier vs. channel transport proteins: Channel proteins provide corridors for molecules, while carrier proteins undergo conformational changes to move substances.

Example: Glucose transporters facilitate the diffusion of glucose into cells.

Concept 7.4: Active Transport Uses Energy to Move Solutes Against Their Gradients

Active Transport

Active transport requires energy (usually from ATP) to move substances against their concentration or electrochemical gradients.

• Membrane potential/electrochemical gradient: The voltage difference across a membrane, influencing the movement of charged substances.

• Electrogenic pumps: Transport proteins that generate voltage across a membrane.

  • Na+/K+ pump: Moves 3 Na+ ions out and 2 K+ ions into the cell, maintaining membrane potential.

  • Proton pump: Moves H+ ions across membranes, important in plants, fungi, and bacteria.

• Cotransport: The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

Example: The Na+/glucose cotransporter in the intestine uses the Na+ gradient to import glucose into cells.

Concept 7.5: Bulk Transport Across the Plasma Membrane Occurs by Exocytosis and Endocytosis

Exocytosis

Exocytosis is the process by which cells export materials in vesicles that fuse with the plasma membrane.

• Example: Secretion of neurotransmitters from nerve cells.

Endocytosis

Endocytosis is the process by which cells import materials by engulfing them in vesicles formed from the plasma membrane.

• Phagocytosis: "Cell eating"; the cell engulfs large particles or cells.

• Pinocytosis: "Cell drinking"; the cell takes in extracellular fluid and dissolved solutes.

• Receptor-mediated endocytosis: Specific molecules are taken in after binding to receptors on the cell surface.

Example: Uptake of cholesterol via LDL receptors in animal cells.