Membrane Structure and Function

Introduction

The plasma membrane is a fundamental structure in all living cells, serving as the boundary that separates the internal environment of the cell from its external surroundings. Its unique structure allows it to regulate the movement of substances into and out of the cell, maintaining homeostasis and enabling communication with the environment.

At the Edge: The Plasma Membrane

Structure and Selective Permeability

• Plasma Membrane: The outer boundary of the cell, composed primarily of a phospholipid bilayer with embedded proteins.

• Selective Permeability: The property of the plasma membrane that allows some substances to cross more easily than others, enabling the cell to control its internal composition.

• Regulation of Traffic: The membrane uses various mechanisms to regulate the movement of molecules, including passive transport, active transport, and bulk transport (endocytosis and exocytosis).

Example: Oxygen and carbon dioxide can diffuse freely across the membrane, while ions and large molecules require specific transport proteins.

Fluid Mosaic Model

Membrane Structure and Dynamics

• Fluid Mosaic Model: Describes the plasma membrane as a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids.

• Phospholipids: Amphipathic molecules with hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, forming a bilayer.

• Membrane Fluidity: Lipids and some proteins can move laterally within the layer, contributing to membrane flexibility and function.

• Weak Hydrophobic Interactions: Hold the membrane together, allowing for dynamic movement of components.

Example: Human and mouse cell membranes can fuse, and their proteins intermingle, demonstrating membrane fluidity.

Membrane Proteins and Their Functions

Types and Roles of Membrane Proteins

• Peripheral Proteins: Loosely bound to the membrane surface.

• Integral Proteins: Penetrate the hydrophobic core; those that span the membrane are called transmembrane proteins.

• Functions of Membrane Proteins:

  • Transport: Move substances across the membrane.

  • Enzymatic Activity: Catalyze chemical reactions.

  • Signal Transduction: Relay signals from outside to inside the cell.

  • Cell-Cell Recognition: Allow cells to identify each other.

  • Intercellular Joining: Connect adjacent cells.

  • Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix (ECM).

Example: Glycoproteins on the cell surface are involved in immune recognition.

Selective Permeability of the Lipid Bilayer

Transport Across the Membrane

• Hydrophobic (Nonpolar) Molecules: Such as hydrocarbons, can dissolve in the lipid bilayer and cross rapidly.

• Hydrophilic Molecules: Including ions and polar molecules, do not cross the membrane easily and require transport proteins.

• Transport Proteins: Facilitate the passage of hydrophilic substances. Carrier proteins bind to molecules and change shape to shuttle them across the membrane. Each transport protein is specific for the substance it moves.

Example: Aquaporins are channel proteins that facilitate water transport.

Passive Transport

Diffusion and Osmosis

• Diffusion: The tendency for molecules to spread out evenly into the available space, moving down their concentration gradient (from high to low concentration).

• Passive Transport: The diffusion of a substance across a biological membrane without energy expenditure by the cell.

• Osmosis: The diffusion of water across a selectively permeable membrane. Water moves from regions of lower solute concentration to regions of higher solute concentration until equilibrium is reached.

Example: Red blood cells placed in a hypotonic solution will swell and may burst (lyse), while those in a hypertonic solution will shrink (crenate).

Effects of Tonicity on Cells

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

• Isotonic Solution: Equal solute concentration; no net water movement.

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

Example: Plant cells become turgid (firm) in hypotonic solutions, flaccid in isotonic, and plasmolyzed in hypertonic solutions.

Facilitated Diffusion

Passive Transport Aided by Proteins

• Channel Proteins: Provide corridors that allow specific molecules or ions to cross the membrane.

• Gated Channels: Open or close in response to a stimulus.

• Carrier Proteins: Undergo a subtle change in shape to translocate the solute-binding site across the membrane.

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

Active Transport

Movement Against Concentration Gradients

• Active Transport: Uses energy (usually from ATP hydrolysis) to move solutes against their concentration gradients.

• Transport Proteins: Such as pumps, are required for active transport.

Equation:

Example: The sodium-potassium pump moves Na+ out of and K+ into the cell, both against their concentration gradients.

Cotransport

• Cotransport: A transport protein couples the "downhill" diffusion of one solute to the "uphill" transport of another against its own concentration gradient.

Example: Sucrose-H+ cotransporter in plant cells uses the gradient of H+ ions to drive the uptake of sucrose.

Bulk Transport

Exocytosis and Endocytosis

• Bulk Transport: Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles, a process that requires energy.

• Exocytosis: Transport vesicles fuse with the plasma membrane, releasing their contents outside the cell.

• Endocytosis: The cell takes in macromolecules by forming vesicles from the plasma membrane. Types include:

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

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

  • Receptor-Mediated Endocytosis: Specific molecules are taken in after they bind to receptors on the cell surface.

Example: White blood cells use phagocytosis to engulf bacteria.

Summary Table: Types of Membrane Transport