#6 - MEMBRANE DYNAMICS AND TRANSPORT

Cell Membrane (Chapter 3)

Functions of the Cell Membrane

• Physical Barrier: Acts as a barrier that separates the internal components of the cell from the external environment.

• Control of Entry and Exit: Regulates the passage of ions, nutrients, wastes, and secretory products.

• Communication and Support: Facilitates communication between cells and provides structural support.

Structure of the Cell Membrane

• Tri-laminar Appearance: Observed under electron microscopy (EM) as a phospholipid bilayer.

• Phospholipid Bilayer Composition: Consists of hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails, which allow selective permeability.

• Key Components: Includes cholesterol (for fluidity/stability) and carbohydrates (for cell recognition).

Molecules' Ability to Cross the Membrane

• Size: Small molecules pass easily, while larger molecules struggle to cross the membrane.

• Charge/Polarity/Lipid Solubility: Non-polar, lipid-soluble molecules cross the membrane more easily than most polar, lipid-insoluble molecules.

• Examples of Passage:

  • Small, non-polar molecules (e.g., O2, N2) and small polar substances (e.g., water, glycerol) can pass readily through the membrane.

  • Larger polar molecules (e.g., glucose, sucrose) and ions generally have difficulty crossing.

Types of Membrane Proteins

• Membrane Proteins Overview: Each cell can contain 10-50 different types, many functioning to aid in the transport of substances across the membrane that otherwise wouldn’t cross.

Channel Proteins

• Allow selective passage of ions and small molecules into and out of the cell.

• Examples:

  • Water channels (aquaporins) have a diameter of 0.8 nm and facilitate water movement.

• Types of Channels:

  • Gated Channels: Can be opened or closed in response to stimulus (e.g., chemical-gated, voltage-gated, mechanically-gated).

Pathophysiology of Cystic Fibrosis (CF)

• Most common fatal genetic disorder causing thick, viscous fluid accumulation in lungs and pancreas.

• Genetic defect in the CFTR channel protein that transports Cl-, leading to NaCl accumulation and affecting antimicrobial protein functionality.

Carrier Proteins

• Bind specific molecules and transport them across the membrane via conformational changes, thus aiding the translocation of specific substances (e.g., glucose, amino acids).

Membrane Proteins - Functionality

Receptor Sites

• Recognize and bind to specific molecules, causing cellular changes. This is often compared to a lock-and-key mechanism.

Docking-Marker Acceptors

• Specialized proteins (t-SNAREs) on the inner cell membrane that bind to v-SNAREs on secretory vesicles, initiating processes like exocytosis upon binding.

Membrane-bound Enzymes

• Located on the inner or outer cell surfaces; they control specific biochemical reactions (e.g., acetylcholinesterase).

Cell Adhesion Molecules (CAMs)

• Extend from the cell surface to help maintain cell connectivity and structure.

• Examples:

  • Integrins: Form links between the cell membrane and extracellular structures.

  • Cadherins: Ca2+-dependent CAMs that maintain adhesion between similar cells within tissues.

Daily Objectives

• Understand the structure and functions of the seven major types of membrane proteins and three major types of cell junctions.

• Learn mechanisms of passive and active transport.

• Define the term "tissue" and recognize the four major tissue types, including cell structure and function correlates.

Self-Recognition Proteins

• These proteins, along with surface carbohydrates, facilitate immune response by distinguishing self from non-self in cell interactions, which is crucial for tissue compatibility in transplants and preventing autoimmune diseases.

Specialized Cell Junctions

• Adhering Junctions (Desmosomes): Act as spot welds connecting adjacent cells, allowing movement of materials between cells; commonly found in tissues that experience stretching (e.g., skin, heart).

• Tight Junctions: Create impermeable barriers, primarily in epithelial tissues, defining selective barriers (e.g., in the digestive tract).

• Gap Junctions: Act as communication channels through connexon proteins allowing ions/metabolites to pass between adjacent cells (e.g., in cardiac and smooth muscle).

Membrane Transport

Passive Transport

• Definition: Movement of substances without energy input (ATP), mainly down concentration gradients until equilibrium is achieved.

• Types of Passive Transport:

  • Diffusion: Small hydrophobic molecules or water move easily across membranes, as outlined by Fick’s Law of Diffusion, considering factors like concentration, permeability, surface area, and distance.

  • Osmosis: Movement of solvent through a selectively permeable membrane driven by solute concentration.

  • Mediated Transport: Passive transport with the aid of carrier/channel proteins (facilitated transport).

Active Transport

• Definition: Requires energy (ATP) to move substances against their concentration gradients.

• Active Transport Mechanisms:

  • Symport: Transports multiple molecules in the same direction (e.g., Na+/Glucose pump).

  • Antiport: Transports multiple molecules in opposite directions (e.g., Na+/K+ ATPase pump).

• Primary Active Transport: Direct use of energy to move substances against gradients.

• Secondary Active Transport: Indirect use of energy, relying on ion concentration gradients.