#6 - MEMBRANE DYNAMICS AND TRANSPORT

Cell Membrane (Chapter 3)

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.

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).

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.

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.

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).

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.

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

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

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

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

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.

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.

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.

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).

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).

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.