Membrane Structure and Function – Chapter 7 Study Notes

Membrane Structure and Function

Overview of Cellular Membranes

Cellular membranes are essential structures that separate the interior of the cell from its external environment. They are primarily composed of lipids and proteins, forming a dynamic and selectively permeable barrier.

• Fluid Mosaic Model: Describes the 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.

• Proteins: Embedded within or attached to the membrane, responsible for most membrane functions.

• Membrane Fluidity: Maintained by weak hydrophobic interactions; lipids and some proteins can move laterally, rarely flip-flop.

Phospholipid Bilayer Structure

The phospholipid bilayer is the fundamental structure of cellular membranes, providing both fluidity and selective permeability.

• Amphipathic Nature: Phospholipids have hydrophilic heads facing outward toward water and hydrophobic tails facing inward.

• Unsaturated vs. Saturated Fatty Acids: Membranes with unsaturated fatty acids are more fluid; saturated fatty acids make membranes more viscous.

• Temperature Effects: Lower temperatures can cause membranes to solidify; fluidity depends on lipid composition.

Membrane Proteins

Proteins embedded in the membrane perform a variety of functions, including transport, signaling, and structural support.

• Integral Proteins: Penetrate the hydrophobic core; some span the membrane (transmembrane proteins).

• Peripheral Proteins: Bound to the surface of the membrane.

• Functions of Membrane Proteins:

  • Transport: Move substances across the membrane.

  • Enzymatic Activity: Catalyze reactions at the membrane surface.

  • 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).

• Medical Relevance: Example: HIV enters immune cells by binding to cell-surface protein CD4 and co-receptor CCR5.

Membrane Permeability

The plasma membrane is selectively permeable, allowing certain molecules to pass while restricting others.

• Hydrophobic Molecules: (e.g., hydrocarbons, O2, CO2) pass easily through the lipid bilayer.

• Hydrophilic Molecules: (e.g., sugars, water, ions) pass slowly or require transport proteins.

• Transport Proteins: Facilitate the movement of specific molecules across the membrane.

Types of Transport Across Membranes

Transport across membranes can be passive or active, depending on energy requirements and direction relative to concentration gradients.

• Passive Transport: Movement of substances down their concentration gradient without energy input.

  • Simple Diffusion: Movement of particles from high to low concentration.

  • Facilitated Diffusion: Transport proteins (channels and carriers) help hydrophilic substances cross the membrane.

• Active Transport: Movement of substances against their concentration gradient, requiring energy (usually ATP).

  • Carrier Proteins: Change shape to move substances across the membrane.

  • Sodium-Potassium Pump: Maintains higher K+ and lower Na+ inside animal cells.

  • Proton Pump: Used in plants, fungi, and bacteria to transport H+ ions.

• Bulk Transport: Large molecules (e.g., proteins, polysaccharides) are transported via vesicles.

  • Exocytosis: Vesicles fuse with the membrane to release contents outside the cell.

  • Endocytosis: Membrane engulfs material to bring it into the cell.

    • Phagocytosis: Cell engulfs large particles.

    • Pinocytosis: Cell takes in extracellular fluid and solutes.

    • Receptor-Mediated Endocytosis: Specific molecules are taken in after binding to receptors.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane, influenced by solute concentration.

• Osmosis: Water moves from regions of lower solute concentration to higher solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

  • Isotonic: No net water movement; cell remains normal.

  • Hypotonic: Water enters the cell; cell may lyse (burst).

  • Hypertonic: Water leaves the cell; cell shrivels.

Electrochemical Gradients and Membrane Potential

Membrane potential is the voltage across a membrane, created by differences in ion distribution. It influences the movement of ions.

• Electrochemical Gradient: Combination of chemical (concentration) and electrical (charge) forces driving ion movement.

• Electrogenic Pumps: Transport proteins that generate voltage across membranes (e.g., sodium-potassium pump, proton pump).

Transport Mechanisms Table

The following table summarizes the main types of transport across the plasma membrane:

Transport Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down gradient	O2, CO2
Facilitated Diffusion	No	Down gradient	Glucose via carrier protein
Active Transport	Yes (ATP)	Against gradient	Sodium-potassium pump
Bulk Transport (Exocytosis/Endocytosis)	Yes (ATP)	Varies	Secretion of proteins, uptake of macromolecules

Key Equations

• Diffusion: Movement down concentration gradient; no specific equation, but described by Fick's Law: $J = -D \frac{dC}{dx}$ where $J$ is flux, $D$ is diffusion coefficient, $\frac{dC}{dx}$ is concentration gradient.

• Membrane Potential: $V_m = V_{inside} - V_{outside}$ where $V_m$ is membrane potential.

Examples and Applications

• Medical Application: Oral rehydration therapy uses Na+/glucose cotransport to treat dehydration from diarrhea.

• Cell Recognition: HIV resistance in individuals lacking CCR5 co-receptor.

Additional info: Academic context and definitions have been expanded for clarity and completeness.