Membrane Structure and Function – Study Notes

CHAPTER 7: MEMBRANE STRUCTURE AND FUNCTION

Introduction to Membrane Structure

The plasma membrane is a fundamental component of all cells, acting as a selective barrier that separates the living cell from its nonliving surroundings. It regulates the movement of substances into and out of the cell, maintaining homeostasis.

• Selective Permeability: The plasma membrane allows some substances to cross more easily than others, a property known as selective permeability.

• Main Macromolecules: The primary macromolecules in membranes are phospholipids and proteins, with some carbohydrates also present.

• Amphipathic Molecules: Most membrane constituents, such as phospholipids, are amphipathic, meaning they have both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.

• Fluid Mosaic Model: The current model of membrane structure describes it as a fluid mosaic, with proteins embedded or attached to a double layer of phospholipids.

Historical Models of Membrane Structure

Understanding of membrane structure has evolved over time, with several key experiments and models contributing to our current knowledge.

• 1895, Charles Overton: Proposed that membranes are made of lipids because lipid-soluble substances enter cells more rapidly than water-soluble ones.

• 1917, Irving Langmuir: Demonstrated that phospholipids form a film on water, with hydrophilic heads in contact with water and hydrophobic tails away from water.

• 1925, Gorter and Grendel: Concluded that cell membranes are a phospholipid bilayer, two molecules thick, with hydrophobic tails shielded from water and hydrophilic heads facing water.

• 1935, Davson-Danielli Model: Suggested a "sandwich" model with a phospholipid bilayer between two layers of proteins. This model was later revised due to inconsistencies with experimental data.

• 1972, Singer and Nicolson: Proposed the Fluid Mosaic Model, where proteins are dispersed and individually inserted into the phospholipid bilayer, with hydrophilic regions exposed to water and hydrophobic regions shielded inside.

Membrane Fluidity

Membrane fluidity is essential for proper function, affecting the movement of proteins and lipids within the bilayer.

• Lateral Movement: Most lipids and some proteins can move laterally within the membrane; flip-flop between layers is rare.

• Factors Affecting Fluidity:

  • Temperature: Lower temperatures can cause membranes to solidify; higher temperatures increase fluidity.

  • Fatty Acid Composition: Membranes rich in unsaturated fatty acids are more fluid due to kinks in the tails, preventing tight packing.

  • Cholesterol: In animal cells, cholesterol acts as a "fluidity buffer," restraining movement at high temperatures and preventing solidification at low temperatures.

• Adaptation: Organisms can adjust membrane lipid composition in response to temperature changes (e.g., increasing unsaturated phospholipids in cold environments).

Membrane Proteins and Their Functions

Membrane proteins are crucial for the diverse functions of biological membranes.

• Peripheral Proteins: Loosely bound to the membrane surface, often attached to integral proteins or the cytoskeleton.

• Integral Proteins: Penetrate the hydrophobic core of the bilayer; transmembrane proteins span the entire membrane.

• Protein Structure: Hydrophobic regions often consist of nonpolar amino acids arranged in alpha helices; hydrophilic regions are exposed to aqueous environments.

• Functions of Membrane Proteins:

  • Transport: Facilitate movement of substances across the membrane.

  • Enzymatic Activity: Catalyze specific 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.

Membrane Carbohydrates and Cell Recognition

Carbohydrates attached to lipids (glycolipids) or proteins (glycoproteins) on the external surface of the plasma membrane play a key role in cell-cell recognition.

• Oligosaccharides: Short, branched chains of fewer than 15 sugar units.

• Variation: The composition of membrane carbohydrates varies among species, individuals, and cell types, providing unique cellular identities (e.g., blood group antigens).

• Medical Relevance: Surface proteins and carbohydrates are important in immune responses and disease (e.g., HIV entry via CD4 and CCR5 receptors).

Traffic Across Membranes

Selective Permeability and Transport

The plasma membrane's molecular organization results in selective permeability, controlling the movement of ions and molecules.

• Hydrophobic Molecules: Such as hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily.

• Polar Molecules and Ions: Pass through with difficulty and often require transport proteins.

• Transport Proteins: Provide hydrophilic channels or carry molecules across the membrane; each is specific for certain substances.

Passive Transport: Diffusion and Osmosis

Passive transport involves the movement of substances down their concentration gradients without energy input from the cell.

• Diffusion: The tendency of molecules to spread out evenly; each substance diffuses down its own concentration gradient.

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

• Solution Terms:

  • Hypertonic: Higher solute concentration.

  • Hypotonic: Lower solute concentration.

  • Isotonic: Equal solute concentration.

• Effects on Cells:

  • Animal cells: Swell and burst in hypotonic, shrink in hypertonic, stable in isotonic solutions.

  • Plant cells: Turgid (firm) in hypotonic, flaccid in isotonic, plasmolyzed in hypertonic solutions.

Facilitated Diffusion

Facilitated diffusion is passive transport aided by transport proteins, allowing polar molecules and ions to cross the membrane more efficiently.

• Channel Proteins: Provide corridors for specific molecules or ions (e.g., aquaporins for water).

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

• Specificity: Each transport protein is specific for the substance it moves.

Active Transport

Active transport moves substances against their concentration gradients, requiring energy (usually from ATP).

• ATP-Powered Pumps: Transfer a phosphate group to the transport protein, inducing a conformational change that moves the solute.

• Sodium-Potassium Pump: Maintains high K+ and low Na+ inside animal cells by pumping 3 Na+ out and 2 K+ in per ATP hydrolyzed.

• Membrane Potential: The voltage across a membrane, typically -50 to -200 mV, due to unequal distribution of ions.

• Electrochemical Gradient: The combined effect of the concentration gradient and membrane potential on ion movement.

• Electrogenic Pumps: Generate voltage across membranes (e.g., Na+-K+ pump in animals, proton pump in plants, fungi, and bacteria).

Cotransport (Secondary Active Transport)

Cotransport involves the coupling of the "downhill" diffusion of one solute to the "uphill" transport of another against its gradient.

• Symport: Both substances move in the same direction.

• Antiport: Substances move in opposite directions.

• Example: Plants use the proton gradient generated by proton pumps to drive the uptake of nutrients like sucrose via symporters.

Bulk Transport: Exocytosis and Endocytosis

Large molecules and particles are transported across the membrane via vesicles in processes requiring energy.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (e.g., secretion of insulin).

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

  • Phagocytosis: "Cell eating"; engulfing large particles.

  • Pinocytosis: "Cell drinking"; uptake of extracellular fluid.

  • Receptor-Mediated Endocytosis: Uptake of specific molecules via receptor proteins (e.g., LDL cholesterol uptake).

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction	Example
Simple Diffusion	No	Down concentration gradient	O2, CO2
Facilitated Diffusion	No	Down concentration gradient	Glucose, ions via channels
Active Transport	Yes (ATP)	Against concentration gradient	Na+/K+ pump
Cotransport	Indirect (uses gradient)	Against gradient (one solute)	H+/sucrose symport
Exocytosis	Yes	Out of cell	Secretion of hormones
Endocytosis	Yes	Into cell	Phagocytosis, pinocytosis

Key Equations

• Membrane Potential: $V_m = V_{inside} - V_{outside}$

• Electrochemical Gradient: $\Delta G = RT \ln \left( \frac{[C]_2}{[C]_1} \right) + zF\Delta \Psi$ Where $[C]_2$ and $[C]_1$ are concentrations on either side, $z$ is ion charge, $F$ is Faraday's constant, and $\Delta \Psi$ is membrane potential.

Example: Sodium-Potassium Pump Cycle

1. Na+ binds to the pump protein inside the cell.

2. ATP phosphorylates the pump, causing a conformational change.

3. Na+ is released outside; K+ binds from outside.

4. Phosphate group is released, pump returns to original shape, K+ is released inside.

Additional info:

• Membrane structure and function are central to many physiological processes, including nerve impulse transmission, nutrient uptake, and immune responses.

• Defects in membrane proteins can lead to diseases such as cystic fibrosis and familial hypercholesterolemia.