Membrane Transport and Permeability Notes

Membrane Structure and Permeability

• Phospholipid Bilayer: Biological membranes consist of hydrophilic phosphate headgroups and a hydrophobic interior of fatty acid chains.
• Permeability Barrier: Membranes are selectively permeable (semipermeable), controlling the movement of specific molecules and ions into and out of cells and organelles.

Overview of Transport Mechanisms

• Passive Transport: Movement along a concentration gradient without net energy input ($-\Delta G$).     * Simple Diffusion: Unassisted movement of gases ($O_2$, $CO_2$), small nonpolar molecules, and small polar molecules ($H_2O$, glycerol).     * Facilitated Diffusion: Protein-mediated movement down a gradient using carrier or channel proteins.
• Active Transport: Movement against a concentration gradient requiring energy input ($+\Delta G$) and intrinsic directionality.

Simple Diffusion and Osmosis

• Factors: Regulated by solute size, polarity, and charge. Nonpolar and smaller molecules diffuse faster.
• Osmosis: The diffusion of water across a selectively permeable membrane toward regions of higher solute concentration.
• Turgor Pressure: In plants, inward water movement creates pressure against the cell wall; in hypertonic solutions, the membrane pulls away (plasmolysis).
• Osmolarity Management: Animal cells, lacking walls, pump out inorganic ions to avoid swelling or bursting (lysis) in hypotonic environments.

Facilitated Diffusion Processes

• Carrier Proteins: Allosteric proteins like GLUT1 (glucose uniport) that alternate between conformational states ($T_1$ and $T_2$) to transport solutes.
• Anion Exchange Protein: An antiport carrier in erythrocytes that facilitates a 1:1 reciprocal exchange of $Cl^-$ and $HCO_3^-$ based on concentration gradients.
• Channel Proteins: Form hydrophilic transmembrane channels for rapid passage.     * Ion Channels: Highly selective pores (e.g., for $Na^+$, $K^+$, $Ca^{2+}$, or $Cl^-$). Gating mechanisms include ligand-gated, mechanically-gated, and voltage-gated.     * Porins: Less specific multipass proteins (with $\beta$-barrels) found in bacteria, mitochondria, and chloroplasts.     * Aquaporins (AQP): Specialized channels for the rapid movement of water.

Active Transport and ATPases

• Direct Active Transport: Coupled directly to exergonic chemical reactions, typically ATP hydrolysis.     * P-type ATPases: Phosphorylated during transport (e.g., $Na^+/K^+$ pump, $Ca^{2+}$ pump).     * V-type ATPases: Pump protons into vacuoles, lysosomes, and the Golgi apparatus.     * F-type ATPases: Proton pumps in bacteria, mitochondria, and chloroplasts; also function as ATP synthases.     * ABC-type ATPases: "ATP-binding cassettes" transporting various solutes; includes MDR (multidrug resistance) proteins.
• Indirect Active Transport: Driven by ion gradients (e.g., $Na^+$ in animals or $H^+$ in plants) rather than direct ATP hydrolysis. Example: the $Na^+$/glucose symporter.
• The $Na^+/K^+$ ATPase: Maintains electrochemical gradients by pumping $3\,Na^+$ out and $2\,K^+$ in per cycle, transitioning between $E_1$ and $E_2$ conformations.

Energetics of Transport

• Uncharged Solutes: Movement is determined solely by the concentration gradient.
• Charged Solutes: Movement depends on the electrochemical potential, combining the concentration gradient and the membrane potential ($V_m$).
• Membrane Potential: Usually negative, favoring the inward movement of cations and opposing their outward movement.