week 2-Solute and Water Movement Across Membranes

Movement of Solutes and Water Across Membranes

• Plasma Membrane: Separates intracellular environment from extracellular fluid.
  • Essential for the transport of ions and molecules (e.g., glucose, oxygen).
• Homeostasis: Controlled movement of solutes and water is vital for maintaining physiological balance.
  • Example: Oxygen and glucose must reach muscle cells during physical exertion.

Simple Diffusion

• Definition: Random thermal movement of molecules from high to low concentration until equilibrium is reached.
  • Advantages: Critical for oxygen transport from blood to cells.
• Factors Influencing Diffusion:
  1. Concentration gradient
  2. Temperature
  3. Molecular size/weight
  4. Surface area of the membrane
  5. Medium of transport (air vs. water)
  6. Distance of diffusion

Diffusion Mechanics

• Fick's First Law: Rate of diffusion is given by: $J = PA (C<em>o - C</em>i)$
  • Where:
  • $J$ = Rate of diffusion
  • $P$ = Permeability coefficient
  • $A$ = Surface area
  • $C_o$ = Concentration outside the cell
  • $C_i$ = Concentration inside the cell
• Influences on Rate:
  • Larger surface area and thinner membranes increase diffusion.
  • Higher concentration gradients enhance diffusion rate.

Membrane Permeability

• Polar vs. Non-Polar Molecules:
  • Polar molecules: Low permeability, do not diffuse well.
  • Non-polar molecules: Higher permeability, diffuse rapidly.
• Ion Channels: Permit ions like Na+, K+, Cl-, Ca2+ to diffuse down their concentration gradient.
  • Selectivity: Channels can be specific for different ions (e.g., Na+ selective channels).

Regulation of Ion Channels

• Gating Mechanisms:
  • Ligand-gated: Opens upon binding of a molecule.
  • Voltage-gated: Opens in response to changes in voltage across the membrane.
  • Mechanically-gated: Opens due to physical deformation of the membrane.

Membrane Potential and Ion Movement

• Membrane Potential: Difference in electrical charge across the membrane, typically negative inside.
• Electrochemical Gradient: Drives ion movement; positive ions enter cells, negative ions exit due to opposite charge attraction.

Mediated-Transport Systems

• Transporters: Proteins aiding in transport of larger or polar molecules across membranes via conformational changes.
  • Can become saturated, limiting transport rate.

Types of Transport

• Facilitated Diffusion: Moves molecules down concentration gradient without ATP usage (e.g., glucose transporters).
• Active Transport: Requires ATP to move substances against their gradient.
  • Primary Active Transport: Directly uses ATP; e.g., Na+/K+ ATPase.
  • Secondary Active Transport: Uses energy from gradients established by primary transport (cotransport of other molecules).

Osmosis

• Definition: Diffusion of water across membranes, typically through aquaporins.
• Osmolality: Total solute concentration of a solution, affecting water concentration.
  • Example: 1 mol NaCl = 2 osmol due to dissociation into ions.

Tonic Solutions

• Isotonic: Same concentration of non-penetrating solutes as extracellular fluid; no volume change.
• Hypotonic: Lower concentration of non-penetrating solutes; cells swell.
• Hypertonic: Higher concentration of non-penetrating solutes; cells shrink.

Endocytosis and Exocytosis

• Endocytosis: Process where membrane folds to form vesicles, bringing substances into the cell.
  • Types:
  • Pinocytosis: Cell drinking of fluids.
  • Phagocytosis: Cell eating of large particles.
  • Receptor-mediated endocytosis: Specific uptake via receptor binding.
• Exocytosis: Vesicles fuse with the plasma membrane, releasing contents outside the cell, important for neurotransmitter secretion in neurons.