Chapter 3_Membrane PhysiologyDGp2 (1)

3.2 Membrane Transport

Osmosis

• Water Movement: Water, being a polar molecule, moves across a selectively permeable membrane via osmosis.

  • Direction: From areas of lower solute concentration (hypoosmotic) to higher solute concentration (hyperosmotic).

  • Driving Force: The concentration gradient of water molecules.

Membrane Composition

• Types of Solutions:

  • Side 1: Higher concentration of H2O, lower concentration of solute.

  • Side 2: Lower concentration of H2O, higher concentration of solute.

• Movement:

  • H2O moves through the membrane from side 1 to side 2 while solute cannot move in either direction.

• Steady State: Equal concentrations of water and solute lead to no further net diffusion.

3.2 Membrane Properties

Hydrostatic and Osmotic Pressure

• Hydrostatic Pressure: Opposes the movement of water during osmosis.

• Osmotic Pressure: The required pressure to stop osmotic flow, proportional to the concentration of impermeable solutes.

  • Solutions with higher solute concentrations exert greater osmotic pressure.

Colligative Properties of Solutes

• Definition: Properties that depend on the number of solute particles in a solution, not the type of particles.

  • Osmotic Pressure

  • Elevation of Boiling Point

  • Depression of Freezing Point

  • Reduction of Vapor Pressure

3.2 Tonicity

Definitions of Tonicity

• Isotonic Solution: Equal concentration of nonpenetrating solutes as in normal cells; cell volume remains constant.

• Hypotonic Solution: Lower solute concentration than inside cells; can cause cells to swell or lyse.

• Hypertonic Solution: Higher solute concentration than inside cells; can lead to cell shrinkage (crenation).

3.3 Assisted Membrane Transport

Membrane Permeability

• Impenetrable to:

  • Large molecules (proteins, nucleic acids).

  • Small, poorly lipid-soluble molecules (e.g., glucose).

  • Small, charged ions.

Transport Mechanisms

• Channel Transport:

  • Formed by transmembrane proteins, allowing selective passage of ions or water (e.g., aquaporins).

  • Channel types: Gated (open/close) and Leak (always open).

• Carrier-Mediated Transport:

  • Transmembrane proteins that change shape to transport substances.

  • Types: Facilitated diffusion (passive) and Active transport (energy required).

Active vs. Passive Transport

• Facilitated Diffusion:

  • Passive process, moves substances from high to low concentration without energy.

• Active Transport:

  • Moves substances against their concentration gradient and requires energy (e.g., ATP).

3.3 Na+/K+-ATPase

Functionality

• Transport Mechanism:

  • Pumps 3 Na+ ions out of the cell and 2 K+ ions into the cell.

  • Helps maintain concentration gradients and regulates cell volume.

Mechanism Steps

• Phosphorylation of the carrier proteins aids in ion transport, with changes in shape facilitating movement across the membrane.

3.4 Intercellular Communication and Signal Transduction

Communication Types

• Direct: Gap junctions, transient surface marker connect, nanotubes.

• Indirect: Most common; uses intercellular chemical messengers (e.g., hormones).

Types of Chemical Messengers

• Paracrine Signals: Local messages affecting neighboring cells.

• Neurotransmitters: Chemicals used for neuron communication with target cells.

• Hormones: Long-range messengers traveling in the bloodstream.

• Neurohormones: Hormones released into circulation by neurons.

• Pheromones: Signals in the environment for other animals.

• Cytokines: Regulatory peptides for development and immunity.

3.4 Signal Transduction Pathways

Mechanisms

• Receptor Binding: Drugs and ligands modify pathways.

• Second Messenger Systems: Amplify signals.

• Phosphorylation Actions: Protein kinases alter protein function upon activation.

Examples of Second Messengers

1. Cyclic AMP: Activated by hormone binding, triggering further signaling cascades.

2. Inositol Trisphosphate: Leads to the release of intracellular calcium stores.

3.5 Membrane Potential

Definition and Characteristics

• Membrane Potential: The charge difference across the plasma membrane, capable of doing work.

• Key Ions: Na+, Cl- in extracellular fluid; K+ in intracellular fluid.

Resting Membrane Potential

• Determined by ion concentration and permeance; typically around -70 mV.

• Greater permeability to K+ influences resting potential closer to K+'s equilibrium potential.

Equilibrium Potentials

• K+ Equilibrium Potential (EK): -90 mV.

• Na+ Equilibrium Potential (ENa): +60 mV.

3.5 Conclusion on Resting Potential and Active Transport

• Resting membrane potential arises from the balance of ion movements and active transport activities (Na+/K+ pump) to maintain gradients.