class 3/11/25

Membrane Properties

• Hydrophobic Interactions

  • Acid tails align to form a hydrophobic region.

  • Interaction with nonpolar molecules like glucose and sucrose due to carbon-oxygen bonds, despite being large molecules.

• Passive Diffusion

  • Small molecules like nitrogen gas can pass through membranes unhindered.

  • After achieving equilibrium across membranes, molecules will continue to move due to diffusion.

Glycoproteins and Cellular Interaction

• Role of Glycoproteins

  • Serve in mechanical protection and cell recognition.

  • Formed by sugars linked to lipids/proteins (glycosylation).

Water Movement and Concentration Gradients

• Polar Molecules

  • Small polar molecules (e.g., water) can cross membranes depending on concentration differences.

  • Despite cells being 70% water, extracellular environments often have much higher water concentrations.

• Water Movement

  • Water tends to move into cells due to concentration gradients.

  • Concentration differences dictate the movement of water into cells despite large intracellular concentrations.

Transport Mechanisms

• Large Molecules

  • Uncharged large molecules like glucose struggle to move across cell membranes without assistance.

  • Special protein channels aid in transporting substances like glucose across membranes.

• Ions and Membrane Permeability

  • Fully charged ions cannot pass through biological membranes easily.

  • Ionic forms of salts prevent crossing under normal conditions without damaging the membrane.

Electrochemical Gradients

• Forces Influencing Transport

  • Chemical Gradient: Concentration differences drive movement across membranes.

  • Electrical Gradient: Charges influence ion movement, preventing certain ions from moving in unfavorable directions.

  • The combined influence is known as the Electrochemical Gradient.

Active Transport Mechanisms

• Diffusion Driven Transport

  • Passive and assisted diffusion is used to move substances along concentration gradients.

• Active Transport

  • Movement against gradients requires energy, typically derived from ATP.

  • Types include uniporters, antiporters, and symporters for different transport strategies.

Sodium-Potassium Pump

• Functionality

  • The pump maintains osmotic balance by transporting 3 sodium ions out and 2 potassium ions into the cell.

  • Requires ATP for energy as both ions are moved against their gradients.

• Results of Pumping Process

  • Creates a more negative intracellular environment.

  • Results in a higher concentration of sodium outside the cell and potassium inside.

Potassium Leak Channels

• Role in Cellular Function

  • Allow potassium to leak out once a concentration threshold is met, balancing concentrations and maintaining membrane potential.

Membrane Potential and Equilibrium

• Membrane Potential

  • Defined as the electrical difference across the membrane, measured in millivolts.

  • Resting Membrane Potential: Achieved when the chemical force of potassium moving out equals the electrical force keeping it in.

Action Potentials in Excitable Cells

• Mechanism

  • Action potentials are electrical signals generated by the influx of sodium, followed by depolarization in response to stimuli.

  • They result in a wave of signal propagation through nerve fibers.

• Sodium Channels

  • Voltage-gated channels facilitate rapid influx of sodium ions during depolarization, propelling the action potential down the axon.

Myelination and Signal Propagation

• Role of Glial Cells

  • Myelinated nerve fibers have glial cells wrapped around them, providing insulation and speeding up signal propagation.

• Nodes of Ranvier

  • Gaps between myelinated segments where sodium channels are concentrated to boost the action potential quickly.

Synaptic Transmission

• Conversion of Signals

  • At synapses, electrical signals convert to chemical signals to cross the gap between neurons, then back to electrical signals in the receiving cell.