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.

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