2.1-2.2

Cellular Membrane and Ion Transport

  • Transmembrane Proteins & Ion Concentration

    • Charged particles around cellular membranes:

    • Higher potassium concentration inside vs. sodium outside.

    • Negatively charged chloride ions are also more concentrated outside.

    • Sodium-potassium ATPase (Na+/K+ ATPase)

    • Utilizes energy (ATP hydrolysis) to move sodium and potassium against their gradients.

    • This is an example of primary active transport.

Types of Protein Transport

  • Carrier Proteins

    • Undergo conformational changes to transport ions.

    • Contrasted with Pore Proteins (e.g., potassium channels) which facilitate passive movement down concentration gradients (no energy required).

    • Facilitated Diffusion: Movement of ions down their concentration gradient through channel proteins.

Epithelial Cells and Transport Mechanisms

  • Importance of epithelial cells in nutrient absorption:

    • Apical Surface: Faces gut lumen; contains microvilli to increase surface area for absorption.

    • Basal Surface: Faces the interior of the body; includes proteins for active transport (like glucose).

    • Tight Junctions: Prevent diffusion of glucose back into the lumen, securing a strong barrier for nutrient absorption.

Glucose Transport Process

  • Glucose transport into epithelial cells must against its concentration gradient:

    • Na+/K+ ATPase creates sodium gradient that stores potential energy.

    • Sodium-Glucose Symporter: Co-transports sodium (moving down its gradient) and glucose (moving against its gradient) into the cell.

    • This is secondary active transport since it utilizes the sodium gradient but does not directly use ATP.

    • Once inside epithelial cells, glucose can exit via GLUT transporter through facilitated diffusion into interstitial fluid for circulation.

Control of Ion Transport and Membrane Potential

  • Dynamic Equilibrium: Restoration of equilibrium affected by sodium-potassium ATPase and potassium channels.

  • Membrane Potential (Vm): Describes the voltage across the membrane influenced by ion distribution and permeability.

    • Can be measured using microelectrodes (e.g., voltage meter techniques).

Nernst Equation & Equilibrium Potential

  • The Nernst equation predicts the equilibrium potential for a single ion based on its concentration gradient and charge.

  • General Equation for equilibrium potential Eion = ( \frac{RT}{zF} \ln \left( \frac{[ion]{outside}}{[ion]_{inside}} \right) ) where:

    • R is the gas constant, T is absolute temperature, z is charge of the ion, and F is Faraday's constant.

  • Example Calculation:

    • For potassium at typical concentrations, the Nernst calculation leads to approx. -90 mV equilibrium potential.

    • For sodium, positive equilibrium potential due to higher concentration outside.

Goldman Equation

  • Goldman-Hodgkin-Katz Equation: Extends the Nernst equation to consider permeability of multiple ions to determine resting membrane potential.

  • Key differences arise from selective permeability through leak channels:

    • Higher permeability for potassium than sodium at rest, affecting potentials shown via Nernst vs. reality measurements (approx. -70 mV).

Types of Gated Channels for Ion Permeability

  • Ligand-Gated Channels: Open in response to binding of neurotransmitters or other chemicals.

  • Mechanically-Gated Channels: Respond to mechanical changes (e.g., touch or sound).

  • Voltage-Gated Channels: Open in response to changes in membrane potential, allowing rapid shifts in permeability for signaling purposes in neurons and muscles.

These notes capture the cellular mechanisms, transport proteins, and math calculations surrounding cellular function focusing on epithelial cells and subsequent nervous system effects. Understanding these fundamental principles is key for further studies in physiology and biochemistry.