Processes at Membranes and Plasma Membrane Resting Potential

Comprehensive vocabulary flashcards covering membrane transport mechanisms, diffusion kinetics, carrier proteins, active transport, and resting membrane potentials from the lecture notes.

Factors Affecting Membrane Permeability

1. Lipid solubility of the diffusing substance, 2. Size and shape of the diffusing substance, 3. Temperature, 4. Membrane Thickness.

Simple Diffusion

The result of the movement of molecules due to their thermal motion, occurring through the lipid bilayer or selective protein channels.

Fick's Law of Diffusion Flux ($J_{diff}$)

$J_{diff} = k \times A \times D \times \frac{\Delta C}{\Delta x}$ measured in $\text{mol/sec}$, where $J$ is diffusion flux, $D$ is the diffusion coefficient, $\Delta C$ is the concentration gradient, and $\Delta x$ is the change in position (thickness).

Ion Channels

Narrow, highly selective pores that can open and close, allowing approximately $100 \times 10^6$ ions to pass through per second; they always facilitate passive transport.

Voltage-Gated Ion Channels

Channels sensitive to the voltage across the cell membrane that open when voltage changes to a trigger or threshold level.

Lidocaine

A local anaesthetic that acts by blocking voltage-gated $Na^+$ channels.

Facilitated Diffusion

Transport for molecules with low permeability that requires carrier proteins to move molecules down their chemical or electrochemical gradient without energy.

Carrier Proteins

Ubiquitous integral membrane proteins (usually with 12-14 transmembrane helices) that are selective, reversible, saturable, and transport at a medium velocity of $1000$ molecules/sec.

Uniporter

A carrier type that transports only one molecule type, such as the Glucose uniporters ($GLUT1$, $GLUT2$, $GLUT4$).

Symporter

A coupled transporter that moves two different molecular types in the same direction, such as the $Na^+$-glucose symporter.

Antiporter

A coupled transporter that moves two different molecular types in opposite directions, such as the Band 3 Protein ($HCO_3^-/Cl^-$) in erythrocytes.

Band 3 Protein

A multi-pass facilitated diffusion anion antiporter in RBCs that exchanges $Cl^-$ for $HCO_3^-$ to transport $CO_2$ to the lungs.

Primary Active Transport

A process that uses an energy source ($ATP$) directly to transport molecules against their concentration gradient.

Secondary Active Transport

A process that uses the energy of a concentration or electrochemical gradient created by primary active transport, such as the $SGLT1$ $Na^+$-glucose symporter.

The $Na^+\text{-}K^+$ Pump

A primary active transporter that pumps $3\,Na^+$ ions out and $2\,K^+$ ions into the cell per cycle using one $ATP$, maintaining osmotic balance and gradients.

Ouabain

A substance that blocks the $Na^+\text{-}K^+$ ATPase pump.

Electrogenic Transport

A transport system that affects the cell charge, such as the $Na^+/Ca^{2+}$ Exchanger with a stoichiometry of $3:2$.

Resting Membrane Potential

The voltage of a resting cell, typically between $-40$ and $-90\,mV$, determined by ion gradients and open leak channels.

Leak Channels

Channels that are open all the time; the best-known type is $K^+$ channels, which allow $K^+$ to go down its concentration gradient out of the cell.

Nernst Equation

$V = \frac{RT}{zF} \ln\left(\frac{C_o}{C_i}\right)$, used to calculate the equilibrium potential ($V$) for a single ion given its outside ($C_o$) and inside ($C_i$) concentrations.

Potassium Equilibrium Potential ($V(K)$)

Approximately $-100\,mV$ when the concentration ratio of $[K]_o/[K]_i$ is $3.5\,mM/150\,mM$.

Ionotropic Receptors

Ligand-gated ion channels that provide fast (millisecond) responses by changing conformation to allow ions to enter or exit the cell.

Metabotropic Receptors

$G$ protein-coupled receptors that mediate intracellular signaling and provide slow (second) responses.

Nicotinic Acetylcholine Receptor ($nAChR$)

A prototypic cationic ionotropic ligand-gated ion channel.

Anionic Ionotropic Receptors

Examples include $GABA_A$ and Glycine receptors.

Metabotropic Receptor Examples

Muscarinic acetylcholine ($mAChR$), $GABA_B$, Adrenergic, Histaminergic, and Dopaminergic receptors.