Membrane potential and electrochemical gradients are key players in cellular transport. These electrical and chemical forces drive the movement of ions and molecules across cell membranes, shaping cellular functions and communication.

Understanding these concepts is crucial for grasping how cells maintain their internal environment and respond to external stimuli. They form the foundation for various transport mechanisms, from passive diffusion to active pumping of molecules against concentration gradients.

Membrane potential and cellular processes

Significance of membrane potential

• Membrane potential is the electrical potential difference across a cell's plasma membrane
  • Inside of the cell is typically more negative relative to the outside
• Crucial for various cellular processes
  • Transport of ions and molecules across the membrane
  • Cell signaling
  • Generation and propagation of electrical signals in excitable cells (neurons, muscle cells)

Resting and changing membrane potentials

• Resting membrane potential is the steady-state potential difference across the membrane when the cell is not stimulated or actively transporting ions
• Changes in membrane potential can trigger specific cellular responses and regulate cellular activities
  • Depolarization decreases the magnitude of the membrane potential, making the inside of the cell less negative
  • Hyperpolarization increases the magnitude of the membrane potential, making the inside of the cell more negative

Electrochemical gradients in membrane transport

Components of electrochemical gradients

• Electrochemical gradients are the combined effects of the electrical potential difference (membrane potential) and the chemical concentration difference of ions across a cell membrane
• Two main components of an electrochemical gradient
  • Electrical gradient driven by the membrane potential
  • Chemical gradient driven by the difference in ion concentrations across the membrane

Role of electrochemical gradients in membrane transport

• Electrochemical gradient of an ion determines the direction and magnitude of its net movement across the membrane
  • Ions tend to move down their electrochemical gradient
• Essential for facilitating various types of membrane transport
  • Passive transport (diffusion, facilitated diffusion)
  • Active transport (primary and secondary active transport)
• Sodium-potassium pump (Na+/K+ ATPase) maintains the electrochemical gradients of Na+ and K+ ions
  • Primary active transporter that pumps Na+ out of the cell and K+ into the cell against their concentration gradients

Factors influencing membrane potential

Selective permeability and ion concentrations

• Main factors contributing to the establishment and maintenance of membrane potential
  • Selective permeability of the plasma membrane
  • Concentration gradients of ions across the membrane
  • Activity of ion channels and pumps
• Plasma membrane is selectively permeable, allowing certain ions to pass through more easily than others
  • K+ passes more easily compared to Na+, Ca2+, and Cl-
  • Contributes to the unequal distribution of ions across the membrane
• Concentration gradients of ions, particularly K+ and Na+, are maintained by the sodium-potassium pump and selective membrane permeability

Ion channels and the resting membrane potential

• Ion channels allow passive movement of ions across the membrane based on their electrochemical gradients and gating properties
  • Leak channels are always open
  • Voltage-gated channels open or close in response to changes in membrane potential
• Resting membrane potential is primarily determined by the concentration gradient and permeability of K+ ions
  • Membrane is most permeable to K+ at rest
• Goldman-Hodgkin-Katz (GHK) equation describes the relationship between membrane potential and permeabilities and concentrations of major ions (K+, Na+, Cl-) across the membrane

Membrane potential vs ion distribution

Unequal ion distribution and the resting potential

• Membrane potential is directly related to the distribution of ions across the cell membrane
  • Unequal distribution of ions is the primary reason for the existence of a membrane potential
• At resting membrane potential, there is a higher concentration of K+ inside the cell and a higher concentration of Na+ outside the cell
  • Maintained by selective permeability of the membrane and activity of the sodium-potassium pump

Nernst equation and equilibrium potentials

• Nernst equation calculates the equilibrium potential for a specific ion based on its concentration gradient across the membrane
  • Assumes the membrane is permeable only to that ion
• Resting membrane potential is typically closer to the equilibrium potential of K+ because the membrane is most permeable to K+ at rest

Changes in membrane potential and ion distribution

• Changes in membrane potential (depolarization, hyperpolarization) are caused by the net movement of ions across the membrane
  • Alters the distribution of ions and the membrane potential
• During an action potential in excitable cells, rapid changes in membrane potential occur due to the opening and closing of voltage-gated Na+ and K+ channels
  • Results in a temporary reversal of the membrane potential and a redistribution of ions