Electric Properties of Cell Membrane

Introduction

All cell membranes exhibit distinct electrical properties due to the unequal distribution of ions between the intracellular and extracellular fluids. This ionic imbalance generates an electrical potential across the membrane, referred to as the membrane potential. This characteristic is essential for numerous physiological processes, particularly in excitable tissues such as nerves and muscles.

Structure of the cell membrane and its electrical properties

The basic structure of the cell membrane

The cell membrane consists of a phospholipid bilayer embedded with proteins. Some of its basic components has a critical role for its electric properties (described below). The combination of these properties causes the cell membrane to function analogously to an electrical circuit. Those is fundamental for understanding the generation and regulation of membrane potential.

Capacitance

Capacitance is the ability to store electrical charge.

Phospholipids function as electrical insulators, preventing the free movement of ions and resulting in charge accumulation. This separation of charge underlies both voltage generation and the capacitance properties of the membrane.

In biological membranes, capacitance is typically about 1 µF/cm².

This property is important because it determines how quickly the membrane potential can change in response to ionic currents.

Conductance

Membrane proteins such as ion channels and transporters facilitate selective ion movement across the membrane. The resulting flow of charged particles generates electrical currents that determine membrane conductance. Meaning - conductance is describing the ease with which ions traverse the membrane.

It depends on the number of open ions channles and their type. Higher conductance allows ions to move more freely, resulting in more rapid changes in membrane potential.

Resistance

Resistance is the inverse of conductance. A membrane exhibiting high resistance restricts ion movement, whereas low resistance permits easier ion flow.

Ion distribution across the membrane

The unequal distribution of ions is a primary factor in generating membrane potential. Potassium ions (K⁺), negatively charged proteins, and phosphates are present at higher concentrations inside the cell compared to the extracellular fluid. Conversely, sodium ions (Na⁺) and chloride ions (Cl⁻) are more concentrated in the extracellular fluid than within the cell.

Meaning - In most cells, the intracellular environment is negatively charged relative to the extracellular space.

These ions exhibit significant concentration gradients across the membrane. The maintenance of these gradients requires active transport mechanisms, which are powered by ATP hydrolysis.

Types of Membrane Potentials

Cells do not maintain a single membrane potential state. Instead, they exhibit various states, including the resting membrane potential, equilibrium potential, and action potential, each with distinct properties.

Resting Membrane Potential

The resting membrane potential is measured when the cell is not actively transmitting signals. In neurons, this value is typically around -70 mV. It is primarily determined by the membrane's permeability to potassium ions relative to other ions, particularly sodium ions.

Equilibrium Potential - Nernst Potential

The Nernst Equation Describes the Relationship of Diffusion Potential to the Ion Concentration Difference Across a Membrane

The equilibrium potential is established when there is no net movement of a specific ion across the membrane. This state depends on the ion's concentration inside and outside the cell, as well as its electrical charge.

The equilibrium potential can be calculated using the Nernst equation:

• E - equilibrium potential
• R - gas constant
• T - temperature, Kelvin unit
• z - charge of the ion
• F - Faraday constant

By using this equations, we get the typical valuse for the main ions determining the electrical properties of the cell -

• Potassium (K⁺): approximately −90 mV
• Sodium (Na⁺): approximately +60 mV
• Chloride (Cl⁻): approximately −70 mV

Action potential

The action potential refers to the rapid change in membrane potential that initiates cellular activity. It encompasses distinct stages and characteristics that may vary among different cell types.

Mechanisms Responsible for Membrane Potential

Selective Permeability of the Membrane

The cell membrane exhibits selective permeability to various ions.

At rest, the membrane is significantly more permeable to potassium ions than to sodium ions. As potassium ions exit the cell, negatively charged proteins and other anions remain, resulting in a negative intracellular charge. Although sodium ions tend to enter the cell, the low permeability to sodium limits their influence on the resting membrane potential.

Na⁺/K⁺ ATPase

The Na⁺/K⁺ ATPase functions as an active transport mechanism that is essential for maintaining ionic gradients across the cell membrane.

For each cycle, 3 sodium ions are transported out of the cell, and 2 potassium ions are transported into the cell. This process results in a net outward movement of positive charge, which contributes modestly to the negative membrane potential.

Intracellular Negative Charges

Negatively charged molecules, such as proteins and phosphates, are present inside the cell and are impermeable to the membrane. These molecules contribute to the negative charge of the intracellular environment.

Their presence enhances the electrical gradient and influences ion movement across the membrane.

Calculation of the electrical properties of the cell membrane

There are two main methods for calculation of the elcetrical properties of cell membrane.

One of them is already menthined and it is the Neanst Equetion. It allow as to calculate the electrical potential for only one ion. The second concept is the Goldman-Hodgkin-Katz concept.

Goldman-Hodgkin-Katz Concept

The resting membrane potential does not correspond to the equilibrium potential of a single ion, since there are multiple ions simultaneously contribute to the over all value.

The Goldman-Hodgkin-Katz equation describes this relationship by accounting for each ion's membrane permeability and the concentration gradients of those ions.

Ions with greater membrane permeability exert a more substantial influence on the membrane potential.

Functional Significance

The electrical properties of the cell membrane are essential for the function of excitable cells. These properties enable the generation of action potentials in neurons, facilitate muscle contraction, and regulate various cellular activities.

These electrical changes occur rapidly compared to other signaling mechanisms within the body.

References

Guyton and Hall Textbook of Medical Physiology - Chapters: Chapters 4 and 5