Principles Of Ion Channel Mechanisms A The Electrochemical Gradient

The electrochemical gradient determines the direction that ions will flow through an open ion channel and is a combination of two types of gradients: a concentration gradient and an electrical field gradient. We can consider these two gradients separately. Figure 1a shows two compartments that contain an aqueous solution of ions separated by a membrane. It is apparent that there is a concentration gradient, since the left side contains more ions than the right. Assuming the membrane is permeable to the ion, there will be a net movement of ions from the right to left side until the concentrations of ions on both sides are the same. In this case, when the concentrations on both sides equalize, the solution will have reached

Figure 1 The electrochemical gradient is a combination of the concentration gradient and the electrical potential. The panels show two compartments that contain a solution of ions (circles). An ion-permeable membrane separates the two compartments. (a) There is a higher concentration of ions on the left side; therefore, ions will tend to diffuse from the left to the right (direction of arrow). It is also apparent that there is an osmotic gradient. Initially, water will tend to flow from the right to the left. At equilibrium, however, the two sides will be isoosmotic. (b) A voltage has been applied across the membrane. As a consequence, positively charged ions are drawn to the left and negatively charged ions to the right.

Figure 1 The electrochemical gradient is a combination of the concentration gradient and the electrical potential. The panels show two compartments that contain a solution of ions (circles). An ion-permeable membrane separates the two compartments. (a) There is a higher concentration of ions on the left side; therefore, ions will tend to diffuse from the left to the right (direction of arrow). It is also apparent that there is an osmotic gradient. Initially, water will tend to flow from the right to the left. At equilibrium, however, the two sides will be isoosmotic. (b) A voltage has been applied across the membrane. As a consequence, positively charged ions are drawn to the left and negatively charged ions to the right.

equilibrium. At equilibrium, an equal number of ions will diffuse across the membrane in both directions, and the concentrations of ions on either side will not change.

The electrical field gradient takes into account the charge on the ion. In Fig. 1b, an electrical potential has been applied so that the left side is negatively charged and the right side is positively charged. Ions that are positively charged will flow into the left compartment until it reaches a new equilibrium, in which the electrostatic forces that pull the cations into the left side are balanced by the tendency for the ions to move down its concentration gradient. Negatively charged ions will tend to flow into the right compartment. In this equilibrium, the final concentrations of ions on both sides are not equal.

The relationship between the electrical potential and the magnitude of the concentration gradient that is created is intuitive: The stronger the electrical potential, the greater the concentration gradient. The Nernst equation describes this relationship:

where Ex is the electrical potential with units of millivolts, R is the gas constant, Tis the temperature, z is the valence of the ion, and F is Faraday's constant. In words, the Nernst equation states that an electrical potential, Ex, will produce a concentration gradient with the ratio [X]out/[X]in when the membrane is permeable to the ion. The converse is also true; a concentration gradient, [X]out/[X]in, will generate an electrical potential, Ex. Near room temperature (20°C), the Nernst equation simplifies to

Ex is also referred to as the equilibrium potential or the Nernst potential.

The cell is similar to the compartments in Fig. 1, only more ions need to be considered. The membrane potential is determined by the permeability of the membrane to a given ion. Figure 2 gives the concentrations of Na+, K+, Cl", and Ca2+ inside and outside a typical cell. Two processes work to maintain the concentration gradients of these ions. The action of ion pumps helps keep the cytoplasmic concentrations ofNa + , Cl", and Ca2+ lowandtheK + concentration high. Second, the presence of macromolecular anions inside the cell, such as proteins, tends to produce gradients of ions on their own. The redistribution of ions due to fixed charges in the cell is referred to as the Donnan effect.

At rest, the membrane is permeable to K+; therefore, the resting potential of the cell is near EK (in Fig. 2, approximately —84mV). The flow of ions through the membrane is not large enough to affect changes in the ionic composition inside or outside the cell. Experimentally, currents can be elicited by changing the electrical potential across the membrane in short pulses. At membrane potentials of approximately —84 mV, there will be small K+ currents since the membrane potential is near EK, at which there is no net flow of K+ across the membrane. Above EK, there will be a net flow of K+ outward because the electrical field gradient is not large enough to balance the concentration gradient. This results in an outward current (Fig. 3a). Below EK, there will be a net inward current because the stronger electrical field gradient will tend to pull more K+ into the cell. With the opening of sodium channels, the membrane becomes predominantly permeable to Na+ rather than K+. The low concentration of Na+ inside the cell and the negative

Figure 2 The concentration of ions inside and surrounding a typical mammalian cell. The equilibrium potentials were calculated using Eq. (2).

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