Ion pumps and channels generate resting and action potentials

The plasma membranes of neurons, like those of all other cells, are lipid bilayers that are impermeable to ions. However, these impermeable lipid bilayers contain many protein molecules that serve as ion channels and ion pumps (see Chapter 5). Ion pumps and channels are responsible for resting and action potentials.

Ion pumps use energy to move ions or other molecules against their concentration gradients. A major ion pump in the plasma membranes of neurons (and of all other cells) is the sodium-potassium pump. The action of this pump expels Na+ ions from inside the cell, exchanging them for K+ ions from outside the cell (Figure 44.5a; see also Figure 5.13). The sodium-potassium pump keeps the concentration of K+ inside the cell greater than that of the extracellular fluid, and the concentration of Na+ inside the cell less than that of the extracellular fluid. The concentration differences established by the pump mean that K+ would diffuse out of the cell and Na+ would diffuse in if the ions could cross the lipid bilayer.

Ion channels are pores formed by proteins in the lipid bi-layer (see Chapter 5). These water-filled pores allow ions to pass through a membrane, but they are generally selective— they allow some types of ions to pass through more easily than others (Figure 44.5b). Thus, there are potassium chan-

44.5 Ion Pumps and Channels (a) The sodium-potassium pump actively moves K+ ions to the inside of a neuron and Na+ ions to the outside. (b) Ion channels allow specific ions to diffuse down their concentration gradient; K+ ions tend to leave neurons when potassium channels are open,and Na+ ions tend to enter neurons when sodium channels are open.

Na+ and K+ channels

Na+ and K+ channels

Ion Pumps Ion Channels

The Na+-K+ pump moves Na+ and K+ ions against their concentration gradients.

K+ and Na+ ions tend to diffuse down their concentration gradients through ion-specific channels.

The Na+-K+ pump moves Na+ and K+ ions against their concentration gradients.

K+ and Na+ ions tend to diffuse down their concentration gradients through ion-specific channels.

nels, sodium channels, chloride channels, and calcium channels, and there are many different kinds of each. Ions move through channels by diffusion, and can move in either direction. The direction and magnitude of net movement of ions through a channel depends on the concentration gradient of that ion type across the plasma membrane, as well as the voltage across that membrane.

Potassium channels are the most common open channels in the plasma membranes of resting (non-stimulated) neurons. As a consequence, resting neurons are more permeable to K+ than to any other ion. As Figure 44.6 shows, this characteristic explains the resting potential. Because the potassium channels make the plasma membrane permeable to K+, and because the sodium-potassium pump keeps the concentration of K+ inside the cell much higher than that outside the cell, K+ tends to diffuse out of the cell through the channels. As positively charged K+ ions diffuse out of the cell, they leave behind unbalanced negative charges (mostly Cl- ions and protein molecules), generating an electric potential across the membrane that tends to pull positively charged K+ ions back into the cell.

The membrane potential at which the tendency of K+ ions to diffuse out of the cell is balanced by the negative electric potential pulling them back in is called the potassium equilibrium potential. The value of the potassium equilibrium potential can be calculated from the concentrations of K+ on the two sides of the membrane using an equation called the Nernst equation, which is derived from the laws of physical chemistry (Figure 44.7). In general, the resting potential is a bit less negative than this equation predicts because resting neurons are also slightly permeable to other ions, such as Na+ and Cl-.

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Responses

  • Pauliina Katajisto
    What stops k and na diffuse down their concentration gradients at equilibrium?
    8 years ago
  • prima
    Which ions are most likely to cross the plasma membrane of a resting neuron?
    7 years ago
  • Dieter
    Can pumps move ions faster than channels?
    7 years ago
  • cody
    Which ions are most responsible for generating membrane potentials?
    7 years ago
  • mathew mcdermott
    Are ion pumps found in the plasma membrane?
    6 years ago
  • BILBO
    What are ions channels and concentration gradients?
    6 years ago

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