Figure 1. The forces that control the movement of molecules across the cell membrane. (a) Chemical forces control the movement of nonionic molecules across the membrane. At time zero a cell is placed into a bath containing 1 mmol L^–1 urea. The concentration gradient across the membrane is initially large, which creates a large chemical force to move urea into the cell. At equilibrium, the concentration of urea is equal across the membrane and the chemical force has dropped to zero. The two headed arrow indicates that the urea still crosses the membrane, but at equilibrium efflux equals influx. (b) In this example, the cell membrane is permeable to  K⁺, but not  Cl^–. At time zero, the cell containing 100 mmol L^–1  KCl is placed into a bath containing 10 mmol L^–1  KCl. Initially, the concentration gradient generates a concentration force that drives  K⁺ out of the cell. With time (a few milliseconds), a negative potential is created inside the cell by the  Cl^– left behind by  K⁺ efflux. This build-up of negative charge creates an electrical force that pulls  K⁺ back into the cell. At equilibrium the negative membrane potential generates an electrical force that exactly balances the chemical force. Thus, the net  K⁺ flux (summation of inward and outward flux) equals zero. Note that, unlike urea, the concentration of  K⁺ is virtually unchanged between the initial conditions and equilibrium. This is because only a small number of  K⁺ ions need to cross the membrane to generate a voltage large enough to balance the chemical force.

Figure 2. Ion channels and pumps move ions across the plasma membrane. (a) Ion channels form water-filled pores that permit the hydrophilic ions to pass through the hydrophobic lipid bilayer membrane. The direction of ion flux through the open channel is governed by electrochemical forces. (b) Ion pumps move ions by using energy from ATP hydrolysis. In this cartoon the pump will simultaneously transport two different ions in opposite directions (analogous to the  Na⁺/ K⁺ ATPase). The pumps move the ions against their concentration gradients. In other words, the ions are moved from an area of low concentration to an area of high concentration. From left to right: ions bind to sites in the pump (left). This binding triggers the hydrolysis of ATP (middle), which alters the conformation of the pump to release the ions on the opposite side of the membrane (right). Once the ions are released the pump returns to its resting position, ready to bind and transport more ions.

Figure 3. Relative permeability of ions determines the membrane potential for a given set of ion concentrations. (a) Two examples where either  K⁺ (left) or  Na⁺ (right) is the only permeant ion. In each case, the membrane potential equals the equilibrium potential for the permeant ion. (b) In a real cell the membrane is permeable to  K⁺,  Na⁺ and  Cl^–. The Goldman equation is required to determine the membrane potential, as it considers both the ion concentrations across the membrane and the permeability of each ion.