The sodium-potassium ion exchange pump
A) must reestablish ion concentrations after each action potential.
B) transports sodium ions into the cell during depolarization.
C) transports potassium ions out of the cell during repolarization.
D) moves sodium and potassium opposite to the direction of their electrochemical gradients.
E) depends on a hydrogen gradient for energy.
D
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Senescence of the immune system makes older people more susceptible to cancer and infectious disease because of a decline in ________
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