Chapter 25, Problem 43EAP

A proton’s speed as it passes point A is 50,000 m/s. It follows the trajectory shown in FIGURE P25.43. What is the proton’s speed at point B?

Living cells “pump” singly ionized sodium ions, Na⁺, from the inside of the cell to the outside to maintain a membrane potential $\Delta V_{\text{membrane}}=V_{\text{in}}-V_{\text{out}}=-70\text{mV}$ . It is called pumping because work must be done to move a positive ion from the negative inside of the cell to the positive outside, and it must go on continuously because sodium ions “leak” back through the cell wall by diffusion.

a. How much work must be done to move one sodium ion from the inside of the cell to the outside?

b. At rest, the human body uses energy at the rate of approximately 100 W to maintain basic metabolic functions. It has been estimated that 20% of this energy is used to operate the sodium pumps of the body. Estimate- to one significant figure-the number of sodium ions pumper per second.