The Na⁺/K⁺ ATPase pump moves 3 Na⁺ out of the cell and 2 K⁺ into the cell per ATP hydrolyzed. What is the net electrical effect on the cell interior?
ANo net change, because ions are exchanged in both directions
BThe interior becomes slightly more negative (hyperpolarization)
CThe interior becomes slightly more positive (depolarization)
DThe effect depends entirely on the current membrane potential
3 positive charges leave and 2 positive charges enter per cycle, for a net outward movement of 1 positive charge. This makes the cell interior slightly more negative, contributing to the resting membrane potential. The pump is electrogenic — it moves charge, not just concentration.
Question 2 True / False
Active transport usually moves molecules from a region of low concentration to a region of high concentration.
TTrue
FFalse
Answer: False
Active transport moves substances against their electrochemical gradient, which combines both concentration and electrical charge. For charged ions, a molecule can be at lower concentration inside the cell but the electrical gradient may favor its entry. The relevant driving force is the electrochemical potential, not concentration alone.
Question 3 Short Answer
Explain the difference between primary and secondary active transport, and give an example of each.
Think about your answer, then reveal below.
Model answer: Primary active transport directly couples ATP hydrolysis to ion movement (e.g., the Na⁺/K⁺ ATPase pump). Secondary active transport uses the electrochemical gradient established by primary transport to drive a second molecule against its gradient (e.g., the sodium-glucose cotransporter SGLT, which uses the inward Na⁺ gradient to pull glucose into intestinal cells).
Secondary active transport does not directly consume ATP, but it is still 'active' because it ultimately depends on the energy invested in creating the ion gradient by primary transport. This is why inhibiting the Na⁺/K⁺ pump eventually halts secondary transport as well.