In order for a muscle cell to contract, sodium ions (Na+) must be removed from the cell and potassium ions (K+) must enter the cell. Both of these lons travel
against the cell's concentration gradient. This process is most likely achieved
througn-
A. Simole diffusion across the cell membrane using no energy
b. A channel protein using no eneray
C. A protein pump using energy
d. A channel protein using energy

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