Muscle cells constantly need to remove sodium ions, na+, out of the cell and transport potassium ions, k+, into the cell in order to maintain homeostasis and for the muscles to work properly. both ions are moved against their concentration gradient. which of the following scenarios is depicting the sodium (na+)-potassium(k+) pump? a because polar molecules can pass through the bilipid layer. d because molecules cannot cross through the bilipid layer. d because a protein is needed for nonpolar molecules to cross the membrane. d because the molecules move from a low concentration to a high concentration.