The cellular process known as the sodium-potassium pump was discovered in the 1950s by Jens Christian Skou, a Danish scientist. This process is a form of active transport that moves three sodium ions to the outside of a cell for every two potassium ions that it moves into the cell. Which of these best explains why energy is needed for active transport? A. Ions are moved against the concentration gradient. B. Ions are negatively charged. C. Ions are attached to large proteins. D. Ions are trapped inside the plasma membrane.

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