Which of the following is the best description of active transport? - A protein pump in the cytosol pumps a molecule up it's concentration gradient by binding to it and changing shape.

- A protein pump embedded in the cell membrane pumps a molecule up it's concentration gradient by binding to it and changing shape. Active transport requires energy (ATP).

- A protein channel embedded in the cell membrane allows molecules to diffuse through it, down it's concentration gradient. No energy (ATP) is required.

- A protein pump embedded in the cell membrane pumps a molecule up it's concentration gradient. The protein pump does not bind to the molecule being transported and does not require energy.