Active transport in cells depends on the availability of energy to enable molecules and ions to move through the cell membrane. On the other hand, passive transport depends on kinetic energy.
Proteins, which are situated in the cell’s lipid bilayer, are responsible for much of the energy needed. Once the proteins set themselves up across the membrane, the molecules and ions can move in and out of the cell. One portion of the protein is inside the membrane while the other portion is outside the membrane. The many proteins are very specific as to what they move. If a protein moves calcium it does not move other substances such as glucose or magnesium across the membrane. Active transport generally moves the molecules and ions from an area of low concentration to an area of high concentration meaning it is crossing the concentration gradient. This is where the energy is needed because the substances meet with resistance as the move through the barrier.