How do substances (ions, proteins, ligands etc. etc.) cross

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When thinking about movement across the cell membrane whether, it is from the outside of the cell or the inside, it is important to remember the fundamental principle of energy conservation. Essentially, energy can be transferred from one form or another but cannot be lost or destroyed.

In dealing with the transport across the cell membrane it is important to understand that any movement of substances requires energy. This energy can come from establishing a gradient or using molecules (such as ATP) to transfer energy.
A gradient can be established through concentration or by electrical potential. This means that on one side of the membrane there is a higher amount of substance or charge and on the other side there is a lower amount of substance or charge. Only by having the membrane in place do we ...