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This tutorial will help you to understand how a concentration gradient across a membrane is used. When a molecule or an ion is moved across a membrane from an area of low concentration to an area of high concentration then a gradient is generated. This gradient can be chemical or it can also create a difference in electrical charge across the membrane if ions are involved. The proton pump generates an electrical and chemical gradient that can be used to create ATP which can drive a large number of different biochemical reactions.