What is a concentration gradient and how does it work with active transport?

Active potential uses energy to pump molecules against a concentration gradient. A difference in concentration of molecules in physical space is called a concentration gradient. Even in the presence of a concentration gradient, many molecules can't move across the cellular membrane. Large molecules cannot fit through the double layer of phospholipids, charged molecules are repelled, and hydrophilic molecules have trouble crossing the hydrophobic interior of the membrane. This often results in higher concentrations of chemicals on one side of the membrane than on the other.

Active transport is carried out by membrane proteins. Pumps are proteins that use energy to carry substances accross the cell membrane. Pumps can transport substances from a low concentration to a high concentration. Active transport requires ATP energy to function.