Active and passive transport are the movement of chemical species from one area of a cell to another. The main difference between the two is that active transport requires chemical energy in the form of ATP while passive transport requires no outside energy. The biggest similarity between the two is that they both involve the movement of chemicals through a membrane. Another similarity between the two is that they both involve working with a concentration gradient. Active transport builds up a concentration gradient, or moves species from areas of lower concentration to areas of higher concentration. Passive transport moves materials along a concentration gradient, or from areas of higher concentration to lower concentration.