The concentration gradient refers to the difference in concentration between two areas
or substances, for example the intracellular and interstitial fluids. The movement from a high to low concentration is referred to as movement with or down the concentration gradient, while movement from an area of low to high concentration is referred to as movement against or up the concentration gradient (Tortora and Derrickson 2009a). There are two general types of movement: passive transport and active transport (Thibodeau and Patton 2012).