ACTIVE TRANSPORT
Active transport uses energy (in the form of ATP), and materials flow against the concentration gradient.

Carrier-mediated active transport systems use energy and membrane proteins to "pump" certain substances against a concentration gradient. This causes the substance to accumulate on one side of the plasma membrane. An example of this type of active transport is the sodium-potassium pump (Figure 5-15 in your textbook), which consists of a membrane protein that uses ATP to pump Na⁺ (sodium) out of the cell and K⁺ (potassium) into the cell. These unbalanced accumulations of Na⁺ and K⁺ are needed for proper nerve functioning.