In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellular energy to achieve this movement. There are two types of active transport: primary active transport that uses adenosine triphosphate (ATP), and secondary active transport that uses an electrochemical gradient.

Some examples of active transport include:

• Phagocytosis of bacteria by macrophages
• Movement of calcium ions out of cardiac muscle cells
• Transportation of amino acids across the intestinal lining in the human gut
• Secretion of proteins such as enzymes, peptide hormones, and antibodies from various cells
• Functioning of white blood cells to defend invading diseases