Active transport is the movement of dissolved molecules into or out of a cell through the cell membrane, from a region of lower concentration to a region of higher concentration. The particles move against the concentration gradient, using energy released during respiration.
Sometimes dissolved molecules are at a higher concentration inside the cell than outside, but, because the organism needs these molecules, they still have to be absorbed. Carrier proteins pick up specific molecules and take them through the cell membrane against the concentration gradient.
Examples of active transport include:
|Down a concentration gradient||✓||✓||✗|
|Against a concentration gradient||✗||✗||✓|
|Substance moved||Dissolved solutes||Water||Dissolved solutes|
|Notes||Gases and dissolved gases also diffuse||Partially permeable membrane needed||Carrier protein needed|