Active transport

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.

Movement of ions through cells. Shows outside cell and inside cell. Small number of carrier molecules outside, double number inside. Cells travel from outside to inside.

Examples of active transport include:

  • uptake of glucose by epithelial cells in the villi of the small intestine
  • uptake of ions from soil water by root hair cells in plants

Active transport vs diffusion and osmosis

DiffusionOsmosisActive transport
Down a concentration gradient
Against a concentration gradient
Energy needed
Substance movedDissolved solutes WaterDissolved solutes
NotesGases and dissolved gases also diffusePartially permeable membrane neededCarrier protein needed