Section 2

Active Transport

• Active transport moves molecules across the cell membrane from an area of lower concentration to an area of higher concentration. Unlike passive transport, active transport requires cells to expend energy.

• Some types of active transport are performed by carrier proteins called cell membrane pumps.

• One example of a cell membrane pump is the sodium-potassium pump. It moves three Na+ ions into the cell's external environment for every two K+ ions it moves into the cytosol. ATP supplies the energy that drives the pump.

• Endocytosis and exocytosis are active transport mechanisms in which large substances enter or leave cells inside vesicles.

In endocytosis, the cell membrane folds around something in the external environment and forms a pouch. The pouch then pinches off and becomes a vesicle in the cytoplasm.

Endocytosis includes pinocytosis, in which the vesicle contains solutes or fluids, and phagocytosis, in which the vesicle contains large particles or cells.

In exocytosis, vesicles made by the cell fuse with the cell membrane, releasing their contents into the external environment.

Vocabulary active transport (p. 103) sodium-potassium pump (p. 103)

endocytosis (p. 105) vesicle (p. 105)

pinocytosis (p. 105) phagocytosis (p. 105)

phagocyte (p. 105) exocytosis (p. 106)

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