Section 1

Passive Transport

• Passive transport involves the movement of molecules across the cell membrane without an input of energy by the cell.

• Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, driven by the molecules' kinetic energy until equilibrium is reached.

• Molecules can diffuse across a cell membrane by dissolving in the phospholipid bilayer or by passing through pores in the membrane.

• Osmosis is the diffusion of water across a membrane. The net direction of osmosis is determined by the relative solute concentrations on the two sides of the membrane.

• When the solute concentration outside the cell is lower than that in the cytosol, the solution outside is hypotonic to the cytosol, and water will diffuse into the cell.

• When the solute concentration outside the cell is higher than that in the cytosol, the solution outside is hypertonic to the cytosol, and water will diffuse out of the cell.

• When the solute concentrations outside and inside the cell are equal, the solution outside is isotonic, and there will be no net movement of water.

• To remain alive, cells must compensate for the water that enters the cell in hypotonic environments and leaves the cell in hypertonic environments.

• In facilitated diffusion, a molecule binds to a carrier protein on one side of the cell membrane. The carrier protein then changes its shape and transports the molecule down its concentration gradient to the other side of the membrane.

• Ion channels are proteins, or groups of proteins, that provide small passageways across the cell membrane through which specific ions can diffuse.

Vocabulary passive transport (p. 97) diffusion (p. 97) concentration gradient (p. 97) equilibrium (p. 97)

osmosis (p. 98) hypotonic (p. 98) hypertonic (p. 98) isotonic (p. 99)

contractile vacuole (p. 99) turgor pressure (p. 100) plasmolysis (p. 100) cytolysis (p. 100)

facilitated diffusion (p. 101) carrier protein (p. 101) ion channel (p. 102)

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