Cell Transport

*A cell must exchange materials with its surroundings, a process controlled by the cell membrane

*Cell membranes are selectively permeable, regulating what enters/leaves the cell

Passive transport – substances can cross through a cell membrane without using any energy

I. Diffusion – movement of molecules from a high concentration to a

low concentration

a) a difference in concentration = concentration gradient

b) Diffusion will continue until the concentrations are equal =

dynamic equilibrium (movement continues, but equilibrium is

maintained)

c) Diffusion depends on the size, type, and chemical nature

(polarity) of molecule

Ex: Cell membrane is made of phospholipids, which are

nonpolar. Therefore, nonpolar substances will diffuse

through a cell membrane. II. Osmosis – diffusion of water across a cell membrane

a) Isotonic solution: Solute concentration is the same as that

inside the cell; no net water movement across the plasma

membrane (water will move in either direction.)

b) Hypertonic solution: Solute concentration is greater than that

inside the cell (water conc. less than that inside); cell loses

water

*In plant cells, results in a loss of turgor pressure.

1. Hypotonic solution: Solute concentration is less than that inside

the cell (water conc. is greater than that inside); cell gains water

*Cells may burst = cytolysis;

*In a plant cell, the cell membrane is push out against the

cell wall = turgor pressure III. Facilitated Diffusion – diffusion assisted by specific proteins

called carrier proteins

Steps in Facilitated Diffusion

a) Carrier protein binds to molecule

b) Carrier protein changes shape, protecting molecule from the

interior of cell membrane

c) Molecule is released on other side, protein returns to original

shape

*Proteins are specific (or picky) as to what molecule they will

bind to

IV. Ion Channels – proteins that act as small passageways through the

cell membrane for ions (charged atoms)

a) Ion channels are specific to one type of ion

* Sodium ion channels will only let sodium ions through

b) Some are always open, while others have “gates”