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”