TRANSPORT
I. Passive Transport
 Does not require energy
 Moves from high concentration to low concentration (downhill)
 3 types: 1. Diffusion and osmosis, 2. Ion channels, 3. Facilitated diffusion
A. Random Motion
 All molecules move randomly because they all have energy
 This is the basis for passive transport and diffusion
B. Diffusion
 Movement of molecules across a semipermeable membrane from an area
of high concentration to an area of low concentration
 Works because where molecules are in a higher concentration, they have a
much better chance of passing through the pores in the membrane
 Movement does occur in both directions, but the NET MOVEMENT is
from high to low
1. NET MOVEMENT: movement in one direction minus
movement in the other direction
 Always occurs from high concentration to low
concentration
 Always specify net movement in terms of its direction
2. Equilibrium
 If movement is allowed to continue, the solutions on both
sides will eventually reach equilibrium
 Equilibrium occurs when the concentrations on both sides
of the membrane are equal
 When this happens, molecules are still diffusing across the
membrane from both sides, but the NET MOVEMENT is
ZERO
3. Osmosis
 Diffusion of water (net movement from high to low, no
energy required)
4. Tonicity
 Describe the flow of water in osmosis
a. Hypotonic – water concentration outside the cell is
greater than the water concentration inside the cell, so the
net movement is from the outside of the cell to the inside of
the cell (high to low) – which could cause the cell to burst
b. Isotonic – water concentration outside the cell is equal
to the water concentration inside the cell, so the net
movement from the outside of the cell to the inside of the
cell is ZERO – best case scenario
c. Hypertonic – water concentration inside the cell is
greater than the water concentration outside the cell, so the
net movement is from the inside of the cell to the outside of
the cell (high to low) – could cause the cell to shrivel up
C. Cell Membrane


Phospholipid heads point to the exterior and interior of the cell and are
hydrophilic (mix well with water)
Phospholipid tails point to the interior of the membrane (repel water)
2 Problems
1. your cells need ions, but ions are repelled by the hydrophobic part
of the membrane
2. your cells need molecules that are too large to fit through spaces
between phospholipids in the cell membrane
D. Solutions
1. Ion channels – proteins that allow ions to diffuse into and out of the
cell (still passive, high to low)
2. Facilitated Diffusion – transport proteins that allow large molecules to
diffuse into and out of the cell (still passive, high to low)
II. Active Transport
 Requires energy, net movement from low concentration to high concentration
A. Pumps – Ex. Sodium-Potassium Pump
B. Endocytosis
 Cell membrane pinches in and takes in a large molecule
1. Phagocytosis – “cell eating”
2. Pinocytosis – “cell drinking
3. Receptor-Mediated Endocytosis – works when receptors are full
C. Exocytosis
 Vesicle fuses with the cell membrane and dumps its contents outside the
cell