5-2: Passive Transport
PASSIVE TRANSPORT
Concentration Gradient = regular, graded concentration change over a distance in a particular
direction
Passive Transport = diffusion of a substance across a biological membrane

Moves particles down their concentration gradient (that is, from [Hi] to [Lo])

Does NOT require cellular energy

Is regulated by the permeability of the membrane
Net Directional Movement = overall movement away from the center of concentration, which
results from kinetic molecular movement in all directions
Diffusion = the net movement of a substance down a concentration gradient

Results from the intrinsic kinetic energy of molecules

Result from random molecular motion even though net movement may appear directional

Continues until dynamic equilibrium is reached = the molecules continue to move, but
there is NO net directional movement

A substance diffuses down its own concentration gradient and is not affected by the
gradients of other substances

Differs from bulk flow = when all the molecules move in the same direction at the same
time
o Ex. waterfalls or blood in the body
Water diffuses freely across membranes

Osmosis = diffusion of water down its gradient

Hypertonic Solution = the solution with a higher solute concentration

Hypotonic Solution = the solution with a lower solute concentration

Isotonic Solution = solution with an equal solute concentration
o If two solutions are isotonic, no net movement of water is perceived

Water Potential (ψ)
o is often used to predict the movement of water between two areas of differing
concentrations.
o Water Potential = Pressure Potential + Solute Potential
ψ
=

For open systems, ψp = 0

Ψs = - iCRT

Where:
ψp
+
ψs
o i = ionization constant, C = concentration, R = pressure constant, and T =
temperature in K

So the equation for open systems becomes: ψ = ψs = - iCRT

For pure water, the ψ = 0

Once solutes are added, the solution becomes more concentrated (ψ lowers). {more -}

Water always moves from areas of HIGH ψ to LOW ψ. (That is, from areas of high
water to areas of low water OR from less concentrated areas to more concentrated areas.)
Water and Cells
Animal Cells lack a cell wall.

In isotonic environments, animal cells remain of concentrated volume.

In hypertonic environments, animal cells lose water by osmosis and shrivel = crenate

In hypotonic environments, animal cells gain water by osmosis and swell
o Danger: could perhaps lyse (burst)

Prevent excessive water loss or uptake by:
o Living in isotonic environments (ex. Marine invertebrates make “compatible”
solutes inside their cells)
o Osmoregulation = removing water in hypotonic environments and retaining
water/pumping out salts in hypertonic environments
Organisms with Cell Walls (Plants, Fungi, Prokaryotes, and some Protists)

In a hypotonic environment, water moves into plant cell until the internal pressure against
the cell wall will not allow more to enter
o Turgor = firmness or tension in walled cells that are in hypotonic environments
o Ideal state for most plant cells
o Provides mechanical support for plants (helps them stand up)
o Requires cells to be hypertonic to their environment.

In isotonic environments, turgor is lost and they become flaccid (limp)  wilt

In hypertonic environments, they lose water by osmosis and plant cells may plasmolyze.
o Plasmolysis = phenomenon where the loss of water causes the plasma membrane
to pull away from the cell wall
Facilitated Diffusion = diffusion of (large) molecules and ions across a membrane with the aid
of transport proteins

Is a form of passive transport

Have a specific binding site

Can become saturated or inhibited

May cause a conformational change that translocates the binding site and the attached
solute

May provide a selective channel across a membrane

Gated Channels = protein channels that open in response to an impulse or chemical
stimulus