Lecture 7: Membrane Transport Essential Cell Biology Fourth Edition Chapter 12

the bilayer (eg. OXYGEN, CO2)

2. PASSIVE (FACILITATED) TRANSPORT – Substances

Carried via a protein, molecule by Molecule. The
TRANSPORT PROTEIN must have a binding site for the
specific molecule transported.

3. ION FLOW THROUGH CHANNELS – ION CHANNELS are
Membrane proteins that can open and close. When
Open they act like a tunnel that can let through
A stream (thousands) of ions.

4. ACTIVE TRANSPORT – Membrane proteins called
PUMPS move molecules against a concentration
Gradient; a source of chemical energy is required (ATP).

at each in more detail…

some things
And not to
others?
SIMPLE
DIFFUSION

is put in pure water, water flows down
Its concentration into

the cell, bursting it. Such
Bursting implies a membrane that allows water
To diffuse but not intracellular materials (until it
Breaks).

How
Do we
Prevent
This
Disaster
In our
body?

– only specific
molecules are transported by a given transport
protein. The

protein has a binding site for
the molecule transported much like an enzyme
has a binding site for substrate.

FACILITATED DIFFUSION

proteins
can move either
ONE OR TWO
specific molecules
at a time

Glucose From the Gut

Be Used in Many Ways

One use is to power facilitated

diffusion. Take
Glucose absorption from the gut:
Can you describe how this transporter is able
To move glucose into the body even against a
Concentration gradient?

Glucose From the Gut

The most famous is the sodium-potassium
Pump found at the

plasma membrane. Here is how it works.












As a result of using ATP (chemical) energy, sodium
and potassium gradients are formed.

Only specific
ions are let through
the “SPECIFICITY
FILTER.” Channels also
close

to stop the flow of
ions; their opening
and closing can
be controlled
by voltage,
chemicals or
mechanical forces

Driving Force! Two possible
Forces are diffusion (conc. gradient) or a

Voltage across the membrane

plasma membrane. First,
there are sodium and potassium
ion chemical

gradients across
the membrane (WHY?). Thus,
when the Na+ and K+ channels are
open, sodium and potassium
flow in opposite directions (why?)
Cells always have more open
K+ channels at rest, thus
Potassium ion flow is most
important in “resting” cells.
But K+ ion flow creates a mismatch of charge resulting in a
voltage. The voltage continues to increase until K+ flow out is
countered by just enough voltage. This “just enough voltage”
is the “resting membrane potential!

plasma membrane. First,
there are sodium and potassium
ion chemical

gradients across
the membrane (WHY?). Thus,
when the Na+ and K+ channels are
open, sodium and potassium
flow in opposite directions (WHY?)
Cells always have more open
K+ channels at rest, thus
Potassium ion flow is most
important in “resting” cells.
But K+ ion flow creates a mismatch of charge resulting in a
voltage. The voltage continues to increase until K+ flow out is
countered by just enough voltage. This “just enough voltage”
is the “resting membrane potential!

cells have many more potassium
Channels open than sodium channels.

Potassium

ions flow down their gradient until
An electrochemical EQUILIBRIUM is reached.

At equilibrium, flow of potassium out and flow
In become equal so they are balanced –
A STEADY STATE.

But this balance will not occur until potassium
Ion flow has moved a little bit of charge across
The membrane to set up a voltage difference
Capable of providing a force that balances
Diffusion. This voltage across the membrane
Can be measured! (How do you think this is done?)

of channels open this will change the “resting”
membrane potential! This

is exactly what happens during an ACTION POTENTIAL in a nerve axon.

A new set of sodium channels open (“activation”) and after
a few milliseconds they close (“inactivation”). Sodium will
Flow into the cell and this will change the membrane
potential temporarily. Why?

This temporary change reduces the membrane potential
from -80 mV past 0 to +40 mV. This temporary
“depolarization” is called the ACTION POTENTIAL.

The Action Potential is temporary because as soon as the
Na+ channels close, K+ channels open: what happens then?

Potential
State of K+

open closed
channels

of the
action potential detect the voltage change at a

distance and begin to open, thus producing an
Even greater
Voltage change.

This cycle
repeats itself
over and over
and the action
potential
moves as a
result.

change triggers a release
Of neurotransmitter by fusion of vesicles with
The

plasma membrane.

and ion flow
Creates a potential change in the next cell.