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Transcript
Chapter 12 Membrane Transport
Not responsible for:
Nernst Equation, other than to know what it’s used for.
Questions in this chapter you should be able to answer:
Chapter 12: 2-14, 16, 17, 19, 20, 21A, B, 22, 23
Membrane Transport
1
To what are membranes permeable?
Where are membrane transport proteins found?
Aquaporin transporters can
facilitate H2O transport in
some cells
Membrane Transport
2
How do we describe the properties of membrane transport proteins?
1. Symmetry of transport
2. Mechanism of transport
Transport symmetry
Membrane Transport
3
3. Energy requirements
What defines ‘active’
transport?
What potential sources of energy for active transport?
Concentration gradients
ATP
Light
Electrical attraction
Membrane Transport
4
What is “membrane potential”
Outside
low [ K+]
High [ Na+]
Inside
High [K+]
Low [Na+]
What ions are commonly involved?
-- Na+, K+, fixed ions
Read text of
Figure 12-21
pg 399
Movement of K+ through the
‘K Leak Channel’ establishes
the cell’s ‘resting potential’
outside
-- concentration gradient
-- electrical attraction
-- -20 to -200 mA
inside
Membrane Transport
5
ECG can drive
or impede transport
What is an
electrochemical
gradient?
concentration gradient
+
membrane potential
What are examples of
transport coupled to
electrochemical gradients?
Na+ gradients
H+ gradients
called 2O active transport
Membrane Transport
6
What are the properties and functions of the Na-pump (Na/K pump)
-- 3 Na+ out for 2 K+ in
What are its…
Symmetry?
Mechanism?
Energy requirement?
What are its functions?
maintenance of tonicity
maintenance of the Na+ gradient
Na/K pump
Question 12-2, pg 395
Membrane Transport
7
How can cells alter membrane potential
and respond to its changes?
-- ‘gated’ ion channels
Nerve impulse;
muscle contraction
-- Voltage-gated and ligand-gated
Sense of hearing;
Plant movements
-- Mechanically-gate channels
Dirurnal cycling;
Phototaxis
-- Light-gated channels
Membrane Transport
8
At sensory cells, opening of ion transporters creates
nerve impulse
Stimulus opens Na+ channels
e.g., stereocilia of
inner ear
Na+ rushes into cell
Changes membrane potential
Membrane Transport
9
Nerve impulses travel from
sensory neurons
 motor neurons
Membrane Transport
10
What is a nerve impulse??
A nerve impulse is an
altered membrane
potential
= “action potential”
Wave-like movement
Electrodes can show movement
of the action potential
Membrane Transport
11
The cellular foundation of “neurobiology”
-- the giant neuron of the squid
Used to study factors that
affect action potential
-- e.g., effect of [Na+]
Membrane Transport
12
An action potential can be studied
experimentally?
Schematic of Voltage Clamp Device
“Voltage clamping” can be used to
manipulate membrane potential
-- & study effects
Measure
current
Voltage-gated ion gates respond
Threshold potential
Refractory period
Stimulating
voltage
Measuring electrode:
-- measures current (ion) flow
axon
Membrane
Potential (mV)
Stimulating electrode:
-- alters membrane potential
Set (‘clamp’)
membrane
potential
Membrane Transport
13
During propagation of Action Potential, waves of Na+ and K+ ions
move back and forth across membrane
Nerve impulse ion flow
Due to opening and closing of ion channels
-- why does A.P. move??
Membrane Transport
14
How can the properties of
different ion channels be
studied?
Patch Clamping
Can measure current
through a single channel
Question 12-4, p 405
Membrane Transport
15
Action Potential triggers
opening and closing of
“voltage-gated Na+ channels”
“voltage-gated K+ channels”
Na+ flows
into cell
Action
Potential
K+ then flows
out of cell
restores
Resting
Potential
Membrane Transport
16
Why do the Na+ and K+ channels open and close at different times?
-- Membrane potential alters their state
voltage-gated Na+ channels
-- 3 states
voltage-gated K+ channels
-- 2 states
State of Na+
Channels
State of K+
Channels
closed
open
closed
The “wave”
Membrane Transport
17
Flow of Na+ ions opens Voltage-gated Na+ channels
Action
Potential
voltage-gated
Na+ channels
Propagate
impulse
voltage-gated
K+ channels
Restore
Resting potential
Membrane Transport
18
Neurons pass signals
at a synapse
“Neuro transmitters”
Common examples
-- Dopamine
-- Serotonin
-- Acetylcholine
Some are inhibitory
-- GABA
Bind to receptors
-- Ligand-gated Channels
Membrane Transport
19
What happens when an action potential reaches the synaptic complex?
Presynaptic membrane
Post-synaptic membrane
Voltage-gated Ca++ channels
Exocytosis of vesicles
Release of neurotransmitters
Ligand-gated channels
Voltage-gated Na+ channels
Synaptic Signaling
Membrane Transport
20
Excitatory Neurotransmitters
 ligand-gated Na+ channels
Inhibitory Neurotransmitters
 ligand-gated Cl-- channels
Membrane Transport
21
Adapted form question 19-19.
The inside of endosomes is acidic, which is achieved by the presence
of a H+ pump in the endosome membrane. The endosome membrane
also contains a transporter which pumps Cl- into the endosome. If a
mutation eliminates the Cl- pump, acidification of the endosome is
impaired.
A, Draw a diagram of the endosome showing the two transporters and
the movement of ions.
B. Why does a mutation to the Cl- pump impair endosome acidification?
C. Could a Ca++ pump substitute for the Cl- pump? Why or why not?
Membrane Transport
22