Membrane Transport 1

Not responsible for:Nernst Equation, other than to know what it’s used for.

Chapter 12 Membrane Transport

Questions in this chapter you should be able to answer:Chapter 12: 2-14, 16, 17, 19, 20, 21A, B, 22, 23

Membrane Transport 2

To what are membranes permeable?

Where are membrane transport proteins found?

Aquaporin transporters can facilitate H2O transport in some cells

Membrane Transport 3

How do we describe the properties of membrane transport proteins?

1. Symmetry of transport

2. Mechanism of transport

Transport symmetry

Membrane Transport 4

3. Energy requirements

What defines ‘active’transport?

What potential sources of energy for active transport?

Concentration gradients

ATP

Light

Electrical attraction

Membrane Transport 5

What is “membrane potential”

What ions are commonly involved?-- Na+, K+, fixed ions

Outside Insidelow [ K+] High [K+]High [ Na+] Low [Na+]

Read text of Figure 12-21pg 399

Movement of K+ through the‘K Leak Channel’ establishes the cell’s ‘resting potential’

-- concentration gradient-- electrical attraction-- -20 to -200 mA

inside

outside

Membrane Transport 6

What are examples of transport coupled to electrochemical gradients?

Na+ gradientsH+ gradients

called 2O active transport

What is an electrochemical gradient?

concentration gradient+

membrane potential

ECG can drive or impede transport

Membrane Transport 7

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

Question 12-2, pg 395Na/K pump

Membrane Transport 8

How can cells alter membrane potentialand 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

Tick and Sick-4 9

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

Tick and Sick-4 10

Nerve impulses travel from sensory neurons

motor neurons

Membrane Transport 11

What is a nerve impulse??

A nerve impulse is an altered membrane potential

= “action potential”

Wave-like movement

Electrodes can show movementof the action potential

Membrane Transport 12

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 13

An action potential can be studied experimentally?

“Voltage clamping” can be used to manipulate membrane potential

-- & study effects

Stimulating electrode:-- alters membrane potential

Measuring electrode:-- measures current (ion) flow

Voltage-gated ion gates respond

Threshold potential

Refractory period

Measure current Set (‘clamp’)

membrane potential

Schematic of Voltage Clamp Device

axon

Stim

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volta

geM

embr

ane

Pot

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V)

Tick and Sick-4 14

Nerve impulse ion flow

During propagation of Action Potential, waves of Na+ and K+ ions move back and forth across membrane

Due to opening and closing of ion channels-- why does A.P. move??

Membrane Transport 15

How can the properties of different ion channels be studied?

Patch Clamping

Can measure current through a single channel

Question 12-4, p 405

Tick and Sick-4 16

K+ then flows out of cell restores Resting

Potential

Na+ flows into cell Action Potential

Action Potential triggers opening and closing of“voltage-gated Na+ channels” “voltage-gated K+ channels”

Membrane Transport 17

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

closed open closedState of K+Channels

State of Na+Channels

The “wave”

Tick and Sick-4 18

Flow of Na+ ions opens Voltage-gated Na+ channels

voltage-gated Na+ channels

voltage-gated K+ channels

Propagateimpulse

RestoreResting potential

ActionPotential

Tick and Sick-4 19

What happens when nerve impulse reaches the end of axon?

Neurons pass signals

at a synapse

Membrane Transport 20

What happens when an action potential reaches the synaptic complex?

Presynaptic membrane Post-synaptic membrane Voltage-gated Ca++ channels Ligand-gated Na+ channels Exocytosis of vesicles Voltage gated Na+ channels

Synaptic Signaling

Tick and Sick-4 21

“Neuro transmitters” are the chemical signal passed from cell-to-cell

Common examples-- Dopamine-- Serotonin-- Acetylcholine

Some are inhibitory-- GABA

Bind to receptors-- Ligand-gated Na+ Channels

Tick and Sick-4 22

Excitatory Neurotransmitters open Na+ channels -- Na+ flows through open channels

-- alters membrane potential-- create action potential in next cell

Synaptic Signaling

Tick and Sick-4 23

Cell process both excitatory and inhibitory signals

-- may signal or not

Membrane Transport 24

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?