lecture 2 - membrane transport

8/12/2019 Lecture 2 - Membrane Transport

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Phospholipids in Archaea



Lipid bilayers are permeable to:

• Gases (O2, CO2, CO, anaesthetics)

• Small uncharged molecules (urea,ethanol)

• Hydrophobic molecules (steroidhormones)

• Water (only slightly)

But impermeable to:

• ions (Na+, K+, H+, Ca2+, Cl – , etc)

• larger polar molecules (glucose)• Most biologically relevant molecules

(ATP, amino acids, proteins etc.)



• There is an asymmetric distribution of ions and

other molecules across cellular membranes

• Examples:



Proteins can assist or are required for

movement of molecules across membranes

FACILITATED TRANSPORT



Passive movement:molecules move down a concentration gradient

- Energetically favorable cells can use energy derived fromthe concentration gradient.

- Solute movement can occur with or without a specific

transporter.

- Passive transport relies on diffusion.

Net diffusion Net diffusion

Net diffusion Net diffusion

Equilibrium

Equilibrium



Animal

cell

Plant

cell

Turgid

(normal)

Flaccid Shriveled

(plasmolyzed)

Plasma

membrane

Lysed Normal Shriveled

Hypotonic solution Isotonic solution Hypertonic solution

H2O

H2O H2O H2O

H2O H2O H2O

Water potential is

higher outside

Water potential is

equal betweeninside and outside

Water potential is

higher inside



• Protein transporters involved in diffusion permit

charged and/or large polar molecules access across

the membrane using aqueous filled channels

•Hydrophobic amino acids associate with membranelipids

• Hydrophilic amino acids line channel through

which molecules move

-

H

-

-

- -

-

-

H

HHH

HH

Na+ Protein channel in membrane



Facilitative transport: movement of molecules down

a concentration gradient assisted by proteins.

Examples:

1) glucose transporter glucose uptake

2) anion transporter (phosphate, carbonate, etc.)

Transporters will bind molecules on the side of themembrane where its concentration is highest andrelease the molecule on the other side (lowerconcentration)



The movementof a single

molecule

across the

membrane by a

protein

transporter is

referred to asUNIPORT



Protein-mediated vs simple movement

can be passive or

active



Active movement:molecules move against a concentration gradient

Energy for movement of a charged or highly polarmolecule is provided by:

- Coupling with the hydrolysis of ATP ATPases.

- Coupling with the concentration gradient formed

across the membrane.



Two types of mechanisms responsible for

active transport

1. Primary active transport (pump): involving ATP directly2. Secondary active transport: 2nd molecule required



Primary active transport: P-type pumps

• P-type pumps are transiently phosphorylatedduring the transport of ions across the membrane .

• Examples:

1) Na+/K+ ATPases can pump Na+ from thecytoplasm in exchange for K+

2) Ca2+ ATPase will pump Ca2+ out of the cell but

will also pump Ca2+ into internal membrane

bound stores

3) H+/K+ ATPase acidification of stomach



Na+ /K+ ATPase transport cycle

OUT

IN



• Multi-subunit pumps foundin membranes of vesiclesand vacuoles (membraneenclosed organelles)

• Phosphate from ATP is nottransferred to the pumpduring ATP hydrolysis

• Generally pump H+ essential for numerouscellular processes

Primary active transport: V-type pumps



• In laboratories: all pumps can be made to "run backwards"

resulting in the production of ATP from an ion (H+, Na+, K+,Ca2+) gradient

• F-type ATPase = backward V-type ATPase

Primary active transport: F-type pumps



• The driving ion andtransported moleculemove in the samedirection:

• Example:- Na+/ glucose transporter:2 molecules of Na+ and 1molecule of glucose (Glc)

moved from outside of cellinto cytoplasm

2Na+ GlcNa+

Glc

OUT

IN (cytoplasm)

Driving Ion = Na+

Secondary active transport: Symport



• The driving ion moves inone direction down its conc.gradient (usually into thecell) but transportedmolecule moves in theopposite direction, againstits conc. gradient. – Eg. Na+/ H+ antiport and Na+/

Ca2+ antiport

Na+Na+Ca2+

OUT

IN (cytoplasm)

Ca2+

Driving Ion = Na+

Secondary active transport: Antiport



• Uptake of glucose

by the gut requires

the activity of

several differenttransporters

32



MEMBRANE POTENTIAL



Figure 11-4b Molecular Biology of the Cell (© Garland Science 2008)

Why are these arrows (flow) different?



The asymmetric distribution of ions across themembrane produces:

1- Chemical gradient

Example: Na+ gradient used for symport/ antiporttransporters.

2- Electrical gradient

Example: Na

+

/K

+

ATPase is an electrogenic pump.

Gradients across membrane



Na+ /K+ ATPase transport cycle

OUT

IN



Measurement of resting membrane potential



• Membrane potential: difference in electric chargeacross the plasma membrane. – Negative inside ranges from –15 to –100 mV

(generally –70 mV) – Resting potential: capability of cells to undergo dramatic

electrical changes• Examples: nerve and muscle cells

• Nernst equation: contribution of each ion to themembrane potential of a cell.

=

[ ]

[ ]= 2.303

log

1

[ ]

[ ]

V : equlibrium potential in Volts (internal potential minus external potential)

R : gas constant (2 cal mol-1 °K –1)

T : absolute temperature (°K)

F : Faraday’s constant (2.3 x 104 cal V –1 mol –1)

z : valence (charge) of the ion



• Non-excitable cells: membrane potential is fairly

constant (unable to change rapidly in response to a

stimulus).

• Excitable cells: membrane potential can change rapidly

in response to a stimulus.

– Rapid electrical change is due to ion flow down the

concentration gradient through ion channels (usually

selective for one type of ion such as K+, Ca2+, Na+)



Ligand-gated ion channels open upon binding a

chemical messenger.

• Example: chemical synapse binding of acetyl

choline to its receptor opens the Na+ channel

allowing influx of Na+.

Ligand

Closed Open

IN

OUT OUT

IN

Ion channel class 1: Ligand-gated



Voltage-gated ion channels open in response todepolarization of the membrane.

• Examples: voltage-gated Na+, K+, Ca2+, Cl – channels

membrane

potential

Closed Open

++++ + + +

----- - - - -IN

OUT OUT

IN

----- - - - -

++++ + + +

Ion channel class 2: Voltage-gated



Mechano-gated channels:• Open in response to stretching, pressure changes or

deformation of the cell surface.

• help sense external changes to its membrane that have

resulted from damage

Stretching

Open

IN

OUT OUT

IN

Closed

Cytoskeleton

filament

Ion channel class 3: Mechano-gated

Ion channel class 4:



• Phoshorylation-dependent channels open whenphosphorylated.

• Phosphorylation is caused by binding to external signals.

Closed Open

IN

OUT OUT

INPP

kinase

phosphatase

Ion channel class 4:

Phosphorylation-dependent

A ti t ti l



Action potential

• Excitatory cells are able

to generate and

propagate an actionpotential.

• The membrane potential

voltages change fromnegative to positive

depolarization.



Action potential occurs

due to sequential

changes in Na+ and K+

permeability.

channel open/

closures cause

charge differences to

build up on the

membrane.



• Action potential is usually initiated at the

chemical synapse via ligand-gated ion channels

and propagated along the axon by opening of

voltage-gated channels

Net charge

displacement

is critical.



• Refractory period of ion channels ensures that

action potential is propagated in one direction

channels cannot re-open for several milliseconds

after inactivation.



In vertebrates, myelinated neurons increase the speed at

which electrical impulse is propogated in vertebrates dielectric property of myelin (80% lipid and 20% protein)

insulates the axon

Myelinated neurons



• Voltage-gated Na+ channels are concentrated at unwrapped

gaps (nodes) along the myelin sheath

• Nodes are located between adjacent Schwann cells.

• Channels open in response to voltage change.• Saltatory movement Na+ propagation of current.

Monospermic fertiliation



Membrane depolarization is a physiological block to

polyspermy. • Sea urchin eggs: depolarization by influx of Na+ ions

• Frog eggs: depolarization by efflux of Cl – ions.

• Human eggs: depolarization by Ca2+ efflux.

Monospermic fertiliation

lecture 2 - membrane transport

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