Essential Biochemistry · 2016-03-09 · KEY CONCEPTS: Section 9-1 • During a nerve impulse, ion...

Lecture Notes for

Chapter 9 Membrane Transport

Essential Biochemistry Third Edition

Charlotte W. Pratt | Kathleen Cornely

Copyright © 2014 John Wiley & Sons, Inc. All rights reserved.

KEY CONCEPTS: Section 9-1

•  During a nerve impulse, ion movements alter membrane potential, producing an action potential that travels along the axon.

•  Transporters obey the laws of

thermodynamics, providing a way for solutes to move down their concentration gradients or using ATP to move substances against their gradients.

© 2014 John Wiley & Sons, Inc. All rights reserved.

Na+ and K+ concentrations are inversely proportional in cells.

•  [Na+] is greater outside of the cell than inside.

•  [K+] is greater inside of the cell than outside.

•  Proteins are required to transport ions across the membrane!


The voltage across a membrane caused by ion transport is the

membrane potential.

Δψ has units of volts. R = Gas Constant = 8.3145 J �K-1 � mol-1 F = Faraday Constant = 96,485 J �V-1 � mol-1

Z = Net charge per ion T = Temperature in Kelvin


Ion movements alter membrane potential.


Propagation of a Nerve Impulse


Action potentials propagate rapidly because axons in mammals are

insulated by a myelin sheath.

Electron micrograph of a myelinated axon © 2014 John Wiley & Sons, Inc. All rights reserved.


•  Porins are β barrel channels with some solute selectivity.

•  Ion channels include a selectivity filter and may be gated.

•  Aquaporins allow only water molecules to pass through.

•  Transport proteins alternate between conformations to expose binding sites on each side of the membrane.


Porins are trimers composed of β sheets.

Each subunit forms a 16-18 stranded membrane-spanning β barrel

Ribbon Diagram Stick Figure


One of the loops in the β barrel constricts the

core and makes the porin specific for small cationic

solutes.


The high selectivity for K+ reflects the geometry of the selectivity filter.


A closer view of the K+ selectivity filter shows a backbone lined with carbonyl

groups with a geometry suitable

for coordinating a K+ ion (purple).


Gated channels undergo conformational changes.


In bacteria, α helices alter their packing arrangements in mechanosensitive ion

channels.


Aquaporins are water specific pores.

Hydrophobic residues and two key Asn residues line aquaporin pores to prevent proton transport.


Some transport proteins can bind more than one type of ligand.

•  Uniport: moves one substance at a time •  Symport: transports two different substances •  Antiport: moves two different substances in different

directions across the membrane


Lactose permease works by a rocking mechanism.

Lactose analog in dark gray spheres



•  Conformational changes resulting from ATP hydrolysis drive Na+ and K+ transport in the Na,K-ATPase.

•  Secondary active transport of a

substance is driven indirectly by the ATP-dependent formation of a gradient of a second substance.


Na,K-ATPase changes conformation as it pumps ions across the membrane.


The Structure of the

Na,K-ATPase


Glucose transport is coupled with Na+ and K+ transport.

Energetically favorable

Energetically unfavorable



•  Neurotransmitters are released by the process of exocytosis.

•  Membrane fusion, driven by the action of

SNARE proteins, requires changes in bilayer curvature.


Acetylcholine is a common neurotransmitter.

Many neurotransmitters are derivatives of amino acids


Acetylcholine is degraded at a neural synapse by acetylcholinesterase.


Events at the Nerve-Muscle Synapse


SNAREs link vesicle and plasma proteins.

•  Soluble N-ethylmaleimide-sensitive-factor attachment protein receptor

•  SNAREs are integral membrane proteins.

•  Complex formation – Two SNAREs from plasma

membrane – One SNARE from synaptic vesicle


Membrane fusion of SNAREs is spontaneous.


Membrane fusion requires changes in bilayer curvature.

How might bilayer

curvature be facilitated?

Removal of an acyl chain could convert shape.


Schematic of Membrane Fusion


Essential Biochemistry · 2016-03-09 · KEY CONCEPTS: Section 9-1 • During a nerve impulse, ion...

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