Molecular Biology of the CellFifth Edition

Molecular Biology of the CellFifth Edition

Chapter 10Membrane Structure

Chapter 10Membrane Structure

Copyright © Garland Science 2008

Alberts • Johnson • Lewis • Raff • Roberts • WalterAlberts • Johnson • Lewis • Raff • Roberts • Walter

Figure 10-1 Molecular Biology of the Cell (© Garland Science 2008)

Biological membranes tend to form bilayers studded with various membrane-bound & transmembrane proteins

The fluid nature of the cell membrane allows lateral movement of many lipid and protein components; a feature which is described in the Fluid Mosaic Model

Overview of Membrane Functions

• Compartmentalization – Membranes form continuous sheets that enclose intracellular compartments.

• Scaffold for biochemical activities – Membranes provide a framework that organizes enzymes for effective interaction.

• Selectively permeable barrier – Membranes allow regulated exchange of substances between compartments.

Overview of Membrane Functions

• Transporting solutes – Membrane proteins facilitate the movement of substances between compartments.

• Responding to external signals – Membrane receptors transduce signals from outside the cell in response to specific ligands.

• Intracellular interaction – Membranes mediate recognition and interaction between adjacent cells.

• Energy transduction – Membranes transduce photosynthetic energy, convert chemical energy to ATP, and store energy.

Membrane & storage lipids

Storage lipids = energy storage; dietary lipids (i.e., triglycerides)

Phospholipid structure

Phospholipids are the major lipid found in biological membranes

Figure 10-2 Molecular Biology of the Cell (© Garland Science 2008)

Fatty acids

Saturated Unsaturated

Effects on membrane fluidity?…….

Lipid-orderedstate

Lipid-disorderedstate

What effectdoes saturation vs.unsaturation of FAhave on natural fats?On phospholipid membrane structure?

Steroids

Shape of lipids can determine Overall structure of “membrane”

Figure 10-8 Molecular Biology of the Cell (© Garland Science 2008)

Lipid content reflects function

What kind of structures allow for transmembrane domains?

Membrane Proteins

Membrane Proteins

Alpha-helix or barrel are found in membrane proteins

Bacteriorhodopsin

TM: Transmembrane Domain (-barrel)

-hemolysintoxin

Glycophorin

Bacteriorhodopsin

Hydrophobicity plot: predicts location of transmembrane

domains in proteins

helices

sheets, i.e. barrels

Some typesof proteinlipidation: importanceis in localization of specific proteins to the membrane (i.e., duringsignal transduction pathway activation)

ER lumenCytosol

Microdomains (lipid rafts) in plasma membrane

Idea of localizedsignal transductionmodules

Molecular Biology of the CellFifth Edition

Molecular Biology of the CellFifth Edition

Chapter 11Membrane Transport of Small Molecules and the Electrical

Properties of Membranes

Chapter 11Membrane Transport of Small Molecules and the Electrical

Properties of Membranes

Copyright © Garland Science 2008

Alberts • Johnson • Lewis • Raff • Roberts • WalterAlberts • Johnson • Lewis • Raff • Roberts • Walter

Figure 11-1 Molecular Biology of the Cell (© Garland Science 2008)

For simple diffusion: solutes will have different rates of diffusion depending upon solute polarity, size, & solute concentration gradient

Figure 11-2 Molecular Biology of the Cell (© Garland Science 2008)

Figure 11-3a Molecular Biology of the Cell (© Garland Science 2008)

Types of membrane transport proteins

Figure 11-3b Molecular Biology of the Cell (© Garland Science 2008)

Types of membrane transport proteins

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

Active & Passive Transport

Figure 11-5 Molecular Biology of the Cell (© Garland Science 2008)

Movement of electrically neutral solutes occurs “down” its concentration gradient (fromhigh [S] to low [S]) until equilibrium is reached

Equilibrium without an electrical potential across the membrane has equal particles and equal charge on both sides

Net movement of electrically chargedsolutes is determinedby a combination ofelectrical potential(Vm) & the chemicalconcentration differenceacross the membrane

Just like M&M view of enzymes:T+Sout↔TS↔T+Sin

T = transporterS = solute

Simple diffusion: process ofhydration shell removal is endergonic, so activation energy for diffusion throughthe membrane is high

Transporter protein reduces activationenergy for solute transport by formingnoncovalent bonds with dehydratedsolute to replace H-bonding with water and by creating a hydrophilictransmembrane passageway

Figure 11-6 Molecular Biology of the Cell (© Garland Science 2008)

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

Membrane Potentials

Figure 11-7 Molecular Biology of the Cell (© Garland Science 2008)

3 ways of driving active transport

Figure 11-8 Molecular Biology of the Cell (© Garland Science 2008)

3 types of transporter-mediated movement

Transporters (consider 3 examples)

•Glucose transporters: –GLUT1 uniporter

–Facilitated diffusion (works with [glucose] gradient)

•Na+/K+ ATPase:–P-Type ATPase (channel becomes phosphorylated during transport)

–Antiporter

–Primary active transport (couples ATP hydrolysis [exergonic] with simultaneous movement of Na+ & K+ against their electrochemical gradients [endergonic])

•ATP synthase:–F-Type ATPases (reversible, ATP-driven proton pumps: protons can either move against concentration gradient (i.e. certain bacteria) or with gradient (i.e., ATP synthesis through oxidative phosphorylation in mitochondria)

–ATP synthase refers to scenario in which protons movement occurs with its gradient!

GLUT1: glucose transporter

A helical wheel diagramreveals an amphipathic-helix

Amphipathic -helices

-helical supermolecular structures.

[Glucose] high outside

cell

[Glucose]Low inside

cell

No energy needed!

Facilitated diffusion

Uniporter:glucose

permease

Insulin regulates glucose transporter expressionon the cell surface

Equilibrium without an electrostatic potential across the membrane has equal particles and equal charge on both sides. However, if there is an electrostatic potential difference, then the steady state will have unequal number of particles and charges on each side.

Vm is the electrostatic potential difference across the membrane. Also denoted or

Na+/K+-ATPase: The Electrogenic Pump

The term,electrogenic, refers to a transport-generated electrical potential.

This usually results when movement of an ion withoutan accompanying counteriontakes place.

The Na+/K+ ATPase pumps Na+ outward to maintain the Na+ gradientthat drives glucose uptake into the bloodstream

Energy required To pump glucoseFrom two sources:

1) [Na+]outside>>[Na+]inside

2) Transmembrane potential (inside-neg., so draws Na+ inward)

…this means[glucose]inside/[glucose]outside ~ 9,000!

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

K+ binding constant

Low

Low

High

Low

Na+ binding constant

High

High

Low

High

ATP Synthase is the last step in the electron transport chain

ATP Synthase

A multisubunit

transmembrane protein

(450 kD)

Two functional units,

F1 and Fo

Fo is a water-insoluble

transmembrane

proton pore

F1 is a water-soluble

peripheral membrane

protein complex

ATP Synthase

Generates 1 ATP for every 3 protonsthat pass through it

ENERGY COUPLING! Couples ATP synthesis (endergonic) with passive diffusion of protons through inner mito. Membrane (exergonic).

Visualizing ATP Synthase

Norbert Dencher and Andreas Engel

AtomicForceMicroscopy

C-subunits of F0 complex

From chloroplasts

Hibernation

The uncoupling of ETC from ATP synthesis

Occurs in brown fat:many mitochondriaand cytochromes

Oxidation of NADHuncoupled from ATP synthesis

The energy of ETC is released as heat!

Also found in mostnewborn mammals

The uncoupling of ETC from ATP synthesis

Oxidation of NADHuncoupled from ATP synthesis

Pore protein calledthermogeninallows protonsto flow down gradient

The energy of ETC is released as heat!(instead of ATP)