CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION …lhsteacher.lexingtonma.org/Pohlman/08A-MembraneStructure.pdf · Section A: Membrane Structure 1.Membrane models have evolved to ﬁt new

CHAPTER 8MEMBRANE STUCTURE AND

FUNCTION

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section A: Membrane Structure1. Membrane models have evolved to fit new data2. Membranes are fluid3. Membranes are mosaics of structure and function4. Membrane carbohydrates are important for cell-cell recognition

• The plasma membrane separates the living cell fromits nonliving surroundings.

• This thin barrier, 8 nm thick, controls traffic into andout of the cell.

• Like other membranes, the plasma membrane isselectively permeable, allowing some substances tocross more easily than others.

Introduction


• The main macromolecules in membranes are lipidsand proteins, but include some carbohydrates.

• The most abundant lipids are phospholipids.

• Phospholipids and most other membraneconstituents are amphipathic molecules.• Amphipathic molecules have both hydrophobic regions

and hydrophilic regions.

• The phospholipids and proteins in membranes createa unique physical environment, described by thefluid mosaic model.• A membrane is a fluid structure with proteins embedded

or attached to a double layer of phospholipids.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Models of membranes were developed long beforemembranes were first seen with electron microscopesin the 1950s.• In 1895, Charles Overton hypothesized that membranes

are made of lipids because substances that dissolve inlipid enter cells faster than those that are insoluble.

• Twenty years later, chemical analysis confirmed thatmembranes isolated from red blood cells are composed oflipids and proteins.

1. Membrane modes have evolved to fit newdata


• Attempts to build artificial membranes providedinsight into the structure of real membranes.• In 1917, Irving Langmuir discovered that phosphilipids

dissolved in benzene would form a film on water whenthe benzene evaporated.

• The hydrophilic heads were immersed in water.


Fig. 8.1a


Fig. 8.1b

• In 1925, E. Gorter and F. Grendel reasoned thatcell membranes must be a phospholipid bilayer,two molecules thick.

• The molecules in the bilayer are arranged such thatthe hydrophobic fatty acid tails are sheltered fromwater while thehydrophilic phosphategroups interactwith water.

• Actual membranes adhere more strongly to waterthan do artificial membranes composed only ofphospholipids.

• One suggestion was that proteins on the surfaceincreased adhesion.

• In 1935, H. Davson andJ. Danielli proposed asandwich model inwhich the phospholipidbilayer lies between twolayers of globularproteins.


Fig. 8.2a

• Early images from electron microscopes seemed tosupport the Davson-Danielli model and until the1960s, it was considered the dominant model.

• Further investigation revealed two problems.• First, not all membranes were alike, but differed in

thickness, appearance when stained, and percentage ofproteins to lipids.

• Second, measurements showed that membrane proteinsare actually not very soluble in water.

• Membrane proteins are amphipathic, with hydrophobicand hydrophilic regions.

• If at the surface, the hydrophobic regions would be incontact with water.


• In 1972, S.J. Singer and G. Nicolson presented arevised model that proposed that the membraneproteins are dispersed and individually insertedinto the phospholipid bilayer.• In this fluid mosaic

model, the hydrophilicregions of proteinsand phospholipids arein maximum contactwith water and thehydrophobic regionsare in a nonaqueousenvironment.


Fig. 8.2b

• A specializedpreparation technique,freeze-fracture, splits amembrane along themiddle of thephospholid bilayerprior to electronmicroscopy.

• This shows proteinparticles interspersedwith a smooth matrix,supporting the fluidmosaic model.


Fig. 8.3

• Membrane molecules are held in place by relativelyweak hydrophobic interactions.

• Most of the lipids and some proteins can driftlaterally in the plane of the membrane, but rarelyflip-flop from one layer to the other.

1. Membranes are fluid


Fig. 8.4a

• The lateral movements of phospholipids are rapid,about 2 microns per second.

• Many larger membrane proteins move more slowlybut do drift.• Some proteins move in very directed manner, perhaps

guided/driven by the motor proteins attached to thecytoskeleton.

• Other proteins never move, anchored by the cytoskeleton.


Fig. 8.5

• Membrane fluidity is influenced by temperatureand by its constituents.

• As temperatures cool, membranes switch from afluid state to a solid state as the phospholipids aremore closely packed.

• Membranes rich in unsaturated fatty acids are morefluid that thosedominated by saturatedfatty acids because thekinks in the unsaturatedfatty acid tails preventtight packing.


Fig. 8.4b

• The steroid cholesterol is wedged betweenphospholipid molecules in the plasma membraneof animals cells.

• At warm temperatures, it restrains the movementof phospholipids and reduces fluidity.

• At cool temperatures, it maintains fluidity bypreventing tight packing.


Fig. 8.4c

• To work properly with active enzymes andappropriate permeability, membrane must be fluid,about as fluid as salad oil.

• Cells can alter the lipid composition of membranesto compensate for changes in fluidity caused bychanging temperatures.• For example, cold-adapted organisms, such as winter

wheat, increase the percentage of unsaturatedphospholipids in the autumn.

• This allows these organisms to prevent their membranesfrom solidifying during winter.


• A membrane is a collage of different proteinsembedded in the fluid matrix of the lipid bilayer.

3. Membranes are mosaics of structure andfunction


Fig. 8.6

• Proteins determine most of the membrane’sspecific functions.

• The plasma membrane and the membranes of thevarious organelles each have unique collections ofproteins.

• There are two populations of membrane proteins.• Peripheral proteins are not embedded in the lipid

bilayer at all.

• Instead, they are loosely bounded to the surface of theprotein, often connected to the other population ofmembrane proteins.


• Integral proteins penetrate the hydrophobic core of thelipid bilayer, often completely spanning the membrane(a transmembrane protein).

• Where they contact the core, they have hydrophobicregions with nonpolar amino acids, often coiled intoalpha helices.

• Where they are incontact with theaqueous environment,they have hydrophilicregions of amino acids.


Fig. 8.7

• One role of membrane proteins is to reinforce theshape of a cell and provide a strong framework.• On the cytoplasmic side, some membrane proteins

connect to the cytoskeleton.

• On the exterior side, some membrane proteins attach tothe fibers of the extracellular matrix.


• Membranes have distinctive inside and outsidefaces.• The two layers may differ

in lipid composition, andproteins in the membranehave a clear direction.

• The outer surface also hascarbohydrates.

• This asymmetricalorientation begins duringsynthesis of new membranein the endoplasmicreticulum.


Fig. 8.8

• The proteins in the plasma membrane may providea variety of major cell functions.


Fig. 8.9

• The membrane plays the key role in cell-cellrecognition.• Cell-cell recognition is the ability of a cell to distinguish

one type of neighboring cell from another.• This attribute is important in cell sorting and organization

as tissues and organs in development.• It is also the basis for rejection of foreign cells by the

immune system.• Cells recognize other cells by keying on surface

molecules, often carbohydrates, on the plasma membrane.

4. Membrane carbohydrates are importantfor cell-cell recognition


• Membrane carbohydrates are usually branchedoligosaccharides with fewer than 15 sugar units.

• They may be covalently bonded either to lipids,forming glycolipids, or, more commonly, toproteins, forming glycoproteins.

• The oligosaccharides on the external side of theplasma membrane vary from species to species,individual to individual, and even from cell type tocell type within the same individual.• This variation marks each cell type as distinct.

• The four human blood groups (A, B, AB, and O) differin the external carbohydrates on red blood cells.


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CHAPTER 8 MEMBRANE STUCTURE AND FUNCTION …lhsteacher.lexingtonma.org/Pohlman/08A-MembraneStructure.pdf · Section A: Membrane Structure 1.Membrane models have evolved to ﬁt new