The Cell Membrane

• AP Chapter 7

Overview: Life at the Edge

• The plasma membrane is the boundary that separates the living cell from its surroundings

• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

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Composition of the cell membrane: lipids (phospholipids and cholesterol) and proteins

• Phospholipids are the most abundant lipid in the plasma membrane

- they are amphipathic molecules, contain hydrophobic and hydrophilic regions.• The fluid mosaic model states that a

membrane is a fluid structure with a “mosaic” of various proteins embedded in it

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Development of the Fluid Mosaic Model• In 1935, Hugh Davson and James Danielli proposed a

sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

• In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

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Sandwich model of the cell membrane

What is wrong with this model?

Proteins are mostly hydrophobic.All phospholipids are not alike.

Fluid Mosaic Model

Fig. 7-2

Hydrophilichead

WATER

Hydrophobictail

WATER

This slide shows how the hydrophobic andhydrophilic regions are set up.

Fig. 7-3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Double phospholipid layer

Different types of phospholipids

Different types of phospholipids making up the membrane

Fig. 7-4

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane

Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

The Fluidity of Membranes

• Phospholipids in the plasma membrane can move within the bilayer

• Most of the lipids, and some proteins, drift laterally (very rarely transverse flip-flops)

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fluidity of membrane

fluidity of cell membrane

Fig. 7-5

Lateral movement(~107 times per second)

Flip-flop(~ once per month)

(a) Movement of phospholipids

(b) Membrane fluidity

Fluid Viscous

Unsaturated hydrocarbontails with kinks

Saturated hydro-carbon tails

(c) Cholesterol within the animal cell membrane

Cholesterol

Biology Animations

Fig. 7-6

RESULTS

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteinsafter 1 hour

This experiment shows how the proteins can move about the membrane.

The fluidity of the membrane depends on temperature and types of lipids making up the membrane.

• Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid

as salad oil.

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Fig. 7-5b

(b) Membrane fluidity

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

• Would you expect an amoeba that lives in a pond in a cold northern climate to have a higher or lower percentage of saturated fatty acids in its membranes during the summer as compared to the winter?

• The steroid cholesterol has different effects on membrane fluidity at different temperatures

• At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids

• At cool temperatures, it maintains fluidity by preventing tight packing

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-5c

Cholesterol

(c) Cholesterol within the animal cell membrane

Membrane Proteins and Their Functions

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

• Proteins determine most of the membrane’s specific functions

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

• Peripheral proteins are bound to the surface of the membrane

• Integral proteins penetrate the hydrophobic core, are called transmembrane proteins

• The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-8

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

• Six major functions of membrane proteins:– Transport– Enzymatic activity– Signal transduction– Cell-cell recognition– Intercellular joining– Attachment to the cytoskeleton and extracellular

matrix (ECM)

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-9

(a) Transport

ATP

(b) Enzymatic activity

Enzymes

(c) Signal transduction

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Intercellular joiningE-selectin is a transmembrane protein expressed by endothelial cells

that binds to an oligosaccharide expressed on the surface of leukocytes

Construct a cell membraneTry this at home!

constructing a cell membrane

The Role of Membrane Carbohydrates in Cell-Cell Recognition

• Cells recognize each other by binding to surface molecules, usually carbohydrates

• Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

• Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual

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Membranes are bifacial• Carbohydrates (making glycoproteins and

glycolipids) on outer surface• Peripheral proteins generally on cytoplasmic

surface• Proteins have a distinct orientation, ie…

receptor proteins oriented at surface, enzyme proteins oriented toward cytoplasm

Synthesis and Sidedness of Membranes

• Membranes have distinct inside and outside faces – determined when the membrane is built by the ER and Golgi.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-10

ER1

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

2Golgiapparatus

Vesicle

3

4

Secretedprotein

Transmembraneglycoprotein

Plasma membrane:

Cytoplasmic face

Extracellular face

Membrane glycolipid

Can you guess where you would find this cell?

This cell?

Notice how thin-walledthey are.

Need a hint?

These cells?