Chapter 7

Membrane Structure and Function

• In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins

• Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions

• 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

Fig. 7-2

Hydrophilichead

WATER

Hydrophobictail

WATER

amphipathic molecules

Fig. 7-3

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Fig. 7-4

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane

Freeze fracture

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

The fluidity of membrane

• 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

Cholesterol

Fig. 7-6

RESULTS

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteinsafter 1 hour

Fig. 7-7

Fibers ofextracellularmatrix (ECM)

Glyco-protein

Microfilamentsof cytoskeleton

Cholesterol

Peripheralproteins

Integralprotein

CYTOPLASMIC SIDEOF MEMBRANE

GlycolipidEXTRACELLULARSIDE OFMEMBRANE

Carbohydrate

Fig. 7-8

N-terminus

C-terminus

HelixCYTOPLASMICSIDE

EXTRACELLULARSIDE

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)

•Six major functions of membrane proteins:

Synthesis and sideness of membranes

Fig. 7-10

ER1

Transmembraneglycoproteins

Secretoryprotein

Glycolipid

2Golgiapparatus

Vesicle

3

4

Secretedprotein

Transmembraneglycoprotein

Plasma membrane:

Cytoplasmic face

Extracellular face

Membrane glycolipid

• Concept 7.2: Membrane structure results in selective permeability

Fig. 7-11Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

(a) Diffusion of one solute

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

(b) Diffusion of two solutes

Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Osmosis is the diffusion of water

across a selectively permeable membrane

Hypotonic solution

(a) Animal cell

(b) Plant

cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

Fig. 7-13

Tonicity is the ability of a solution to cause a cell to gain or lose water

Fig. 7-14 Osmoregulation

Filling vacuole 50 µm

(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.

Contracting vacuole

(b) When full, the vacuole and canals contract, expelling fluid from the cell.

Fig. 7-15

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein (Shape change)

Some diseases are caused by malfunctions in specific transport systems, for example the

kidney disease cystinuria胱胺酸尿症 (#)

• Cystinuria is characterized by the inadequate reabsorption of cystine during the filtering process in the kidneys, thus resulting in an excessive concentration of this amino acid. Cystine may precipitate out of the urine, if the urine is neutral or acidic, and form crystals or stones in the kidneys, ureters, or bladder.

• Cystine?

2

Fig. 7-16-7 Active transportEXTRACELLULAR

FLUID [Na+] high [K+] low

[Na+] low

[K+] high

Na+

Na+

Na+

Na+

Na+

Na+

CYTOPLASM ATP

ADP P

Na+ Na+

Na+

P 3

K+

K+ 6

K+

K+

5 4

K+

K+

P P

1(Phosphoryla

tion)

(dephosphorylation)

Fig. 7-17Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

How Ion Pumps Maintain Membrane Potential

• Membrane potential is the voltage difference across a membrane

• Voltage is created by differences in the distribution of positive and negative ions

• Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:– A chemical force (the ion’s concentration gradient)

– An electrical force (the effect of the membrane potential on the ion’s movement)

•An electrogenic pump is a transport protein that generates voltage across a membrane•The sodium-potassium pump is the major electrogenic pump of animal cells

• The main electrogenic pump of plants, fungi, and bacteria is a proton pump

Fig. 7-18

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

Cotransport: Coupled Transport by a Membrane Protein

• Cotransport occurs when active transport of a solute indirectly drives transport of another solute

• Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

Fig. 7-19

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and

endocytosis• In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents– Many secretory cells use exocytosis to export their products

• In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane

– There are three types of endocytosis:– Phagocytosis (“cellular eating”)– Pinocytosis (“cellular drinking”)– Receptor-mediated endocytosis

Fig. 7-20PHAGOCYTOSIS

EXTRACELLULARFLUID

CYTOPLASM

Pseudopodium

“Food”orother particle

Foodvacuole

PINOCYTOSIS

1 µm

Pseudopodiumof amoeba

Bacterium

Food vacuole

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Plasmamembrane

Vesicle

0.5 µm

Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedvesicle

Coatedpit

Ligand

Coatprotein

Plasmamembrane

A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)

0.25 µm

Fig. 7-UN3

Environment:0.01 M sucrose

0.01 M glucose

0.01 M fructose

“Cell”

0.03 M sucrose

0.02 M glucose

Fig. 7-UN4

You should now be able to:1. Define the following terms: amphipathic

molecules, aquaporins, diffusion2. Explain how membrane fluidity is

influenced by temperature and membrane composition

3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions

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

4. Explain how transport proteins facilitate diffusion

5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps

6. Explain how large molecules are transported across a cell membrane

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