Membrane Structure and Function

Chapter 7

• The plasma membrane selectively permeable

Overview: Life at the Edge

• Phospholipids most abundant lipids in plasma membrane

• amphipathic = hydrophobic and hydrophilic regions– Polar head– Hydrocarbon tails

Cell membranes

• Phospholipid

Hydrophilichead

WATER

Hydrophobictail

WATER

Membrane Models: Scientific Inquiry

• 1935Davson and Danielli - bilayer model

• 1972 Singer and Nicolson - fluid mosaic model– membrane mosaic of proteins dispersed within bilayer

• Current model: mosaicism

TECHNIQUE

Extracellularlayer

KnifeProteins Inside of extracellular layer

RESULTS

Inside of cytoplasmic layer

Cytoplasmic layerPlasma membrane

• Freeze-fracture splits membrane along middle of bilayer (lipid layer is weak)

proteins

Phospholipidbilayer

Hydrophobic regionsof protein

Hydrophilicregions of protein

Proteins embedded in bilayer

Phospholipids in membrane move laterally within bilayer

rarely flip flop• Lipids, proteins, may move laterally

The Fluidity of Membranes

(a) Movement of phospholipids

Lateral movement(107 times per second)

Flip-flop( once per month)

RESULTS

Membrane proteins

Mouse cellHuman cell

Hybrid cell

Mixed proteinsafter 1 hour

1. Type of phospholipid

Membrane fluidity affected by:

Fluid

Unsaturated hydrocarbontails with kinks

Viscous

Saturated hydro-carbon tails

2. Temperature

• cool gel– Tightly packed tails

• warm fluid

3. Cholesterol • Stabilizes membrane fluidity with changing

temperature

FYICholesterol can compose ½ of the membrane

Bacterial cell membranes do not contain cholesterol

Plant cells do not contain much

Cholesterol

(c) Cholesterol within the animal cell membrane

• Mosaic of proteins embedded in lipid bilayer• Proteins determine most of membrane’s

functions

Membrane Proteins and Their Functions

Membrane Proteins

Peripheral proteins – bound to _____________ of membrane

Integral proteins – penetrate hydrophobic region– Transmembrane proteins

N-terminus

C-terminus

• ex. Insulin receptor

1. Receptor proteins for signal transduction

• Hydrophilic core• Ex. Aquaporins

2. Channel proteins for passage of molecules hydrophilic core

Ex. Glucose transporter shuttles glucose across membrane

3. Transport proteins

(a) Transport (b) Enzymatic activity (c) Signal transduction

ATP

Enzymes

Signal transduction

Signaling molecule

Receptor

(d) Cell-cell recognition

Glyco-protein

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

• Glycoproteins – Carbohydrates attached to proteins

mucin

4. Cell-cell recognition

Ex. gap junctions allow passage of ions and small molecules from cell to cell

5. Intercellular joining proteins

6. Extracellular matrix proteins

7. Membrane enzymes

– Transport– Enzymatic activity– Signal transduction– Cell-cell recognition– Intercellular joining– Attachment to the cytoskeleton and extracellular matrix

(ECM)

Six major functions of membrane proteins:

• Plasma membrane regulates cell molecular traffic

Selective permeability

• Hydrophobic molecules dissolve in bilayer and pass through membrane rapidly– O2, CO2, NO, steroids, nanoparticles

O2

CO2

Permeability of Lipid Bilayer

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

• U-M scientists used a manmade nanoparticle less than 5 nm. They are small enough to cross cell membranes. One nanometer equals one-billionth of a meter, which means it would take 100,000 nanometers lined up side-by-side to equal the diameter of a human hair.

• Folate molecules on the nanoparticle bind to receptors on cancer tumor cell membranes and the cell immediately internalizes it, because it thinks it's getting folate, which it needs to grow. Folate is a vitamin

• But while it's bringing folate across the cell membrane, the cell also draws in the methotrexate, a poison that will kill it.

Nanoparticles can cross cell membranes!

• Hydrophilic (polar) molecules do not cross easily– sugar, water, ions

molecules move randomly

A. Diffusion

• Molecules diffuse down their concentration gradient from high to lower concentration until equilibrium

Passive transport = no energy used

(b) Diffusion of two solutes

Net diffusion

Net diffusion

Net diffusion

Net diffusion

Equilibrium

Equilibrium

What molecules can diffuse across the cell membrane?

Answer: O2, CO2, urea

• B. Osmosis is diffusion of water across a selectively permeable membrane

• Water diffuses across membrane from region of higher water concentration to the region of lower water concentration until equilbrium

Lowerconcentrationof sugar)

H2O

Higher Concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

• Tonicity =ability of solution to cause cell to gain or lose water

• Isotonic solution: Solute concentration same as in cell– no net water movement across membrane

• Hypertonic solution: Solute concentration greater than inside cell– cell loses water

• Hypotonic solution: Solute concentration is less than inside cell– cell gains water

Water Balance of Cells Without Walls

Hypotonic solution

(a) Animal cell

H2O

Lysed

H2O H2O

Normal

Isotonic solution

H2O

Shriveled

Hypertonic solution

Solution type? Isotonic, hypotonic, hypertonic?

• Osmoregulation= control of water balance

• Ex. Paramecium video – pond water is _____________ to the protista– contractile vacuole

How do cells deal with changing external water concentrations?

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.

• Isotonic solution no net movement of water into cell– flaccid (limp)

Water Balance in plants (cell wall)

• Hypotonic solution cell (vacuole) swells – cell wall opposes uptake turgid (firm)

• Hypertonic lose water; membrane pulls away from wall – plasmolysis (lethal)

Hypotonic solution

(b) Plant cell

H2O

Turgid (normal)

H2O H2O

Isotonic solution

Flaccid

H2O

Plasmolyzed

Hypertonic solution

C. Facilitated Diffusion: Passive Transport aided by proteins

• Channel proteins• Carrier proteins

What molecules use facilitated diffusion to cross membrane?

Answer: glucose, sodium ions, chloride ions, water

EXTRACELLULAR FLUID

Channel protein

(a) A channel protein

Solute CYTOPLASM

Solute Carrier protein

(b) A carrier protein

• Energy (ATP) required to move solutes against their gradients (from lower to higher conc. !)

Pumps are membrane proteins Ex. sodium-potassium pump (an enzyme)

FYI:All animalsNobel prize 1997 (Jens Skou)Uses 1/3 of cells total energy productionProvides driving force for other cell processes (secondary

transport, volume, gradients)

Active transport

EXTRACELLULAR

FLUID [Na+] high [K+] low

Na+

Na+

Na+ [Na+] low[K+] high CYTOPLASM

Cytoplasmic Na+ binds to

the sodium-potassium pump. 1

Examine the figure:

Na+ high outside cellK+ low

Na+ low inside cellK+ high

According to diffusion?

1. Cytoplasmic Na+ binds to pump

Na+ binding stimulatesphosphorylation by ATP.

Na+

Na+

Na+

ATP P

ADP

2

2. ADP phosphorylated to ATP

What is ATP?

Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.

Na+

P

Na+ Na+

3 Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.

Na+

P

Na+ Na+

3

3. Na+ out of cell

shape change ofpump

Against conc. grad.

K+ binds on theextracellular side andtriggers release of thephosphate group.

P P

K+

K+

4

4. Extracellular K+ binds to pump

ATP used

Shape change

K+

K+

5 + 6 K+ inside cell Pump animation

Step by step

Passive transport

Diffusion Facilitated diffusion

Active transport

ATP

Review the differencepassive vsactive transport

NO ATP IS USED

WHICH ONE REPRESENTSTHE CHANNEL PROTEIN?

a. Maintain membrane potential = voltage difference across membrane

Inside of cell more electronegative than out

= negative membrane potential

Why do cells need pumps?

EXTRACELLULARFLUID

H+

H+

H+

H+

Proton pump

+

+

+

H+

H+

+

+

H+

–

–

–

–

ATP

CYTOPLASM

–

– chemical = concentration gradient

– electrical = membrane potential

b. Maintain electrochemical gradients

• c. Cotransport

• Ex. Sucrose transporter sodium driven

Proton pump

–

–

–

–

–

–

+

+

+

+

+

+

ATP

H+

H+

H+

H+

H+

H+

H+

H+

Diffusionof H+

Sucrose-H+

cotransporter

Sucrose

Sucrose

• Exocytosis – To secrete products from cell– Vesicles fuse with membrane

Bulk transport

2. Endocytosis cell takes in macromolecules by forming vesicles from membrane

a. Phagocytosis – for large particle

Vesicle fuses with lysosome to digest particle

PHAGOCYTOSIS

CYTOPLASM EXTRACELLULARFLUID

Pseudopodium

“Food” orother particle

Foodvacuole Food vacuole

Bacterium

An amoeba engulfing a bacteriumvia phagocytosis (TEM)

Pseudopodiumof amoeba

1 µm

b. Pinocytosis – for fluids/small molecules

PINOCYTOSIS

Plasmamembrane

Vesicle

0.5 µm

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

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor Coat protein

Coatedpit

Ligand

Coatedvesicle

c. receptor-mediated endocytosis, ligand binds to receptor vesicle