05 lecture ppt membrance structure and Function

BiologySylvia S. Mader

Michael WindelspechtChapter 5

Membrane Structure and Function

Lecture Outline

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Outline• 5.1 Plasma Membrane Structure and

Function

• 5.2 Passive Transport Across a Membrane

• 5.3 Active Transport Across a Membrane

• 5.4 Modification of Cell Surfaces

5.1 Plasma Membrane Structure and Function

• The plasma membrane is common to all cells• Separates:

Internal cytoplasm from the external environment of the cell

• Phospholipid bilayer: External surface lined with hydrophilic polar heads Cytoplasmic surface lined with hydrophilic polar heads Nonpolar, hydrophobic, fatty-acid tails sandwiched in

between

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Plasma Membrane Structure and Function

• Components of the Plasma Membrane Three components:

• Lipid component referred to as phospholipid bilayer• Protein molecules

– Float around like icebergs on a sea– Membrane proteins may be peripheral or integral

» Peripheral proteins are found on the inner membrane surface

» Integral proteins are partially or wholly embedded (transmembrane) in the membrane

• Cholesterol affects the fluidity of the membrane

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Membrane Proteins

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hydrophobicregion

phospholipidWater outside cell

integralprotein

hydrophilicregions

Water inside cell

peripheralproteins

cholesterol


• Carbohydrate Chains Glycoproteins

• Proteins with attached carbohydrate chains Glycolipids

• Lipids with attached carbohydrate chains These carbohydrate chains exist only on the

outside of the membrane• Makes the membrane asymmetrical

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Plasma Membrane of an Animal Cell

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Plasma membrane

filaments of cytoskeleton Inside cell

integral protein

cholesterol

peripheral protein

glycolipid

carbohydratechain Outside cell

hydrophilicheads

phospholipidglycoprotein

extracellularMatrix (ECM)

phospholipidbilayer

hydrophobictails


• Functions of Membrane Proteins Channel Proteins:

• Allow passage of molecules through membrane via a channel in the protein

Carrier Proteins:• Combine with the substance to be transported• Assist passage of molecules through membrane

Cell Recognition Proteins:• Glycoproteins• Help the body recognize foreign substances

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• Functions of Membrane Proteins (continued) Receptor Proteins:

• Bind with specific molecules• Allow a cell to respond to signals from other cells

Enzymatic Proteins:• Carry out metabolic reactions directly

Junction Proteins:• Attach adjacent cells

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Channel Protein:Allows a particularmolecule or ion tocross the plasmamembrane freely.Cystic fibrosis, aninherited disorder,is caused by afaulty chloride (Cl–)channel; a thickmucus collects inairways and inpancreatic and liver ducts.

a.

Membrane Protein Diversity

b.

Carrier Protein:Selectively interactswith a specificmolecule or ion sothat it can cross theplasma membrane.The inability of somepersons to useenergy for sodium-potassium (Na+–K+)transport has beensuggested as thecause of their obesity.



Cell RecognitionProtein:The MHC (majorhistocompatibilitycomplex) glycoproteinsare different for eachperson, so organtransplants are difficultto achieve. Cells withforeign MHCglycoproteins areattacked by white bloodcells responsible forimmunity.

c.


Receptor Protein:Is shaped in such away that a specificmolecule can bind toit. Pygmies are short,not because they donot produce enoughgrowth hormone, butbecause their plasmamembrane growthhormone receptorsare faulty and cannotinteract with growthhormone.

d.

Enzymatic Protein:Catalyzes a specificreaction. The membraneprotein, adenylatecyclase, is involved inATP metabolism. Cholerabacteria release a toxinthat interferes with theproper functioning ofadenylate cyclase;sodium (Na+) and waterleave intestinal cells, andthe individual may diefrom severe diarrhea.

e.


Junction Proteins:Tight junctions joincells so that a tissuecan fulfill a function, aswhen a tissue pinchesoff the neural tubeduring development.Without thiscooperation betweencells, an animalembryo would have nonervous system.

f.



Channel Protein:Allows a particularmolecule or ion tocross the plasmamembrane freely.Cystic fibrosis, aninherited disorder,is caused by afaulty chloride (Cl–)channel; a thickmucus collects inairways and inpancreatic andliver ducts.

Carrier Protein:Selectively interactswith a specificmolecule or ion sothat it can cross theplasma membrane.The inability of somepersons to useenergy for sodium-potassium (Na+–K+)transport has beensuggested as thecause of their obesity.

Receptor Protein:Is shaped in such away that a specificmolecule can bind toit. Pygmies are short,not because they donot produce enoughgrowth hormone, butbecause their plasmamembrane growthhormone receptorsare faulty and cannotinteract with growthhormone.

Enzymatic Protein:Catalyzes a specificreaction. The membraneprotein, adenylatecyclase, is involved inATP metabolism. Cholerabacteria release a toxinthat interferes with theproper functioning ofadenylate cyclase;sodium (Na+) and waterleave intestinal cells, andthe individual may diefrom severe diarrhea.

Junction Proteins:Tight junctions joincells so that a tissuecan fulfill a function, aswhen a tissue pinchesoff the neural tubeduring development.Without thiscooperation betweencells, an animalembryo would have nonervous system.

Cell RecognitionProtein:The MHC (majorhistocompatibilitycomplex) glycoproteinsare different for eachperson, so organtransplants are difficultto achieve. Cells withforeign MHCglycoproteins areattacked by white bloodcells responsible forimmunity.

a. b.

d. e.

c.

f.

Membrane Protein Diversity

How Cells Talk to One Another

• Signaling molecules serve as chemical messengers allowing cells to communicate with one another

Cell receptors bind to specific signaling molecules Once the signaling molecule and the cell receptor

bind a cascade of events occurs that elicits a cellular response

• Signal transduction pathway

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Cell Signaling

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Cellularresponse:

Altered shapeor movementof cell

Alteredmetabolismor cellularfunction

Altered geneexpressionand the typesand amountof proteinsproduced

generegulatory

protein

Nucleus

b.

Cytoplasm

unactivatedreceptorprotein

Nuclearenvelope

enzyme

structuralproteinreceptor

activation

signalingmolecule

Targetedprotein:plasma

membrane

newborna. egg embryo

Left: © Anatomical Travelogue/Photo Researchers, Inc.; Middle: © Neil Harding/Stone/Getty Images; Right: © Photodisc Collection/Getty RF

2.Transduction pathway: Series of relay proteins that ends when a protein is activated.

3. Response: Targeted protein(s) bring about a cellular response.

1. Receptor: Binds to a signaling molecule, becomes activated and initiates a transduction pathway.


• Permeability of the Plasma Membrane The plasma membrane is selectively permeable

• Allows some substances to move across the membrane• Inhibits passage of other molecules

Small, non-charged molecules (CO2, O2, glycerol, alcohol) freely cross the membrane by passing through the phospholipid bilayer

• These molecules follow their concentration gradient– Move from an area of high concentration to an area of

low concentration.

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• Permeability of the Plasma Membrane Water moves across the plasma membrane

• Specialized proteins termed aquaporins speed up water transport across the membrane

The movement of ions and polar molecules across the membrane is often assisted by carrier proteins

Some molecules must move against their concentration gradient with the expenditure of energy

• Active transport Large particles enter or exit the cell via bulk transport

• Exocytosis• Endocytosis

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How Molecules Cross the Plasma Membrane

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water inside cell phospholipidmolecule

protein

water outside cell

nonpolar,hydrophobic core


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+

–+


protein

water outside cellcharged moleculesand ions–




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+

–+


protein

charged moleculesand ions

H2O

–




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+

–+


protein

nonchargedmolecules

water outside cellcharged moleculesand ions

H2O

–




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+

–+


protein

macromolecule

nonchargedmolecules

water outside cellcharged moleculesand ions

H2O

–



Passage of Molecules Into and out of the Cell

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5.2 Passive Transport Across a Membrane

• A solution consists of: A solvent (liquid), and A solute (dissolved solid)

• Diffusion Net movement of molecules down a concentration

gradient Molecules move both ways along gradient, but net

movement is from high to low concentration Equilibrium:

• When NET movement stops• Solute concentration is uniform – no gradient

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Process of Diffusion

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crystaldye

a. Crystal of dye is placed in water


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time

crystaldye

a. Crystal of dye is placed in water b. Diffusion of water and dye molecules


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time time

crystaldye

a. Crystal of dye is placed in water b. Diffusion of water and dye molecules c. Equal distribution of molecules results

Gas Exchange in Lungs

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O2

oxygen

O2

O2

O2

O2

O2

O2O2

O2 O2

O2

O2O2

O2

highO2

concentrationlowO2

concentration

bronchiole

capillaryalveolus


Passive Transport Across a Membrane

• Osmosis: Special case of diffusion Focuses on solvent (water) movement rather than

solute Diffusion of water across a selectively permeable

membrane• Solute concentration on one side is high, but water

concentration is low• Solute concentration on other side is low, but water

concentration is high Water can diffuse both ways across membrane but

the solute cannot Net movement of water is toward low water (high

solute) concentration• Osmotic pressure is the pressure that develops

due to osmosis

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Osmosis Demonstration

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a.

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

10%

5%

<10%

>5%

solute

differentiallypermeablemembrane

water

b.

c.

less water (higherpercentage of solute)

more water (lowerpercentage of solute)

beaker

thistletube


• Isotonic Solutions Solute and water concentrations are equal on

both sides of membrane No net gain or loss of water by the cell

• Hypotonic Solutions Concentration of solute in the solution is lower

than inside the cell Cells placed in a hypotonic solution will swell

• Causes turgor pressure in plants• May cause animal cells to lyse (rupture)

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• Hypertonic Solutions Concentration of solute is higher in the

solution than inside the cell Cells placed in a hypertonic solution will

shrink • Crenation in animal cells• Plasmolysis in plant cells

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Osmosis in Animal and Plant Cells

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In a hypertonic solution, watermainly leaves the cell, whichshrivels (crenation).

In a hypertonic solution, vacuoleslose water, the cytoplasm shrinks(plasmolysis), and chloroplastsare seen in the center of the cell.

In a hypotonic solution, vacuolesfill with water, turgor pressuredevelops, and chloroplasts areseen next to the cell wall.

In an isotonic solution, there is nonet movement of water.

In an isotonic solution, there is no netmovement of water.

In a hypotonic solution, watermainly enters the cell, which mayburst (lysis).

plasmamembrane

Animalcells

nucleus

Plantcells

centralvacuole

chloroplast

nucleus

cellwall

plasmamembrane


Facilitated Transport• Movement of molecules that cannot pass directly

through the membrane lipids• These molecules must combine with carrier

proteins to move across the membrane• Follow concentration gradient, moving from high

concentration to low concentration

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solute

Outside

Inside

plasma membrane

carrier protein

Facilitated Transport


solute

Outside

Inside

plasmamembrane

carrier protein


solute

Outside

Inside

plasma membrane

carrier protein



5.3 Active Transport Across a Membrane

Active Transport• The movement of molecules against their

concentration gradient– Movement from low to high concentration

• Movement is facilitated by carrier proteins • Requires the expenditure of energy in the form of

ATP• Ex: sodium-potassium pump

– Uses ATP to move sodium ions out of the cells and potassium ions into the cell against their concentration gradients.

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The Sodium-Potassium Pump

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carrierprotein

1. Carrier has a shape that allowsit to take up 3 Na+

Outside

Inside

K+

K+

Na+

K+

K+

Na+

Na+ Na+

Na+


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carrier

protein

1. Carrier has a shape that allowsit to take up 3 Na+.

2. ATP is split, and phosphategroup attaches to carrier

Outside

Inside

ATP

K+

P

Na+

Na+

K+

K+

K+K+

K+

Na+Na+ Na+

Na+

K+

K+

Na+

Na+

Na+


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carrierprotein


3. Change in shape results andcauses carrier to release 3 Na+

outside the cell.

Outside

Inside

ATP

K+

K+

K+

P

P

Na+

Na+

Na+

Na+

K+

K+

K+

Na+

Na+ Na +

K+ K+

K+

Na+

Na+

Na+

Na+

2. ATP is split, and phosphategroup attaches to carrier

K+

Na+Na+

K+

K+

Na +


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carrierprotein


4. Carrier has a shape thatallows it to take up 2K+.


outside the cell.

Outside

Inside

ATP

K+

K+

K+

K+

K+

K+K+

K+

P

P

P

Na+Na+

Na+

Na+

Na+

Na+

Na+

K+

K+

K+

Na+

Na+ Na +

Na+

Na+

Na+

Na+

Na +

Na+

K+K+

K+

2. ATP is split, and phosphategroup attaches to carrier.

K+Na+Na+

K+

Na+

Na +



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carrierprotein


4. Carrier has a shape thatallows it to take up 2 K+.



outside the cell.

5. Phosphate group is releasedfrom carrier.

Outside

Inside

ATP

K+

K+

K+

K+P

P

P

P

Na+

Na+

Na+

Na+ Na+

Na +

Na+

Na+

Na +

Na+

Na+

K+

K+K+

Na+

Na+ Na +

Na+

K+K+

K+

Na+

Na+

Na+

Na+

K+K+

K+

Na+

K+

K+

K+

Na+ Na+

K+

K+

Na+Na+

K+

K+

Na+



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carrierprotein


4. Carrier has a shape thatallows it to take up 2 K+.



outside the cell.

5. Phosphate group is releasedfrom carrier.

Outside

Inside

ATP

K+

K+

K+

K+P

P

P

P

Na+

Na+

Na+

Na+ Na+

Na +

Na+

Na+

Na +

Na+

Na+

K+

K+K+

Na+

Na+ Na +

Na+

K+K+

K+

Na+

Na+

Na+

Na+

K+K+

K+

Na+

K+

K+

K+

Na+ Na+

K+

K+

Na+Na+

K+

K+

Na +


6. Change in shape results andcauses carrier to release 2K+

inside the cell.

K+

K

K+Na+

Na+

Na +

Na+ Na+

K+

Active Transport Across a Membrane

• Macromolecules are transported into or out of the cell inside vesicles via bulk transport Exocytosis – Vesicles fuse with plasma membrane and

secrete contents Endocytosis – Cells engulf substances into a pouch

which becomes a vesicle• Phagocytosis – Large, solid material is taken in by endocytosis• Pinocytosis – Vesicles form around a liquid or very small

particles• Receptor-Mediated Endocytosis– Specific form of pinocytosis

using receptor proteins and a coated pit

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Exocytosis

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OutsidePlasma membrane

Insidesecretoryvesicle

Three Methods of Endocytosis

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pseudopod

paramecium

vacuoleforming

vesiclesforming

coatedpit

coatedvesicle

solute

solute

a. Phagocytosis

b. Pinocytosis

vacuole

coated vesicle

plasma membrane

receptor protein

coated pit

c. Receptor-mediated endocytosis

vesicle

0.5 μm

399.9 μm

5.4 Modifications of Cell Surfaces

• Cell Surfaces in Animals Extracellular Matrix (ECM)

• Meshwork of proteins and polysaccharides in close connection with the cell that produced them

– Collagen – resists stretching

– Elastin – provides resilience to the ECM

– Integrin – play role in cell signaling

– Proteoglycans – regulate passage of material through the ECM to the plasma membrane

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Animal Cell Extracellular Matrix

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collagenproteoglycan

actin filament

fibronectin

elastin

integrin

Outside (extracellular matrix)

Inside (cytoplasm)

Modifications of Cell Surfaces

• Cell Surfaces in Animals Junctions Between Cells

• Adhesion Junctions - Intercellular filaments between cells

– Desmosomes – internal cytoplasmic plaques

– Tight Junctions – form impermeable barriers

• Gap Junctions– Plasma membrane channels are joined (allows

communication)

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Junctions Between Cells of the Intestinal Wall

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plasmamembranescytoplasmic

plaque

Filamentsofcytoskeleton

adhesionproteins

intercellularspace

a. Adhesion junction

b. Tight junction

c. Gap junction

plasmamembranes

light junctionproteins

intercellularspace

plasmamembranes

intercellularspace

membranechannels

a: From Douglas E. Kelly, J. Cell Biol. 28 (1966): 51. Reproduced by copyright permission of The Rockefeller University Press; b: © David M. Phillips/Visuals Unlimited;c: Courtesy Camillo Peracchia, M.D.

20 nm

50 nm

100 nm

Modifications of Cell Surfaces • Plant Cell Walls

Plants have a freely permeable cell wall, with cellulose as the main component

• Plasmodesmata penetrate the cell wall• Each contains a strand of cytoplasm• Allow passage of material between cells

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Plasmodesmata

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cell wall

plasmodesmata

cell wall

Cell 1 Cell 2

plasmamembrane

cell wall cell wall

cytoplasm

plasmamembrane

cytoplasm

middle lamella

plasmodesmata

0.3mm

© E.H. Newcomb/Biological Photo Service

05 lecture ppt membrance structure and Function

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