MEMBRANE STRUCTURE AND TRAFFIC

AP BiologyAP Biology

MEMBRANE MEMBRANE STRUCTURE AND TRAFFICSTRUCTURE AND TRAFFIC


Membrane ModelsMembrane Models

General Features of the Plasma MembraneGeneral Features of the Plasma Membrane – A A boundaryboundary that separates living cell from that separates living cell from

nonliving environment. nonliving environment.

– A A control devicecontrol device for chemical traffic into for chemical traffic into and out of the cell. and out of the cell.

– Is Is selectively permeableselectively permeable. .

– Has a Has a unique structureunique structure which determines which determines its function and solubility characteristics. its function and solubility characteristics.

– Intimately involved in cell cell recognition. Intimately involved in cell cell recognition.



A. Early Membrane ObservationsA. Early Membrane Observations1. 1. Lipid and lipid soluble materials enter Lipid and lipid soluble materials enter

cells more rapidly than substances that cells more rapidly than substances that are insoluble in lipids. are insoluble in lipids.

– Deduction: Membranes are made of Deduction: Membranes are made of lipids. lipids.

– Deduction: Fat-soluble substance Deduction: Fat-soluble substance move through the membrane by move through the membrane by dissolving in it ("like dissolves like"). dissolving in it ("like dissolves like").



A. Early Membrane ModelsA. Early Membrane Models2.2. Evidence: Amphipathic Evidence: Amphipathic phospholipidsphospholipids will form will form

an artificial membrane on the surface of water an artificial membrane on the surface of water with only the hydrophilic heads immersed in with only the hydrophilic heads immersed in water water

– Amphipathic = Condition where a molecule has Amphipathic = Condition where a molecule has both a hydrophilic region and a hydrophobic both a hydrophilic region and a hydrophobic region. region.

– Deduction: Because of their molecular Deduction: Because of their molecular structure, phospholipids can form membranes. structure, phospholipids can form membranes. They form either layers or micellesThey form either layers or micelles. .

http://home.earthlink.net/~dayvdanls/CarbonChem2.html#Lipids

http://home.earthlink.net/~dayvdanls/CampOLs/Micelles.html

head(hydrophilic)

tails(hydrophobic)

C

CHCH2

CH2

CH2

CHC

O

O

N OPH3C

CH3

O

O

OH2C

O

HC

CH2

O

CH2

CH2

CH2

CH2

CH3

CH2CH2CH2CH2CH2CH2CH2

CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

CH2CH2

CH3

phospholipid

hydrophilicheads

hydrophobictails

hydrophilicheads

extracellular fluid(watery environment)

cytoplasm(watery environment)

bilayer



A. Early Membrane ObservationA. Early Membrane Observation3.3. Phospholipid content of membranes Phospholipid content of membranes

isolated from red blood cells is just enough isolated from red blood cells is just enough to cover the cells with two layers. to cover the cells with two layers.

– Deduction: Cell membranes are actually Deduction: Cell membranes are actually phospholipid bilayers, two molecules thick. phospholipid bilayers, two molecules thick.

4.4. Membranes isolated from red blood cells Membranes isolated from red blood cells contain proteins as well as lipids. contain proteins as well as lipids.

– There is protein in biological membranes. There is protein in biological membranes.



B. The Davson-Danielli ModelB. The Davson-Danielli Model (1935) (1935)

– Phospholipid bilayer sandwiched between two Phospholipid bilayer sandwiched between two layers of globular protein. layers of globular protein.

– Polar heads are oriented towards protein layers Polar heads are oriented towards protein layers forming a hydrophilic zone. forming a hydrophilic zone.

– Nonpolar tails are oriented in between polar Nonpolar tails are oriented in between polar heads forming a hydrophobic zone. heads forming a hydrophobic zone.



B. The Davson-Danielli ModelB. The Davson-Danielli Model– Not all membranes are identical or symmetrical. Not all membranes are identical or symmetrical. – Membranes with different functions also differ in Membranes with different functions also differ in

chemical composition and structure. chemical composition and structure. – Membranes are bifacial with distinct inside and Membranes are bifacial with distinct inside and

outside faces. outside faces. – A membrane with an outside layer of proteins A membrane with an outside layer of proteins

would be an unstable structure. would be an unstable structure. – Membrane proteins are not soluble in water, and, Membrane proteins are not soluble in water, and,

like phospholipid, they are amphipathic. like phospholipid, they are amphipathic. – Protein layer not likely because its hydrophobic Protein layer not likely because its hydrophobic

regions would be in an aqueous environment, regions would be in an aqueous environment, and it would also separate the hydrophilic and it would also separate the hydrophilic phospholipid heads from water. phospholipid heads from water.



C. The Singer-Nicolson Model (1972)C. The Singer-Nicolson Model (1972)



D. The Fluid Mosaic ModelD. The Fluid Mosaic Model

1. 1. Membranes are Fluid Membranes are Fluid



D. The Fluid Mosaic ModelD. The Fluid Mosaic Model1.1. Membranes are Fluid Membranes are Fluid

2.2. Membranes are Mosaics of Structure Membranes are Mosaics of Structure and Functionand Function





3.3. Membranes are bifacial. Membranes are bifacial.





3.3. Membranes are bifacial. Membranes are bifacial.

4.4. Membrane Carbohydrates recognize Membrane Carbohydrates recognize other cells.other cells.


Membrane ModelsMembrane ModelsD. The Fluid Mosaic ModelD. The Fluid Mosaic Model2.2. Membrane proteins drift more slowly than Membrane proteins drift more slowly than

lipids. The fact that proteins drift laterally was lipids. The fact that proteins drift laterally was established experimentally by fusing a human established experimentally by fusing a human and mouse cell: and mouse cell:

Membrane proteins of a human and mouse Membrane proteins of a human and mouse cell were labeled with different green and red cell were labeled with different green and red fluorescent dyes. fluorescent dyes.

Cells were fused to form a hybrid cell with a Cells were fused to form a hybrid cell with a continuous membrane. continuous membrane.

Hybrid cell membrane had initially distinct Hybrid cell membrane had initially distinct regions of green and red dye. regions of green and red dye.

In less than an hour, the two colors were In less than an hour, the two colors were intermixed. intermixed.



D.D. The Fluid Mosaic ModelThe Fluid Mosaic Model3.3. Some membrane proteins are tethered to the Some membrane proteins are tethered to the

cytoskeleton and cannot move far. cytoskeleton and cannot move far. 4.4. Membranes solidify if the temperature Membranes solidify if the temperature

decreases to a critical point. Critical decreases to a critical point. Critical temperature is lower in membranes with a temperature is lower in membranes with a greater concentration of unsaturated greater concentration of unsaturated phospholipids.phospholipids.


Membrane ModelsMembrane ModelsD. The Fluid Mosaic ModelD. The Fluid Mosaic Model5.5. Because they hinder close packing of Because they hinder close packing of

phospholipids, the steroid cholesterol and phospholipids, the steroid cholesterol and unsaturated hydrocarbon tails (with kinks at unsaturated hydrocarbon tails (with kinks at the carbon-to-carbon double bonds) enhance the carbon-to-carbon double bonds) enhance membrane fluidity. membrane fluidity.

6.6. Membranes must be fluid to work properly. Membranes must be fluid to work properly. Solidification may result in permeability Solidification may result in permeability changes and enzyme deactivation. changes and enzyme deactivation.

7.7. Organisms adapt to cold temperatures by Organisms adapt to cold temperatures by altering membrane lipid composition (e.g. altering membrane lipid composition (e.g. winter wheat increases concentration of winter wheat increases concentration of membrane unsaturated phospholipids and membrane unsaturated phospholipids and some hibernating animals enrich membranes some hibernating animals enrich membranes with cholesterol).with cholesterol).

glycoprotein

Extracellular Fluid (outside)

Cytoplasm (inside)

cholesterol

phospholipid bilayer phospholipid

recognition protein

receptor protein

transport protein

protein filaments

carbohydrate

binding site

water molecule

drop of dye


4.2 How Do Substances Move 4.2 How Do Substances Move Across Membranes?Across Membranes?

4.2.2 Movement Across Membranes 4.2.2 Movement Across Membranes Occurs by Both Passive and Active Occurs by Both Passive and Active TransportTransport– Table 4.1 Transport Across Membranes (p. Table 4.1 Transport Across Membranes (p.

62) 62)



4.2.3 Passive Transport Includes Simple 4.2.3 Passive Transport Includes Simple Diffusion, Facilitated Diffusion, and OsmosisDiffusion, Facilitated Diffusion, and Osmosis– 4.2.3.1 Plasma Membranes Are Selectively 4.2.3.1 Plasma Membranes Are Selectively

Permeable to Diffusion of MoleculesPermeable to Diffusion of Molecules– 4.2.3.2 Some Molecules Move Across Membranes 4.2.3.2 Some Molecules Move Across Membranes

by Simple Diffusionby Simple Diffusion Figure 4.3 (Hide/Reveal) Diffusion through the plasma Figure 4.3 (Hide/Reveal) Diffusion through the plasma

membrane (p. 63) membrane (p. 63)

lipid-soluble molecules(O2, CO2, H2O)

ions

amino acids,sugars,

small proteins

(extracellular fluid)

(cytoplasm)

Simple diffusion

channelprotein

Facilitated diffusion through a channel

(cytoplasm)

Facilitated diffusion through a carrier (extracellular fluid)

carrierprotein

lipid-soluble molecules(O2, CO2, H2O)


(cytoplasm)

Simple diffusion

ions

channelprotein

Facilitated diffusion through a channel

amino acids,sugars,

small proteins

carrierprotein

Facilitated diffusion through a carrier

(cytoplasm)


amino acids,sugars,

small proteins

carrierprotein

(cytoplasm)

Facilitated diffusion through a carrier (extracellular fluid)

amino acids,sugars,

small proteins

Facilitated diffusion through a carrier

carrierprotein

amino acids,sugars,

small proteins



– 4.2.3.3 Other Molecules Cross the Membrane 4.2.3.3 Other Molecules Cross the Membrane by Facilitated Diffusion, with the Help of by Facilitated Diffusion, with the Help of Membrane Transport ProteinsMembrane Transport Proteins

– 4.2.3.4 Osmosis Is the Diffusion of Water 4.2.3.4 Osmosis Is the Diffusion of Water Across MembranesAcross Membranes Figure 4.4 Osmosis (p. 64) Figure 4.4 Osmosis (p. 64)

selectivelypermeablemembrane

sugarmoleculewatermolecule


pore

H2O

sugar


pore

H2O

sugar


sugarmolecule

watermolecule



– 4.2.3.5 Osmosis Across the Plasma Membrane 4.2.3.5 Osmosis Across the Plasma Membrane Plays an Important Role in the Lives of CellsPlays an Important Role in the Lives of Cells Figure 4.5 The effects of osmosis (p. 65) Figure 4.5 The effects of osmosis (p. 65)

Isotonic solution10 micrometers

Hypertonic solution Hypotonic solution

Isotonic solution10 micrometers

Hypertonic solution

Hypotonic solution



4.2.4 Active Transport Uses Energy to 4.2.4 Active Transport Uses Energy to Move Molecules Against Their Move Molecules Against Their Concentration GradientsConcentration Gradients– Figure 4.6 (Hide/Reveal) Active transport (p. Figure 4.6 (Hide/Reveal) Active transport (p.

66) 66)

P

ADP

The protein releases the ion and the remnants of ATP (ADP and P) and closes.

3 The transport proteinbinds both ATP and Ca2+.1

ATP

Ca2+

ATPbindingsite

recognitionsite


(cytoplasm)

Energy from ATP changesthe shape of the transportprotein and moves the ion across the membrane.

2


(cytoplasm)


3

ADP

P


2 The transport proteinbinds both ATP and Ca2+.

ATPbindingsite

ATP

Ca2+

1

recognitionsite

The transport proteinbinds both ATP and Ca2+.

ATPbindingsite

ATP

Ca2+


(cytoplasm)

1

recognitionsite


(cytoplasm)


2


(cytoplasm)


3

ADP

P



4.2.5 Cells Engulf Particles or Fluids by 4.2.5 Cells Engulf Particles or Fluids by EndocytosisEndocytosis– Figure 4.7 Three types of endocytosis (p. 67) Figure 4.7 Three types of endocytosis (p. 67)

The plasma membrane extends pseudopods toward an extracellular particle (for example, food). The ends of the pseudopods fuse, encircling the particle. A vesicle calleda food vacuole is formed containing the engulfed particle.


(cytoplasm)

pseudopods

food vacuole

food particle

12

3

12

3

Phagocytosis

Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. The receptors bindthe molecules and the membrane dimples inward. The coatedpit region of the membrane encloses the receptor-bound molecules. A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm.


(cytoplasm)

1

1

2

3 4

4

32

1

Receptor-mediated endocytosis

nutrientsreceptors

coated pit

coated vesicle

A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle.

Pinocytosis

3

3


(cytoplasm)

vesicle containing extracellularfluid

2

2

1

1

A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle.

Pinocytosis

3

3


(cytoplasm)


2

2

1

1

A dimple forms in the plasma membrane, which

Pinocytosis

1

1


(cytoplasm)

deepens and surrounds the extracellular fluid.

Pinocytosis

2

2


(cytoplasm)

The membrane encloses the extracellular fluid,forming a vesicle.

Pinocytosis

3

3


(cytoplasm)


Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. The receptors bindthe molecules and the membrane dimples inward. The coatedpit region of the membrane encloses the receptor-bound molecules. A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm.


(cytoplasm)

1

1

2

3 4

4

32

1


nutrientsreceptors

coated pit

coated vesicle


(cytoplasm)

1

nutrientsreceptors

coated pit


Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites.

1


(cytoplasm)

2

nutrientsreceptors

coated pit

coated vesicle


The receptors bind the molecules and the membrane dimples inward.

2


(cytoplasm)

3

nutrientsreceptors

coated pit

coated vesicle


The coated pit region of the membrane encloses the receptor-bound molecules.

3


(cytoplasm)

nutrientsreceptors

coated pit

coated vesicle


A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm.

4

4

The plasma membrane extends pseudopods toward an extracellular particle (for example, food). The ends of the pseudopods fuse, encircling the particle. A vesicle calleda food vacuole is formed containing the engulfed particle.


(cytoplasm)

pseudopods

food vacuole

food particle

12

3

1 2

3

Phagocytosis

The plasma membrane extends pseudopods toward an extracellular particle (for example, food).


(cytoplasm)

pseudopods

food vacuole

food particle

1

1

(c) Phagocytosis

The ends of the pseudopods fuse, encircling the particle.


(cytoplasm)

pseudopods

food vacuole

food particle

2

(c) Phagocytosis

2

A vesicle called a food vacuole is formed containing the engulfed particle.


(cytoplasm)

pseudopods

food vacuole

food particle

3

3

(c) Phagocytosis



– 4.2.5.1 Pinocytosis Moves Liquids into the Cell4.2.5.1 Pinocytosis Moves Liquids into the Cell– 4.2.5.2 Receptor-Mediated Endocytosis Moves 4.2.5.2 Receptor-Mediated Endocytosis Moves

Specific Molecules into the CellSpecific Molecules into the Cell Figure 4.8 Receptor-mediated endocytosis (p. 67) Figure 4.8 Receptor-mediated endocytosis (p. 67)

coated vesicle

0.1 micrometerproteincoating

coated pit

extracellular particlesbound to receptors

plasma membrane

(cyto-plasm)


proteincoating

coated pit

extracellular particlesbound to receptors

plasma membrane

(cyto-plasm)


coated vesicle

0.1 micrometer



– 4.2.5.3 Phagocytosis Moves Large 4.2.5.3 Phagocytosis Moves Large Particles into the CellParticles into the Cell

4.2.6 Exocytosis Moves Material Out of 4.2.6 Exocytosis Moves Material Out of the Cellthe Cell– Figure 4.9 Exocytosis (p. 68) Figure 4.9 Exocytosis (p. 68)

plasma membrane

plasma membrane

(cytoplasm)

vesicle

0.2 micrometer

secretedmaterial


plasma membrane

(cytoplasm)

vesicle

secretedmaterial


plasma membrane

vesicle

0.2 micrometer

secretedmaterial


4.3 How Are Cell Surfaces 4.3 How Are Cell Surfaces Specialized?Specialized?

4.3.1 Various Specialized Junctions 4.3.1 Various Specialized Junctions Allow Cells to Connect and Allow Cells to Connect and CommunicateCommunicate– 4.3.1.1 Desmosomes Attach Cells 4.3.1.1 Desmosomes Attach Cells

TogetherTogether Figure 4.10 Cell attachment structures (p. 68) Figure 4.10 Cell attachment structures (p. 68)

liver

liver cells

plasma membrane

Gap junctions

cell wall

rootcells

root

Plasmodesmata


4.3 How Are Cell Surfaces 4.3 How Are Cell Surfaces Specialized?Specialized?

– 4.3.2 Some Cells Are Supported by Cell 4.3.2 Some Cells Are Supported by Cell WallsWalls Figure 4.12 Plant cell walls (p. 69) Figure 4.12 Plant cell walls (p. 69) Figure 4.13 Caribou browse on the frozen Figure 4.13 Caribou browse on the frozen

Alaskan tundra (p. 70) Alaskan tundra (p. 70)

primarycell wall

secondarycell wall

plasmamembrane

cytoplasmmiddlelamella

MEMBRANE STRUCTURE AND TRAFFIC

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