Structure, Function and Chemistry of Structure, Function and Chemistry of Membrane Membrane

The Function of Membranes• Membrane define boundaries and serve as

permeability barriers – plasma (or cell) membrane is the permeability barrier– intracellular membrane serve to compartmentalize function

within eukaryotic cells• Sites of specific proteins , specific functions

– marker En. in organelles • Glucose phosphatase ER

• regulation of transport– transport protein : passive transport (facilitated

transport); active transport (direct and indirect )– Endocytosis and exocytosis

The Function of Membranes

• detection and transmission of signals– Signal transduction

– Receptors

cell-to-cell communication– gap junction; plasmodesmata

Charles Overton(1890) --Lipophilic

Irving Langmuir(1902) --- benzene-lipid solution onto the surface of the water

Gorter and Grendel(1925): --- lipid bilayer,7 m in diameter --- twice monolayer of lipid --- erythrocytesDavson and Danielli(1935)

---Differential permeability --- glucose, galactose --- the presence of proteins in membranes

History of the Plasma Membrane• 1665: Robert Hooke • 1895: Charles Overton - composed of lipids • 1900-1920’s: must be a phospholipid • 1925: E. Gorter and G. Grendel - phospholipid

bilayer• 1935: J.R. Danielli and H. Davson – proteins

also part, proposed the Sandwich Model • 1950’s: J.D. Robertson – proposed the Unit

Membrane Model • 1972: S.J. Singer and G.L. Nicolson – proposed

Fluid Mosaic Model

Plasma Membrane is made of Phospholipids

• Gorter + Grendel – Red Blood Cells analyzed

– Enough for Phospholipid bilayer

– Polar heads face out and Nonpolar tails face in

– Does not explain why some nonlipids are permeable

Plasma Membrane Models

Sandwich Model(Danielli + Davson)2 layers of globular proteins with phospholipid inside to make a layer and then join 2 layers together to make a channel for molecules to pass

Unit Membrane Model (Robertson)Outer layer of protein with phospholipid bilayer inside, believed all cells same composition, does not explain how some molecules pass through or the use of proteins with nonpolar parts, used transmission electron microscopy

Fluid Mosaic Model (Singer + Nicolson)Phospholipid bilayer with proteins partially or fully imbedded, electron micrographs of freeze-fractured membrane

• Robertson (1950s, EM)– The unit membrane, 7.5 m (6-8),– Trilaminar ; hydrophobic, do not readily stain – Osmium: proteins– Davson and Danielli ：

• Proteins alpha helix

• Protein/lipid ratio not the same: 3 in bacterial cells and 0.23 for the myelin sheath

• phopholipase, only 75% lipid were degraded; membrane lipids

protected by protein

Robertson -- unit membrane

Which membrane model is correct?

1) Rapidly freeze specimen

2) Use special knife to cut membrane in half

3) Apply a carbon + platinum coating to the surface

4) Use scanning electron microscope to see the surface

According to the electron micrograph which membrane model is correct?

Why?

Fluid-Mosaic Model

• Singer and Nicolson (1972)– The fluid mosaic model : A mosaic of proteins

discontinuously embedded in a fluid lipid bilayer.• Integral membrane proteins, embedded; • peripheral proteins on the surface • Lipid anchored proteins • Lipid lateral mobility

– Explanations for Davson-Danieli model– Universally accepted; the essential features p. 166 1st

para. • Unwin and Henderson (1975)

– Integral membrane protein has a transmembrane segment, bacteriorhodopsin

Fluid-Mosaic Model• Fluid – the plasma membrane is the consistency of olive oil at body

temperature, due to unsaturated phospholipids. (cells differ in the amount of unsaturated to saturated fatty acid tails)

• Most of the lipids and some proteins drift laterally on either side. Phospholipids do not switch from one layer to the next.

• Cholesterol affects fluidity: at body temperature it lessens fluidity by restraining the movement of phospholipids, at colder temperatures it adds fluidity by not allowing phospholipids to pack close together.

• Mosaic – membrane proteins form a collage that differs on either side of the membrane and from cell to cell (greater than 50 types of proteins), proteins span the membrane with hydrophilic portions facing out and hydrophobic portions facing in. Provides the functions of the membrane

Membrane Structure• A boundary separating the cell from its environment• About 8 nm thick• Controls chemical traffic in and out of the cell• Selectively permeable• Unique structure determines function and solubility

characteristics

Nitrogen-Nitrogen-containing containing

groupgroup

Phosphate Phosphate groupgroup

Fatty acid tailFatty acid tail

Three major classes of membrane lipids

Three major classes of membrane lipids

The phospholipid bilayer and isolation

1. Impermeable to water-soluble and polar molecules, ions

2. Permeable to small and nonpolar molecules

3. Lipids oriented with polar heads facing out

TLC

• Glass or metal plate coated with a layer of silicic acid

• Solvent system: chloroform, methanol, water

• Separated by their polarity : nonpolar lipids , move fast because it do not adhere strongly to the silicic acid

Membrane Asymmetry

• Movements of phospholipid– flip-flopping = transverse diffusion ;rotation;

lateral diffusion

The Fluidity of Membranes

Membranes rarely flip-flop – too unstableFrequent lateral movement

Double bonds in unsaturated lipid tails cause kinks which contribute to fluidity

Cholesterol reduces fluidity at high temps. Increases fluidity at low temps.

The inner and outer leaflet is reversed in ER membrane

IN ER: PC present in both leaflet or dominant in inner leaflet

PS, PE, PI are confined to the cytosolic leaflet (outer leaflet in ER)

Lipid Bilayers are Dynamic Structures

• Lipid movements– Lateral diffusion, fast

• 37°C, 2m long, 2-dimensional solution

– Transverse diffusion (flip-flop)• 109 very slow, large activity energy

– Membrane-bound protein• Not phosphatidyl serine, phosphatidyl ethanolamine

• Move PC, sphingomyelin

• Using ATP

• Called “flippase” or “translocase”(mammals)

Membrane fluidity

• Demonstration : fluorescence recovery after photobleaching

• Tm

• chain length (number of carbon atoms ) ; saturation (number of double bonds) ; cholesterol

Membrane fluidity

• Demonstration : fluorescence recovery after photobleaching

• T ; chain length (number of carbon atoms ) ; saturation (number of double bonds) ; cholesterol

C 數 16:0 – 41oC ; 14:0 –23oC

Double bonds:

18:0 – 54oC; 18:1 -- -20oC

Note: Oleate --18:1; sterate– 18:0

Rigid ring– mobility decrease

-fluidity decrease

Disrupt the ordered packing

van der waal force decrease

fluidity increase

37oC

25oC

The inner and outer leaflet is reversed in ER membrane

IN ER: PC present in both leaflet or dominant in inner leaflet

PS, PE, PI are confined to the cytosolic leaflet (outer leaflet in ER)

The Regulation of Membrane Fluidity : Poikilotherms

• Poikilotherms – 變溫動物— homeoviscous adaptation – Bacteria, fungi, protists, plant , cold-blooded animals;

paralyzed by temperatures much above 45oC– Homeotherm : warm-blooded

• Acholeplasma– Cannot manufacture their own fatty acids, – unsaturated fatty acids, membranes remain fluid and

the bacteria grow, – only saturated fatty acid – the bacteria grow only until

membranes undergo a phase transition to the gel state.

The Regulation of Membrane Fluidity : Homeoviscous Adaptation

• Homeoviscous adaptation : compensating for the effects of temperature

The Regulation of Membrane Fluidity : Homeoviscous Adaptation• Micrococcus

– T increase in the proportion of 16-carbon rather than 18-carbon fatty acids, increase in the activity of an En for removing two terminal carbon form 18-carbon hydrocarbon tails

• E. coli– T -- synthesis of a desaturase En that introduces

double bonds into the hydrocarbon chains of fatty acids – decrease the transition temperature of the membrane

The Regulation of Membrane Fluidity : Homeoviscous Adaptation• Yeasts, Plants

– Lower T, increased solubility of oxygen, O2 is a substrate for an desaturase En. System involved in the generation of unsaturated fatty acids; more O2 available at lower T, unsaturated fatty acids are synthesized at a greater rate and membrane fluidity increase.

• Poikilotherms animals– Increasing the proportion of unsaturated fatty acids

and cholesterol

Plasma membrane protein

• Transport proteins and regulationControls exchange of water-soluble molecules

– Energy production and ion pumps

• Receptor proteins and communication– Nervous system; Endocrine system

• Recognition proteins and identification– The immune system

• Membrane proteins, genetic disease, and evolution of biocide resistance

Membrane Proteins

• Freeze fracture technique

• SDS PAGE

• Functions : transport proteins, channels, ATPases, receptors, et al.

• Mobility : cell fusion; patching and capping

Membrane Proteins (p. 175)

• Freeze fracture technique (Fig 16)

• E face: exterior; P face: protoplasmic face

Membrane protein

• Integral protein : – single pass :glycophorin [p.178] – multipass (2,3-20): band 3 protein, anion

exchange protein

• Peripheral protein

• Lipid-anchored protein

Peripheral proteins are on the membrane surface.

They are water-soluble, with mostly hydrophilic surfaces.

Often peripheral proteins can be dislodged by conditions that disrupt ionic & H-bond interactions, e.g., extraction with solutions containing high concentrations of salts, change of pH, and/or chelators that bind divalent cations.

– Electrostatic forces– Hydrophilic portions---lipids– Extraction---pH, ionic strength

Peripheral membrane proteins-anchored to a phospholipid in one layer of the membrane-possess nonpolar regions that are inserted in the lipid bilayer-are free to move throughout one layer of the bilayer

Integral membrane proteins• Integral proteins possess at least one transmembrane

domain i.e span the lipid bilayer (transmembrane proteins)• have domains that extend into the hydrocarbon core of the

membrane• nonpolar regions of the protein are embedded in the interior

of the bilayer i.e. region of the protein containing hydrophobic amino acids

• polar regions of the protein protrude from both sides of the bilayer

• Intramembrane domains have largely hydrophobic surfaces, that interact with membrane lipids.

• single pass :glycophorin

• multipass (2,3-20): band 3 protein, anion exchange protein (2 polypeptides, 6 times; N-and C-terminal on the same side

A membrane-spanning -helix is the most common structural motif found in integral proteins.

membrane

N

C

Integral protein structure 

Atomic-resolution structures have been determined for a small (but growing) number of integral membrane proteins.

Integral proteins are difficult to crystallize for X-ray analysis.

Because of their hydrophobic transmembrane domains, detergents must be present during crystallization.

In an -helix, amino acid R-groups protrude out from the helically coiled polypeptide backbone. The largely hydrophobic R-groups of a membrane-spanning -helix contact the hydrophobic membrane core, while the more polar peptide backbone is buried. Colors: C N O R-group (H atoms not shown).

-helix R-groups in magenta

Particular amino acids tend to occur at different positions relative to the surface or interior of the bilayer in transmembrane segments of integral proteins.

Residues with aliphatic side-chains (leucine, isoleucine, alanine, valine) predominate in the middle of the bilayer.

H3N+ C COO

CH3

H

H3N+ C COO

CH CH3

CH2

CH3

H

H3N+ C COO

CH2

CH CH3

CH3

H

H3N+ C COO

CH CH3

CH3

H

alanine (Ala, A) isoleucine (Ile, I) leucine (Leu, L) valine (Val, V)

amino acids: non-polar aliphatic R-groups

It has been suggested that the polar character of the tryptophan amide group and the tyrosine hydroxyl, along with their hydrophobic ring structures, suit them for localization at the polar/apolar interface.

tryptophan tyrosine

H2N C COO

CH2

HN

H

H3N+ C COO

CH2

OH

H

Tyrosine and tryptophan are common near the membrane surface.

Lysine & arginine are often at the lipid/water interface, with the positively charged groups at the ends of their aliphatic side chains extending toward the polar membrane surface. 

H3N+ C COO

CH2

CH2

CH2

NH

C

NH2

NH2

H

H3N+ C COO

CH2

CH2

CH2

CH2

NH3

H

lysine arginine

Cytochrome oxidase is an integral protein whose intra-membrane domains are mainly transmembrane -helices.

membrane

Cytochrome oxidase dimer (PDB file 1OCC)

Hydropathy plot

Hydropathy index of amino acid• 20-30 amino acids

• 10 amino acids at a time • Hydropathy index : Hydrophobicity values for the

various amine acids – • Hydropathy scale for amino acid side chains• Ile 4.5 Val 4.2 Ala 1.8• Pro -1.6 Arg -4.5

A 20-amino acid -helix just spans a lipid bilayer.

Hydropathy plots are used to search for 20-amino acid stretches of hydrophobic amino acids in the primary sequence of a protein for which a crystal structure is not available.

Putative hydrophobic transmembrane -helices have been identified this way in many membrane proteins.

Hydropathy plots alone are not conclusive.

Protein topology studies are used to test the transmembrane distribution of protein domains predicted by hydropathy plots.

If a hydropathy plot indicates one 20-amino acid hydrophobic stretch (1 putative transmembrane -helix), topology studies are expected to confirm location of N & C termini on opposite sides of membrane.

If two transmembrane -helices are predicted, N & C termini should be on the same side. The segment between the -helices should be on the other side.

N C

membrane

N

C

Transmembrane topology is tested with impermeant probes, added on one side of a membrane. For example:

Protease enzymes. Degradation a protein segment indicates exposure to the aqueous phase on the side of the membrane to which a protease is added.

Monoclonal antibodies raised to peptides equivalent to individual segments of the protein. Binding indicates surface exposure of a protein segment on the side to which the Ab is added.

Such studies have shown that all copies of a given type of integral protein have the same orientation relative to the membrane. Flip-flop of integral proteins does not occur.

An -helix lining a water-filled channel might have polar amino acid R-groups facing the lumen, & non-polar R-groups facing lipids or other hydrophobic -helices.

Such mixed polarity would prevent detection by a hydropathy plot.

A “helical wheel” looks down the axis of an -helix, projecting side-chains onto a plane.

Simplified helical wheel diagram of four -helices lining the lumen of an ion channel. Polar amino acid R-group Non-polar amino acid R-group

74

Membrane Proteins

Extensive nonpolar regions within a transmembrane protein can create a pore through the membrane.

- sheets in the protein secondary structure form a cylinder called a -barrel

--barrel interior is polar and allows water and small polar molecules to pass through the membrane

75

Porin -barrelWhile transmembrane -helices are the most common structural motif for integral proteins, a family of bacterial outer envelope channel proteins called porins have instead barrel structures.

A barrel is a sheet rolled up to form a cylindrical pore.

At right is shown one channel of a trimeric porin complex.

Porin Monomer

PDB 1AOS

In a -sheet, amino acid R-groups alternately point above & below the sheet.

Much of porin primary structure consists of alternating polar & non-polar amino acids. • Polar residues face the aqueous lumen. • Non-polar residues are in contact with membrane lipids.

polar R group, non-polar R group

Many proteins have a modular design, with different segments of the primary structure folding into domains with different functions.

Some cytosolic proteins have domains that bind to polar head groups of lipids that transiently exist in a membrane.

The enzymes that create or degrade these lipids are subject to signal-mediated regulation, providing a mechanism for modulating affinity of a protein for a membrane surface.

O P

O

O

H2C

CH

H2C

OCR1

O O C

O

R2

OH

H

OPO32

H

H

OPO32H

OH

H

O

H OH

1 6

5

43

2

PIP2 phosphatidylinositol- 4,5-bisphosphate

E.g., pleckstrin homology (PH) domains bind to phosphorylated derivatives of phosphatidylinositol.

Some PH domains bind PIP2 (PI-4,5-P2).

Other pleckstrin homology domains recognize and bind phosphatidylinositol derivatives with Pi esterified at the 3' OH of inositol. E.g., PI-3-P, PI-3,4-P2, PI-3,4,5-P3.

O P

O

O

H2C

CH

H2C

OCR1

O O C

O

R2

OH

H

OH

H

H

OHH

OPO32

H

O

H OH

1 6

52

3 4

phosphatidyl-inositol-

3-phosphate

• Lipid-anchored protein– Fatty acid

• Mristic acid (14C); palmitic acid (16 C)– Prenylated (prenyl group: isoprene derivative )

• 15C farnesyl group or 20 C geranylgeranyl group– GPI----PLC

• Some proteins are bounded to the membrane by covalently attached hydrocarbon chains:– glycosylphosphatidylinositol(GPI)-anchored :alkaline

phosphatease, Thy-1– myristate-anchored proteins : p60v-src (or V-Src)– farnesyl-anchored proteins : p21ras (Ras)

Some proteins bind to membranes via a covalently attached lipid anchor, that inserts into the bilayer.

A protein may link to the cytosolic surface of the plasma membrane via a covalently attached fatty acid (e.g., palmitate or myristate) or an isoprenoid group.

Palmitate is usually attached via an ester linkage to the thiol of a cysteine residue.

A protein may be released from plasma membrane to cytosol via depalmitoylation, hydrolysis of the ester link.

lipid anchor

membrane

H3C (CH2)14 C

O

S CH2 CH

C

NH

O

palmitate

cysteine residue

An isoprenoid such as a farnesyl residue, is attached to some proteins via a thioether linkage to a cysteine thiol.

C H C H 2CH 3 C

C H 3

C H C H 2CC H 2

C H 3

C H C H 2 S P r o t e i nCC H 2

C H 3

f a r n e s y l r e s i d u e l i n k e d t o p r o t e i n v i a c y s t e i n e S

lipid anchor

membrane

Glycosylphosphatidylinositols (GPI) are complex glycolipids that attach some proteins to the outer surface of the plasma membrane.

The linkage is similar to the following, although the oligosaccharide composition may vary:

protein (C-term.) - phosphoethanolamine – mannose - mannose - mannose - N-acetylglucosamine – inositol (of PI in membrane)

The protein is tethered some distance out from the membrane surface by the long oligosaccharide chain.

GPI-linked proteins may be released from the outer cell surface by phospholipases.

Membrane Carbohydrates• Found only on the outside of the membrane.• Function in cell to cell recognition. • Sorting embryonic cells into tissues. • Immune defense.• Usually oligosaccharides

(15 or less sugar units)• Glycolipids or Glycoproteins

• Glycoproteins and glycolipids are Glycoproteins and glycolipids are proteins/lipids with short chain carbohydrates proteins/lipids with short chain carbohydrates attached on the extracellular side of the attached on the extracellular side of the membrane.membrane.

Membrane Proteins

• Freeze fracture technique • SDS PAGE (polyacrylamide gel

electrophoresis) • Molecular biology—hydropathy plot

Functions : transport proteins, channels, ATPases, receptors, et al.

• Mobility : cell fusion; patching and capping

Hydrophobic domains of detergents substitute for lipids, coating hydrophobic surfaces of integral proteins.

Polar domains of detergents interact with water.

If detergents are removed, purified integral proteins tend to aggregate & come out of solution. Their hydrophobic surfaces associate to minimize contact with water.

Amphipathic detergents are required for solubilization of integral proteins from membranes.

detergentsolubilization

Protein withbound detergent

polarnon-polar

membrane

Sidedness of the Plasma Membrane

Membranes have distinct cytoplasmic and extracellular sides.

Complex sphingolipids tend to separate out from glycerophospholipids & co-localize with cholesterol in membrane microdomains called lipid rafts.

Membrane fragments assumed to be lipid rafts are found to be resistant to detergent solubilization, which has facilitated their isolation & characterization.

Lipid rafts

Differences in molecular shape may contribute to a tendency for sphingolipids to separate out from glycerophospholipids in membrane microdomains.

• Sphingolipids usually lack double bonds in their fatty acid chains.

• Glycerophospholipids often include at least one fatty acid that is kinked, due to one or more double bonds.

• See diagram (in article by J. Santini & coworkers).

Lipid raft domains tend to be thicker than adjacent membrane areas, in part because the saturated hydrocarbon chains of sphingolipids are more extended.

Hydrogen bonding between the hydroxyl group of cholesterol and the amide group of sphingomyelin may in part account for the observed affinity of cholesterol for sphingomyelin in raft domains.

H2CHC

O

CH

NH CH

C

CH2

CH3

H

OH

( )12

C

R

O

PO O

O

H2C

H2CN+

CH3

H3C

CH3

Sphingomyelin

phosphocholine

sphingosine

fatty acid

C holestero lH O

Proteins involved in cell signaling often associate with lipid raft domains.

• Otherwise soluble signal proteins often assemble in complexes at the cytosolic surface of the plasma membrane in part via insertion of attached fatty acyl or isoprenoid lipid anchors into raft domains.

• Integral proteins may concentrate in raft domains via interactions with raft lipids or with other raft proteins.

• Some raft domains contain derivatives of phosphatidylinositol that bind signal proteins with pleckstrin homology domains.

Caveolin is a protein associated with the cytosolic leaflet of the plasma membrane in caveolae.

Caveolin interacts with cholesterol and self-associates as oligomers that may contribute to deforming the membrane to create the unique morphology of caveolae.

caveolae cytosol

are invaginated lipid raft domains of the plasma membrane that have roles in cell signaling and membrane internalization.

CAVEOLAE

Plasma membrane protein

• En: GPD—glucose catabolism• Electron transport proteins: cytochromes; iron-sulfur

proteins • Transport proteins and regulation

Controls exchange of water-soluble molecules– Energy production and ion pumps

• Receptor proteins and communication– Nervous system; Endocrine system

• Recognition proteins and identification– Junctions; connexons---gap junction

• Membrane proteins, genetic disease, and evolution of biocide resistance

Functions of Membrane

Proteins

Sidedness of the Plasma Membrane

Membranes have distinct cytoplasmic and extracellular sides.

Sellective Permeability of the Lipid Bilayer

Proteins of the Plasma Membrane Provide 6 Membrane Functions:

1) Transport Proteins

2) Receptor Proteins

3) Enzymatic Proteins

4) Cell Recognition Proteins

5) Attachment Proteins

6) Intercellular Junction

Proteins

1) Transport Proteins

Channel Proteins – channel for lipid insoluble molecules and ions to pass freely through

Carrier Proteins – bind to a substance and carry it across membrane, change shape in process

Provide hydrophilic tunnel for ions.They are specific for the substances they transport

2) Receptor Proteins

– Bind to chemical messengers (Ex. hormones) which sends a message into the cell causing cellular reaction

3) Enzymatic Proteins

– Carry out enzymatic reactions right at the membrane when a substrate binds to the active site

4) Cell Recognition Proteins

– Glycoproteins (and glycolipids) on extracellular surface serve as ID tags (which species, type of cell, individual). Carbohydrates are short branched chains of less than 15 sugars

5) Attachment Proteins

- Attach to cytoskeleton (to maintain cell shape and stabilize proteins) and/or the extracellular matrix (integrins connect to both).

- Extracellular Matrix – protein fibers and carbohydrates secreted by cells and fills the spaces between cells and supports cells in a tissue.

- Extracellular matrix can influence activity inside the cell and coordinate the behavior of all the cells in a tissue.

6) Intercellular Junction Proteins

– Bind cells together

– Tight junctions

– Gap junctions

• Materials must move in and out of the cell through the plasma membrane.

• Some materials move between the phospholipids.

• Some materials move through the proteins.

How do materials move into and out of the cell?

Review: Passive Transport

• Diffusion – O2 moves in and CO2 moves out during cell respiration

• Facilitated Diffusion – glucose and amino acids enter cell for cell respiration

• Osmosis – cell removal or addition of water

Osmosis and Tonicity

• Tonicity refers to the total solute concentration of the solution outside the cell.

• What are the three types of tonicity?1) Isotonic

2) Hypotonic

3) Hypertonic

Isotonic• Solutions that have the same concentration of solutes as

the suspended cell.

• What will happen to a cell placed in an Isotonic solution?

• The cell will have no net movement of water and will stay the same size.

• Ex. Blood plasma has high concentration of albumin molecules to make it isotonic to tissues.

Hypotonic• Solutions that have a lower solute concentration than

the suspended cell.

• What will happen to a cell placed in a Hypotonic solution?

• The cell will gain water and swell.• If the cell bursts, then we call this lysis. (Red blood

cells = hemolysis)• In plant cells with rigid cell walls, this creates turgor

pressure.

Hypertonic• Solutions that have a higher solute concentration than a

suspended cell.

• What will happen to a cell placed in a Hypertonic solution?

• The cell will lose water and shrink. (Red blood cells = crenation)

• In plant cells, the central vacuole will shrink and the plasma membrane will pull away from the cell wall causing the cytoplasm to shrink called plasmolysis.

Review Tonicity

• What will happen to a red blood cell in a hypertonic solution?

• What will happen to a red blood cell in an isotonic solution?

• What will happen to a red blood cell in a hypotonic solution?

Three Forms of Endocytosis1. Pinocytosis

– form of endocytosis that brings into the cell a small volume of extracellular fluid and the materials suspended in it• “cell drinking”• Harbors are created• Pinched off and becomes a

vesicle 2. Receptor-mediated endocytosis

– Depends on receptors, whose role is to bind to specific molecules and then hold on to them• Receptors move laterally• Congregate in an area• Are pinched off

3. Phagocytosis– The process of bringing relatively

large materials into a cell by means of wrapping extensions of the plasma membrane around the material and fusing the extensions together• “cell eating”• Immune cells ingests a whole

bacteria• Single celled organisms obtain

their food• Psuedopods”false feet” aid in

capturing food• Lysosomes help break down

organism or food particle

Active Transport 2: Exocytosis

• Movement of large molecules bound in vesicles out of the cell with the aid of ATP energy. Vesicle fuses with the plasma membrane to eject macromolecules.

• Ex. Proteins, polysaccharides, polynucleotides, whole cells, hormones, mucus, neurotransmitters, waste

Active Transport 3: Endocytosis

• Movement of large molecules into the cell by engulfing them in vesicles, using ATP energy.

• Three types of Endocytosis:– Phagocytosis– Pinocytosis– Receptor-mediated endocytosis

Phagocytosis

• “Cellular Eating” – engulfing large molecules, whole cells, bacteria

• Ex. Macrophages ingesting bacteria or worn out red blood cells.

• Ex. Unicellular organisms engulfing food particles.

Pinocytosis• “Cellular Drinking” – engulfing liquids and

small molecules dissolved in liquids; unspecific what enters.

• Ex. Intestinal cells, Kidney cells, Plant root cells

Receptor-Mediated Endocytosis• Movement of very specific

molecules into the cell with the use of vesicles coated with the protein clathrin.

• Coated pits are specific locations coated with clathrin and receptors. When specific molecules (ligands) bind to the receptors, then this stimulates the molecules to be engulfed into a coated vesicle.

• Ex. Uptake of cholesterol (LDL) by animal cells

Cholesterol Enters Cells by Receptor Mediated Endocytosis

• In the blood, cholesterol is bound to lipid and protein complexes called LDLs

• The LDLs bind to LDL receptors on cell membranes initiating endocytosis.

• In the disease, familial hypercholesterolemia, the LDL receptors are defective, cholesterol cannot enter the cell and accumulates in the blood causing atherosclerosis.

Aquaporin – facilitates water diffusion (osmosis)

Tetrameric protein, four identical subunits

Each subunit forms a water channel

Review Types of Endocytosis

• What is phagocytosis?

• What is pinocytosis?

• What is receptor-mediated endocytosis?

• Recognition Proteins Recognition Proteins - identify type of cell - identify type of cell andand identify a cell as “self” versus foreignidentify a cell as “self” versus foreign– Most are glycoproteins Most are glycoproteins

• Carbohydrate chains vary between species, individuals, Carbohydrate chains vary between species, individuals, and even between cell types in a given individual.and even between cell types in a given individual.

• Glycolipids also play a role in cell recognitionGlycolipids also play a role in cell recognition

IntegrinsIntegrins

• IntegrinsIntegrins are a type of integral protein are a type of integral protein– The cytoskeleton attaches to integrins on the The cytoskeleton attaches to integrins on the

cytoplasmic side of the membranecytoplasmic side of the membrane– Integrins strengthen the membraneIntegrins strengthen the membrane

• Cell JunctionCell Junction proteins proteins - help like cells - help like cells stick together to form tissuesstick together to form tissues

Types of Cell Junctions

In Animal Cells:

• Tight Junctions

• Desmosomes

• Gap Junctions

In Plant Cells:

• Plasmodesmata

Tight Junctions• Transmembrane Proteins of opposite cells attach

in a tight zipper-like fashion

• No leakage

• Ex. Intestine, Kidneys, Epithelium of skin

Desmosomes• Cytoplasmic plaques of two cells bind with the

aid of intermediate filaments of keratin

• Allows for stretching

• Ex. Stomach, Bladder, Heart

Gap Junctions• Channel proteins of opposite cells join together

providing channels for ions, sugars, amino acids, and other small molecules to pass.

• Allows communication between cells.

• Ex. Heart muscle, animal embryos

Plasmodesmata• Channels between the cell walls of plant cells that

are lined with the plasma membranes of adjacent cells and smooth ER runs through.

• Allows for the exchange of cytosol between adjacent cells; moving water, small solutes, sugar, and amino acids.

• Ex. Xylem and Phloem in Plants

Review Types of Cell Junctions• What is the difference between a

plasmodesmata, tight junction, gap junction, and desmosome?

References

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. (1999). Biology. Reading: Addison Wesley Longman, Inc.

Mader, Sylvia S. (1996). Biology. Boston: Times Mirror Higher Education Group, Inc.

Raven, Peter H. and Johnson, George B. (1989). Biology. Boston: TimesMirror/Mosby College Publishing.

Diffusion of Salts Across Membranes

Each dye diffuses down its own concentration gradient

Summary: The nature of the Plasma membrane

• Constantly interacting with the outer world

• Very thin 10,000 0f them stacked would equal a sheet of paper.

• Fluid and fatty makeup.

• Stable

• Flexible

• Constantly reformed– Materials moving in and out

– Every 30 minutes

Important components of the plasma membrane

1. PHOSPHOLIPIB BILAYER– Largest component of the cell membrane– Fatty Acid tails face each other and Phosphate-Glyceral heads point in

the opposite directions

2. CHOLESTERAL– “patching” material– Keeps the membrane fluid

3. PROTEINS– Intergrated in the membrane– Are exposed at both sides– Serve as

• Support• Structure• Signaling• Identification markers• Cellular passageways

4. GLYCOCALYX– Carbohydrate chain that is layered outside the membrane

First Component: The Phospholipid Bilayer• Recall: Phospholipid=Hydrophillic head

(Phosphate)+ Hydrophobic tails (fatty Acid)– Phospholipid bilayera chief component of the plasma

membrane, composed of two layers of phospholipids, arranged with their fatty acid chains pointing toward each other.

• Consequences1. Oil like

2. Two hydrophobic layers only allow other hydrophobic substances to cross

Second Component: Cholesterol• Lies between two phospholipid molecule• “Patch up” keep small molecules from getting through• Helps the membrane maintain an optimum level of fluidity

– Accomplishes this by not allowing the chain to lie to close together• Without it at low temperatures the membrane would change from flexible

substance hard substance

• At low temperatures keeps the membrane from becoming too fluid

Third Component:Proteins• Found

– Embedded in– Lying on

• Two types1. Integral proteins are plasma membrane proteins

that are bound to the membrane’s hydrophobic interior• Can extend to either side of the membrane

2. Peripheral proteins plasma membrane proteins that lie on either side of the membrane but that are not bounded to the hydrophobic interior.

Role of Proteins1. STRUCTURAL SUPPORT

– Peripheral proteins are attached to the cytoskeleton• Anchors

• Gives cells their shape

2. RECOGNITION– Integral proteins

• Have binding site to tell Immune cells what kind of cell it is– How does this apply to blood type?

Role of Proteins cont.3. COMMUNICATION– Cells need to send messages to one another– Accomplished by

• Hormones– Signals are channeled through receptor proteins

• Plasma membrane proteins that binds with a signaling molecule1. Shape is specific to molecule (hormone)2. The signal is transferred internally a cascade of reactions follow

– Insulin receptor Increases production of protein channels for glucose

4. TRANSPORT– Integral proteins Transport protein proteins that facilitate the

movement of molecules or ions from one side of the plasma membrane to the other• Have channels• Hydrophillic opening

Fourth Component: The Glycalyx

• Carbohydrate chains – short branched extensions protruding from the phospholipds and the

proteins• Binding site for many signals insulin• Lubricates cells

– Allows them to stick together

• Collectively all these chains form the glycalyxan outer layer of the plasma membrane composed of short carbohydrate chains that attach to membrane proteins and phospholipid molecules

The Fluid-Mosaic Membrane Model

• Plasma membrane a membrane, forming the outer boundary of many cells, composed of a phospholipid bilayer that is interspersed with proteins and cholesterol and coated on its exterior face with carbohydrate chains.

• Fluid Mosaic model a conceptualization of the plasma membrane as a fluid, phospholipid bilayer that has, moving laterally within it, a mosaic of proteins.