Cell Membrane Final-Dr.urvashi

8/12/2019 Cell Membrane Final-Dr.urvashi

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Cell Membrane

Dr.Urvashi Kumar Kohli


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crucial to the life of the cell

encloses the cell,

defines its boundaries, and

maintains the essential differences between the cytosol and the

extracellular environment.

Cell Membrane


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1.1 Functions and properties of membrane

Membrane is a selectively permeablebarrierbetween the cell and the external environment.

Its function is to maintainHomeostasis. Its selectivepermeability allows the cell to maintain aconstant internal environment.

Plasma membranes formcompartments(compartmentation) within cells.


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Functions of Cell Membrane are to:

1. regulate cell volume

(control water in/ out)

2. maintain intracellular pH(H+regulation)

3. selectively regulate ionic

composition(e.g. Na+

, K+

)4. concentrate metabolic fuel

(nutrients, ATP, etc)

,


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Membrane transport system ought to (continued):

5. concentrate and move building blocks (amino

acids, etc)

6. remove toxic compounds (detoxification, trap

and/or pump out)7. generate ionic gradientsto maintain excitability

of nerve and muscle cells

8. control theflow of informationwithin the cell,

between cells and their environment.


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A BRIEF HISTORY OF STUDIES ON

PLASMA MEMBRANE STRUCTURE

Ernst Overton(1890s) - Chemical nature of the outer boundary layer

To test the permeability of the outer boundary layer, Overton placed plant root hairs

into hundreds of different solutions containing a diverse array of solutes. He discovered

that the more lipid soluble the solute, the more rapidly it would enter the root hair

cells

Remarks------dissolving power of the outer boundary layer of the cell

matched that of a fatty oil


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-- the polar groups of each molecular layer (or leaflet) were directed outward

toward the aqueous environment

E. Gorter and F. Grendel(1925)

Ratio of the surface area of water covered by the extracted lipid to the surface area

calculated for the red blood cells from which the lipid was extracted varied between1.8 to 1 and 2.2 to 1 and concluded that the plasma membrane contained a

bimolecular layer of lipids, that is, a lipid bilayer


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In 1935, Hugh Davson and James Danielli proposed that the plasmamembrane was composed of a lipid bilayer that was lined on both its inner and

outer surface by a layer of globular proteins

Davson and Danielli suggested that, in addition

to the outer and inner protein layers, the lipid

bilayer was also penetrated by protein-lined

pores, which could provide conduits for polar

solutes and ions to enter and exit the cell.


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Fluid-mosaic model proposed in 1972 by S. Jonathan Singer and Garth

Nicolson

A current representation of the plasma membrane showing the same basic organization as that proposed bySinger and Nicolson

The fluid-mosaic model of membrane structure as initially

proposed by Singer and Nicolson in 1972.


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All biological membranes have a common general

structure:Lipid and protein molecules, held together mainly by noncovalent

interactions.Characteristics:dynamic, fluid structures, and most of their molecules are able to move about in

the plane of the membrane

lipid molecules - double layer about 6 nm thick

- bilayer provides the basic structure of the membrane

- serves as a relatively impermeable barrier to the

passage of most water-soluble molecules.

Protein molecules - transport specific molecules

- catalyzing membrane associated reactions, such as ATP

synthesis- structural links - connect the membrane to the cytoskeleton

and/or to either the extracellular matrix or

an adjacent cell,

- receptors

Carbohydrates -cell membranes are asymmetrical structures


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Figure - 1. Three views of a cell membrane. (A) An electron micrograph of a plasma

membrane (of a human red blood cell) seen in cross-section. (B and C) Schematic

drawings showing twodimensional and three-dimensional views of a cell

membrane.


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Phospholipids(Amphipathic)

Sphingolipids(amphipathic)


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Figure -2. The parts of a phospholipid molecule. Phosphatidylcholine, represented

schematically (A), in formula (B), as a space-filling model (C), and as a symbol (D). The kink

due to the cis-double bond is exaggerated in these drawings for emphasis


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Figure 10-3. A lipid micelle and a lipid bilayer seen in cross-section. Lipid molecules form

such structures spontaneously in water. The shape of the lipid molecule determines

which of these structures is formed. Wedge-shaped lipid molecules (above) form micelles,

whereas cylindershaped phospholipid molecules (below) form bilayers.


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Figure - 4 . Phospholipid mobility. The types of movement possible for phospholipid

molecules in a lipid bilayer.


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The Fluidity of a Lipid Bilayer Depends on Its Composition

Phase Transition

- Hydrocarbon chains(short and Double bond)

- cholestrolInhibit phase transition

Composition A. phospholipids ( 4 major)

- phosphatidylcholine, sphingomyelin,

phosphatidylserine, and

phosphatidylethanolamine.

- inositol phospholipidsB. Cholestrol

C. Glycolipids


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Figure - 5. Influence of cis-double bonds in hydrocarbon chains. The double bonds

make it more difficult to pack the chains together and therefore make the lipid bilayer

more difficult to freeze.


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Figure - 6. The structure of cholesterol. Cholesterol is represented by a formula in

(A), by a schematic drawing in (B), and as a space-filling model in (C).


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Figure - 7. Cholesterol in a lipid bilayer. Schematic drawing of a cholesterol molecule

interacting with two phospholipid molecules in one leaflet of a lipid bilayer.

The Lipid Bilayer Is Asymmetrical


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Figure - 8. Four major phospholipids in mammalian plasma membranes. Note that different

head groups are represented by different symbols in this figure and the next. All of the lipid

molecules shown are derived from glycerol except for sphingomyelin, which is derived fromserine



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Figure - 9. The asymmetrical distribution of phospholipids and glycolipids in the lipidbilayer of human red blood cells. The symbols used for the phospholipids are those

introduced in Figure- 8. In addition, glycolipids are drawn with hexagonal polar head

groups (blue). Cholesterol (not shown) is thought to be distributed about equally in both

monolayers.



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Plasma membrane, are synthesized in the endoplasmic reticulum (ER), andit is here that the phospholipid asymmetry is generated by the phospholipid

translocators in the ER that move specific phospholipid molecules from one

monolayer to the other.


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Sphingolipids (amphipathic)


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Glycolipids Are Found on the Surface of All Plasma Membranes

5% of the lipid molecules in the outer monolayer

most complex of the glycolipids, the gangliosides - sialic acid

- most abundant in the plasma membrane of nerve cells, where they

constitute 5-10% of the total lipid mass,


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Figure 10-12. Glycolipid molecules. Galactocerebroside (A) is called a neutral glycolipid because

the sugar that forms its head group is uncharged. A ganglioside (B) always contains one or more

negatively charged sialic acid residues (also called N-acetylneuraminic acid, or NANA), whose structure is shown in (C). Whereas in bacteria

and plants almost all glycolipids are derived from glycerol, as are most phospholipids, in animal cells they are almost always produced from

sphingosine, an amino alcohol derived from serine, as is the case for the phospholipid sphingomyelin (see Figure- . Gal = g


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functions of glycolipids

In the plasma membrane of epithelial cells, -glycolipids protect the membrane from

the harsh conditions (such as low pH and degradative enzymes).

Charged glycolipids, such as gangliosides, (electrical effects): their presence will alter

the electrical field across the membrane and the concentrations of ions- especially

Ca2+ - at its external surface.

Glycolipids may also play a role in electrical insulation, since in the myelin

membrane, which electrically insulates nerve cell axons, the noncytoplasmic half of

the bilayer is filled with them.

They are also thought to function in cell-recognition processes.

Nature and Importance of the Lipid Bilayer


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Table -1 .Approximate Lipid Compositions of Different Cell Membranes

Nature and Importance of the Lipid Bilayer


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The Nature and Importance of the Lipid Bilayer

1. diversity in composition in different bacteria---physical state depends upon lipid

composition

2.Thickness - 60 Angstron

3.Continuous unbroken structure

4.self resealing property

5.flexible-shape changes (during locomotion n cell division)

6.lipid bilayer- facilitate regulated fusion and budding of membranes

7.maintaining the proper internal composition of a cell,

8. in separating electric charges across the plasma membrane

9. Self Assemble( PC Liposomes )

2 Proteins


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2. Proteins

THE STRUCTURE AND FUNCTIONS OF MEMBRANE PROTEINS

- Asymmetry of Proteins orientation in Membrane(membrane "sidedness")

- membrane protein- has a defined orientation relative to the cytoplasm.

-Therefore properties of one surface of a membrane are very different from those

of other surface the other surface

Three Classes of Proteins

a. Integral membrane

Proteins

b. Peripheral Proteins

c. GPI anchored Proteins

2 Proteins


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Figure - Six ways in which membrane proteins associate with the lipid bilayer. Most transmembrane proteins are

thought to extend across the bilayer as a single helix (1) or as multiple helices (2); some of these "single-pass"

and "multipass" proteins have a covalently attachedfatty acid chain inserted in the cytoplasmic monolayer (1).

Other membrane proteins are attached to the bilayer solely by a covalently attached lipid - either a fatty acid

chain or prenyl group - in the cytoplasmic monolayer (3) or, less often, via an oligosaccharide, to a minor

phospholipid, phosphatidylinositol, in the noncytoplasmic monolayer (4). Finally, many proteins are attached to

the membrane only by noncovalent interactions with other membrane proteins (5) and (6). How

the structure in (3) is formed is illustrated in next Figure

Membrane Proteins Can Be Associated with the Lipid Bilayer in Various Ways

2. Proteins

Integral Membrane Protein


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Figure 10-14. The covalent attachment of either of two types of lipid groups can help localize a

water-soluble protein to a membrane after its synthesis in the cytosol. (A) A fatty acid chain

(either myristic or palmitic acid) is attached via an amide linkage to an amino-terminal glycine. (B)

A prenyl group (either farnesyl or a longer geranylgeranyl group - both related to cholesterol) is

attached via a thioether linkage to a cysteine residue that is four residues from the carboxyl

terminus. Following this prenylation, the terminal three amino acids are cleaved off and the new

carboxyl terminus is methylated before insertion into the membrane. The structures of two lipid

anchors are shown underneath: (C) a myristyl anchor (a 14-carbon saturated fatty acid chain),

and (D) a farnesyl anchor (a 15-carbon unsaturated hydrocarbon chain).

In Most Transmembrane Proteins the Polypeptide Chain Is Thought to Cross the Lipid


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Figure 10-15. A segment of a transmembrane polypeptide chain crossing the lipid

bilayer as an alpha helix. Only the - carbon backbone of the polypeptide chain is

shown, with the hydrophobic amino acids in green and yellow.

yp p g p

Bilayer in an alpha-helical Conformation


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Figure . The three-dimensional structure of a porin trimer of Rhodobacter capsulatus

determined by x-ray crystallography. (A) Each monomer consists of a 16-stranded antiparallel b-

barrel that forms a transmembrane water-filled channel. (B) The monomers tightly associate to

form trimers, which have three separate channels for the diffusion of small solutes through the

bacterial outer membrane. A long loop of polypeptide chain (shown in red), which connects two b-strands, protrudes into the lumen of each channel, narrowing it to a cross-section of 0.6 x 1 nm.

Porins Are Pore-forming Transmembrane Proteins That Cross the Lipid Bilayer as a

beta- Barrel

Synthesis of TMM (Asymmetry of proteins)


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Figure 10-17. A typical single-pass transmembrane protein. Note that the polypeptide chain

traverses the lipid bilayer as a right-handed alpha- helix and that the oligosaccharide chains and

disulfide bonds are all on the noncytosolic surface of the membrane. Disulfide bonds do not form

between the sulfhydryl groups in the cytoplasmic domain of the protein because the reducingenvironment in the cytosol maintains these groups in their reduced (-SH) form.


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location of proteins- reflects the functionIMM(TM)- Functions on both sides- transport molecules across it.e.g. cell-surface receptors-TM proteins.Peripheral membrane proteins-e.g. protiens involved in intracellular signaling (cytosolic half).

TMM(IMM)Unique Orientation(Text)

---non-Cytoplasmic, membrane- spanning domain and cytoplasmic domain


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Figure 10-18. A detergent micelle in water, shown in cross-section. Because

they have both polar and nonpolar ends, detergent molecules are

amphipathic.

Membrane Proteins Can Be Solubilized and Purified in Detergents


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Figure 10-19. Solubilizing membrane proteins with a mild detergent. The detergent disrupts the

lipid bilayer and brings the proteins into solution as protein-lipid-detergent complexes. The

phospholipids in the membrane are also solubilized by the detergent.


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Figure 10-20. The structures of two commonly used detergents. Sodium dodecyl sulfate (SDS) is

an anionic detergent, and Triton X-100 is a nonionic detergent. The hydrophobic portion of each

detergent is shown in green, and the hydrophilic portion is shown in blue. Note that the bracketedportion of Triton X-100 is repeated about eight times


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Figure . The use of mild detergents for solubilizing, purifying, and reconstituting functional

membrane protein systems. In this example functional Na+-K+ ATPase molecules are purified and

incorporated into phospholipid vesicles. The Na+-K+ ATPase is an ion pump that is present in the

plasma membrane of most animal cells; it uses the energy of ATP hydrolysis to pump Na+ out ofthe cell and K+ in,

Cells Can Confine Proteins and Lipids to Specific Domains


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Figure . Diagram of an epithelial cell showing how a plasma membrane protein is restricted

to a particular domain of the membrane. Protein A (in the apical membrane) and protein B (in the

basal and lateral membranes) can diffuse laterally in their own domains but are prevented from

entering the other domain, at least partly by the specialized cell junction called a tight junction.

Lipid molecules in the outer (noncytoplasmic) monolayer of the plasma membrane are likewise

unable to diffuse between the two domains; lipids in the inner (cytoplasmic) monolayer, however,

are able to do so (not shown).

Within a Membrane


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Six major functions of membrane proteins:

Transport

Enzymatic activity

Signal transduction

Cell-cell recognition

Intercellular joining

Attachment to the cytoskeleton and extracellular

matrix (ECM)

LE 7-9a


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EnzymesSignal

ReceptorATP

Transport Enzymatic activity Signal transduction

LE 7-9b


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Glyco-protein

Cell-cell recognition Intercellular joining Attachment to the

cytoskeleton and extra-cellular matrix (ECM)


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The Role of Membrane Carbohydrates


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The Role of Membrane Carbohydratesin Cell-Cell Recognition

Cells recognize each other by binding to surface

molecules, often carbohydrates, on the plasma

membrane

Membrane carbohydrates may be covalently bonded to

lipids (forming glycolipids) or more commonly toproteins (forming glycoproteins)

Carbohydrates on the external side of the plasma

membrane vary among species, individuals, and even

cell types in an individual

Plasma membrane is a semi-permeable selective)


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Lipid bilayer

Small hydrophobic molecules: O2,CO2, N2, benzene

Small uncharged polar molecules: H2O,ethanol, glycerol

Larger uncharged polar molecules:glucose, amino acid, nucleotides

membrane/ Membrane structure results inselective permeability

Ions: H+, Na+, HCO3-, K+, Ca+,Mg2+, CL-, etc. Transporters or

channels

The process of diffusionof water is called osmosis.

But why some can diffuse while some cannot? What criteria are controlling the diffusion of

molecules across the membrane ?

Plasma membrane is a semi-permeable (selective)
http://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Diffusion


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Plasma membrane is a semi-permeable (selective) membrane

The process of diffusionof

water is called osmosis.

But why some can diffuse

while some cannot? What

criteria are controlling the

diffusion of moleculesacross the membrane ?

Figure 11-1. The relative permeability of a synthetic lipid bilayer to different classes of molecules.

The smaller the molecule and, more important, the fewer hydrogen bonds it makes with water, the

more rapidly the molecule diffuses across the bilayer.

Protein - free Lipid Bilayers Are Highly

Impermeable to Ions
http://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Diffusion


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Simple diffusion : cell membranes allow water and non-polar molecules to

permeate

Channel protein- Passive diffusion

Carrier Protein- Active or Passive

Two classes of membrane transport proteins


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Figure 11-3. A schematic view of the two classes of membrane transport proteins. A carrier

protein is thought to alternate between two conformations, so that the solute binding site is

sequentially accessible on one side of the bilayer and then on the other. In contrast, a channel

protein is thought to form a water-filled pore across the bilayer through which specific ions can

Diffuse.

Two classes of membrane transport proteins


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Figure 11-4. Comparison of passive transport down an electrochemical gradient with active

transport against an electrochemical gradient. Whereas simple diffusion and passive transport by

membrane transport proteins (facilitated diffusion) occur spontaneously, active transport requires

an input of metabolic energy. Only carrier proteins can carry out active transport, but both carrier

proteins and channel proteins can mediate facilitated diffusion.

Three types of carrier-mediated transport26


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Figure - Three types of carrier-mediated transport. The schematic diagram shows carrier

proteins functioning as uniports, symports, and antiports.

Uniport-

Glucose carriers in

animal cells take upglucose from

extracellular fluid.

(passive transport)

Symport -glucose with Na+

intestinal cells and Kidneycellstake up glucose from

the lumen of the intestine

and kidney tubules(low)

Antiport

Band 3 protein

(anion carrier)

In human RBC- exchange Cl-

for HCO3-)

Conformational change in a carrier protein could mediate the facilitated diffusion of a


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Figure . A hypothetical model showing how a conformational change in a carrier protein could

mediate the facilitated diffusion of a solute. The carrier protein shown can exist in two

conformational states: in state "pong" the binding sites for solute A are exposed on the outside of

the bilayer; in state "ping" the same sites are exposed on the other side of the bilayer. The

transition between the two states is proposed to occur randomly and to be completely reversible.

Therefore, if the concentration of A is higher on the outside of the bilayer, more A will bind to the

carrier protein in the pong conformation than in the ping conformation, and there will be a net

transport of A down its electrochemical gradient.

solute


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Table 11-1. Comparison of Ion Concentrations

Inside and Outside a Typical Mammalian Cell

Pumping cycle of the Na+-K+ ATPasePrimary Acitve Transport


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Figure . A schematic model of the pumping cycle of the Na+-K+ ATPase. The binding of Na+ (1) and the subsequent phosphorylation by ATP of

the cytoplasmic face of the ATPase (2) induce the protein to undergo a conformational change that transfers the Na+ across the membrane and

releases it on the outside (3). Then the binding of K+on the extracellular surface (4) and the subsequent dephosphorylation (5) return the protein

to its original conformation, which transfers the K+ across the membrane and releases it into the cytosol (6). These changes in conformation are

analogous to the ping pong transitions shown in Figure - except that here the Na+- dependent phosphorylation and the K+-dependent

dephosphorylation of the protein cause the conforma-tional transitions to occur in an orderly manner, enabling the protein to do useful work.

Although for simplicity only one Na+- and one K+-binding site are shown, in the real pump there are thought to be three Na+- and two K+-

binding sites. Moreover, although the ATPase is shown as alternating between two conformational states, there is evidence that it goes through

a more complex series of conformational changes during the actual pumping cycle.


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Figure 11-10. The Na+-K+ ATPase. This carrier protein actively pumps Na+ out of and K+ into a

cell against their electrochemical gradients. For every molecule of ATP hydrolyzed inside the cell,

three Na+ are pumped out and two K+ are pumped in. The specific pump inhibitor ouabain and K+

compete for the same site on the external side of the ATPase.


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Secondary Active Transport: Ion Gradients

Na+Glucose

Cl-

HClO3-


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Cells maintain voltage across


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Cells maintain voltage acrossplasma membranes

Cells maintain voltage across plasmamembranes.

Cytoplasm negative compared toopposite side of membrane(membrane potential - ranges from

-50 to -200 millivolts)


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Membrane potential favors passivetransport of cations (positive ions)into cell and anions (negative ions)out of cell.

Creates an electrochemical

gradient across membrane.


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Some organisms have proton pumpsthat actively pump H+out of cell(i.e. plants, bacteria, and fungi)


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Maintenance of Membrane Potential


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Maintenance of Membrane Potential

by Ion Pumps

Membrane potential is the voltage difference across amembrane

Two combined forces, collectively called the

electrochemical gradient, drive the diffusion of ions

across a membrane: A chemical force (the ions concentration gradient)

An electrical force (the effect of the membrane

potential on the ions movement)


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An electrogenic pump is a transport protein thatgenerates the voltage across a membrane

The main electrogenic pump of plants, fungi, and

bacteria is a proton pump

Ionophores


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Figure . A mobile ion carrier and a channel-forming ionophore. In both cases net ion flow

occurs only down an electrochemical gradient.

Valinomycin

(K+)Gramicidin A

Ionophores


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Figure 11-6. The structure of a gramicidin channel. The channel is formed by the association of

two identical peptides at their amino-terminal ends. Each chain is folded into a b helix, which

resembles a rolled-up b pleated sheet. (A) is a side view and (B) a top view. The peptide

backbones that line the channel are shown in blue and dark green, while the light green

represents the protruding hydrophobic side chains. The lipid bilayer is shown in gray. (C) shows

the size of unhydrated K+ions, while (D) shows a membrane-spanning a helix in top view for

comparison with the b helix.

Bulk transport across the plasma


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p p

membrane occurs by exocytosis and

endocytosis

Small molecules and water enter or leave the cell

through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins,

cross the membrane via vesicles


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Exocytosis In exocytosis, transport vesicles migrate to the

membrane, fuse with it, and release their contents

Many secretory cells use exocytosis to export their

products


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Endocytosis In endocytosis, the cell takes in macromolecules by

forming vesicles from the plasma membrane

Endocytosis is a reversal of exocytosis, involving

different proteins


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Three types of endocytosis:

Phagocytosis (cellular eating): Cell engulfs particlein a vacuole

Pinocytosis (cellular drinking): Cell creates vesiclearound fluid

Receptor-mediated endocytosis: Binding of ligandsto receptors triggers vesicle formation

LE 7-20c


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Receptor

RECEPTOR-MEDIATED ENDOCYTOSIS

Ligand

Coated

pit

Coated

vesicle

Coat protein

Coat

protein

Plasma

membrane

0.25 m

A coated pit

and a coated

vesicle formed

during

receptor-

mediated

endocytosis(TEMs).


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Synthesis and Sidedness of Membranes

Asymmetry of plasma membranes


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Asymmetry of plasma membranes

1. Membrane separates interior and exterior side of the cell

and the 2 faces of lipid bilayer are different in compositionand structure, with different proteins and phospholipids.2. The plasma membrane core contain 1/3 cholesterol and 2/3

phospholipids and sphingolipids, the outer leaflet contains5% glycolipids.

3. Oligosaccharide chains are attached at outer face of lipidsor proteins.

4. Shingomyelin and phosphatidylcholine are mainly at theouter face of the bilayer.

5. Phosphotidylethanolamine and phosphatidylserine aremainly in the inner face.

6. Asymmetry of proteins- Oligosaccharides and Disulphidebonds in the protein on non-cytoplasmis side

Ch i l d h i l ti f l


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1. Sheet-likestructure consists of proteins,lipids, with carbohydrates attached to them.

2. Non-covalent assemblies of lipids and proteins.3. The proteins serve the functions as

transporters, channels, enzymes, signaltransducers, etc.

4. The lipid molecules areamphipathicmolecules, they have both hydrophilic (polar)

and hydrophobic (non-polar) moieties.5. Barriersto the flow of charged molecules.

Chemical and physical properties of plasma

membranes

Cell Membrane Final-Dr.urvashi

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