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Plasma Membrane Chapter 13: The Plasma Membrane P529-P565

Structure of Plasma Membrane

Lipid Bilayer Proteins

-  Forms a stable barrier between two aqueous compartments; -  Impermeable to water-soluble mole- cules including ions and most biological molecules; - The bilayer lipids are mobile, not solid.

-  Selective transport of molecules; -  Cell-cell, Cell/Matrix interactions; - Cell signaling.

Fluid mosaic model

Lipid Bilayer

PS PI PE

PC SM

-

+ Outer leaflet:

SM & PC, Glyco- Lipid, cholesterol

Inner leaflet:

PS & PE, PI, cholesterol

The outer leaflet and inner leaflet are asymmetrical

Phospholipid

Sphingomyelin - SM Phosphatidylcholine - PC Phosphatidylserine - PS Phosphatidylethanolamine - PE Phosphatidylinositol - PI

Phospholipids

-  Account for more than half of the lipid in most membrane; -  Responsible for the basic function of membranes as barriers; -  The long fatty acid chains can move freely in the interior of the membrane.

phosphotidylinositol

Glycolipids

-  Constitute about 2% of the lipids of most plasmas membrane; -  Exclusively in the outer leaflet; -  Its carbohydrate portions exposed on the cell surface play protective role, as well as involved in cell-cell recognition.

(N-acetylneuraminic Acid)

Cholesterol

-  A major constituent of animal cell membrane: about equal molar amounts as the phospholipids; -  It fills the gap between kinked long fatty acid chains; Help control membrane fluidity: at high Tm, reduces membrane mobility and permeability; at low Tm, prevents membrane from freezing.

Synthesis of Phospholipid

-  All major lipids of eukaryotic mem- branes are synthesized by the smooth ER or Golgi;

-  Phospholipids are synthesized from water soluble cytosolic precursors on the cytosolic side of the ER memrane;

-  New phospholipids are added only to the cytosolic half of the ER membrane;

-  Smooth ER is also the major site for the synthesis of Cholesterol and Ceramide. Ceramide is converted to glycolipids or sphingomyelin.

(Major function of smooth ER: lipid Metabolism)

How are newly synthesized phospholipids transferred to the lumenal half of the ER membrane in order to maintain a stable membrane?

-  Phospholipid flippase catalyze the translocation of phospholipids across the ER membrane resulting in even growth of both halves of the bilayer;

-  Lipids are exported from ER in vesicles to Golgi, and subsequently to the plasma membrane.

Inside -in

Inside -out

Membrane Proteins

- Roughly 30% of all animal proteins are membrane proteins. - Proteins make up from 25-75% of membrane mass with a typical membrane containing 50% of its mass as protein.

Integral proteins Peripheral and lipid anchored proteins

- Released by membrane disruption -  Peripheral proteins are proteins attached to other membrane proteins by ionic bonds and can be released by extreme PH or high salt concentration

Transmembrane proteins

Extracellular domain

Transmembrane domain

Intracellular domain

- The transmembrane domains are usually alpha helices of 20-25 hydrophobic amino acids

- The transmembrane domains in some cases are beta-sheets. One such example is the porin family proteins, which form barrel-like channels.

-  The process of adding carbohydrates to protein peptides is called glycosylation; - Glycosylated proteins are called glycoprotein and most ER Proteins Are glycosylated; - Glycosylation is initiated in ER before translation is completed; - A common oligosaccharide unit is added to acceptor asparagine residues in the consensus sequence Asn-X-Ser/Thr by a membrane-bound enzyme called oligosaccharyl transferase;

Glycosylation

Oligosaccharide unit

review

glycocalyx

Plasma membrane

nucleus

cytosol

Glycocalyx

-  A carbohydrate coat on the outer face of the plasma membrane, formed by the oligosac- charides of glycolipids and glycoproteins;

-  Protects cell surface, also involved in cell-cell recognition.

ABO blood type determination by oligosaccharide groups in glyolipid on red blood cell membrane.

ABO locus

A allele

B allele

O allele Glycosyltransferase)

Lipid or Glycolipid Attached Proteins

-  Covalent bond -  Lipid attached proteins are synthesized on free cytosolic ribosomes and modified by the addition of lipids; -  Glycosylphosphotidylinositol (GPI) anchored proteins are synthesized in the ER and transported to the outside of the plasma membrane.

GPI anchored

Lipid anchored

Attachment of Glycolipid •  Glycolipid (lipid linked to oligosaccharides), also named as glycosyl-phosphotidylinositol (GPI) due to the presence of phosphatidylinositol, is synthesized inside the ER;

•  The pre-assembled GPI is added to the carboxy terminus of some proteins that are retained in the membrane by a C-terminal hydrophobic sequence;

•  The C-terminal sequence is cleaved and exchanged for the GPI anchor, so the proteins remain attached to the membrane only by the GPI anchor;

•  Glycolipid modification helps to attach some proteins to the external face of the PM.

review

Are membrane proteins mobile?

(Larry Frye and Michael Edidin, 1970)

-  protein mobility can be restricted by membrane protein-protein interaction, protein interaction with cytoskeleton, interaction with extracellular matrix and interaction with proteins of neighboring cells.

http://www.dnatube.com/video/360/Fluid-Mosaic-Model

- Transporters carry a molecule (such as glucose) from one side of the plasma membrane to the other.

-  Receptors can bind an extracellular molecule (triangle), and this activates an intracellular process.

-  Enzymes in the membrane can do the same thing they do in the cytoplasm of a cell: transform a molecule into another form.

-  Anchor proteins can physically link intracellular structures with extracellular structures.

Function of Membrane Proteins

Transportation across the plasma membrane

(no external source of energy used)

Passive diffusion Facilitated Diffusion

Passive Diffusion

- Through the lipid bilayer

-  No external energy required

-  Concentration gradient dependent

urea

Facilitated Diffusion/Passive Transport -  Concentration gradient dependent (from high to low); - No external energy required; -  Membrane proteins mediated.

Carrier proteins Channel proteins - Transport specific molecule by conformational change

-  Allow free diffusion of any molecules of appropriate size and charge through its open pore

Porins Gap junctions Ion channels

Ligand gated Voltage gated GLUT1-13

Glucose Transporters

http://sites.sinauer.com/cooper6e/animation0202.html

Ligand gated Voltage gated

(-40mV)

Presynaptic cell

Postsynaptic cell

http://sites.sinauer.com/cooper6e/animation1301.html

Active Transport

ATP hydrolysis driven Ion gradient driven

- Transportation against concentration gradient

Na+-K+ pump (energy derived from coupled transport of a second molecule in the energetically favorable direction)

http://sites.sinauer.com/cooper6e/animation1302.html

Symport Antiport Na+/glucose symporters"

(Apical membrane of intestinal or kidney epithelial cells) Na+/Ca+ antiporters"Na+/H- exchange protein"

Co-existence of active transport and facilitated diffusion

Endocytosis - The process of a cell taking up macromolecules and particles from its surrounding medium

Phagocytosis Pinocytosis - Ingestion of large particles such as bacteria - Ingestion of fluids or macromolecules

in small vesicles

pinocytosis

http://sites.sinauer.com/cooper6e/animation1303.html

Endocytosis vs. Exocytosis

http://sites.sinauer.com/cooper6e/video1301.html http://www.dnatube.com/video/364/Exocytosis-role-of-plasma-membrane

Human macrophages destroy ~ 1 x 1011 RBC every day

Phagocytosis

Protist

Amoeba

Red blood cells

Macrophage

Receptor Mediated Pinocytosis

1500 cholesteryl esters

800 phospholipid

1 apoprotein B100 Cholesterol uptake

LDL: low density lipoprotein

http://www.dnatube.com/video/275/How-does-LDL-enter-the-cell http://www.dnatube.com/video/98/Triskelion-clathrin

(liver cells, 70% LDL uptake)

LDL receptor pathology: Familial Hypercholesterolemia

The children were born with blood cholesterol levels over 1000 mg/dl, and they began having heart attacks before age 5.

Development of Atherosclerosis: a leading cause of cardiovascular disease

http://www.dnatube.com/video/8619/Development-of-atherosclerosis

LDL-C: low protein high cholesterol lipoprotein – “bad”

HDL-C: high protein low cholesterol lipoprotein - “good”

Cellular Composition

Water Inorganic ions Organic molecules

Carbohydrates

Lipids Proteins Nucleic Acids

Monosaccharide

Polysaccharide

Oligosaccharide

Energy storage Cell structure Cell recognition and interaction

Functions:

Energy storage Membrane structure Cell signaling

Fatty acid

Phospholipids

Triacylglycerols

Cholesterol Glycolipids

Nucleotide

Polynucleotide

Oligonucleotide

DNA RNA

Amino acid

Polypeptide

Cell structure Cellular transportation Cell signaling Cellular defense Enzymatic activities ……

Genetic information

(Chapter 2 The Composition of Cells p43-p63)

http://sites.sinauer.com/cooper6e/animation0201.html

A carbohydrate is a biological molecule consisting of carbon C, hydrogen (H) and oxygen (O) atoms, usually with a H:O ratio of 2:1 as in water.

Carbohydrates

Monosaccharides: Glucose, ribose (RNA), deoxyribose (DNA) Galactose (component of lactose), Fructose (fruit sugar).

Disaccharides: Two joined monosaccharides linked by glycosidic bond. Ex.

Sucrose – glucose+fructose (common table sugar); Lactose – galactose + glucose (major sugar in milk); Maltose - glucose + glucose (product of starch digestion)

2 C6H12O6 − H2O = C12H22O11

Oligosaccharides: 3-9 monosaccharides. Part of glycoprotein or glycolipid and serve as chemical markers (M6P modification for lysosome targeting; ABO blood type specificity etc.)

Polysaccharides: long chains of monosaccharide units bound together by glycosidic linkages and on hydrolysis (catalyzed by Amylases) give the constituent monosaccharides or oligosaccharides.

Storage Polysaccharides: Starch & Glycogen (polymers of glucose). Starch is made up of a mixture of Amylose (linear chain) and Amylopectin (branched chains). Ex. Rice, potato, wheat and maize. Animals store excess glucose by polymerizing it to form glycogen, which is structurally similar to amylopectin. The liver and skeletal muscles are major depots of glycogen. http://sites.sinauer.com/cooper6e/video0202.html

starch

Structural Polysaccharides: Cellulose (plants) & chitin (crustaceans).

Cellulose is a polymer of glucose bonded by beta-linages which cannot be broken down by human cells (lack of cellulase). Wood is largely cellulose while cotton and paper are almost pure cellulose.

Differences between cellulose and starch: -  The orientation of the glycosidic bonds linking the glucose residues are different. - There are no side chains in cellulose as there are in starch, allowing these linear molecules to lie close together to form a rigid structure (perfect for plant cell wall).

Chitin is a structural polysaccharide of an amino sugar (N-acetyl glucoseamine). Found in the wall of hyphae (fungi) and in the exoskeletons of all anthropods (insects and crustaceans). Beta-1,4 link is broken by chitinase.

http://sites.sinauer.com/cooper6e/video0203.html

Lipids Include fatty acids, triglycerides (fats), phospholipids; sphingolipid, glycolipid & steroid lipid (cholesterol and hormones, ex. Estrogen, testosterone, progesterone etc..). There are 3 essential fatty acids that have to be incorporated in the diet: linoleic acid, linolenic acid and arachidonic acid.

How does too much of carbohydrates lead to fat accumulation?

Glucose

Acetyl-CoA

Fatty acid

glycerol

Triglycerides (fats)

Fatty acid synthase Lipogenesis

Protein

“unnatural amino acids”

What nutrients are necessary in order to culture animal cells In vitro (outside the body)?

1.  All 20 of the amino acids from which proteins are synthesized;

2.  A purine (hypoxanthine) and a pyrimidine (thymidine) for the synthesis of nucleotides and their polymers, DNA and RNA;

3.  2 precursors (choline and inositol) needed to synthesize some of the phospholipids in the cell;

4.  8 vitamins, all of which serve as parts of coenzymes;

5.  The coenzyme lipoic acid;

6.  Glucose as a source of energy and carbon atoms;

7.  The inorganic ions: Na+, K+, Ca2+, Cu2+, Zn2+, and Co2+

8.  Serum (provides growth factors and hormones for cell survival and division).