Evaluation Seminar OnThe Plasma Membrane And Gap

JunctionBY

MALLAPPA. SHALAVADI.

M-PHARM-I

HSK. COLLEGE OF PHARMACY,

BAGALKOT.

CONTENTS

INTRODUCTIONSTRUCTURE OF PLASMA MEMBRANEFUNCTIONS OF PLASMA MEMBRANEGAP JUNCTION.

Introduction

• Cell membrane are crucial to the life of the cell.

• The plasma membrane encloses the cell, defines its boundaries, and maintains essential difference betn cytosol and extra cellular environment.

• separates the living cell from its surroundings.• 5 nm thick, controls traffic into and out of the

cell.• selectively permeable, allowing some

substances to cross more easily than others• Major macromolecules in membranes are

lipids, proteins, and some carbohydrates• Made of a bilayer of phospholipids. With polar

heads, hydrophilic, and non-polar tails, hydrophobic.

STRUCTUREAll biological membrane has common general

structure.Thin film of lipid and protein held together by

non covalent interactions.

Fluid mosaic model• The membrane is represented as a fluid mosaic model, a fluid

environment with a mosaic of proteins and carbs. embedded or attached that serve several functions.

• On the basis of the dynamic properties of proteins in membranes, S. Jonathan Singer and Garth Nicolson proposed the concept of a fluid mosaic model for the overall organization of biological membranes in 1972.

• Membrane proteins are free to diffuse laterally in the lipid matrix unless restricted by special interactions.

Membrane Movement and Cholesterol

• Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other.

• Cholesterol is wedged between phospholipids molecules in the plasma membrane of animals cells. It restrains the movement of the phospholipids in warm temps. and maintains fluidity by preventing tight packing at cold temps.

Cells can Change their Membrane Composition

• Cells can modify the lipid make-up of membranes to compensate for changes in fluidity caused by changing temperatures.– Ex, winter wheat, increases the percentage of

unsaturated phospholipids in the autumn.– This lets them prevent their membranes from

solidifying during winter.

MEMBRANE LIPIDS ARE AMPHIPATHICMOLECULS

Approximately 50,00,000 lipid molecules present in 1x1 um area of lipid bilayer.

Amphipathic –hydrophilic and hydrophobic Most of lipid is phospholipids.Have polar head group and 2 non polar tail.Tail is fatty acids differ in length (14 and 24

carbon atoms).1or2 cis double bonds creates small kink in tail

Individual units arewedge-shaped(cross section of headgreater than thatof side chain)

Individual units arecylindrical (cross sectionof head equals that of side chain)

(a) Micelle (b) Bilayer (c) Liposome

Aqueouscavity

Amphipathic lipid aggregates that form in water. (a) In micelles, the hydrophobic chains of the fatty acids are sequestered at the core of the sphere. There is virtually no water in the hydrophobic interior. (b) In an open bilayer, all acyl side chains except those at the edges of the sheet are protected from interaction with water. (c) When a two-dimensional bilayer folds on itself, it forms a closed bilayer, a three-dimensional hollow vesicle (liposome) enclosing an aqueous cavity.

• TYPES OF MEMBRANE LIPIDS

1.Phospholipid

2.Glycolipid

3.Cholesterol

PHOSPHOLIPIDS• Phospholipids are abundant in all biological

membranes• Four components

fatty acids, glycerol,

phosphate, alcohol.

Schematic structure of Phospholipid

• Phospholipids are built from glycerol,

3-carbon alcohol, or sphingosine, a more complex alcohol.

A)PHOSPHOGLYCERIDES

• Glycerol is back bone to which two fatty acid chain and a phosphorylated alcohol are attached.

• Simplest phosphoglycerides

Common phosphoglycerides

Sphingomyelin• Sphingomyelin is a phospholipid found in

membranes that is not derived from glycerol. Instead, the backbone in sphingomyelin is sphingosine, an amino alcohol that contains a long, unsaturated hydrocarbon chain

• In sphingomyelin, the amino group of the sphingosine backbone is linked to a fatty acid by an amide bond. In addition, the primary hydroxyl group of sphingosine is esterified to phosphoryl choline

Structures of Sphingosine and Sphingomyelin

• Glycolipids,• Glycolipids, as their name implies, are sugar-

containing lipids. Glycolipids in animal cells are derived from sphingosine.

• The amino group of the sphingosine backbone is acylated by a fatty acid

• In Glycolipids, one or more sugars are attached to this group.

• The simplest glycolipid, called a cerebroside, contains a single sugar residue, either glucose or galactose.

CHOLESTROL

• Cholesterol is a lipid with a structure quite different from that of phospholipids. It is a steroid, built from four linked hydrocarbon rings.

• It constitutes almost 25% of the membrane lipids in certain nerve cells but is essentially absent from some intracellular membranes.

Lipid Bilayers Are Highly Impermeable to Ions and Most Polar Molecules

• lipid bilayer membranes have a very low permeability for ions and most polar molecules.

• Water is a conspicuous exception to this generalization; it readily traverses such membranes because of its small size, high

concentration, and lack of a complete charge.

Permeability Coefficients (P) of Ions and Molecules in a Lipid Bilayer

Proteins

• Membranes are very complex and dynamic containing many different parts.

• Proteins decide most of the membrane’s functions.• Contain lipids and carbohydrates also• The collection of molecules in the membrane vary from

membrane to membrane• All of the structures in the membrane serve various

functions like cell recognition proteins.• Typically contains 50% of proteins.

2 Types of Proteins• Peripheral proteins are not embedded in the lipid bilayer,

they are loosely bounded to the surface. • Integral proteins penetrate, often completely spanning the

membrane (a transmembrane proteins)

Integral and Peripheral Membrane Proteins.

• Peripheral membrane proteins are bound to membranes primarily by electrostatic and hydrogen-bond interactions with the head

groups of lipids.

Many peripheral membrane proteins are bound to the surfaces of integral proteins, on either the cytosolic or the extracellular side of the

membrane.

Others are anchored to the lipid bilayer by a covalently attached hydrophobic chain, such as a fatty acid.

Integral proteins• The firm attachment of integral proteins to

membranes is the result of hydrophobic interactions between membrane lipids and hydrophobic domains of the protein.

NH3

–OOC

Type I

Type II

Type III

Type IV

Type VI

Type VInside Outside

• Types I and II have only one transmembrane

helix; the amino-terminal domain is outside the cell in type I proteins and inside in type II. Type III proteins have multiple transmembrane helices in a single polypeptide.

In type IV proteins, transmembrane domains of several different polypeptides assemble to form a channel through the membrane

Type V proteins are held to the bilayer primarily by covalently linked lipids

type VI proteins have both transmembrane helices and lipid (GPI) anchors

Type Description Examples

Integral proteins

or transmembrane proteins

Span the membrane and have a hydrophilic cytosolic domain, which interacts with internal molecules, a hydrophobic membrane-spanning domain that anchors it within the cell membrane, and a hydrophilic extracellular domain that interacts with external molecules. The hydrophobic domain consists of one, multiple, or a combination of α-helices and β sheet protein motifs.

Ion channels, proton pumps, G protein-coupled receptor

Lipid anchored proteins

Covalently-bound to single or multiple lipid molecules; hydrophobically insert into the cell membrane and anchor the protein. The protein itself is not in contact with the membrane.

G proteins

Peripheral proteins

Attached to integral membrane proteins, or associated with peripheral regions of the lipid bilayer. These proteins tend to have only temporary interactions with biological membranes, and, once reacted the molecule, dissociates to carry on its work in the cytoplasm.

Some enzymes, some hormones

FUNCTIONS• The cell membrane physically separates the

intracellular components from the extracellular environment, thereby serving a function similar to that of skin

• The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell,

• The barrier is selectively permeable and able to regulate what enters and exits the cell

• The membrane also maintains the cell potential

Protein Functions

Carbohydrates in the Membrane

• Used for cell to cell recognition, the ability of a cell to distinguish one type of neighboring cell from another.– important in cell sorting and organization as tissues and

organs in development.

• Basis of immune response. Ex. WBC and T-cell response • Membrane carbohydrates are usually branched

oligosaccharides with fewer than 15 sugar units• vary from species to species, individual to individual, and

even from cell type to cell type within the same individual.

Crossing the Membrane

• steady traffic of small molecules and ions moves across the plasma membrane in both directions– Ex, sugars, amino acids, and other nutrients enter a

muscle cell and waste products leave• membranes are selectively permeable so all this traffic is

under some control. Esp. the large molecules. • Passage is controlled in part due to the hydrophobic core

of the membrane. So other hydrophobic molecules cross easily while polar molecules and ions have difficulty.

• Proteins assist and control the transport of ions and polar molecules.

Transport Proteins

• ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane.– Some transport proteins have a hydrophilic channel– Others bind molecules and carry passengers across the

membrane physically• Each transport protein is specific– Ex. Gluclose transport in liver. Not fructose.

Examples

Exocytosis and Endocytosis

• Ways of getting large molecules in and out of the cell.

• Phagocytosis is cell eating and involves solids.• Pinocytosis is cell drinking and involves liquids.

GAP JUNCTION• Gap junctions, also known as cell-to-cell

channels, serve as passageways between the interiors of contiguous cells.

• Gap junctions are clustered in discrete regions of the plasma membranes of apposed cells. Electron micrographs of sheets of gap junctions show them tightly packed in a regular hexagonal array

Gap Junctions

Comprised ofconnexons

Connexons madeof connexins

• A cell-to-cell channel is made of 12 molecules of connexin, one of a family of transmembrane proteins with molecular masses ranging from 30 to 42 kd.

• Each connexin molecule appears to have four membrane-spanning helices.

• Six connexin molecules are hexagonally arrayed to form a half channel, called a connexon or hemhannel

• Two connexons join end to end in the intercellular space to form a functional channel between the communicating cells

• Cell-to-cell channels differ from other membrane channels in three respects:

(1) they traverse two membranes rather than

one

(2) they connect cytosol to cytosol, rather than to the extracellular space or the lumen of an organelle

(3) The connexons forming a channel are synthesized by different cells.

• Gap junctions form readily when cells are brought together. A cell-to-cell channel, once formed, tends to stay open for seconds to minutes

• They are closed by high concentrations of calcium ion and by low pH.

• The closing of gap junctions by Ca 2 + and H + serves to seal normal cells from traumatized or dying neighbors.

• They also controlled by membrane potential and Harmon induced phosphorylation.

• All polar molecules with a mass of less than about 1 kd can readily pass through these cell-to-cell channels.

• Thus, inorganic ions and most metabolites (e.g., sugars, amino acids, and nucleotides) can flow between the interiors of cells joined by gap junctions.

• In contrast, proteins, nucleic acids, and polysaccharides are too large to traverse these channels.

• Gap junctions are important for intercellular communication.

REFERANCES

1.Biochemistry by Lubert Sryer, Jeremy M. Berg.

2.Principles of biochemistry by Lehninger , Nelson, Cox.

3.www.Google.com

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