PLASMA MEMBRANE - The Gateway To The Cell

Shilpa S UMSc. Biochemistry &

Molecular biology

Plasma membrane or cell membrane or cytoplasmic membrane is the biological membrane that seperates the interior of all cells from the outside environment.

Thin delicate structures.

Only 5 -10 nm wide

Provides framework in which components can be organized.

Selectively permeable to ions and organic molecules & controls the movement of substances in and out of the cells.

Basic function- protect the cell from its surroundings

“Around 5000 plasma membranes stacked

one on top of the other is equal to the thickness of a single

page of a book!!!!!!!!”

MEMBRANE FUNCTIONS–an overview

Compartmentalization Scaffold for biochemical activities Selectively permeable barrierTransporting solutes Responding to external signalsAttachment to the cytoskeleton & ECM Intercellular interactionEnergy transduction

HISTORYModels of membranes were developed long before membranes were first seen with the electron microscopes in the 1950s. In 1985 ERNST OVERTON (University of Zurich) hypothesized that

membranes are made of lipids because substance that dissolve in lipid enters cells faster than those that are insoluble.

He placed plant root hairs into 100’s of different solutions containing a diverse array of solutes

He discovered that the more lipid soluble solute rapidly enter the root hair cells.

Attempts to build artificial membranes provided insight into the structure of real membranes In 1917 ,IRVING LANGMUIR discovered that phospholipids dissolved in

benzene would form a film on water when the benzene evaporated. The hydrophilic heads were immersed in water

The first proposal that cell membranes contain a lipid bilayer was made by two Dutch scientists E.GROTEL and F. GRENDEL in 1925

They extracted the lipid from human RBC’s & measured the amount of surface area the lipid would cover over the surface of water

Surface area of water covered by extracted lipid = 2

Surface area of the RBC’s Concluded that plasma membrane contained a

bimolecular layer of lipids ie,lipid bilayer.

PROPERTIES OF LIPID BILAYER Lipid bilayer is a universal component of all cell membranes It composed of two layers of fatty acids organized in two

sheets. The lipid bilayer is typically about 5 nm to 10 nm thick. that contain both a hydrophilic and a hydrophobic moiety. With the hydrophobic tails of each individual sheet interacting

with one another, a hydrophobic interior is formed and this acts as a permeability barrier.

The hydrophilic head groups interact with the aqueous medium on both sides of the bilayer. The two opposing sheets are also known as leaflets.

The membrane is composed of lipids and proteins and sometimes even carbohydrates

Impermeable barrier: passage of only selective molecules like water and small hydrophilic molecules.

In 1935 H DAVSON and J DANIELLI proposed a sandwich model in which the phospholipid bilayer was lined on both of its inner & outer surface by a layer of globular proteins.

Early images from electron microscopes seemed to support the Davson – Danielli model and until the 1960s it was considered the dominant model.

In 1950,they revised their model and said that in addition to the outer & inner protein layers , it was also penetrated by protein lined pores for the polar solutes and ions to enter and exit the cell.

In 1972, S JONATHAN SINGER and GARTH NICOLSON of University of California proposed ‘FLUID MOSAIC MODEL’ ,

Revised model that proposed that the membrane proteins are dispersed and invidually inserted into the phospholipid bilayer.

In this fluid mosaic model, the hydrophilic regions of proteins and phospholipids are in maximum contact with water and the hydrophobic regions are in a nonaqueous environment.

STRUCTURAL ORGANIZATION

Bimolecular layer of amphipathic lipids (phospholipid bilayer) 2 segments with very different chemical properties Polar heads (hydrophilic)face outside 5 to 10 nm thick Contain phospholipids, proteins, and steroid molecules.

WHY “FLUID MOSAIC”???

A membrane is a mosaic Proteins and other molecules are embedded in a framework of phoshoplids. A membrane is a fluid Most proteins and phospholipid molecules can move move laterally.

PHOSPHOLIPID BILAYER Lipid bilayer is a universal component of all cell membranes. The structure is called a "lipid bilayer" because it composed of two

layers of fatty acids organized in two sheets. The lipid bilayer is typically about five nanometers to ten

nanometers thick and surrounds all cells providing the cell membrane structure.

With the hydrophobic tails of each individual sheet interacting with one another, a hydrophobic interior is formed and this acts as a permeability barrier.

The hydrophilic head groups interact with the aqueous medium on both sides of the bilayer. The two opposing sheets are also known as leaflets.

The three main structures phospholipids form in solution; the liposome (a closed bilayer), the micelle and the bilayer.

Surfaces of a cellular membrane

Two surfaces of a cellular membrane CYTOSOLIC FACE :faces the cytosol EXOPLASMIC FACE: faces the exterior

environment

In the case of cellular membranes exoplasmic face is towards the interior and cytosolic face towards the exterior of the organelle. Exceptions are mitochondria , chloroplast, and nucleus.

Chemical composition of membrane

Proteins •Integral•Lipid anchored•peripheral

Polar lipids

• Phospholipids

• Glycerolipids• Spingolipds• Sterol• cholesterol

Carbohydrates

•Glycolipids•glycoproteins

Membranes are lipid protein assemblies ,held together by non covalent bonds.

Lipid bilayer is the structural backbone of the membrane.

It provides the barrier that prevents random movements of water soluble materials in and out of the cell.

The ratio of lipid to protein in a membrane varies depending on the type of cellular membranes, type of organism, & the type of cell.

MEMBRANE LIPIDS The plasma membrane contains three principal classes of

amphipathic lipids (contain both hydrophobic and hydrophilic ends)

1. Phosphoglycerides; most abundant 2. Sphingolipids 3. Steroid: cholesterol

Phosphoglycerides Most abundant lipid They are built on a glycerol backbone ,so called as

‘phosphoglycerides’. Membrane glycerides are diglycerides Derivative of glycerol 3 phosphate Contain a hydrophobic tail composed of 2 fatty acyl chains

esterified to the hydroxyl group in glycerol phosphate and a polar head attached to phosphate group (fatty acyl chains can differ in no of carbon they contain and degree of saturation)

Classified according to nature of head group Without any additional substitutions beyond the phosphate

and the two fatty acyl chains – phosphatidic acid

GLYCEROl

phosphat

eAmino alcohol

Fatty acid

Fatty acid

GENERAL STRUCTURE OF PHOSPHOGLYCERIDES

Choline – forms phosphatidylcholine Ethanolamine - phosphatidylethanolamine Serine – phosphatidylserine Inositol – phosphatidylinositol

Each of these groups are small & hydrophilic and together with the –vely charged phosphate to which it is attached , forms a highly water soluble domain at one end of the molecule called the head group

A membrane fatty acid may be fully saturated or monounsaturated or diunsaturated.

Phoshphoglycerides often contains 1 saturated and 1 unsaturated fatty acyl chains.

Negatively charged phosphate group and hydroxyl group on head groups interact strongly with water

Sphingolipids Derived from amino alcohol sphingosine Contains long hydrocarbon chain. Long chain fatty acid attached by amide linkage to

sphingosine amino group - ceramide Amphipathic

SPHINGOMYLEIN – most abundant sphingolipid ,in which phosphocholine is attached to terminal –OH group of sphingosine

They are similar to the phosphoglycerides &can form mixed bilayers with them

If carbohydrate is attached to sphingosine we get GLYCOLIPIDS They consume 2 -10 % of total lipid in plasma membrane If carb: is a simple sugar – cerebrosides If cluster of sugars – gangliosidesThey have crucial role in cell functions, nervous system is rich in glycolipidsMyelin sheath contains a high content of a glycolipid called galactocerebroside which is formed when a galactose is added to ceramide

MEMBRANE STEROIDS

Consist of cholesterol and its analogues. Abundant in mammalian cell, but absent in

prokaryotes and plant cells. About 50-90% of cholesterol in mammalian cells is

present in plasma membrane and associated vesicles. Oriented with their small hydrophilic -OH group

towards the membrane surface & remaining in the lipid bilayer.

Although almost entirely hydrocarbon, it is amphipathic because its –OH group can interact with water.

Interfere with the movements of the fatty acid tails of the phospholipids

MEMBRANE PROTEINS Associated with each membrane is a set of membrane proteins that

enables the membrane to carry out its distinctive activities . The types of proteins attached to a membrane varies depending on cell

type and subcellular location. Proteins associated with particular membrane are responsible for its

distinctive function. Amount of protein also vary depending on subcellular location and cell

type. eg: inter mitochondrial membrane contain 76% protein, myelin

membrane contain 18% protein. 3 different types of protein: based on their intimacy of their

relationship to the lipid bilayer. a) INTEGRAL MEMBRANE PROTEIN b) LIPID ANCHORED MEMBRANE PROTEIN c) PERIPHERAL MEMBRANE PROTEIN

Peripheral membrane proteins

INTEGRAL MEMBRANE PROTEINS Trans membrane proteins( they pass entirely through the

lipid bilayer) Span a lipid bilayer and comprises 3 segments. Cytosolic and exoplasmic face domains have hydrophillic

exterior surface that interact with aqueous solution on the cytosolic and exoplasmic faces of the membrane.

In contrast the membrane spanning segment contain more hydrophobic amino acids whose side chains protrude outward and interact with the hydrophobic hydrocarbon core.

Membrane spanning segment consist of 1 or more α helices or multiple β strands.

PERIPHERAL MEMBRANE PROTEINS Do not directly contact the hydrophobic core of phospholipid

bilayer. Associated with the membrane by weak electrostatic bonds Can be bound to either cytosolic or exoplasmic face of plasma

membrane. Cytoskeleton can be loosely associated with cytosolic face by

one or more peripheral proteins. Such interactions provide support for various cellular

membranes, helping to determine cell shape and much properties, and 2 way communication between all exterior and interior faces.

LIPID ANCHORED MEMBRANE PROTEINS Also known as lipid-linked proteins  Located on the surface of the cell membrane that are covalently

 attached to lipids by a small oligosaccharide linked to a molecule of phosphatidyl inositol embedded within the cell membrane(GPI –anchored proteins).

Hydrophobic segment embedded in one segment of the membrane. These lipids insert and assume a place in the bilayer structure of the

membrane alongside the similar fatty acid tails. Polypeptide chain itself does not enter phospholipid bilayer. The lipid-anchored protein can be located on either side of the cell

membrane. Thus, the lipid serves to anchor the protein to the cell membrane.[1][2]

MEMBRANE CARBOHYDRATES

Depending on species & cell type carbohydrate conc: varies from 2 – 10% by weight

More than 90% of membrane carbohydrate covalently linked to proteins to form glycoproteins and rest to form glycolipids.

Carbohydrates of the cell membranes face outside

into the extracellular space. Carbohydrates of glycoproteins is present as

short , branched hydrophilic oligosaccharides Has an imp role in mediating proteins to

different cellular compartments Carbohydrates of glycolipids of RBC plasma

membrane determine the blood type of a person.

MEMBRANE FLUIDITY Physical state of a lipid membrane is determined by its fluidity. Membrane fluidity is influenced by temperature, saturated or

unsaturated, fatty acid chain length, presence of cholesterol etc. If the temperature of the bilayer is kept relatively warm,the lipid

exists in a fluid state. As temperatures lowers, membranes switch from a fluid state to a

solid state as the phospholipids are more closely packed(frozen crystalline gel).

Saturated fatty acids have the shape of a straight flexible rod ,unsaturated fatty acids have crooks/kinks in the chain at the site of a double bond.

Consequently phospholipids with saturated chains pack more tightly than those containing unsaturated chains .

Shorter the fatty acyl chains of a phospholipids ,the lower its melting temperature.

The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells

At warm temperatures , it restrains the movement of phospholipids and reduces fluidity

At cool temperatures it maintains fluidity by preventing tight packing.

LATERAL MOBILITY OF LIPIDS Thermal motion allows lipid molecules to rotate freely around

long axes and also to diffuse laterally within each leaflet. Because movements are lateral, fatty acyl chains remain in

hydrophobic interior. A typical lipid molecule exchange places with its neighbors in a

leaflet about 10^7 times/sec and diffuse several micrometers /sec at 37˚C.

These diffusions indicate that viscosity of plasma membrane is 100 times that of water- approx. that of olive oil.

A lipid molecule can diffuse the length of a bacteria in 1sec and that of animal cell in 20secs.

Movement can be observed by Fluorescence Recovery after Photobleaching (FRAP) technique.

FLOURESCENCE RECOVERY AFTER PHOTOBLEACHING  Method for determining the kinetics of diffusion through tissue or

cells. It is capable of quantifying the two dimensional lateral diffusion of a

molecularly thin film containing fluorescently labeled probes, or to examine single cells.

This technique is very useful in biological studies of cell membrane diffusion and protein binding.