Paper VIII Unit I- Cell Wall and Plasma...

D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 1

CELL WALL

The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some

types of cells. It is located outside the cell membrane and provides these cells with structural

support and protection, and also acts as a filtering mechanism. A major function of the cell wall

is to act as a pressure vessel, preventing over-expansion when water enters the cell. They are

found in plants, bacteria, fungi, algae, and some archaea. Animals and protozoa do not have cell

walls.

Plant cell walls are thick walls that encase the cell, which can be numerous micrometers thick.

Cell walls are made of microfibrils of cellulose set in a base of proteins and other

polysaccharides. The wall itself consists of a primary cell wall, a secondary cell wall, and a

middle lamella. The plant cell also has many holes on its perimeter as well.

The material in the cell wall varies between species, and can also differ depending on cell type

and developmental stage. In bacteria, peptidoglycan forms the cell wall. Archaean cell walls

have various compositions, and may be formed of glycoprotein S-layers, pseudopeptidoglycan,


or polysaccharides. Fungi possess cell walls made of the glucosamine polymer chitin, and algae

typically possess walls made of glycoproteins and polysaccharides. Unusually, diatoms have a

cell wall composed of silicic acid. Often, other accessory molecules are found anchored to the

cell wall.

PLANT WALL LAYERS

Many plant cells have walls that are strong enough to

withstand the osmotic pressure from the difference in solute

concentration between the cell interior and distilled water.

Up to three strata or layers may be found in plant cell walls:

1. The middle lamella, a layer rich in pectins. This

outermost layer forms the interface between adjacent

plant cells and glues them together.

2. The primary cell wall, generally a thin, flexible and extensible layer of cellulose formed

while the cell is growing.

3. The secondary cell wall, a thick layer formed inside the primary cell wall after the cell is

fully grown. It is not found in all cell types. In some cells, such as found xylem, the

secondary wall contains lignin, which strengthens and waterproofs the wall.

The secondary cell wall consists mainly of cellulose, but also other polysaccharides, lignin, and

glycoproteins. It sometimes consists of three distinct layers - S1, S2 and S3 - where the direction

of the Cellulose microfibrils differs between the layers. Apparently there are no Structural

proteins or enzymes in the secondary wall.

The secondary cell wall has different ratios of wall constituents compared to the primary wall.

An example of this is that wood secondary walls contain xylans, whereas the primary wall

contains xyloglucans and the cellulose fraction is higher in the secondary wall. Pectins may also

be absent from the secondary wall and apparently it contain no Structural proteins or enzymes.


The Cellulose microfibrils give tensile strength, whereas lignification in addition to making the

secondary wall impermeable to water also give a "brittle" texture. Conceptually this give

lignified secondary wall properties resembling armored concrete, where the cellulose

microfibrils act as the armoring and the lignin as concrete.

Lignification of the secondary wall confer resistance to pathogens by two mechanisms. As lignin

repel water, hydrolytic enzymes are less likely to attack and successfully penetrate the wall and it

lowers the nutritional value of the wall, providing less energy to pathogens.

Wood consists mostly of secondary cell wall, and holds the plant up against gravity.

Some secondary cell walls store nutrients, such as those in the cotyledons and the endosperm.

These contain little cellulose, and mostly other polysaccharides

COMPONENTS OF THE CELL WALL (CHEMISTRY OF CELL WALL)

Multiple layers of the cell wall possesses different components. Broadly one can classify them as

follows –

1. Carbohydrates: Cellulose (23%), Hemicellulose (24%), Pectin (34%), etc.

2. Proteins (19%)

3. Lignin

4. Lipids: Suberin, wax, cutin

5. Water


The composition of the cell wall varies greatly amongst the plant species and even in the

different cell types of the same plant organ. Ex. Anatomical studies of the stem shows various

types of cells viz. parenchyma, collenchymas, sclerenchyma, xylem elements and phloem

elements; all these cells from the same location of plant organ has different chemical

composition.

1. CARBOHYDRATES: Cellulose, Hemicellulose, Pectin, etc.

The main ingredient in cell walls are polysaccharides (or complex carbohydrates or complex

sugars) which are built from monosaccharides (or simple sugars). Eleven different

monosaccharides are common in these polysaccharides including glucose and galactose.

Carbohydrates are good building blocks because they can produce a nearly infinite variety of

structures. There are a variety of other components in the wall including protein, and lignin.

Let's look at these wall components in more detail:

a. Cellulose

β1,4-glucan, made of as many as 25,000 individual glucose molecules. A cellulose chain will

form hydrogen bonds with about 36 other chains to yield a microfibril. Microfibrils are 5-12 nm

wide and give the wall strength - they have a tensile strength equivalent to steel. Some regions of

the microfibrils are highly crystalline while others are more "amorphous".

Properties of Cellulose:

• Cellulose is the structural component of the primary cell wall of green plants, many forms

of algae and the oomycetes.

• Cellulose has no taste, is odourless, is hydrophilic with the contact angle of 20–30, is

insoluble in water

• Some species of bacteria secrete it to form biofilms.

• Cellulose is the most common organic compound on Earth.

• About 33% of all plant matter is cellulose (the cellulose content of cotton is 90% and that

of wood is 40–50%


• Cellulose is derived from D-glucose units, which condense through β(1→4)-glycosidic

bonds.

• A glycosidic bond is a type of covalent bond that joins a carbohydrate molecule to another

group, which may or may not be another carbohydrate.

b. Hemicellulose (Cross-linking glycans)

Diverse group of carbohydrates that used to be called hemicellulose. Hemicellulose is composed

of carbohydrates based on pentose sugars, mainly xylose, as well as hexose sugars, such as

glucose and mannose. Hemicellulose comprises 25 to 35 percent of the dry weight of wood

residues; they are second only to cellulose in abundance among carbohydrates. While use of

hemicellulose is currently limited, quantities of hemicelluloses, pectins, and various other plant

polymers are abundant in crop residues and have great potential in the production of chemicals

and materials. During the pulping process, hemicellulose is pooled with lignin to become the

wood-processing residue, black liquor.

Hemicellulose contains many different sugar monomers. In contrast, cellulose contains only

anhydrous glucose. For instance, besides glucose, sugar monomers in hemicellulose can include

xylose, mannose, galactose, rhamnose, and arabinose. Hemicelluloses contain most of the D-

pentose sugars, and occasionally small amounts of L-sugars as well. Xylose is always the sugar

monomer present in the largest amount, but mannuronic acid and galacturonic acid also tend to

be present.


They are linear (straight), flat, with a β-1,4 backbone and relatively short side chains. Two

common types include xyloglucans and glucuronarabinoxylans. Other less common ones include

glucomannans, galactoglucomannans, and galactomannans. The main feature of this group is

that they don’t aggregate with themselves - in other words, they don’t form microfibrils.

However, they form hydrogen bonds with cellulose and hence the reason they are called "cross-

linking glycans". There may be a fucose sugar at the end of the side chains which may help keep

the molecules planar by interacting with other regions of the chain.

c. Pectin

Pectin is a structural heteropolysaccharide contained in the primary cell walls of terrestrial

plants. It was first isolated and described in 1825 by Henri Braconnot. It is produced

commercially as a white to light brown powder, mainly extracted from citrus fruits, and is used

in food as a gelling agent particularly in jams and jellies. It is also used in fillings, medicines,

sweets, as a stabilizer in fruit juices and milk drinks and as a source of dietary fiber.

These are also a diverse group of polysaccharides and are particularly rich in galacturonic acid

(galacturonans = pectic acids). They are polymers of primarily β 1,4 galacturonans

(=polygalacturonans) are called homogalacturons (HGA) and are particularly common. These

are helical in shape. Divalent cations, like calcium, also form cross-linkages to join adjacent

polymers creating a gel.

Although most pectic polysaccharides are acidic, others are composed of neutral sugars

including arabinans and galactans. The pectic polysaccharides serve a variety of functions

including determining wall porosity, providing a charged wall surface for cell-cell adhesion - or

in other words gluing cells together (i.e,. middle lamella), cell-cell recognition, pathogen

recognition and others.


2. PROTEINS

Wall proteins are typically glycoproteins (polypeptide backbone with carbohydrate side chains).

The proteins are particularly rich in the amino acids hydroxyproline (hydroxyproline-rich

glycoprotein, HPRG), proline (proline-rich protein, PRP), and glycine (glycine-rich protein,

GRP). These proteins form rods (HRGP, PRP) or beta-pleated sheets (GRP). Extensin is a

well-studied HRGP. HRGP is induced by wounding and pathogen attack. The wall proteins also

have a structural role since: (1) the amino acids are characteristic of other structural proteins such

as collagen; and (2) to extract the protein from the wall requires destructive conditions. Protein

appears to be cross-linked to pectic substances and may have sites for lignification. The proteins

may serve as the scaffolding used to construct the other wall components.

Another group of wall proteins are heavily glycosylated with arabinose and galactose. These

arabinogalactan proteins, or AGP's, seem to be tissue specific and may function in cell signaling.

They may be important in embryogenesis and growth and guidance of the pollen tube.

3. LIGNIN

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an

integral part of the secondary cell walls of plants and some algae. The term was introduced in

1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is one of

the most abundant organic polymers on Earth, exceeded only by cellulose, employing 30% of

non-fossil organic carbon and constituting from a quarter to a third of the dry mass of wood. As a

biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary


structure. Its most commonly noted function is the support through strengthening of wood

(xylem cells) in trees. It is a polymer of phenolics, especially phenylpropanoids. Lignin is

primarily a strengthening agent in the wall. It also resists fungal/pathogen attack.

Lignin fills the spaces in the cell wall between cellulose, hemicellulose, and pectin

components, especially in tracheids, sclereids and xylem. It is covalently linked to

hemicellulose and, therefore, crosslinks

different plant polysaccharides, conferring

mechanical strength to the cell wall and by

extension the plant as a whole. It is particularly

abundant in compression wood but scarce in

tension wood.

Lignin plays a crucial part in conducting water

in plant stems. The polysaccharide components of plant cell walls are highly hydrophilic and

thus permeable to water, whereas lignin is more hydrophobic. The crosslinking of

polysaccharides by lignin is an obstacle for water absorption to the cell wall. Thus, lignin makes

it possible for the plant's vascular tissue to conduct water efficiently. Lignin is present in all

vascular plants, but not in bryophytes, supporting the idea that the original function of lignin was

restricted to water transport.

4. LIPIDS : SUBERIN, WAX, CUTIN

A variety of lipids are associated with the wall for strength and waterproofing.

• Cutin – polymeric network of oxygenated C16 and C18 fatty acids

• Inelastic and hydrophobic but NOT a significant barrier to water loss – pathogen defense

• Suberin – similar but longer fatty acids, less oxygenated and linked to phenolics – more

hydrophobic than cutin

• Aerial surfaces covered with waxes – extremely long Chain fatty acids – prevents water loss

5. WATER

The wall is largely hydrated and comprised of between 75-80% water. This is responsible for

some of the wall properties. For example, hydrated walls have greater flexibility and

extensibility than non-hydrated walls.


ULTRASTRUCTURE OF CELL WALL

Electron Microscopy has shown that the cell wall is constructed on the same architectural

principle which applied well in the construction of animal bones and such common building

materials as fibre glass (plastic + glass) or reinforced concrete (concrete + metal framework).

The strong fibres (cellulose microfibrils) resistance to tension embedded in an amorphous matrix

(comprising hemicelloulose, pectins and proteins). The plant cell wall is ≈ 0.2 micrometer thick

and completely coats the outside of the plant cell’s plasma membrane. This structure serves some

of the same functions as those of the extracellular matrix produced by animal cells, even though

the two structures are composed of entirely different macromolecules and have a different

organization. Like the extracellular matrix, the plant cell wall connects cells into tissues, signals

a plant cell to grow and divide, and controls the shape of plant organs. Just as the extracellular

matrix helps define the shapes of animal cells, the cell wall defines the shapes of plant cells.

When the cell wall is digested away from plant cells by hydrolytic enzymes, spherical cells

enclosed by a plasma membrane are left. In the past, the plant cell wall was viewed as an

inanimate rigid box, but it is now recognized as a dynamic structure that plays important roles in

controlling the differentiation of plant cells during embryogenesis and growth.

Because a major function of a plant cell wall is to withstand the osmotic turgor pressure of the

cell, the cell wall is built for lateral strength. It is arranged into layers of cellulose microfibrils—

bundles of long, linear, extensively hydrogenbonded polymers of glucose in β glycosidic

linkages. The cellulose microfibrils are embedded in a matrix composed of pectin, a polymer of

D-galacturonic acid and other monosaccharides, and hemicellulose, a short, highly branched

polymer of several five- and six-carbon monosaccharides.

The mechanical strength of the cell wall depends on crosslinking of the microfibrils by

hemicellulose chains. The layers of microfibrils prevent the cell wall fromstretching laterally.

Cellulose microfibrils are synthesized on the exoplasmic face of the plasma membrane from

UDPglucose and ADP-glucose formed in the cytosol. The polymerizing enzyme, called cellulose

synthase, moves within the plane of the plasma membrane as cellulose is formed, in directions

determined by the underlying microtubule cytoskeleton. Unlike cellulose, pectin and

hemicellulose are synthesized in the Golgi apparatus and transported to the cell surface where

they form an interlinked network that helps bind the walls of adjacent cells to one another and


cushions them. When purified, pectin binds water and forms a gel in the presence of Ca2+ and

borate ions—hence the use of pectins in many processed foods. As much as 15 percent of the cell

wall may be composed of extensin, a glycoprotein that contains abundant hydroxyproline and

serine. Most of the hydroxyproline

residues are linked to short chains of

arabinose (a five-carbon

monosaccharide), and the serine

residues are linked to galactose.

Carbohydrate accounts for about 65

percent of extensin by weight, and its

protein backbone forms an extended

rodlike helix with the hydroxyl or O-

linked carbohydrates protruding outward. Lignin—a complex, insoluble polymer of phenolic

residues—associates with cellulose and is a strengthening material. Like cartilage proteoglycans,

lignin resists compression forces on the matrix.

The cell wall is a selective filter whose permeability is controlled largely by pectins in the wall

matrix. Whereas water and ions

diffuse freely across cell walls, the

diffusion of large molecules,

including proteins larger than 20

kDa, is limited. This limitation may

account for why many plant

hormones are small, water-soluble

molecules, which can diffuse across

the cell wall and interact with receptors in the plasma membrane of plant cells. The cell wall

microfibrils are linked with plasma membrane in its lipid bilayer.


FUNCTIONS OF THE CELL WALL:

The cell wall serves a variety of purposes including:

1. Maintaining/determining cell shape (analogous to an external skeleton for every cell). Since

protoplasts are invariably round, this is good evidence that the wall ultimately determines the

shape of plant cells.

2. Support and mechanical strength (allows plants to get tall, hold out thin leaves to obtain

light)

3. It prevents the cell membrane from bursting in a hypotonic medium (i.e., resists water

pressure)

4. It controls the rate and direction of cell growth and regulates cell volume

5. Cell wall is ultimately responsible for the plant architectural design and controlling plant

morphogenesis since the wall dictates that plants develop by cell addition (not cell migration)

6. Cell wall components has a metabolic role (i.e., some of the proteins in the wall are enzymes

for transport, secretion)

7. It is a main physical barrier to: (a) pathogens; and (b) water in suberized cells

8. Cell wall is a carbohydrate storage - the components of the wall can be reused in other

metabolic processes (especially in seeds). Thus, in one sense the wall serves as a storage

repository for carbohydrates. The cell wall carbohydrates reserve can be used dire/starvation

situations.

9. Signaling - fragments of wall, called oligosaccharins, act as hormones. Oligosaccharins,

which can result from normal development or pathogen attack, serve a variety of functions

including: (a) stimulate ethylene synthesis; (b) induce phytoalexin (defense chemicals

produced in response to a fungal/bacterial infection) synthesis; etc.

10. Economic products - cell walls are important for products such as paper, wood, fiber, energy,

shelter, and even roughage in our diet.


PLASMA MEMBRANE

The plasma membrane is the outermost boundary of the prokaryotic and eukaryotic cell. It

separates cytoplasm from its surrounding. Plasma membranes are used to compartmentalize the

cells. It is a ultrathin, elastic, living, dynamic and selective transport barrier. It is fluid mosaic

assembly of molecules of lipid (phospholipid and cholesterol), proteins and carbohydrates.

Plasma membrane controls the entry of nutrients and exit of waste products, and generates

difference in ion concentration between interior and exterior of the cell. It also acts as a sensor of

external signals and allows the cells to react or change in response to the environmental signals.

All membranes including plasma membrane and internal membranes of eukaryotic cells like

bounding membranes of Nucleus, Mitochondrion, Chloroplasts, Endoplasmic reticulum, Golgi

bodies, etc. are same in structure and selective permeability but differing in other functions and

compositions.

The plasma membrane is also called as Cytoplasmic Membrane, Cell Membrane, Cell Membrane

or Plasmalemma. The term Cell Membrane was firstly used by Nageli and Cramer (1955) and

Plasmalemma by Plowe (1931)

The plasma membrane and other cellular membranes are composed primarily of two layers of

phospholipid molecules. These bipartite

molecules have a “water-loving”

(hydrophilic) end and a “water-hating”

(hydrophobic) end. The two phospholipid

layers of a membrane are oriented with all

the hydrophilic ends directed toward the

inner and outer surfaces and the

hydrophobic ends buried within the

interior. Smaller amounts of other lipids,

such as cholesterol, and many kinds of proteins are inserted into the phospholipid framework.

The lipid molecules and some proteins can float sidewise in the plane of the membrane, giving

membranes a fluid character. This fluidity allows cells to change shape and even move.

However, the attachment of some membrane proteins to other molecules inside or outside the

cell restricts their lateral movement.


CHEMICAL COMPOSITION

Overall plasma membrane composed of 20 % Water with 80 % Organic Contents. The entire

amount of organic contents share following amount (%) of chief constituents -

CONSTITUENTS CELL (R. B. C.) CELL ORGANELLES (MITOCHONDRION)

Proteins 18 76

Lipids 79 24

Carbohydrates 03 00

PROTEINS

Membrane proteins are defined by their location within or at the surface of a phospholipid

bilayer. Although every biological membrane has the same basic bilayer structure, the proteins

associated with a particular membrane are responsible for its distinctive activities. The density

and complement of proteins associated with biomembranes vary, depending on cell type and

subcellular location.

The lipid bilayer presents a unique two-dimensional hydrophobic environment for membrane

proteins. Some proteins are buried within the lipid-rich bilayer; other proteins are associated with

the exoplasmic or cytosolic leaflet of the bilayer. Protein domains on the extracellular surface of

the plasma membrane generally bind to other molecules, including external signaling proteins,

ions, and small metabolites (e.g., glucose, fatty acids), and to adhesion molecules on other cells

or in the external environment. Domains within the plasma membrane, particularly those that

form channels and pores, move molecules in and out of cells. Domains lying along the cytosolic

face of the plasma membrane have a wide range of functions, from anchoring cytoskeletal

proteins to the membrane to triggering intracellular signaling pathways.

Membrane proteins can be classified into three categories— integral, lipid-anchored, and

peripheral—on the basis of the nature of the membrane–protein interactions.


I. Integral membrane proteins, also

called transmembrane proteins,

span a phospholipid bilayer and are

built of three segments.

II. Lipid-anchored membrane

proteins are bound covalently to

one or more lipid molecules. The

hydrophobic carbon chain of the

attached lipid is embedded in one leaflet of the membrane and anchors the protein to the

membrane. The polypeptide chain itself does not enter the phospholipids bilayer.

III. Peripheral membrane proteins do not interact with the hydrophobic core of the

phospholipid bilayer. Instead theyare usually bound to the membrane indirectly by

interactions with integral membrane proteins or directly by interactions with lipid head

groups. Peripheral proteins are localized to either the cytosolic or the exoplasmic face of the

plasma membrane. Following are some examples of the proteins -

Peripheral proteins (Cytoskeleton formation) :

Spectrin, Ankyrins, Actin, etc.

Integral Proteins (Surface Reacting – Transport, Reception, Recognition)

Glycophorin A, Glycophorin B, Glycophorin C, etc.

In addition to these proteins, which are closely associated with the bilayer, cytoskeletal filaments

are more loosely associated with the cytosolic face, usually through one or more

FORMS OF PROTEINS

1. Transmembrane Proteins

a. Single Pass Transmembrane

b. Multipass Transmembrane

2. Covalently Linked Cytosolic Extrinsic Proteins

3. Covalently Linked Non Cytosolic Extrinsic Proteins

4. Non Covalently Linked Extrinsic Proteins


LIPIDS

Phospholipids of the composition present in cells spontaneously form sheetlike phospholipid

bilayers, which are two molecules thick. The hydrocarbon chains of the phospholipids in each

layer, or leaflet, form a hydrophobic core that is 3–4 nm thick in most biomembranes. Electron

microscopy of thin membrane sections stained with osmium tetroxide, which binds strongly to

the polar head groups of phospholipids, reveals the bilayer structure (Figure).

A cross section of all single membranes stained with osmium tetroxide looks like a railroad

track: two thin dark lines (the stain–head group complexes) with a uniform light space of about 2

nm (the hydrophobic tails) between them. The lipid bilayer has two important properties. First,

the hydrophobic core is an impermeable barrier that prevents the diffusion of water-soluble

(hydrophilic) solutes across themembrane. Importantly, this simple barrier function is modulated

by the presence of membrane proteins that mediate the transport of specific molecules across this

otherwise impermeable bilayer. The second property of the bilayer is its stability. The bilayer

structure is maintained by hydrophobic and van der Waals interactions between the lipid chains.

Even though the exterior aqueous environment can vary widely in ionic strength and pH, the

bilayer has the strength to retain its characteristic architecture.

A typical biomembrane is assembled from phosphoglycerides, sphingolipids, and steroids. All

three classes of lipids are amphipathic molecules having a polar (hydrophilic) head group and

hydrophobic tail.


Although the common membrane lipids have this amphipathic character in common, they differ

in their chemical structures, abundance, and functions in the membrane.

Phosphoglycerides, the most abundant class of lipids in most membranes, are derivatives of

glycerol 3-phosphate. A typical phosphoglyceride molecule consists of a hydrophobic tail

composed of two fatty acyl chains esterified to the two hydroxyl groups in glycerol phosphate

and a polar head group attached to the phosphate group. The two fatty acyl chains may differ in

the number of carbons that they contain (commonly 16 or 18) and their degree of saturation (0, 1,

or 2 double bonds). A phosphogyceride is classified according to the nature of its head group. In

phosphatidylcholines, the most abundant phospholipids in the plasma membrane, the head group

consists of choline, a positively charged alcohol, esterified to the negatively charged phosphate.

In other phosphoglycerides, an OH-containing molecule such as ethanolamine, serine, and the

sugar derivative inositol is linked to the phosphate group. The negatively charged phosphate

group and the positively charged groups or the hydroxyl groups on the head group interact

strongly with water.

A second class of membrane lipid is the sphingolipids. All of these compounds are derived from

sphingosine, an amino alcohol with a long hydrocarbon chain, and contain a long-chain fatty acid

attached to the sphingosine amino group.

Cholesterol and its derivatives constitute the third important class of membrane lipids, the

steroids. The basic structure of steroids is a four-ring hydrocarbon. Cholesterol, the major

steroidal constituent of animal tissues, has a hydroxyl substituent on one ring

CARBOHYDRATES

Carbohydrates are present as a short, unbranched or branched chains of sugars (oligosaccharides)

attached either to proteins or lipids forming glycoproteins or glycolipids respectively. All types

of oligosaccharides are formed by various combinations of six principle sugars viz. D-Galactose,

D-Mannose, L-Fucose, Sialic acid, N-Acetyl Glucosamine, N-Acetyl-D-Galactosamine.


PLASMA MEMBRANE ULTRASTRUCTURE

The plasma membrane does the major function of regulating transportation of substances from

inside the cell to the outside and vice versa. The specificity of plasma membrane structure plays

a crucial role in the overall functioning of the cell. In simple terms, it acts in a similar manner to

the skin of animals. Various scientific hypotheses have been proposed to explain the structure of

the plasma membrane, out of which the most popularly accepted theory is the fluid mosaic

model.

I. THE PHOSPHOLIPID BILAYER

The fundamental part of the plasma membrane structure is the lipid bilayer. Types of lipids

present in the plasma membrane are phospholipids, cholesterol and glycolipids. However, as

majority of the molecules are of phospholipid type (containing a phosphate group), the two lipid

layers are better known as phospholipid layers.

The plasma membrane is the most thoroughly studied of all cell membranes, and it is largely

through investigations of the plasma membrane

that our current concepts of membrane structure

have evolved. In 1925, two Dutch scientists (E.

Gorter and R. Grendel) extracted the membrane

lipids from a known number of red blood cells,

corresponding to a known surface area of plasma

membrane. They then determined the surface area

occupied by a monolayer of the extracted lipid

spread out at an air-water interface. The surface

area of the lipid monolayer turned out to be twice that occupied by the erythrocyte plasma

membranes, leading to the conclusion that the membranes consisted of lipid bilayers rather than

monolayers.

The bilayer structure of the erythrocyte plasma membrane is clearly evident in high-

magnification electron micrographs. The plasma membrane appears as two dense lines separated

by an intervening space—a morphology frequently referred to as a “railroad track” appearance.

This image results from the binding of the electron-dense heavy metals used as stains in


transmission electron microscopy to the polar head groups of the phospholipids, which therefore

appear as dark lines. These dense lines are separated by the lightly stained interior portion of the

membrane, which contains the hydrophobic fatty acid chains. The lipid tails are water repelling

(hydrophobic), while phosphate heads are water-attracted (hydrophilic). The phospholipid

bilayer is arranged in a specific fashion, with the hydrophobic tails orienting towards the inside

(facing each other) and the hydrophilic head aligning to the outside. Thus, both sides of the

plasma membrane, one that faces the cytosol and the other facing the outside environment, are

hydrophilic in nature.

II. PROTEIN LIPID BILAYER MODEL

In 1935, Hugh Davson and James Danielli proposed a model of the cell membrane in which the

phospholipid bilayer lay between two layers of globular protein. It is also called as Davson-

Danielli sandwich model. The phosopholipid bilayer had already been proposed by Gorter and

Grendel in 1925, but the

Davson–Danielli model's

flanking proteinaceous layers

were novel and intended to

explain Danielli's observations

on the surface tension of lipid

bilayers. (It is now known that

the phospholipid head groups

are sufficient to explain the

measured surface tension.)

The Davson–Danielli model predominated until Singer and Nicolson advanced the fluid mosaic

model in 1972. The fluid mosaic model expanded on the Davson–Danielli model by including

transmembrane proteins, and eliminated the previously-proposed flanking protein layers that

were not well-supported by experimental evidence.

Limitation: The model was considering stable nature of plasma membrane and hence dynamic

nature was not explained.


III. FLUID MOSAIC MODEL OF THE PLASMA MEMBRANE

Dissecting "Fluid Mosaic" revealed that -

"fluid" = soluble, constantly changing movement.

"mosaic" = composed of a plethora of different macromolecules (ie proteins, phospholipids, and

fats).

While lipids are the fundamental structural elements of membranes, proteins are responsible for

carrying out specific membrane functions. Most plasma membranes consist of approximately

50% lipid and 50% protein by weight, with the carbohydrate portions of glycolipids and

glycoproteins constituting 5 to 10% of the membrane mass. Since proteins are much larger than

lipids, this percentage corresponds to about one protein molecule per every 50 to 100 molecules

of lipid. In 1972, Jonathan Singer and Garth Nicolson proposed the fluid mosaic model of

membrane structure, which is now generally accepted as the basic paradigm for the organization

of all biological membranes. In this model, membranes are viewed as two-dimensional fluids in

which proteins are inserted into lipid bilayers


Singer and Nicolson distinguished two classes of membrane-associated proteins, which they

called peripheral and integral membrane proteins. Integral membrane proteins are inserted into

the lipid bilayer, whereas peripheral proteins are bound to the membrane indirectly by protein-

protein interactions.

What affects the membrane's "fluidity"? (More Properties)

1. The membrane is held together by hydrophobic interaction (fatty acids and protein parts)

which is much weaker than covalent bonding.

2. Lipids drift laterally and flip-flopping (although it occurs) is rare because the 'phobic fatty

acid chains would touch HOH (and they resist this). It moves laterally at 2 micrometers per

second. Some membrane proteins drift as well, but most are anchored in the membrane.

3. The membrane remains fluid as temperature lowers until (at a critical temp) it solidifies.

4. The Steroid Cholestoral

1. helps stabilize fluidity.

2. at body temperature, it restrains the movements of phospholipids because it hinders

close packing together by its presence;

2. therefore, it raises the membranes tolerance of colder temperatures.

5. If it solidifies, the proteins become inactive.

6. A cell CAN alter lipid composition (saturated to unsaturated and vice versa).

What constitutes "mozaic"-ness?

1. Many different proteins in the bilayer proteins determine specific functions

2. Types of proteins

� integral

1. can be trans-membrane or just partway

� peripheral

1. not embedded in the membrane

2. attached to the surface of the membrane; sometimes to integral proteins


FUNCTIONS OF THE PLASMA MEMBRANE

Although the lipid composition of a membrane largely determines its physical characteristics, its

complement of proteins is primarily responsible for a membrane’s functional properties. We

have alluded to many functions of the plasma membrane in the preceding discussion and briefly

consider its major functions here.

1. In all cells, the plasma membrane acts as a permeability barrier that prevents the entry

of unwanted materials from the extracellular milieu and the exit of needed metabolites.

2. Specific membrane transport proteins in the plasma membrane permit the passage of

nutrients into the cell and metabolic wastes out of it; others function to maintain the

proper ionic composition and pH (Η7.2) of the cytosol.

a. Some of the transport process happens "passively" without the cell needing to expend

any energy to make them happen. These processes are called "passive transport

processes".

b. Other transport processes require energy from the cell's reserves to "power" them.

These processes are called "active transport processes".

3. The plasma membrane is highly permeable to water but poorly permeable to salts and small

molecules such as sugars and amino acids. Owing to osmosis, water moves across such a

semipermeable membrane from a solution of low solute (high water) concentration to one of

high solute (low water) concentration until the total solute concentrations and thus the water

concentrations on both sides are equal.

4. Plasma membrane protects and separate the interior part of cell (protoplasm) from external

environment.

5. Plasma membrane help to adhere with adjacent cells to form tissue and maintains

connection with adjacent cells via pores on membrane known as plasmodesmata (in plants)

and desmosome (in animals).

6. Unlike animal cells, bacterial, fungal, and plant cells are surrounded by a rigid cell wall and

lack the extracellular matrix found in animal tissues. The plasma membrane is intimately

engaged in the assembly of cell walls, which in plants are built primarily of cellulose. The


cell wall prevents the swelling or shrinking of a cell that would otherwise occur when it is

placed in a hypotonic or hypertonic medium, respectively. For this reason, cells surrounded

by a wall can grow in media having an osmotic strength much less than that of the cytosol.

7. In addition to these universal functions, the plasma membrane has other crucial roles in

multicellular organisms. Specialized areas of the plasma membrane contain proteins and

glycolipids that form specific junctions between cells to strengthen tissues and to allow the

exchange of metabolites between cells.

8. Still other proteins in the plasma membrane act as anchoring points for many of the

cytoskeletal fibers that permeate the cytosol, imparting shape and strength to cells.

9. The plasma membranes of many types of eukaryotic cells also contain receptor proteins

that bind specific signaling molecules (e.g., hormones, growth factors, neurotransmitters),

leading to various cellular responses. These proteins, which are critical for cell development

and functioning.

10. Finally, peripheral cytosolic proteins that are recruited to the membrane surface function as

enzymes, intracellular signal transducers, and structural proteins for stabilizing the

membrane.

11. Like the plasma membrane, the membrane surrounding each organelle in eukaryotic cells

contains a unique set of proteins essential for its proper functioning.

12. Plasma membrane also carry out exocytosis (excretion of waste outside the cell),

endocytosis (intake of large particles inside the cell) and pinocytosis (a mechanism by

which cells ingest extracellular fluid and its contents- drinking)

13. Function of plasma membrane of some cells (phagocytes) include important role in

immunity

Paper VIII Unit I- Cell Wall and Plasma...

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