paper viii unit i- cell wall and plasma...
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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,
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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.
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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
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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%
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• 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.
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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.
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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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 8
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.
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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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 10
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.
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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.
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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.
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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.
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 14
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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 15
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.
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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.
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 17
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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 18
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.
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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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 20
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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 21
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
D. D. Khedkar Unit – I : Cell Wall and Plasma Membrane 22
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