Name: Iqra javaid
Department: BS MIC 1
Registration number: UF 17-9103
Course: Cell biology
Date: 11-feb-2018
Assignment no: 1
Submitted to: Dr. Asif Jamal
Topic:
Ultra-structure of Cell membrane.
Cell membrane is very critical component of a cell. The reason to this statement is that it maintains
cell integrity and keeps the outside and inside components of cells separated from each other. If
there is no cell membrane, there is no life. The main structure of cell membrane is given as fluid
mosaic model.
Characteristics of plasma membrane:
Cell membranes also behaves like selective barriers. The cell membrane separates a cell from its
outside environment helping the molecular organization to differ from its surrounding. The plasma
membrane plays a great role in cell communication, transport of molecules, and cell development.
Receptor proteins helps the cell membrane to receive signals from the outside. Transport proteins
in the plasma membrane provide transport of small molecules. The flexibility of the cell membrane
and its expansion capability allows cell growth and maintenance of shape. Despite of all, biological
membranes all are general in structure, each is a very thin layer of lipid (fatty) molecules and
protein molecules, held together by noncovalent bonding.7. Cell membranes are dynamic in
motion and most of their molecules move in plane. The lipid molecules are arranged as a double
layer continuously, which is about 5 nm thick1.This lipid bilayer provides the basic frame of the
cell membrane in which molecules are floating. And is actually a relatively impermeable blockage
to the passage of mostly water-soluble molecules. All other functions like, catalyzing membrane-
linked reactions for example, ATP synthesis is done by the help of proteins in membrane. Some
transmembrane proteins allow the structural connection that links the cytoskeleton with the lipid
bilayer the extracellular environment. while other transmembrane protein allows receptors to
transduce chemical signals in the cell’s internal mechanism. The membrane proteins are encoded
in animal’s genome is considered to be round about 30%.
THE LIPID BILAYER:
The lipid bilayer is frame of cell membrane. It structure can be easily seen by electron microscope.
Phospho-glycerides, Sphingolipids, and Sterols are important constituent of lipid molecules in cell
membrane. Lipid molecules are about 50% of the total composition of most animal cell
membranes, nearly all of the rest are protein molecules. In plasma membrane nearly 5×10^6 lipid
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molecules in a 1 micrometer x1 micrometer area. Or 10^9 lipid molecules in this region is found.
The lipid molecules in cell membranes are amphiphilic—that is, they consist of hydrophilic
(“water-loving”) or polar end and a hydrophobic (“water-fearing”) or nonpolar end2.
Abundant membrane lipids in cell membrane are the phospholipids. In their structure, they have
majorly a polar head group and two hydrophobic hydrocarbon tails. The tails are actually fatty
acids, and their length can vary from 14 to 24 carbon atoms. In all animals, plant or bacterial cell
membrane one tail typically consist of one or more cis-double bonds. These bond is actually
unsaturated in one tail whereas in the other tail it is saturated. Each cis-double bond makes a small
twist in the tail5. How are phospholipid molecules pack effects the differences in the length and
saturation of the fatty acid tails and also causing the mobility of the membrane to be affected. The
main phospholipids are the phosphor-glycerides. Two long-chain fatty acids are connected through
ester bonds next to carbon atoms of the glycerol (3 carbon glycerol is its backbone), whereas the
third carbon atom is linked to a phosphate group, which is further in touch with multiple different
kinds of head group. Phospho-glycerides, phosphatidylserine, Phosphatidylethanolamine, and
phosphatidylcholine are the result of amalgamation of different head groups. Sphingomyelin, is
made from sphingosine. Sphingosine is a long acyl chain consisting of an amino group (NH2) and
two hydroxyl groups (OH) at one end of the molecule. A fatty acid tail is linked to the amino
group, and to the terminal hydroxyl group, with one hydroxyl free, a phosphocholine group is
linked. Free hydroxyl group gives the polar properties. Because it links to water molecules and
also with surrounding polar molecules. Phosphatidylcholine, phosphatidylethanolamine,
phosphatidylserine, and sphingomyelin collectively make 50% of lipid constitution. Cholesterol
and glycolipids, in addition to phospholipid, is also present in cell membrane. Up to one molecule
for every phospholipid molecule, Cholesterol is present in the plasma membrane. Cholesterol is a
sterol. It’s composition is a rigid ring structure, a single polar hydroxyl group to which is ring
attached and nonpolar hydrocarbon chain short in length. Orientation of cholesterol molecule is
just same as phospholipid molecules, adjusting themselves in the bilayer, to adjacent phospholipid
molecules their polar head is attached.
Phospholipids Spontaneously Form Bilayer:
The bilayers formation impulsively in aqueous environments is due to shape and amphiphilic
nature molecules. As it is already known that, hydrophilic molecules dissolve immediately in water
because favorable electrostatic interactions in the charged groups or uncharged polar groups with
water molecules occurs. Hydrophobic molecules are insoluble in water. The reason is all the atoms
in it are uncharged and nonpolar which is never favorable for its bonding with water. Molecules
form ice cage like structure as soon as they are dispersed in the water molecules. This formation
increases the free energy. Energy cost is minimized. Lipid molecules, hydrophobic and hydrophilic
regions behave in the same way. They spontaneously cumulate the hydrophobic region in the
interior and hydrophilic heads to exterior region near water. They can do this depending on their
shape in two ways:
1. spherical micelles, with the tails inward can be formed by them.
2. Double-layered sheets, or bilayers, with the hydrophilic head groups exterior and interior
sandwiched hydrophobic tails groups.
In this energetically most satisfactory composition, exteriorly the hydrophilic heads face the water
at each side of bilayer and interiorly the hydrophobic tails are protected from the water. This also
provide a self-healing property to the phospholipid membrane. When any of the water molecule
or a phospholipid molecule is teared, this is automatically rearranged by any small vacuoles.
Forming a sealed compartment and closing with other group can help the breakage of the structure.
Due to shape and amphiphilic nature of the phospholipid molecule, creation of life is possible. A
remarkable behavior regulates the fundamentals of living cell. Above all, Fluidity and dynamicity
is distinguished and unique character of cell membrane.
The Lipid Bilayer Is a Two-dimensional Fluid:
Around 1970, researchers recognized within lipid bilayers that individual lipid molecules are able
to diffuse freely6.The initial information is brought from studies of synthetic lipid bilayers.
Preparation studies useful are of two types
1. liposomes, which are bilayers made in the form of spherical vesicles can vary in size.
Depending on how they are produced, ranges from about 25 nm to 1 mm in diameter
2. black membranes are known as planar bilayers formed across a hole in a partition between
two aqueous compartments.
To study the motion of individual lipid molecules and their components various techniques are
used. One can make a lipid molecule, with a fluorescent dye or a small gold particle follow the
diffusion of even individual molecules in a membrane and attached to its polar head group.
Alternatively, “spin label,” can be used to modify the lipid such as a nitroxyl group ( N–O);
because this contains an unpaired electron that can be detected by electron spin resonance (ESR)
spectroscopy because of its paramagnetic behavior. ESR spectrum can be used to study its motion
and orientation. Such studies shows that phospholipid molecules in synthetic bilayers very rarely
migrate from the monolayer (also called a leaflet) on one side to that on the other. This process,
known as “flip-flop,”. In opposite side, lipid molecules exchange places with their beside
molecules within a monolayer (~107 times per second). Rapid lateral diffusion occurs because of
this, with a diffusion coefficient (D) of about 10–8 cm2/sec. This means that an average lipid
molecule diffuses the length of a large bacterial cell (~2 mm) in about 1 second. These studies
have also revealed that individual lipid molecules rotate very rapidly and have flexible
hydrocarbon chains. Lipid molecules in membranes are very muddled, presenting an irregular
surface of variously spaced seen by computer stimulation. In isolated biological membranes,
synthetic bilayer gives similar mobility studies on labeled lipid molecules. A two-dimensional
liquid in which the constituent molecules are free to move laterally is actually the lipid component
of a biological membrane. Individual phospholipid molecules are normally confined within
monolayer in synthetic bilayer. Problem is created for their synthesis due to this confinement. In
only one monolayer of a membrane phospholipid molecule is manufactured. New lipid bilayer
could not be made, if none of these newly made molecules could migrate sharply to the non-
cytosolic monolayer. Enzymes called phospholipid translocators, a special class of transmembrane
enzyme, helps to solve the problem by catalyzing the rapid flip-flop of phospholipids from one
monolayer to the other side of the monolayer.
The Fluidity of a Lipid Bilayer Depends on Its Composition
The fluidity of cell membranes is very precisely regulated. When the bilayer viscosity is increased
beyond a threshold level, certain membrane transport processes and enzyme activities ceases. As
is readily described in studies of synthetic bilayers, the fluidity of a lipid bilayer depends on both
its composition and its temperature3. A synthetic bilayer made from a single type of phospholipid,
at a characteristic freezing point, changes from a liquid state to a two-dimensional rigid crystalline
(or gel) state. This phase transition known as the change of state, and the temperature at which it
occurs is lower, if there is a shorter or double bonded hydrocarbon tail. A shorter chain length
helps the interaction of hydrocarbon tails with one another, in both the same and opposite
monolayer, and cis-double bonds produce twists in the hydrocarbon chains which makes more
situation critical, so that the membrane remains fluid at lower temperatures4. Adjustment of the
fatty acid composition of their membrane lipids is done by Bacteria, yeasts, and other organisms
whose temperature fluctuates with that of their environment. For instance, the cells of those
organisms synthesize fatty acids, as the temperature falls, with more cis-double bonds. Cholesterol
controls the properties of lipid bilayers. The permeability-barrier properties of the lipid bilayer is
enhanced by mixing it up with phospholipids. With help of hydroxyl group close to the polar head
groups of the phospholipids, it injects into the bilayer, so that its rigid, platelike steroid rings
interact with—and partly immobilize—those region closest to the polar head groups. cholesterol
makes the lipid bilayer less deformable in this region by decreasing the mobility of the first. few
CH2 group, and thereby decreases the permeability of the bilayer to small water-soluble molecules.
Cholesterol stiffens the packing of the lipids but does not effects the fluidity. Cholesterol also
prevents the hydrocarbon chains from coming together and crystallizing when its concentration is
high in eukaryotic plasma membrane. Bacterial cell membranes are composed of one main type of
phospholipid. They also contain no cholesterol. Mechanical stability is improved by an overlying
cell wall in the bacteria. In archaea, lipids usually contain 20–25-carbon-long prenyl chains and
fatty acid chains are similarly hydrophobic and flexible. Thus, lipid membrane can be made up of
different molecules but have similar features. Containing large amounts of cholesterol and also
containing a mixture of different phospholipids is a basic difference of bacteria, archae and
eukaryotic cell membrane. Typical cell membrane is much more complex than originally thought,
analysis of membrane lipids by mass spectrometry has revealed this. Membranes are composed of
a confusing variety of 500–1000 different lipid species. Complexity is reflected by the
combinatorial variation in head groups, hydrocarbon chain lengths. Membranes also contain many
minor lipids, which have important functions. The inositol phospholipids have crucial functions in
guiding membrane traffic and in cell signaling, which are present in small amount.
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