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Structure and Functions of the Cell Membrane
The cell membrane (also called the plasma membrane or
plasmalemma) is the biological membrane which separates the
interior
of
a
cell
from
the
outside
environment.
Plasma
membranes bounds all plant and animal cells and all single-celled
eukaryotes and prokaryotes. They are fluid mosaics of lipids,
proteins,
and
carbohydrates.
Structurally,
they
show
resemblance to other cellular membranes, but differ in their lipid
composition and more drastically in their protein content from
one cell type to another and from intracellular membranes.
In this topic we will describe these three structures and see
how they organise and function in the cell membrane.
Lipids
Lipids form an important group of organic molecules which
includes fatty acids and their naturally-occurring derivatives like
carotenoids, waxes, sterols and fat-soluble vitamins (such as
vitamins A, D, E and K). Lipid molecules are generally insoluble in
water. They may be also be defined as hydrophobic small
molecules
consisting
of
long,
18-22
carbon,
hydrocarbon
backbones with only a small amount of oxygen-containing
groups. Lipids are the biomolecules which serve innumerable
functions in organisms. Phospholipids, glycolipids and steroids are
very
important
for
the
organisation
membranes in cells.
Basic Lipid Structure
and
functioning
of
Fats (triacylglycerols)
Fats can be defined as a diverse group of compounds that
are generally insoluble in water but soluble in organic solvents.
Chemically, fats are triesters of glycerol and fatty acids. The
fatty acids are long, unbranched hydrocarbons that terminate
with a monocarboxylic acid. Depending upon the double bonds,
fatty acids can be of two types: saturated and unsaturated. In a
typical fatty acid, each carbon atom can be bonded to two
hydrogen atoms. Fatty acids which have no double bonds are
called as saturated fatty acids, because the carbon atoms are
saturated with hydrogen which means that they are bonded to
the maximum number of hydrogen atoms possible. On the other
hand, fatty acids which have one or more double bonds are called
as unsaturated fatty acids. Fatty acids which have only one
double bond are known as "monounsaturated" fatty acids,
whereas those which have more than one double bond are called
as "polyunsaturated" fatty acids.
These double bonds introduce
"kinks" in the carbon chain which has a direct bearing on the
fluidity of lipid membranes. Therefore, a fat molecule can be
made of one, two, or three different types of fatty acids.
Depending upon the type of fatty acid, fats can be saturated or
unsaturated.
A saturated fat does not contain any unsaturated fatty acid
while as an unsaturated fat contains a minimum of one
unsaturated fatty acid esterified with the glycerol.
The former
are typically solid at room temperature and the latter are fluid at
low temperatures because of the kinks introduced by the double
bonds in the fatty acid chains, which do not allow their close
packing.
Phospholipids
Phospholipids are a diverse and important class of lipids
known for their role in cell membrane organisation and structure.
They serve as the major constituent of all cell membranes as they
nicely assemble to form the lipid bilayer of membranes. A
phospholipid molecule consists of two parts: a hydrophilic polar
head and a non polar hydrophobic tail. The phosphate group
along with the glycerol constitute the hydrophilic polar head,
whereas the fatty acid molecules form the hydrophobic tail. Thus
phospholipids are amphipathic, with polar head and non polar tail.
Most of the phospholipids contain a glycerol, two fatty acid
chains, one or more phosphate groups, or also a simple organic
molecule such as choline as shown in the fig.
Phospholipids,
being amphipathic have both water loving and water hating
areas, this property of phospholipids is basic to the formation of
micelles and lipid bilayers.
Phospholipid Structure
When phospholipids are exposed to aqueous environments,
they self assemble into structures called micelles and bilayers, in
which the hydrophobic tails (water-repelling parts) form the core
and remain hidden from the water while as the hydrophilic polar
heads (water-loving regions) remain in contact with water as
shown below in the diagram.
These
specific
properties
prove
selective
in
allowing
phospholipids to play an important role in the phospholipid bilayer
synthesis. Lipid bilayers occur when hydrophobic or non polar
tails of phospholipid molecules line up against one another,
forming a membrane with hydrophilic heads on both sides facing
the water and an inner hydrophobic core. In living systems, the
phospholipids often occur in association with other molecules
(e.g., proteins, glycolipids, cholesterol) in a bilayer, such as a cell
membrane that surrounds the cell and intracellular structures like
chloroplast, mitochondria and other membrane-bound organelles.
There are a few important features of phospholipid bilayer
which are critical to membrane function. To begin with, the
structure of phospholipids is responsible for the basic function of
membranes as barriers between two aqueous compartments,
interior cell compartment and outer environment. Since, the core
of the phospholipid bilayer is occupied by hydrophobic fatty acid
tails, this hydrophobic core lends membrane the property of
impermeability
biological
to
water-soluble
molecules.
Second
molecules,
important
ions
and
feature
of
other
the
phospholipid bilayer is its viscous fluid nature. The phospholipids
have a good degree of unsaturation in their fatty acids, having
one or more double bonds, which cause kinks in the hydrocarbon
chains making them difficult to pack together. This causes the
long hydrocarbon chains of the fatty acids to move freely in the
interior of the membrane, so the membrane itself is soft and
fluid. Because of this fluidity, both phospholipids and proteins are
free to diffuse laterally within the membrane, property which
forms the basis of many membrane functions.
Steroids
In addition to phospholipids, the cell membranes of animal cells
also contain glycolipids and steroids. The steroids are a class of
lipids which do have a molecular structure which comprises of
four fused rings and are fat-soluble organic compounds. This
class of lipids includes many hormones such as androgens of
animals and cholesterol. Cholesterol is an important steroid which
is an integral constituent of animal cell membranes. Cholesterol
functions to enhance the fluidity of membranes by preventing
close packing of fatty acid chains and also enhances the
membrane rigidity. Cholesterol also influences the cell membrane
permeability.
Proteins
Proteins are also the vital components which enter into the
constitution of cell membranes. The number of protein molecules
in
the
membrane
weight/weight basis.
is
less
than
lipids,
but
are
equal
on
Two classes of membrane-associated
proteins distinguished are extrinsic and intrinsic.
1. Extrinsic or Peripheral proteins: They are superficially
located and can be easily extracted from the membrane
following treatments with polar reagents, such as solutions
of extreme pH or high salt concentration that do not disrupt
the phospholipid bilayer. They are soluble in aqueous
solution and constitute about 30% of the protein content of
plasma membrane e.g. Cytochrome c on the mitochondrial
surface.
2. Intrinsic or Integral protein: They constitute about 70%
of the protein content of plasma membrane. They are tightly
held and difficult to remove from the membrane. They are
insoluble in aqueous solutions. To obtain the integral
proteins, the membrane has to be disrupted.
Some of the integral or intrinsic proteins partially
traverse the lipid bilayer and are found inserted on one side
of the membrane only, but many integral proteins called
tunnel proteins or transmembrane proteins completely
traverse the lipid bilayer on both sides. When a protein
crosses
the
lipid
bilayer
it
adopts
an
alpha-helical
configuration. The integral transmembrane proteins may be
called as single-pass proteins, bi-pass protein or multi-pass
proteins depending on whether the protein makes one, two
or multiple turns across the lipid bilayer. Because of their
property
of
spanning
the
membrane
completely,
transmembrane proteins are able to perform functions both
inside and outside of the cell. A good number of transmembrane proteins are also believed to have channels
through which ions and other water-soluble materials are
diffused into the cells.
Membrane Proteins
The transmembrane proteins like the phospholipid
molecules have hydrophobic and hydrophilic groups and
regions. The non polar or hydrophobic regions of the integral
protein are embedded in the hydrophobic interior of the lipid
bilayer while as the hydrophilic regions protrude from the
bilayer surface.
Owing to the semi-permeability of the cell membrane, the
cell depends on special mechanisms for communication with other
cells and exchange of nutrients with the extracellular space.
These special roles are primarily performed by proteins. The
intrinsic proteins embedded in the lipid bilayer of the membrane
serve many of the membrane functions, besides acting as
structural components. Their specific and unique shapes also
allow them to function as receptors and receptor sites, signal
transducers and transporters.
The transport proteins or carrier
proteins help in transporting substances across the membrane.
The
extrinsic
proteins
serve
as
anchoring
sites
for
the
cytoskeleton or extracellular fibres.
Carbohydrates
In addition to the lipids and proteins, carbohydrates also
form an important constituent of the cell membrane. The outer
surface of the protein-lipid membrane bilayer is coated with a
layer of carbohydrate chains. This carbohydrate layer is called as
glyocalyx. The lipids and intrinsic proteins of the membrane are
bound to carbohydrates, forming glycolipids and glycoproteins,
respectively. These carbohydrates which are normally short chain
oligosaccharides give cells their identity and also play the roles of
cell-cell interaction and cell recognition.
MEMBRANE ORGANIZATION
Many models of plasma membrane structure have been
proposed by scientists from time to time. In 1935, Danielli and
Davson proposed a model according to which the plasma
membrane is a trilaminar structure with a phospholipid layer
sandwiched by two protein layers. In 1959, David Robertson
proposed the Unit membrane model which means that the
trilaminar structure is of common occurrence in all biological
membranes. The three layers have a total thickness of 75 A0 to
100 A0. It advocates that all membranes do have a common
structure. This model, however, could not explain the dynamic
nature and functional specificity of the membrane.
Fluid Mosaic Model
The fluid mosaic model of cell membrane was proposed by
two scientists, Jonathan Singer and Garth Nicolson in 1972. This
model of membrane structure is now the most widely accepted,
since
it
explains
the
basic
organization
of
all
biological
membranes. According to this model, cell membrane consists of a
highly viscous fluid matrix made up of phospholipid bilayer to
which proteins are associated. The lipid forms the ocean in which
proteins are immersed as ice bergs. This model came to be
known as fluid mosaic model because it views the membranes as
two-dimensional fluids, in which the phospholipids and membrane
proteins are free to diffuse laterally. The mosaic pattern in the
fluid membrane is attributed to the scattered arrangement of
protein molecules in the fluid of phospholipid matrix; hence the
name mosaic to the model.
According to the fluid mosaic model, lipids are the basic
structural components of membranes while as the proteins within
the phospholipid bilayer carry out specific membrane functions.
Generally,
the
plasma
membranes
are
constituted
of
approximately 50% lipid and 50% protein by weight, while as the
carbohydrates are relatively minor and make up only 5 to 10% of
the membrane weight.
Membrane asymmetry
The plasma membrane shows asymmetry in its structure.
The two layers of the lipid bilayer differ in their lipid and protein
composition. The outer surface of the membrane bilayer, which
faces
the
extracellular
matrix,
contains
oligosaccharides
(glycolipids and glycoproteins) which give identity to the cell. It
also possesses the end of the integral proteins which receive
signals from outside the cell. The inner side of the membrane lies
in contact with the cell cytoplasm. It remains attached to the
cytoskeleton and also contains the end of the integral proteins
which transmit the signals that are received on the extracellular
side.
Membrane Fluidity
The fluidity of cell membrane is because of the phospholipids and
is primary to all the functions of the membrane and thus of the
cell also. Fluidity of membrane allows the constituent lipids and
proteins to mobilise within the bilayer. This mobility is having a
tremendous biological significance since it governs the transport
or exchange of life-driving commodities in and out of the cell. The
fluidity of membranes depends upon the the structure of fatty
acid chain and temperature. Higher degree of unsaturated fatty
acids increases membrane fluidity while as their lower content
decreases it. So membrane fluidity at any temperature is
maintained by the right ratio of saturated to unsaturated fatty
acids. During cold periods, some animals and plants respond to
decreasing
unsaturated
temperatures
fatty
acids
by
in
increasing
cell
the
amount
of
membranes.
Presence
of
cholesterol in animal cell membranes prevents the close packing
of fatty acid tails, thereby lowering the need of unsaturated fatty
acids. This prevents the cell membrane from becoming too liquid
at body temperature and therefore, maintaining the desired
fluidity.
The lipids present in the cell membrane are randomly
moving
at
the
rate
of
22
µm
per
second.
Usually
the
phospholipids present in the same lipid layer of the membrane
move freely but very rarely they flip to the other layer. Flipping is
an energy-dependent process and it occurs rarely because it
requires the hydrophillic head of the phospholipid to traverse the
hydrophobic region of the bilayer.
Membrane Functions or Membrane Transport
Cell membranes are called selectively or differentially
permeable i.e. they permit the passage of certain ions and
molecules while excluding others. Membranes are relatively
permeable to water, some simple sugars, amino acids and lipidsoluble materials but are relatively impermeable to very large
molecules such as proteins, polysaccharides, etc. It is observed
that the negatively charged ions pass rapidly than the positively
charged ions, though non-electrolytes pass most rapidly. This
means that the membrane is positively charged.
There are different processes used by the molecules and
ions to move into and out of the cells through cell membrane.
A. Passive transport:
1. Simple diffusion: In this process of diffusion, the
molecules and ions pass through a membrane along their
concentration gradient, without involving expenditure of
energy.
Lipid-soluble
substances
and
water-soluble
substances can pass by simple diffusion. The smaller
molecules pass more rapidly than the larger ones.
Passage of charged ions leads to change in the potential
across the membrane which is unfavourable for further
passage of the same ion. In order to avoid the effect, a
positively charged ion should accompany a negatively
charged ion. Alternatively, movement of a cation in one
direction should be accompanied by movement of a cation
in the other direction.
2. Facilitated diffusion: Some substances are transferred
across membranes more readily than is expected from a
process of passive diffusion, although no expenditure of
energy is involved in their transport. The process of such
transfers is called the Facilitated diffusion and occurs
according to the concentration gradient. The process
requires participation of a transmembrane protein called
the carrier, transporter or permease. Besides, the carrier
also
binds
the
transported
compounds
specifically.
Specificity, however, may not be absolute. Thus, the
glucose
carrier
for
the
erythrocyte
membrane
has
maximum affinity for glucose, mannose and fructose.
Fructose in small intestine is absorbed by Facilitated
diffusion.
Mechanism of Facilitated diffusion: After binding of
the substance for transport, to the carrier, the molecular
events for transport are clear. In the “ping pong model”
the carrier protein is believed to have two conformations.
In one, the binding site of the carrier molecule faces one
side of the membrane and in the other conformation it
faces
the
other
side.
Exchange
between
the
two
conformations is obligatory but exchange is slow if the
binding sites are empty but becomes fast if these sites are
occupied.
Osmosis: This is a special diffusion of solvent (water).
The
solvent
permeable
molecules
membrane
pass
(cell
through
a
membrane)
selectively
from
dilute
solutions to water by the process of osmosis.
Ion channels: These are membrane proteins that allow
the passage of ions that would ordinarily be stopped by
the
lipid
bilayer
of
the
membrane.
These
small
passageways are specific for one type of ion, such that a
calcium ion could not pass through an iron ion channel.
The ion channels also serve as gates because they
regulate ion flow in response to two environmental
factors: chemical or electrical signals from the cells and
membrane movement. This happens in your body when a
nervous impulse encounters a gap or synaptic cleft
between
nerve
cells.
The
electrical
stimulation
is
continued because ion channels are opened to allow
specific ions to pass through the receiving membrane,
which continues the electrical stimulation to the next
nerve cell.
B. Active transport: There is evidence that dissolved
substances, especially mineral ions, continue to move into
the cells even though there is a greater concentration of
them within the cell than outside. Such a movement of
material against the concentration gradient is called active
transport, which involves the expenditure of metabolic
energy.
Active and Passive Transport Proteins
For example
Sodium-potassium pump
or
Na+-K+
ATPase: It
is
present in the plasma membrane of many cells. Na+-K+
ATPase exchanges 3 Na+ (from cell to outside) with 2 K+
(into the cell from outside).
Steps involved
1. Binding side towards the cell--Na+ binds.
2. Phosphorylation of transporter-binding site gets everted
3. Na+ released, conformation for K+ acquired. Thus, K+
binds.
4. Dephosphorylation of transporter.
5. Original position and process repeats.
Uniport and co-transport
Active transport system may act as Uniport or Cotransport processes. In a uniport process, only a single
molecule
is
transported.
In
Co-transported
processes,
transport of one molecule is linked with transport of another
molecule in the same direction (symport) or in the opposite
direction (antiport).
C. Membrane Transport of Macromolecules (Bulk Flow)
Many cells usually use the processes of exocytosis
and
endocytosis
for
secretion
and
ingestion
of
macromolecules, respectively. In exocytosis, the vesicle
fuses with the cell membrane and releases the contents to
the exterior of the cell while as in endocytosis, the
membrane
invaginates
and
pinches
off,
engulfing
the
molecule. The endocytotic vesicles are of two different sizespinocytotic (which are small and contain the dissolved
solutes) and phagocytic (which are large and contain the
solid
particles).
During
receptor-mediated
endocytosis,
coated vesicles bind to specific receptors on the cell surface,
guiding the cell to select what molecules to take and what to
reject.
D. Membrane Receptors
Receptor proteins are the specialized transmembrane
proteins which act as the communication office of the cell,
thereby allowing the cell to interact with the outside environment.
The exterior end of the receptor protein binds to a specific
messenger molecule. This signal is transmitted, causing the
interior end of the protein to change shape, which triggers a
reaction inside the cell. These receptor proteins are highly
specific.