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PLASMA MEMBRANE
Functions of plasma membrane
Compartmentalization. Membranes are continuous, unbroken sheets and, as such, inevitably
enclose compartments. The plasma membrane encloses the contents of the entire cell,
whereas the nuclear and cytoplasmic membranes enclose diverse intracellular spaces. The
various membrane-bound compartments of a cell possess markedly different contents.
Membrane compartmentalization allows specialized activities to proceed without external
interference and enables cellular activities to be regulated independently of one another.
Scaffold for biochemical activities. Membranes not only enclose compartments but are also a
distinct compartment themselves. As long as reactants are present in solution, their relative
positions cannot be stabilized and their interactions are dependent on random collisions.
Because of their construction, membranes provide the cell with an extensive framework or
scaffolding within which components can be ordered for effective interaction.
Providing a selectively permeable barrier. Membranes prevent the unrestricted exchange of
molecules from one side to the other. At the same time, membranes provide the means of
communication between the compartments they separate. The plasma membrane, which
encircles a cell, can be compared to a moat around a castle: both serve as a general barrier,
yet both have gated “bridges” that promote the movement of select elements into and out of
the enclosed living space.
Transporting solutes. The plasma membrane contains the machinery for physically
transporting substances from one side of the membrane to another, often from a region amino
acids, that are necessary to fuel its metabolism and build its macromolecules. The plasma
membrane is also able to transport specific ions, thereby establishing ionic gradients across
itself. This capability is especially critical for nerve and muscle cells.
Responding to external signals. The plasma membrane plays a critical role in the response of
a cell to external stimuli, a process known as signal transduction. Membranes possess
receptors that combine with specific molecules (or ligands) having a complementary
structure. Different types of cells have membranes with different receptors and are, therefore,
capable of recognizing and responding to different ligands in their environment. The
interaction of a plasma membrane receptor with an external ligand may cause the membrane
to generate a signal that stimulates or inhibits internal activities.
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Intercellular interaction. Situated at the outer edge of every living cell, the plasma
membrane of multicellular organisms mediates the interactions between a cell and its
neighbors. The plasma membrane allows cells to recognize and signal one another, to adhere
when appropriate, and to exchange materials and information.
Energy transduction. Membranes are intimately involved in the processes by which one type
of energy is converted to another type (energy transduction). The most fundamental energy
transduction occurs during photosynthesis when energy in sunlight is absorbed by membranebound pigments, converted into chemical energy, and stored in carbohydrates. Membranes
are also involved in the transfer of chemical energy from carbohydrates and fats to ATP.
Danielli-Davson Structure: In 1935, James Danielli and Hughs Davson proposed that all
the plasma membrane consist of lipid layer coated with protein molecules as continous layer.
This suggest the triliminar or having three layers. The lipid layer is a fluid medium in which
the protein coated or attracted.
Unit Membrane Hypothesis: In 1950‟s J. David Robertson noticed that all cellular
membranes-whether they are plasma, nuclear or cytoplasmic, whether they are taken from
plants, animals or micro organisms showed the same substructure. They appeared as a
trilaminar structure, of 75Ao to 100 Ao in thickness, composed of two dark electron dense
layers, each 20 Ao to 25Ao thick, separated by a light, electron transparent layer, 35 Ao to 50
Ao thick. Robertson termed this structure a „unit membrane‟ and proposed that all cellular
membranes had unit membrane constitution. This generalization is known as unit membrane
hypothesis. According to the unit membrane hypothesis all cellular membranes are trilaminar
lipoprotein structures. Unit membrane concept differed from the Danielli- Davison model in
that the protein layers are asymmetric on both sides. On the outer surface they are
mucoproteins and on the inner surface, non mucoid proteins.
Fluid-Mosaic Model: In 1972, S J. Singer and G. I. Nicholson put forward the “Fluid
Mosaic Model” of membrane structure in which a mosaic protein molecules floats in a fluid
lipid bilayer. This model is proposed that membrane is made up of lipid and protein but the
protein does not form a continuous layer covering both sides of the membrane as proposed by
Danielli and Davson. In mosaic model the protein molecules are either partially (peripheral
protein) or wholly embedded (integral protein). Some of these proteins that float, consist of
pores that allow the passage of particular molecules or ions through the membrane. In
absence of these pores, the polar molecules could be difficult to cross the membrane.
According to this model, the membrane structure is not static, the lipid molecule linked to
one another only by weak bond.
What is the lipid bilayer?
The lipid bilayer (phospholipid bilayer/cell membrane) is a structural component of the cell
that isolates the cell components (organelles, cytoplasm) from the extracellular environment.
Structure of Lipid bilayer
The hydrophilic head is polar allowing it to form hydrogen bonds with water molecules,
whereas the tail region - made from two hydrocarbon chains - is non-polar or hydrophobic. It
is this combination of both a hydrophobic and hydrophilic region (amphiphilic) that gives
phospholipids such an important function within the cell. When placed in water, the
phospholipid molecules naturally align into a bilayer, allowing the hydrophobic tails to avoid
water whilst the hydrophilic heads form hydrogen bonds with water molecules. Interestingly,
the lipid bilayer will form a closed sphere (liposome) to completely exclude water from the
hydophobic tail.
Chemical Composition
Membrane Lipids:
There are three main types of membrane lipids: phosphoglycerides, sphingolipids, and
cholesterol.
Phosphoglycerides : Most membrane lipids contain a phosphate group, which makes them
phospholipids. Because most membrane phospholipids are built on a glycerol backbone, they
are called phosphoglycerides. Unlike triglycerides, which have three fatty acids and are not
amphipathic, membrane glycerides are diglycerides.
Sphingolipids : A less abundant class of membrane lipids, called sphingolipids, are
derivatives of sphingosine, an amino alcohol that contains a long hydrocarbon chain.
Sphingolipids consist of sphingosine linked to a fatty acid by its amino group. This molecule
is a ceramide.
Cholesterol: Another lipid component of certain membranes is the sterol cholesterol, which
in certain animal cells may constitute up to 50 percent of the lipid molecules in the plasma
membrane. Cholesterol is absent from the plasma membranes of most plant and all bacterial
cells. Cholesterol is smaller than the other lipids of the membrane and less amphipathic.
Membrane Carbohydrates:
The plasma membranes of eukaryotic cells contain carbohydrates that are covalently linked to
both lipid and protein components. Depending on the species and cell type, the carbohydrate
content of the plasma membrane ranges between 2 and 10 percent by weight. More than 90
percent of the membrane‟s carbohydrate is covalently linked to proteins to form
glycoproteins; the remaining carbohydrate is covalently linked to lipids to form glycolipids
Membrane proteins can be grouped into three distinct classes distinguished by the intimacy
of their relationship to the lipid bilayer. These are:-
Integral
proteins;
protrude
proteins
proteins that penetrate the lipid bilayer. Integral proteins are transmembrane
that is, they pass entirely through the lipid bilayer and thus have domains that
from both the extracellular and cytoplasmic sides of the membrane. Some integral
have only one membrane-spanning segment, whereas others are multispanning.
Genome sequencing studies suggest that integral membrane proteins constitute about 30
percent of all encoded proteins.
Peripheral proteins that are located entirely outside of the lipid bilayer, on the cytoplasmic
or extracellular side, yet are associated with the surface of the membrane by noncovalent
bonds.
Lipid-anchored proteins that are located outside the lipid bilayer, on either the extracellular
or cytoplasmic surface, but are covalently linked to a lipid molecule that is situated within the
bilayer.
Briefly describe different activities of the lipid bilayer
Maintain an internal environment: the external cellular environment is often different to the
cellular environment (ion, protein concentrations etc). The cell can control its environment by
controling gene expression of transmembrane proteins (see 'crossing the membrane').
Allows the cell to control what enters/exits: transporter proteins and channels enable the cell
to selectively uptake molecules that it requires whilst excluding those that may be harmful.
Protection: the lipid bilayer creates an envelope - housing the cellular components and
offering some protection. Some cells, such as plant cells, have a cell wall which offers further
protection.
Contains important components involved in cell recognition, communication, signalling etc:
the lipid bilayer is the boundary between the internal and external environment and must
mediate communication between the two.
Gives the cell shape (anchors with the cytoskeleton): in some cells the shape is . One example
is the red blood cell. Its concave shape increases its surface area allowing it to bind oxygen
much more efficiently.
Explain with diagram the structure of the lipid bilayer
The lipid bilayer is made up of many phospholipids that align together. Each phospholipid is
made up of a hydrophilic head and a hydrophobic tail. The most common phospholipid is
phosphatidylcholine which contains a choline molecule bound to phosphate and glycerol. The
hydrophilic head is polar allowing it to form hydrogen bonds with water molecules, whereas
the tail region - made from two hydrocarbon chains - is non-polar or hydrophobic. It is this
combination of both a hydrophobic and hydrophilic region (amphiphilic) that gives
phospholipids such an important function within the cell. When placed in water, the
phospholipid molecules naturally align into a bilayer (figure 2), allowing the hydrophobic
tails to avoid water whilst the hydrophilic heads form hydrogen bonds with water molecules.
Interestingly, the lipid bilayer will form a closed sphere (liposome) to completely exclude
water from the hydophobic tail.
The lipid-bilayer isolates the internal components of the cell from the extracellular
environment. Whilst lipid-soluble molecules can cross the membrane directly through the
bilayer, this is not so for large molecules such as glucose, water and other polar molecules.
Transmembrane proteins are anchored throughout the entire bilayer and allow non-polar
molecules to cross the membrane, facillitating the passing of these molecules into or out of
the cell. These proteins not only allow essential components into the cell, they also offer an
opportunity for the cell to control what can and cannot enter. The cell can tightly control what
can and cannot enter the cell by altering the expression of genes encoding transmembrane
proteins.
What roles the protein channels play in the membranes?
The definition of a channel (or a pore) is that of a protein structure that facilitates the
translocation of molecules or ions across the membrane through the creation of a central
aqueous channel in the protein. This central channel facilitates diffusion in both directions
dependent upon the direction of the concentration gradient. Channel proteins do not bind or
sequester the molecule or ion that is moving through the channel. Specificity of channels for
ions or molecules is a function of the size and charge of the substance. The flow of molecules
through a channel can be regulated by various mechanisms that result in opening or closing
of the passageway.