What is Cell Membrane?
The
biological membrane which forms the boundary between the cell and its
external environment in an animal is called as Cell membrane. The cell
membrane is also called as plasma membrane. The cell membrane is not
permeable to all the ions and molecules; rather it is selectively
permeable membrane, which allows only selected molecules to pass through
it.
Functions of the cell membrane:
The functions of cell membrane can be summarized as:
- Protect the integrity of the cell by allowing certain substances into the cell, while keeping other substances out.
- It serves as a base for attachment of the cytoskeleton, receptors, channels, ionic pumps.
- Cell membranes are involved in a variety of cellular processes such as cell adhesion, cell signaling and so on.
Models / Theories explaining the cell structure:
Many
theories were put forward to explain the structure of the cell
membrane. Among those the important ones are the barrier theory by
Moritz traube. He proposed that there is barrier between the interior
and exterior of the cell and that barrier is semi permeable allowing the
passage of only a few substances. He did not have any direct evidence
regarding the composition of that barrier.
The
lipid nature of the membrane was found out by Quincke. In 1935 two
scientists, Davson and Danielli proposed that the cell membrane is
composed of lipid bi-layer, and there was a coating of proteins on
either side of the lipid bilayer. This theory also failed as it could
not explain the mechanism how the substances are transported from one
side to the other.
In
1957 Robertson proposed the unit membrane model by modifying the Davson
Danielle model. He also proposed that the membranes were having a lipid
bilayer. However, membranes vary in composition and function, from
species to species, from cell to cell and from one cell locale to
another. They are dynamic, fluid, plastic, elastic structures in a
constant state of flux and turnover. Even there were arguments against
the Robertson’s unit membrane model.
In
1972 SJ singer and Nicholson proposed fluid mosaic model is the
currently most favored model. This is the model which is accepted
presently. According to this model the membrane components are not
rigidly fixed, but exist in a fluid and dynamic state. Lipids of the
membrane are fluid at physiological temperatures, thus there may be
fluid movement of the lipid molecules and lateral movement (diffusion)
of the proteins that are embedded in the membrane. "Protein icebergs in a
sea of lipid". This forms a heterogeneous mosaic of proteins which may
be continuously rearranged. According to this model proteins are present
on the surface and also we have proteins which traverse through out the
cell membrane.
Composition of Cell membrane:
Cell membrane is mainly formed by three important constituents:
- Phospholipids.
- Carbohydrates.
- Proteins.
Phospholipids:
These
substances basically belong to the class of lipids. These are mainly
seen forming the cell membrane. These lipids contain further is made up 2
fatty acids, a glycerol unit, a phosphate group and a polar molecule.
The 2 fatty acids present in a phospholipid become a part of the tail in
the phospholipid molecule. Since the tail part is occupied by the fatty
acids it is not soluble in water (lipids / fats are water insoluble).
The phosphate group and the polar molecule form the head part in a
phospholipid and it is water soluble. To conclude a phospholipid
molecule has a hydrophilic (water loving) head and hydrophobic (water
hating or water repelling) tail.
When
we place phospholipids in water medium, all the phospholipids arrange
themselves in such a manner that all the heads of phospholipids face the
water and the tail part move inwards. Such an arrangement of
phospholipids is called as phospholipid bilayer (2 layers of
phospholipids). Phospholipid bilayer is shown in the picture below.
There
are four major identifiable phospholipids in an animal cell membrane,
phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and
sphingomyelin. The above mentioned phospholipids are asymmetrically
distributed between the two halves of the membrane bilayer. The outer
leaflet of the plasma membrane consists mainly of phosphatidylcholine
and sphingomyelin, whereas phosphatidylethanolamine and
phosphatidylserine are the predominant phospholipids of the inner
leaflet. A fifth phospholipid, phosphatidylinositol, is also localized
to the inner half of the plasma membrane.
Structure of Cell membrane:
As
mentioned previously phospholipids for the main constituent of the cell
membrane. The fundamental structure of the cell membrane or the main
cytoskeleton for the cell membrane is given the lipid bilayer. All the
other components of cell membrane adhere to the lipid bilayer. The
hydrophobic tails of the phospholipids face each other, where as the
hydrophilic phospholipid head faces the interior and exterior of the
cell.
Apart from phospholipids we have other lipids like cholesterol and
Glycolipids in the cell membrane.
Cholesterol in the cell membrane and its functions:
It
is a major constituent of the cell membrane, especially in the animal
cells. Because of its rigid ring structure, cholesterol plays a distinct
role in membrane structure. Cholesterol will not form a membrane by
itself, but inserts into a bilayer of phospholipids with its polar
hydroxyl group close to the phospholipid head. Depending on the
temperature, cholesterol has distinct effects on membrane fluidity. At
high temperatures, cholesterol interferes with the movement of the
phospholipid fatty acid chains, making the outer part of the membrane
less fluid and reducing its permeability to small molecules. At low
temperatures, however, cholesterol has the opposite effect: By
interfering with interactions between fatty acid chains, cholesterol
prevents membranes from freezing and maintains membrane fluidity.
Glycolipids in the cell membrane and its functions:
Glycolipids
are the substances which contain both carbohydrate moiety and lipid
moiety (In simple words Glycolipids = Carbohydrates + lipids). The
glycolipids are found exclusively in the outer leaflet of the plasma
membrane, with their carbohydrate portions exposed on the cell surface.
They are relatively minor membrane components, constituting only about
2% of the lipids of the cell membrane.
Proteins in the cell membrane:
While
the lipids form the cytoskeleton of the cell membrane, the proteins are
responsible for the optimal functioning of the cell membrane. Proteins
are inserted into the lipid bilayer.
According to the singer and Nicolson model of the cell membrane, there are two types of proteins in the cell membrane.
- Peripheral proteins: The operational definition of a peripheral protein would be, any protein that dissociates from the membrane when treated with polar reagents (Solutions of extreme pH or high concentration of salt). These proteins are present on either end of the cell membrane and not inserted into the hydrophobic interior of the lipid bilayer. Instead, they are indirectly associated with membranes through protein-protein interactions.
- Integral proteins: Peripheral membrane proteins or integral membrane proteins are the proteins which traverse through the entire length of the cell membrane and can be released only by treatments that disrupt the phospholipid bilayer. Portions of these integral membrane proteins are inserted into the lipid bilayer, so they can be dissociated only by reagents that disrupt hydrophobic interactions.
There
is evidence to show that both the phospholipids and proteins have a
lateral movement. The functions of proteins in the cell membrane are:
- Help in transport of substances on either direction of the cell membrane.
- Enzymatic activity: A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution.
- Signal transduction: A membrane protein (receptor) may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone.
- Cell to Cell recognition: Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells.
- Intracellular joining: Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions.
- Attachment to the cytoskeleton and extracellular matrix: Microfilaments or other elements of the cytoskeleton may be non-covalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins.
Carbohydrates in the cell membrane:
Membrane
carbohydrates are usually short, branched chains of fewer than 15 sugar
units. Some are covalently bonded to lipids, forming molecules called
glycolipids. (Recall that glyco refers to the presence of carbohydrate.)
However, most are covalently bonded to proteins, which are thereby
glycoproteins. Carbohydrates in either form are present on the outer
surface of the cell membrane. Cells recognize other cells by binding to
molecules, often containing carbohydrates, on the extracellular surface
of the plasma membrane.
The
carbohydrates on the extracellular side of the plasma membrane vary
from species to species, among individuals of the same species, and even
from one cell type to another in a single individual. The diversity of
the molecules and their location on the cell’s surface enable membrane
carbohydrates to function as markers that distinguish one cell from
another. ABO blood grouping system is again based on the carbohydrates
present on the surface of the red blood cell.
