Cell Membrane: Definition
The
plasma membrane aka Cell membrane, is defined as a lipid and protein double
layer that surrounds the cell. It separates the cell from its external
environment. This structure is available in all cell types whether prokaryote
or eukaryote. The plasma membrane is made up of phospholipids and proteins.
What is the Function of the Cell Membrane
The
cell membrane is among the most important structures of the cell. And it is
structurally adapted to suit its functions. The plasma membrane gives structure
to the cell and is responsible for regulating the substances that enter and
leave the cell. This structure is partially permeable, hence it allows some
materials to pass through while restricting other substances.
What substances does the cell membrane allow to pass through?
The
plasma membrane is a partially permeable membrane made up of lipids and
proteins. It allows small molecules such oxygen, carbon dioxide, water etc. to
easily pass through. These substances are needed by cells for their metabolic
reaction (e.g. Respiration) hence it is important that cell membranes allow
easy passage of these substances into the cell. However water passes the cell
membrane at a slower rate.
Despite
being small, certain highly molecules or ions cannot directly pass through, the
same goes for large molecules (macromolecules) like carbohydrates or amino
acids. Such molecules instead enter the cell through the proteins embedded in
the cell membrane. This allows the cell to able to control the rate at which
these materials are diffusing into or outside the cell. Hence it enables the
cell to maintain a balanced ionic concentration.
Cell
membranes are also able to take in molecules through a process known as
endocytosis. Endocytosis is a process by which the cell takes in molecules by
forming depressions which surround the particle. The depressions eventually
engulf the particle and get pinched off to form small spheres of membrane
referred to as vesicles. The vesicles contain the molecule and carry it to
where it is required for use in the cell. The process of endocytosis includes,
Phagocytosis (cell eating) and pinocytosis (cell drinking).
Endocytosis
makes it possible for the cell membrane to take in large molecules. The
opposite process of endocytosis is exocytosis. During the exocytosis, cells
deliver materials through the cell membrane to the external environment.
Vesicles are formed in the cytoplasm and move to the surface of the plasma
membrane. At the plasma membrane the vesicles merge with the membrane and
contents are delivered to the external environment of the cell. Hence
exocytosis is important for excretion of waste products as well as worn out
cell organelles.
What does the Plasma Membrane Do?
The
cell membrane has an important function in cell signalling and cellular
communication. Within the plasma membrane, there are several proteins embedded
that are capable of binding molecules in the external environment of the cell
and deliver messages to the inside of the cell about its surroundings.
More
importantly, the receptors on the cell membrane can attach to substances which
are produced by other parts of the body like hormones. When such molecules bind
to their target receptor proteins on the plasma membrane, signal transduction
pathways inside the cell are initiated, these transmits the signal to the
appropriate molecules. Hence cells are able to perform
complex actions which are instructed by a signalling molecule such as
synthesising or inhibiting the production of a certain protein.
What is the Cell Membrane made up of?
Structure of the cell membrane
Phospholipid Bilayer
The
plasma membrane consists of a phospholipid bilayer. the phospholipids are
simply lipids. These lipids are made up of a phosphate group head and two fatty
acid tails, this is what gives the cell membrane its hydrophobic nature. This
make up of phospholipid molecules enables the cell membrane to spontaneously
form a double-layered membrane.
The
phospholipid tail is hydrophobic in nature whereas the phosphate group is
hydrophilic in nature, this in turn makes the plasma membrane amphipathic in
nature. This means that the phosphate group is attracted to water while the
tail repels water.
Lipid Molecules
The
cell membrane is also made up of lipid molecules, glycolipids and sterols to be
specific. An important role of the sterols
such as the cholesterol is regulating the fluidity of the plasma
membrane in animal cells. When cholesterol is in low amounts, the membranes
gain more fluid and are made more permeable to molecules. Hence the cholesterol
level in the membranes helps in maintaining cell permeability making sure that
the right amount of molecules enter the cell at the right time.
Proteins
The
cell membrane is also made up of various kinds of proteins. The proteins make
up about half of the plasma membrane. Of these available proteins,
transmembrane proteins are the majority. Transmembrane proteins are proteins
embedded inside the membrane but tend to stick out from both sides of the
membrane.
Among
these proteins, some have roles as receptor proteins responsible for binding
signal molecules. On the other hand others are ion channels serving as the only
means for ions into or out of the cell.
In
conclusion, the plasma membrane has a fluid uniformity because it is made up of
a large part of phospholipids hence this allows proteins to freely move across
its surface. Because of this a variety of cell proteins and lipids make it have
a mosaic look. Hence it is called the fluid mosaic membrane by scientists..
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