Plasma membrane
The plasma
membrane, also known as the cell membrane, is a crucial structure that
separates the cell's interior from its external environment by enveloping each
living cell. The thin, semi-permeable membrane has a significant role in
maintaining the cell's integrity, functionality, and homeostasis.
At first
appearance, the plasma membrane appears to be a static wall, separating the
interior of the cell from the outside world. This seemingly simple structure,
however, is significantly more complicated.
Its primary
function is to operate as a selectively permeable barrier (allowing certain
substances to pass through while preventing others), regulating the passage of
ions, chemicals, and nutrients into and out of the cell. This selective
permeability is a mechanism that allows the cell to regulate its internal
environment by allowing selective components to enter the cell while keeping
the unnecessary and possibly dangerous molecules out.
The lipid
bilayer, a double layer of phospholipid molecules, is the fundamental
structural component of the plasma membrane. These lipids have a hydrophilic
(attractive to water) head and hydrophobic (repelling to water) tails. Because
of this unusual configuration, a durable barrier that is impervious to most
water-soluble compounds is formed.
A vast array
of proteins, each with a specialised purpose, are embedded within this lipid
bilayer. These proteins are divided into two types:
Integral proteins that is present inside the membrane and
peripheral proteins that are present on the surface of membrane. Integral
proteins, also known as transmembrane proteins, is a protein present across the
membrane providing a pathway for ions and molecules to move across the
membrane.
Peripheral proteins are loosely linked to the membrane and
frequently operate as enzymes, receptors, or structural elements.
carbohydrates
are also present on the extracellular side of the membrane either bound to
lipids(glycolipids) or bound to proteins (glycoproteins). These carbohydrates
play significant role in cell communication, cell recognition and cell
adhesion.
Structure of plasma membrane
The plasma
membrane is a complex and versatile structure composed of lipids, proteins, and
carbohydrates.
This
detailed structure can be broken down into several key components:
1) Phospholipid Bilayer: The foundation of the plasma membrane is a
double layer of phospholipid molecules. Each phospholipid consists of a
hydrophilic head(water-loving) and two hydrophobic tails(water-repelling). The
hydrophilic heads face outward toward the aqueous environment, while the
hydrophobic tails are oriented inward, forming a barrier that separates the
internal and external environments of the cell.
2) Proteins: It is present in two form that is:-
· Integral proteins which is incorporated into the lipid bilayer
and span its entire width. They can act as channels and transporters allowing
selected ions and molecules to pass through and therefore they are essential
for controlling ion and chemical transport across the membrane.
· Peripheral proteins are proteins that are present on the surface
of membrane, either on the extracellular or cytoplasmic side. Peripheral
proteins frequently act as enzymes, structural elements, or in cell signalling
and adhesion.
3) Carbohydrates: It is present on the plasma membrane in two forms. It can be
found attached to lipids and proteins.
· Glycolipids: Carbohydrates are covalently bound to lipids (glycolipids) in the
plasma membrane's outer leaflet.
· Glycoproteins: Carbohydrates are also covalently bound to proteins
(glycoproteins), especially on the extracellular side of the membrane. These
carbohydrate chains play critical roles in cell identification, adhesion, and
communication with other cells.
4) Cholesterol: These molecules are interspersed within the lipid bilayer.
Cholesterol contributes to membrane fluidity and stability by preventing
phospholipid fatty acid chains from clustering too tightly together. It is
required for membrane elasticity and function.
5) Lipid Rafts: Lipid rafts are areas of the plasma membrane that are enriched
with certain lipids and proteins. These microdomains help in membrane sorting
and trafficking and also facilitate other cellular functions like signal
transduction.
6) Cytoskeleton Attachment: Integral proteins can be attached to the
cytoskeleton on the cytoplasmic side of the membrane, providing structural
support and aiding intracellular signalling.
7) Asymmetry of the Plasma Membrane: The plasma
membrane has asymmetric lipid and protein contents on the
inner and outer leaflets. This asymmetry is essential for several cellular
activities, such as cell signalling and membrane trafficking.
8) The fluid mosaic concept, which emphasises the membrane's dynamic and
fluid nature, is frequently used to describe the general organisation of the
plasma membrane. Proteins and lipids can migrate laterally within the bilayer,
allowing for adaptation and flexibility.
Function of plasmamembrane
The plasma
membrane performs the following important functions:
1) Selective Permeability: One of the plasma membrane's most essential
tasks is to operate as a selectively permeable barrier. The cell membrane
controls the flow of ions and molecules into and out of the cell. It allows
certain molecules to enter while keeping others out. This selective
permeability is crucial for maintaining the cell's internal environment,
providing vital components needed for the cell's functioning, and regulating
the concentration of ions and molecules within the cell.
2) Transport: The plasma membrane contains transmembrane proteins such as
transporters and channels that allow ions, nutrients, and other substances to
move across the membrane. Active transport systems, such as the
sodium-potassium pump, necessitate the use of energy to move molecules against
concentration gradients.
3) Cell Signalling: Receptors and signalling molecules are
implanted in the plasma membrane, allowing the cell to detect and respond to
external signals. These signals when bound to their receptor on the plasma
membrane can lead to performing specific functions like metabolism of
substances, cell division, gene expression etc.
4) Cell Adhesion: Cell adhesion is mediated by proteins on the
plasma membrane's surface. They enable cells to bind to one another (cell-cell
adhesion) and to the extracellular matrix (cell-matrix adhesion). Cell adhesion
is essential for the creation, maintenance, and overall structural integrity.
5) Cell recognition: Carbohydrate chains connected to proteins
and lipids on the plasma membrane's extracellular surface play an important
function in cell identification and immunological response. They act as the
cell marker, present on the surface of the cell allowing other cell to
recognize it, which is very crucial in immunological activity and tissue
compatibility.
6) Endocytosis and Exocytosis: The plasma membrane is engaged
in processes such as endocytosis, in which the cell engulfs and internalises
material through the formation of vesicles.
Exocytosis,
on the other hand, is the process by which chemicals present inside the
vesicles are released outside the cell by attaching these vesicles to the
plasma membrane. These mechanisms are essential for the intake of nutrients and
elimination of waste substances to the outside of the cell and also the
secretion of various cell produces.
7) Maintaining Cell Shape and Integrity: The plasma
membrane gives the cell structural support and helps it keep its shape. It also
prevents the loss of certain important substances.
8) Protection: The plasma membrane acts as a barrier, preventing potentially
hazardous substances such as poisons and infections from entering the cell.
