2. Plasma Membrane
The plasma membrane is an envelope surrounding the cell.
It forms the cell’s flexible outer surface which separates and protects the cell’s internal
environment from the external hostile environment.
Besides being a protective barrier, plasma membrane provides a connecting system between
the cell and its environment.
It is a selective barrier that regulates the flow of materials into and out of a cell. This selectivity
helps establish and maintain the environment appropriate for normal cellular activities.
It Is responsible for maintenance of shape and size of the cell.
The subcellular organelles such as nucleus, mitochondria, lysosomes are also surrounded by
membranes.
3. Chemical Composition
The membranes are composed of lipids, proteins and carbohydrates.
The actual composition differs from tissue to tissue , but typically it contains, 40 % of the dry
weight is lipids, about 60% proteins and 1 to 10 % carbohydrates.
4. Fluid Mosaic Model
The plasma membrane is best described by using a structural model called the fluid mosaic
model.
Proposed by Singer and Nicolson, is a more recent and acceptable model for membrane
structure. The biological membranes usually have a thickness of 5-8 nm. A membrane is
essentially composed of a lipid bilayer.
Globular proteins are irregularly embedded in the lipid bilayer.
According to this model, the molecular arrangement of the plasma membrane resembles an
ever-moving sea of fluid lipids that contains a mosaic of many different proteins. Some proteins
float freely like icebergs in the lipid sea, whereas others are anchored at specific locations. Some
of the proteins in the plasma membrane allow movement of polar molecules and ions into and
out of the cell. Other proteins can act as signal receptors or adhesion molecules.
The membrane is asymmetric due to the irregular distribution of proteins. The lipid and protein
subunits of the membrane give an appearance of mosaic.
The membrane freely changes, hence the structure of the membrane is considered as fluid
mosaic.
5.
6. Membrane Lipids
The basic structural framework of the plasma membrane is the lipid bilayer, two back-to-back
layers. The major classes of membrane lipids are:
– Phospholipids
– Glycolipids
– Cholesterol
About 75% of the membrane lipids are phospholipids, lipids that contain phosphorus
Present in smaller amounts are cholesterol (about 20%)
A steroid with an attached OH (hydroxyl) group, and various glycolipids (about 5%), lipids with
attached carbohydrate groups
7. Membrane Proteins
Proteins of the membrane are classified into two major categories:
– Integral proteins or intrinsic proteins or transmembrane proteins and – Peripheral or extrinsic
proteins.
• Integral proteins are either partially or totally immersed in the lipid bilayer. Many integral
membrane proteins span the lipid bilayer from one side to the other and are called
transmembrane protein whereas others are partly embedded in either the outer or inner leaflet
of the lipid bilayer. Transmembrane proteins act as enzymes and transport carriers for ions as
well as water soluble substances, such as glucose.
• Peripheral proteins are attached to the surface of the lipid bilayer by electrostatic and
hydrogen bonds. They bound loosely to the polar head groups of the membrane phospholipid
bilayer. Peripheral proteins function almost entirely as enzymes and receptors.
8. Membrane Proteins
Some integral membrane proteins form ion channels, pores or holes through which specific
ions, such as potassium ions (K+), can flow to get into or out of the cell
Other integral proteins act as carriers, selectively moving a polar substance or ion from one side
of the membrane to the other. Carriers are also known as transporters.
Some integral proteins are enzymes that catalyze specific chemical reactions at the inside or
outside surface of the cell.
Integral proteins may also serve as linkers, which anchor proteins in the plasma membranes of
neighboring cells to one another or to protein filaments inside and outside the cell. Peripheral
proteins also serve as enzymes and linkers.
Integral proteins called receptors serve as cellular recognition sites. Each type of receptor
recognizes and binds a specific type of molecule. For instance, insulin receptors bind the
hormone insulin.
A specific molecule that binds to a receptor is called a ligand. Membrane glycoproteins and
glycolipids often serve as cell identity markers. The ABO blood type markers are one example of
cell identity markers.
9. Glycocalyx
The carbohydrate portions of glycolipids and glycoproteins form an extensive sugary coat
complex polysaccharides called the glycocalyx.
Most of the integral proteins are glycoproteins and about one-tenth of the membrane lipid
molecules are glycolipids. The carbohydrate portion of these molecules protrudes to the outside
of the cell, dangling outward from the cell surface. Therefore, it acts like a molecular “signature”
that enables cells to recognize one another.
Many of the carbohydrates act as receptor for hormones.
Some carbohydrate moieties function in antibody processing. For example, a white blood cell’s
ability to detect a “foreign” glycocalyx is one basis of the immune response that helps us destroy
invading organisms.
It enables cells to adhere to one another.
Makes red blood cells slippery as they flow through narrow blood vessels.
10.
11. Membrane Permeability
This property of membranes to pass some substances through it and other not, is called selective
permeability. The lipid bilayer portion of the membrane is permeable to nonpolar, uncharged
molecules, such as oxygen, carbon dioxide, and steroids, but is impermeable to ions and large,
uncharged polar molecules such as glucose. Transmembrane proteins that act as channels and
transporters increase the plasma membrane’s permeability that cannot cross the lipid bilayer
without help.
12. Passive Diffusion
It is a passive process in which the net movement of a substance is from a region of higher
concentration to a region of lower concentration. The substance moves because of its kinetic
energy; diffusion continues until equilibrium is reached, that is, the substance becomes evenly
distributed.
A good example of diffusion in the body occurs in the lungs for intake of oxygen and out take of
carbon dioxide.
13. Facilitated Diffusion
Passive movement of a substance down its concentration gradient through the lipid bilayer by
transmembrane proteins that function as carriers In carrier mediated facilitated diffusion, a carrier
(also called a transporter) is used to move a solute down its concentration gradient across the
plasma membrane.
The solute binds to a specific carrier on one side of the membrane and is released on the other
side after the carrier undergoes a change in shape.
Glucose enters many body cells by carrier-mediated facilitated diffusion as follows;
Glucose binds to a specific type of carrier protein called the glucose transporter (GluT) on the
outside surface of the membrane. As the transporter undergoes a change in shape, glucose passes
through the membrane. The transporter releases glucose on the other side of the membrane. The
selective permeability of the plasma membrane is often regulated to achieve .
14. Active transport
Active transport occurs against a concentration gradient and this is dependent on the supply of
metabolic energy (ATP).
Active transport is also a carrier mediated process like facilitated diffusion. The most important
primary active transport systems are ion-pumps (through the involvement of pump ATPases or ion
transporting ATPases).
Na+-K+ pump The Na+-K+ pump is responsible for the maintenance of high K+ and low Na+
concentrations in the cells.
Na+-cotransport system : The amino acids and sugars are transported into the cells by a Na+-
cotransport system. This process essentially consists of the passage of glucose (or amino acid) into
the cell with a simultaneous movement of Na+.
15.
16.
17. Cotransport system
In cotransport, the transport of a substance through the membrane is coupled to the
spontaneous movement of another substance.
18. Transport of macromolecules
The transport of macromolecules such as proteins, polysaccharides and polynucleotides across
the membranes is equally important.
This is brought about by two independent mechanisms namely
endocytosis—intake of macromolecules by the cells (e.g. uptake of LDL by cells)
and exocytosis—release of macromolecules from the cells to the outside (e.g. secretion of
hormones-insulin, PTH).
19. Osmosis
It is also a passive process in which the net movement of water molecules through a selectively
permeable membrane from an area of higher water concentration to an area of lower water
concentration occurs.
During osmosis, water molecules pass through a plasma membrane in two ways:
1. by moving through the lipid bilayer via simple diffusion, as previously described,
2. by moving through aquaporins(aquawater), Integral membrane proteins that function as
water channels.
20. Types of Receptors
Receptors are protein molecules in the target cell or on its surface that bind ligands.
There are two types of receptors: internal receptors and cell-surface receptors
1. Internal receptors
Internal receptors, also known as intracellular or cytoplasmic receptors, are found in the cytoplasm
of the cell and respond to hydrophobic ligand molecules that are able to travel across the plasma
membrane. Once inside the cell, many of these molecules bind to proteins that act as regulators of
mRNA synthesis to mediate gene expression.
21. 2- Cell surface receptors
Cell-surface receptors, also known as transmembrane receptors, are cell surface, membrane-
anchored, or integral proteins that bind to external ligand molecules.
This type of receptor spans the plasma membrane and performs signal transduction, converting an
extracellular signal into an intracellular signal.
Ligands that interact with cell-surface receptors do not have to enter the cell that they affect. Cell-
surface receptors are also called cell specifIc proteins or markers because they are specific to individual
cell types.
Cell-surface receptors are involved in most of the signaling in multicellular organisms. There are three
general categories of cell-surface receptors:
1. Ion channel-linked receptors
2. G-protein-linked receptors
3. Enzyme-linked receptors
22. Ion channel - linked Receptors
Ion channel-linked receptors bind a ligand and open a channel through the membrane
that allows specific ions to pass through. There is a conformational change in the
protein’s structure that allows ions such as sodium, calcium, magnesium, and hydrogen
to pass through.
Note: A ligand is a molecule that binds another specific molecule, in some cases, delivering a signal in the
process. Ligands can thus be thought of as signaling molecules. Ligands interact with proteins in target cells,
which are cells that are affected by chemical signals; these proteins are called receptors.
23. Enzyme - linked Receptors
• Enzyme-linked receptors are cell-surface receptors with intracellular domains that are
associated with an enzyme. In some cases, the intracellular domain of the receptor
itself is an enzyme or the enzyme-linked receptor has an intracellular domain that
interacts directly with an enzyme.
• An example of this type of enzyme-linked receptor is the tyrosine kinase
receptor. The tyrosine kinase receptor transfers phosphate groups to tyrosine
molecules.
24. G - protein linked Receptors
• G- protein-linked receptors bind a ligand and activate a membrane protein called a G-protein. The
activated G-protein then interacts with either an ion channel or an enzyme in the membrane.
• All G-protein-linked receptors have seven transmembrane domains, but each receptor has its own
specific extracellular domain and G-protein-binding site.
