Eukaryotic plasma membrane structure and function

1. EUKARYOTIC
PLASMA
MEMBRANE
Dr. Akanksha Jain
Ph.D. Biotechnology
Department of Biotechnology
Shri Shankaracharya Mahavidhyalaya
Bhilai

2. Plasma membrane (De Robertis, 1965), Plasma-lemma (J.Q. Plowe, 1931), Unit
membrane (Rorbertson, 1959).
The term cell membrane was originally used by C. Nageli and C. Cramer
(1855). Plasma Membrane of neuron (nerve cell) is called neurolemma while
that of haemolysed RBC is called red cell ghost.
The plasma membrane of muscle cell along with based lamina is called
sarcolemma.
Definition:
Plasma membrane is a living, ultrathin, dynamic elastic semipermeable
membrane that encloses the protoplasm of a cell.
Synonyms of Plasma membrane

3. Location:
It is the outermost boundary of all living cells. But prokaryotes and plant cells
generally have an additional cell wall outside the plasma membrane. In addition to
Plasma membrane, eukaryotic cells contain intracellular membrane surrounding,
the vacuole and organelles. The plasma membrane and the intracellular membranes
together called as biological membranes of bio-membranes.
Chemical Composition:
Chemically plasma membrane is a molecular assembly of lipids (20- 40%), proteins
(60-75%) and carbohydrates (1-5%). The carbohydrates found in the form of
glycoproteins or glycolipids and restricted only to the outer surface of plasma
membrane. The lipids and proteins are held together by non-covalent interactions.
The membrane lipids of plasma membrane are of 3 major types:
(a) Phospholipids,
(b) Glycolipids, and
(C) Sterols.

4. All of them are amphipathic or amphiatic molecules because they
possess both hydrophilic (polar) and hydrophobic (non-polar) ends.
Majority (80%) of the phospholipids are neutral (e.g.
phosphatidylcholine, phosphatidylethalamine and spingomyelin) and
the rest phospholipid s are acidic or negatively charged (e.g.
phophatidylionositol, phosphotidylserine etc.).
The glycolipids may be cerebroside or ganglioside. Sterols found in
the membrane may be cholesterol (in animals), stigmasterol, β-sterol
(in plants) and ergosterol (in microbes). All lipids are symbolically
represented with a polar head and two fatty acid tails.

5. The Membrane proteins are two types: integral or intrinsic (~ 70%)
and peripheral or extrinsic (-30%). Nearly all known integral proteins
span the lipid bilayer, while peripheral proteins are superficially
attached by electrostatic and hydrogen bond interactions. Membrane
proteins have various roles-mechanical, transport, enzymatic.
Carbohydrates are mainly branched or un-branched oligosaccharides
present only on the outer face of plasma membrane. In many protists
and animal cells they form a cell coat (= glycocalyx) on the outer face
of plasma membrane which protect the underline plasma membrane.

7. Structural Models of Plasma Membrane:
1. Lipid bilayer Model (Gorter and Grendell, 1926):
The plasma membrane of erythrocyte is a continuous lipid bilayer structure.
2. Sandwich model or ‘Protein-Lipid-Protein’ model (Danielli and Davson,
1935): According to this model plasma membrane is a trilamellar structure with
a middle lipid bilayer sandwiched between two continuous layers of protein.

8. 3. Unit membrane model (Robertson,
1959):
This model is the interpretation of electron on
microscopic image on myelin along the line of
Danielli- Davson model.
According to this model all biological membranes
have the same basic structure:
(a) The average thickness is about 7.5 nm (75A).
(b) They have a characteristic trilamellar (3-
layered) structure,
(c) The three layers include a central lipid bilayer
(3.5 nm) sandwiched between 2 protein layers
(each-7.5 nm)

9. 4. Fluid mosaic model (Singer and Nicolson, 1972):
The authors described the model as “protein icebergs in a two dimensional lipid
sea”.
This postulates:
(i) That the biological membranes are quasi-fluid (semi-fluid) structures in which
both lipids and integral proteins are free to move laterally as well as within the
bilayer
(ii) That the lipids and proteins are arranged in a mosaic manner.
(iii) The integral or intrinsic proteins are embedded in the lipid bilayer while the
extrinsic or peripheral proteins are superficially attached on both surface of the
membrane.
(iv) The exoplasmic face (E-face) of the cell membrane often possesses
carbohydrate chains or oligosaccharides. They are bound to both proteins and
phospholipids producing glycoproteins and glycolipids respectively. The
carbohydrate coat present on the E-face of plasma membrane constitute
glycocalyx or cell coat. The oligosaccharides gives a negative charge to outer
surface. They act as cell surface markers, receptors, blood grouping etc..

10. Evidences supporting Fluid-mosaic model:
(a) Branton (1968) conformed the mosaic nature of proteins by studying freeze-
fracture electron microscopy of the plasma membrane that revealed randomly
distributed pumps and depressions.

11. Frey and Edidin (1970) experimentally demonstrated the fluid nature of plasma
membrane b) fusion of a mouse cell with a human cell to yield a hybrid cell called
heterokaryon or cybrids. This cell fusion can be induced by agents called fusogen
(e.g., Sendai virus, polyethyleneglycol, electric shock etc.)

12. Functions of Plasma Membrane:
Besides enclosing the cell and protecting it from the external environment, the
plasma membrane has several important functions, such as, regulating the
movement of materials inside and outside the cell, metabolic functions,
communication between different cells and adhesion between cells.
1. Movement of Materials:
Movement (import and export) of materials occur by different
mechanisms, e.g:
(a) Simple diffusion,
(b) Facilitated diffusion,
(c) Active transport, and
(d) Endocytosis and exocytosis.

13. (a) Simple Diffusion:
Simple diffusion refers to the unaided movement of a substance from the
region of its higher concentration to a region of its lower concentration till an
equilibrium is achieved. Some solutes diffuse through the plasma membrane
more readily than the others.
Therefore, the plasma membrane is called a selectively permeable or
differentially permeable membrane. When water molecules move through a
differentially permeable membrane from lower to higher concentration of
solutes, the process is called osmosis.
(b) Facilitated Diffusion:
It is similar to simple diffusion but the rate of the solute movement increases
by interaction with specific membrane transporters. The transporters are
“trans membrane proteins.”
(c) Active Transport:
It is the mechanism by which movement of solutes occurs in one direction
(unidirectional), i.e., from lower to higher concentration. This is an energy
requiring process. The energy is obtained from hydrolysis of ATP and from
other sources.

14. (d) Endocytosis and Exocytosis:
Certain substances are imported within the cell or expelled out of the cell
via membrane “vesicles”. The uptake of the external substances via
vesicles is called endocytosis, while the excretion of substances via
vesicles is called exocytosis.
Endocytosis is divided into two types. The uptake of large particles
through vesicles is called phagocytosis, while the uptake of small
particles and water soluble molecules, such as, enzymes, hormones,
antibiotics etc., is called pinocytosis

15. 2. Metabolic Functions:
Plasma membrane plays an important role in metabolism. Several enzymes
are located on the cell surface, such as, those involved in extracellular nutrient
breakdown and those involved in cell wall biosynthesis. In prokaryotes,
respiratory enzymes are located in the plasma membrane.
3. Communication Recognition and Adhesion:
Some important functions of the plasma membrane are the communication
between cells, recognition and cell to cell adhesion. Such functions are carried
out by “receptors” which are trans membrane proteins or integral proteins.
The extracellular substance, called “ligand” binds to the specific receptors.
This binding triggers a change in the function of the membrane. It can
transduce signal in the cytoplasm, the phenomenon is called “signal
transduction”.