The cell membrane is a phospholipid bilayer 7.5nm thick that envelops the cell. It is composed of phospholipids, cholesterol, and integral and peripheral proteins arranged in a fluid mosaic structure. The cell membrane is selectively permeable and uses three mechanisms for transport - passive diffusion, facilitated diffusion, active transport, and vesicular transport. Vesicular transport involves endocytosis which brings contents into the cell, and exocytosis which releases contents from the cell.

1. CELL MEMBRANE
DR. SAMIA SHAHBAZ

2. All the cells are bounded by a limiting membrane called
CELL MEMBRANE (PLASMA MEMBRANE/PLASMALEMMA).
It totally envelops the cell and acts as a barrier between cell cytoplasm and
extracellular fluid (ECF).

3.  The cell membrane is 7.5nm thick.
 Exhibits TRILAMINAR (3-layered) STRUCTURE i.e. two electron dense
lines separated by an electron lucent central zone.
 Each layer is 2.5nm thick.

4. COMPOSITION OF CELL MEMBRANE
Phospholipids
Cholesterol
Proteins
 Integral
 Peripheral

5. Thus,
it consists of a bimolecular layer of phospholipids (lipid bilayer) in which
proteins and cholesterol are intercalated. It is semi permeable. The lipid
bilayer is FLUID in nature and the large protein molecules embedded in it
exhibit a MOSAIC pattern. This is known as FLUID MOSAIC MODEL.

7. PHOSPHOLIPIDS
Each phospholipid molecule has a:
 polar i.e. hydrophilic head
 non polar i.e. hydrophobic tail
Polar head consists of glycerol conjugated to choline or serine
Non polar head consists of two long chain fatty acids
One of the chain consists of saturated fatty acids and the other one formed by
unsaturated fatty acids.

8. Both these chains are linked covalently to glycerol component of polar head.
The hydrophilic heads lie at the outer and inner faces of the cell membrane
and
the hydrophobic tails project towards the middle of the bilayer.

9. CHOLESTEROL
The cholesterol molecules lie between the long chain fatty acids.
It stiffens the cell membrane.

10. PROTEINS
The protein molecules make up approximately 50% of the total plasmalemma
mass. They are divided into two groups:
 Integral proteins
 Peripheral proteins
The integral proteins are either embedded in the lipid bilayer or pass through
it completely.
The peripheral proteins are not embedded, rather they are loosely associated
with the internal or external surface of the cell membrane

11. Weak electrostatic forces exist between peripheral and integral proteins.
Most of the composition is by integral proteins and majority of them pass
through the entire thickness of the cell membrane hence known as
TRANSMEMBRANE PROTEINS.

12. FUNCTIONS OF CELL MEMBRANE
 Mechanical structure: maintains the physical integrity of the cell.
 Selective permeability: gases and small molecules can easily pass through
it.
 Transportation: certain molecules pass through it passively and other
needed various transporters.
 Communication: contains receptors
which recognize and respond to specific
signals.

13. TRANSPORT ACROSS CELL MEMBRANE
There are three basic transport mechanisms:
1. Passive transport
 Passive diffusion
 Facilitated diffusion
2. Active transport
3. Vesicular transport
 Endocytosis
 Exocytosis

14. PASSIVE TRANSPORT
In this type, the substances move down the concentration gradient (i.e. from
higher concentration to lower concentration). No energy is required.
PASSIVE DIFFUSION:
Fat, fat soluble molecules and small (hydrophobic) molecules can easily cross
the cell membrane by this diffusion. Similarly, dissolved gases like oxygen O2
and Carbon Dioxide CO2 are also a good example.

15. FACILITATED DIFFUSION:
Water and water soluble molecules like ions, glucose and amino acids also
move down the concentration gradient but they require the aid of special kind
of transmembrane proteins called CHANNEL PROTEINS.

17. ACTIVE TRANSPORT
The uphill movement of the substances against the concentration gradient
requiring some kind of energy is known as active transport. It is accomplished
by special transmembrane proteins known as CARRIER PROTEINS.
FOR EXAMPLE:
movement of sodium out of the cell by sodium pump
And
Movement of potassium inside the cell by potassium pump across the cell
membrane.

18. VESICULAR TRANSPORT
This is the mechanism through which large molecules enter, exit or move with
in the cell. Vesicles are formed by budding off from the cell membrane or from
the membranous organelles of the cell. These move to their target sites and
fuse with the target membrane to release their contents.
TYPES:
ENDOCYTOSIS
 Pinocytosis
 Receptor mediated endocytosis
 Phagocytosis
EXOCYTOSIS

19. ENDOCYTOSIS
It is the process by which vesicles enter into the cell
PINOCYTOSIS: (cell drinking)
It is the processes by which macromolecules (e.g. protein molecules) are taken
up by the cell along with small quantity of extracellular fluid. When the
macromolecule to be endocytosed comes in contact with the cell membrane, a
shallow depression is first formed. It gradually deepens to form a flask shaped
invagination. Narrowing and subsequent closure of the neck of invagination
detaches it from the plasmalemma as PINOCYTOTIC VESICLE.
Pinocytosis occurs in almost all cells of the body.

22. RECEPTOR-MEDIATED ENDOCYTOSIS:
This process depends on the presence of receptor proteins called
CARGO PROTEINS in the cell membrane.
The cargo proteins bind to LIGANDS which come in contact with the cell
membrane.
Internally, the cargo receptors become associated with special proteins called
CLATHRIN.
They are assembled beneath the cargo receptors, thus, pulling the cell
membrane and forming a CLATHRIN COATED PIT.
It then becomes a COATED VESICLE, pinching off from the cell membrane
and reaching the cytoplasm. Here the coated proteins are removed.
So, the UNCOATED PINOCYTOTIC VESICLE remains which fuses with
endosome.

23. .

24. PHAGOCYTOSIS: (cell eating)
It is the processes by which large particles like cell debris and bacteria are
engulfed by the phagocytic cells.
When a bacterium comes in contact with the cell membrane, it produces
processes which surround that large particle. The edges of these processes
fuse, forming a vesicle called PHAGOSOME.

25. EXOCYTOSIS
.
The cytoplasmic vesicles merge with the plasma membrane and release its contents.