2. Cell membrane (Plasma membrane ).
• All cells are surrounded by a thin, elastic,
semi- permeable living membrane called Cell
membrane (Plasma membrane or plasma
• It serves as the boundary of the cytoplasm.
• Keeps the cell contents in place
• Gives form to the cell
• The thickness varies from 100 – 215 A°.
3. • Membranes from different sources vary in
• Chemically , it is composed of proteins, lipids
• Proteins represent the main component of
most biological membranes.
4. On the basis of the degree of association with
the membrane , membrane proteins are
i) Peripheral (Extrinsic ) proteins –
Have a weaker association with the
Soluble in aqueous solutions
Usually , they are free of lipids .
5. ii ) Integral (Intrinsic ) proteins
Represent more than 70% of the membrane
Firmly associated with the membrane
Insoluble in aqueous solutions
Associated with lipids – The main lipid
components of plasma membrane -
phospholipids, cholesterol and galactolipids.
6. Lipids are arranged in parallel chains in a
double (bimolecular) layer with their
hydrophobic, non polar ends adjoining each
other and their hydrophilic polar ends
associated with proteins.
More than 30 enzymes have been detected
in isolated plasma membranes like Mg ++
ATPase, Alkaline phosphatase, Adenyl
cyclase, Na+ K+ATPase etc.
7. MOLECULAR MODELS
OF PLASMA MEMBRANE
1. UNIT MEMBRANE MODEL
Proposed by Robertson (1960)
The unit membrane is a trilaminar
membrane with protein – lipid – protein
The outer and inner dense layers represent
the proteins , each about 20 A° thick.
8. The interior less dense region of about 35 A°
thickness represents the bimolecular layer
The total thickness of the plasma membrane
is 75 A° - thickness may vary from
membrane to membrane.
11. • The lipid and protein molecules are held
• At some places in the membrane , there are
10 A° wide pores.
• Trilaminar composition is found in most of
the membranes of cellular organelles like
Endoplasmic reticulum, Golgi complex,
Lysosomes, Plastids etc.
12. Limitations of Unit membrane model
Unit membrane model is not sufficient to
explain the structure and function of all
biological membranes which are much more
13. 2. FLUID MOSAIC MODEL
• Proposed by Singer and Nicholson (1972)
• Generally accepted model
• Described as ‘protein icebergs’ in a sea of
Important Postulates :
1. The lipid and integral proteins are disposed
in a kind of mosaic arrangement.
2. The biological membranes are quasi- fluid
structures in which both the lipids and the
integral proteins are able to perform
translational movements within the bilayer.
15. • The main components of the membrane viz.
lipids and proteins are held together by
relatively weak interactions – ionic and H –
• Fluidity (both lipids and proteins have
freedom of lateral movement within the
bilayer ) is achieved with slightest disturbance.
16. • Intrinsic proteins and lipids are amphipathic
molecules , having both hydrophilic and
hydrophobic groups within the same
• This explains that transmembrane protein
traversing through the membrane will have
hydrophilic groups on either side of the
membrane and hydrophobic group in the
interior of the membrane which is in contact
with the lipids.
17. • Such traversing proteins may be in contact
with the aqueous solvent on both sides of
• The membranes are dynamic not static.
• Small sized pores , with 0.7 nm diameter
18. Functions of Plasma membrane
• Controls the entry and exit of molecules and ions
into and out of the cell - helps to maintain a
balance between the osmotic pressure of the
intracellular fluid and that of the interstitial fluid.
• Osmosis - Plasma membrane is semi- permeable.
The to and fro movement of water molecules
through the plasma membrane occurs due to the
differences in the concentration of the solutes on
19. • Endocytosis:
Infoldings of plasma
membrane help in
the intake of solid