2. OUTLINES
.What is cell membrane?
Chemical composition
Structure of cell membrane
Function of cell membrane
Models of cell membrane
3. WHAT IS CELL MEMBRANE ?
The cell membrane is a biological membrane
separating the inner part of cell from outside
environment.
It is also called plasma membrane or plasma
lemma .
It appears in thin section with the electron
microscope as a triple layered structure about 7.5
to 10 nm thick.
Term coined by C.Nageli and C.Cramer in 1855
and plasma lemma coined by J.Q.PLOWE in 1931.
4. CELL MEMBRANE
The cell membrane is flexible and allows a
unicellular organism to move.
5. CHEMICAL COMPOSITION
1. LIPIDS : Four classes of lipids are commonly present in
plasma membrane. Phospholipids (most
abundant),spingolipids ,glycolipids and sterols
(e.g. cholesterol). All of them are amphipathic.Cholestrol is
abundant in mammalian cell and is absent in prokaryotic
cells.
2 .PROTEINS :It contains about 50 % proteins. Amount
and type is variable. Myelin cells contain about 23 %
protein internal membrane of chloroplast and and
mitochondria contains 50% protein. Protein of plasma
membrane are of two types ectoproteins and
endoproteins. Plasma membrane contains structural
proteins, transport protein and enzymes. Some of them act
as receptors.
6. CONT…….
(enzymes present in the cell membrane are Acetyl
phosphatase, acid phosphatase, ATPase, RNAase ,
maltase, lactase, phospholipase etc.)
3 CARBOHYDRATE. : They are present only in the
plasma membrane and present exterior or polar end
of phospholipids at the external surface of plasma
membrane.
7. PHOSPHOLIPIDS
Phospholipids (PL) has a polar head and non polar
tails.
The polar head is charged and hydrophobic (loves
water ) facing outward, contains a _PO4 group and
glycerol.
The non polar tails are hydrophobic (hates water)
facing inward contain 2 fatty acid chain that are non
polar.
When PL are placed in water it forms spherical
bilayer.
8.
9. MEMBRANE CHOLESTEROL
The amount of cholesterol
may vary with the type of
Membrane.
plasma membrane have
nearly one cholesterol per
PL molecule.
13. FUNCTION OF PLASMA MEMBRANE
Keeps a cell intact.
Protective barrier
Regulate transport in and out of cell (selectively
permeable)
Small lipids molecules can easily pass , e.g.O2
&CO2.
Water can freely cross the membrane.
14. Ions and large molecules cannot pass without
assistance.
Allow cell recognition.
Provide anchoring sites for filaments of
cytoskeleton
Provide a binding site for enzyme.
Interlocking surface binds cells together (junction)
Contains the cytoplasm.
15. MEMBRANE STRUCTURE
Membranes consist of a PL bilayer combined with a
variety of proteins in a fluid mosaic arrangement.
The surface of cell membrane are hydrophobic
(water loving) and the interior of cell membrane are
hydrophobic (hate water).
16. MODELS OF CELL MEMBRANE
1 DANIELLI AND DAVSON MODEL
It is also called sandwich model
In 1935, Danielli and Davson studied
triglyceride lipids bilayers over a
water surface.
They found that they arranged
themselves with the polar heads
facing outward.
It always formed droplets (oil in water
) and the surface tension was much
higher than that of cells.
17.
18. ROBERTSON MODEL
In 1965, Robertson noted the structure of
membranes seen in the electron microscope.
He saw no spaces for pores in the electron
microscope.
He hypothesized that the railroad track appearance
came the binding of osmium tetroxide to proteins
and polar groups of lipids.
proposed unit membrane hypothesis.
19.
20. FLUID MOSAIC MODEL
According to S . J .Singer and garth Nicolson 1972,
the two biological membranes can be considered
as a two dimensional liquid where all lipids and
proteins molecules diffuse more or less freely.
Singer studied PL bilayers and found that they can
form a flattened surface on water, with no
requirement for protein coat.
It occurs in the form of globular protein.
Widely accepted model.
