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(Biomembranes, e
...
Plasmalemma)
Cell membranes or biomembranes are
quasifluid film like partitions of about
75 A° thickness
...
Plasmalemma or plasma
membrane lies on the outside of
protoplast and separates it from
external environment
...
Most of the
eucaryotic cell organells have their own
membrane coverings
...
Chemically, it has
proteins (60-70%), lipids (20- 40%) and
carbohydrates (1-5%)
...
Lipids
are generally phospholipids with polar
hydrophilic heads and nonpolar
hydrophilic tails (amphipathic or
amphiatic)
...
Deniell & Davson Model (1935):
States that a double phospholipid layer
is surrounded on either side by a layer
of hydrated globular proteins or P-L- LP
...
Robertson Model (1959):
States that a lipid bilayer is surrounded
on either side by extended or P-protein
with a difference in their type for outer
and inner side
...
Both the above given models are
lamellar models
...
Cell
membranes are quasifluid with viscous
lipid bilayer having proteins at places
(mosaic) both on the surface and
inside
...
External
proteins are called EXTRINSIC or
PERIPHERAL (30% of total) while
internal proteins are named INTRINSIC
or INTEGRAL (70% of total)
...
Proteins extending from
outside to inside are called 'tunnel
proteins' or 'transmembrane proteins'
...
At places
the external proteins and lipid
molecules bear oligosaccharides
forming glycoproteins and glycolipids
for recognition; attachment and
antigenic properties
...
Lipid nature of the cell membrane can
be destroyed with benzene
...
Fluidity of cell
membranes can be tested by mixing
two types of animal cells treated with
green and red fluorescent dyes in the
presence of polyethylene glycol
...
The cells kept at 0°C do
not show further mixing while the lot
kept at 37°C exhibits complete mixing
...
The
function of microvilli is to increase its
absorptive surface
...
The plasmodesmata were first of
all observed by Tangel (1879) and
finally by Strasburger (1882)
...
It was first
reported by Yamada (1955)
...
(iv) Desmosomes:
In certain cells, the plasma membrane
of adjacent cells become thicker in
certain regionsfand from these
thickened areas arises many fine
filaments known as 'tonofibrils'
...
The intercellular space between
desmosomes contains a coating
material which provides cellular
adhesion to the cell
...
It was given the present name
by Plowe (1931)
...
Antigenic, recognition, attaching and
receptor sites occur over it
...
6-0
...
1 nm diameter)
...
They are called mesosomes
in bacteria (Fitz
...
In animals the plasmalemmae of
adjacent cells may show tight
junctions, desmosomes, gap junction,
interdigitations or intercellular bridges
...
Adjacent
membranes are thickened with disc
shaped adhesive material in between
and tonofibrils radiating out from the
adhesive regions, e
...
, epithelia
subjected to disruption
...
septate
desmosomes possesses parallel septa
in between the two membrances
...
Plasmalemmae of two
cells fused to form impermeable or
occluding junction's e
...
epithelial cells
of capillaries and brain cells
...
Both plasmalemma and
adjacent cytoplasm are thickened
...
They have
intercellular gaps and allow movement
of substances in betweeen two cells
...
Very
common
...
Cytoplasm:
(Strasburger, 1882)
...
Cytoplasm is differentiated into
cytoplasmic matrix, cell organelles and
cell inclusions
...
Cytoplasmic
matrix is often differentiated into outer
gel part called 'ectoplast' (plasmagel)
and inner sol part called
'endoplast' (plasmasol)
...
In rotation type of cyclosis,
cytoplasmic matrix continuously flows
in one direction, (e
...
young cells of
Hydrilla leaf)
...
g
...