cell membrane:
Cell membrane is the outermost boundary of animal cell and is covered by cell wall in a plant cell.
Plasma membrane is about 7nm
thick.
Composition:
Cell membrane or plasma membrane
is composed of
·
Proteins (60-80 %)
·
Lipids (20-40%)
·
Small amount of carbohydrates in the form of
glycolipids or glycoproteins.
It has two
models as proposed
Unit membrane model
· Fluid mosaic model
Unit membrane
model:
This model was
presented by J. David Robertson in 1959
According to this model,
“Cell membrane is composed of lipid bilayer sandwiched
between inner and outer layer of proteins”
Fluid mosaic
model:
This model was
proposed by S.J. Singer and Nicholsen in 1972.
According to
this model,
"Protein layers are not continuous and are not confined to
the surface of membrane but are embedded in lipid layers in a mosaic manner,”
·
This model is the most accepted one.
Role of
different molecules:
·
Phospholipid
Form lipid
bilayer
·
Cholesterol
It helps to
stabilize the lipid bilayer.
·
Channel proteins
Allow a
particular molecule or ion to pass through the plasma membrane freely.
·
Carrier proteins
They selectively
interact with a specific molecule or ion so that it can cross the cell
membrane.
·
Glycoproteins and glycolipids
These are found
on the cell surface and help in recognition.
Transport mechanism
across cell membrane:
Cell membrane is
a differentially permeable or selectively permeable membrane, allowing only the
selective substances to pass through it.
·
Lipid soluble substances pass the cell membrane
easily.
·
Many small gas molecules, water glucose being
neutral can cross easily.
·
Ions being charged particles have some difficulty
in crossing.
Some other
function of cell membrane:
- cell membrane
- cytoplasm
- nucleus
cell membrane:
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