CELL MEMBRANES
Dr. Raheela Jabeen
Assistant Professor
Department of Biochemistry & Biotechnology, WUM
The cell, the basic unit of life, is
essentially came into being when
membrane formed.
Membranes function to organize
biological processes by
compartmentalizing these processes
into different organelles .
2
As in eukaryotes, many subcellular
organelles, such as nuclei, mitochondria,
chloroplasts, the endoplasmic reticulum,
and the Golgi apparatus are likewise
membrane bounded.
Membranes are flexible, self sealing and
selectively permeable to polar solutes.
3
All cell membranes share a characteristic trilaminar
appearance. This erythrocyte (picture below) was stained with
osmium tetroxide and viewed with an electronmicroscope.
The plasma membrane appears as a three-layer structure,
5 to 8 nm (50 to 80 Å) thick. The trilaminar image consists of
two electron-dense layers (the osmium, bound to the inner
and outer surfaces of the membrane) separated by a less
dense central region.
Membrane Bilayer
FUNCTIONS OF MEMBRANE
Protective barrier
Regulate transport in & out of cell
(selectively permeable)
Central to both biological energy
conversion and cell to cell
communication
Allow cell recognition
Provide anchoring sites for filaments
of cytoskeleton
4 4
Provide a binding site for
enzymes
Interlocking surfaces bind cells
together (junctions)
Contains the cytoplasm (fluid in
cell)
6
Membrane Structure and Function
Membrane Structure
Lipids and proteins are the main
components of the membranes,
although carbohydrates are also
important as part of glycoproteins
and glycolipids.
7
The term cell membrane
was coined by C. Nageli and C.
Cramer in 1855 and the
term plasmalemma has been given
by J. Q. Plowe in 1931.
Chemical composition (%) of
some purified membranes
The most abundant lipids in most
membranes are phospholipids and vary
with type of Membrane.
Four major classes of lipids are
commonly present in the plasma
membrane and other membranes :
phospholipids (most abundant),
sphingolipids, glycolipids and sterols
(e.g., cholesterol)
1
0
Phospholipids and most of
proteins of membrane are
amphipathic molecules.(both
hydrophilic and hydrophobic)
Cells clearly have a characteristic
mechanism of membrane lipid
they synthesize and target to
particular organelles.
Proteins synthesized by free ribosomes may be inserted
into the plasma membrane following their completion and
release from the ribosomes.
Proteins of plasma membrane synthesized on attached
ribosomes of rough ER are inserted first into the
membrane of RER and then transferred to the Golgi
apparatus, processed there (e.g., glycosylation) and
ultimately are dispatched to the
plasma membrane via the secretory vesicles.
Likewise, the synthesis of phospholipid molecules of the
plasma membrane takes place by the smooth ER (SER).
Like the proteins, newly synthesized lipids are inserted
into SER membranes, then they are passed to Golgi
apparatus for the processing and ultimately are dispatched
to the plasma membrane via small secretory vesicles.
The cytosol also contains a number of phospholipid
transport proteins that function to transfer phospholipid
molecules from one cellular membrane to another (e.g.,
from ER membranes to plasma membranes
Lipid composition of the plasma membrane and organelle
membranes of a rat hepatocyte.
REFERENCES
1. Voet D. and Voet J. G., “Biochemistry”, John Wiley &
Sons, New York.
2. Zubay G., “Biochemistry”, Macmillan Publishing Co.
3. Stryer L., “Biochemistry”, Freeman & Co.
4. Lehninger A. L., “Principles of Biochemistry”, Worth
Publisher, New York
Any Questions??
