Biomembrane and Cell signalling
BCH 452 (III)
Dr. Samina Hyder Haq
Assistant professor
Dept of biochemistry
Collage of Science
King Saud university
Fluid Mosaic Model The Current
model of Biomembrane.
• Proposed in 1972 by SJ.Singer & G. Nicholson.
• Membrane is a mosaic of protein molecule bobbing
in a fluid bilayer of phospholipids.
• Membrane is held together by hydrophobic
interactions
Proteins interacts with the membrane
in three ways,
1. Integral membrane
Proteins.
2.Lipid anchored membrane
proteins
3. Peripheral
Membrane proteins.
Integral proteins or trans-membrane
proteins
Made up of 3 segments.
outer and inner The cytosolic and exoplasmic
domains have hydrophilic exterior surfaces that
interact with the aqueous solutions on the
cytosolic and exoplasmic faces of the membrane.
They are like water soluble proteins in their amino
acid composition and structure.
middle the 3-nm-thick membrane-spanning
domain contains many hydrophobic amino acids
whose side chains protrude outward and
interacts with the phospholipids layer.
Lipid anchored membrane proteins.
Bound covalently to one or more lipid molecules.
The hydrophobic carbon chain of the attached lipid
is embedded in one leaflet of the membrane and
anchors the protein to the membrane. The
polypeptide chain itself does not enter the
phospholipid bilayer.
Peripheral membrane Proteins.
• They are usually bound to the membrane indirectly by
interactions with integral membrane proteins or directly
by interactions with lipid head groups. Peripheral
proteins are localized to either the cytosolic or the
exoplasmic face of the plasma membrane.
• In addition to these proteins, which are closely
associated with the bilayer, cytoskeletal filaments are
more loosely associated with the cytosolic face, usually
through one or more peripheral proteins. Such
associations with the cytoskeleton provide support for
various cellular membranes.
• They also play a role in the two-way communication
between the cell interior and the cell exterior.
Fluid Nature of membrane.
• Phospholipids can drift laterally within the plane
of membrane bilayer.(LATERAL movement)
• Phospholipids can also move across the
bilayer.(Flip Flop movement)
Movement of phospholipids
Migration of Protein in bilayer
Cholesterol acts as a “temperature
buffer’’.
• The steroid cholesterol is
wedged between phospholipids
molecules in the plasma
membrane of animal cells.
• At warm temperatures (such as
37°C), cholesterol restrains the
movement of phospholipids and
reduces fluidity.
• At cool temperatures, it
maintains fluidity by preventing tight packing.
Fluidity of the membrane
Fluidity of the membrane depends on
temperature and the type of
phospholipids.
At low temperature the membrane
has gel like consistency and when
heated it has fluid like consistency.
Membrane rich in unsaturated fatty
acids are more fluid as compared to
the membrane containing saturated
fatty acids. Because kink in the
fatty acids chain prevent tight
packing.
Membrane are Amphipathic.
• A typical biomembrane is assembled
from phosphoglycerides,
sphingolipids, and steroids. All three
classes of lipids are amphipathic
molecules having a polar
(hydrophilic) head group and
hydrophobic tail.
• The hydrophobic effect and van der
Waals interactions, cause the tail
groups to self-associate into a bilayer
with the polar head groups oriented
toward water.