Biochemical aspects
Learning objectives
At the end of lecture student should be able to
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Describe the structure of cell membrane
Explain molecular basis of cell membrane
Name different types of membrane protein
Correlate functions with structure of membrane
Membranes (highly fluid & Dynamic structure)
Plasma membranes form closed compartments around
cellular protoplasm to define cell boundaries.
• It shows selective permeability
– acts as a barrier, maintaining differences in composition
between the inside and outside of the cell.
– Done by specific proteins named transporters and ion
channels.
• The plasma membrane exchanges material with the
extracellular environment by
– Exocytosis
– endocytosis, and
– gap junctions
Membranes (highly fluid & Dynamic structure)
• plays key roles in cell–cell interactions and in
transmembrane signaling.
• Membranes also form specialized compartments within
the cell. (organelles), eg,
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mitochondria,
ER,
sarcoplasmic reticulum,
Golgi complexes,
secretory granules,
lysosomes, and
nuclear membrane.
• Membranes localize enzymes, function as integral elements
Membranes (highly fluid & Dynamic structure)
Changes in membrane structure affect water balance
and ion flux and therefore every process within the cell.
• Specific deficiencies or alterations of certain
membrane
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Familial Hypercholesterolemia
Cystic Fibrosis
Wilson’s disease
Hereditary Spheriocytosis
Metastasis of Cancer cells
In short, normal cellular function depends on
normal membranes.
Membrane structure is
visible using an electron
microscope.
Transmission electron
microscopes (TEM) can
show the 2 layers of a
membrane.
Freeze-fracturing
techniques separate the
layers and reveal
membrane proteins.
Membrane structure
Cellular membranes have 4 components:
• 1. Phospholipid bilayer
• 2. Transmembrane proteins
• 3. Interior protein network
• 4. Cell surface markers
MEMBRANE STRUCTURE
• The fluid mosaic model of membrane structure
contends that membranes consist of:
• phospholipids arranged in a bilayer
• globular proteins inserted in the lipid bilayer
Phospholipid structure consists of
glycerol – a 3-carbon
polyalcohol acting as a
backbone for the
phospholipids
2 fatty acids attached to
the glycerol
phosphate group
attached to the glycerol
PHOSPHOLIPIDS
The fatty acids are nonpolar chains of carbon and
hydrogen.
– Their nonpolar nature makes them hydrophobic
(“water-fearing”).
– The phosphate group is polar and hydrophilic
(“water-loving”).
PHOSPHOLIPIDS
The partially hydrophilic, partially hydrophobic
phospholipid spontaneously forms a bilayer:
-fatty acids are on the inside
-phosphate groups are on both surfaces of the
bilayer
Phospholipids
Phospholipids bilayers are fluid.
• -hydrogen bonding of water holds the 2 layers
together
• -individual phospholipids and unanchored
proteins can move through the membrane
• -saturated fatty acids make the membrane less
fluid than unsaturated fatty acids
• -warm temperatures make the membrane more
fluid than cold temperatures
MEMBRANE PROTEIN
Membrane proteins have various functions:
1. Transporters
2. Enzymes
3. Cell surface receptors
4. Cell surface identity markers
5. Cell-to-cell adhesion proteins
6. Attachments to the cytoskeleton
(1) a single a helix,
(2) as multiple a helices, or
(3) as a rolled-up b sheet (a barrel). Some of these "single-pass" and "multipass" proteins have a
covalently attached fatty acid chain inserted in the cytosolic lipid monolayer. Other membrane proteins
are exposed at only one side of the membrane.
(4) Some of these are anchored to the cytosolic surface by an amphipathic a helix that partitions into the
cytosolic monolayer of the lipid bilayer through the hydrophobic face of the helix.
(5) Others are attached to the bilayer solely by a covalently attached lipid chain either a fatty acid chain or
a prenyl group in the cytosolic monolayer or,
(6) via an oligosaccharide linker, to phosphatidylinositol in the non cytosolic monolayer.
(7, 8) Finally, many proteins are attached to the membrane only by non-covalent interactions with other
membrane proteins.
MEMBRANE PROTEIN
Peripheral membrane proteins
-Anchored to a phospholipids in one layer of the
membrane
-Possess nonpolar regions that are inserted in
the lipid bilayer
-Are free to move throughout one layer of the
bilayer
MEMBRANE PROTEIN
Integral membrane proteins
-span the lipid bilayer (transmembrane proteins)
-nonpolar regions of the protein are embedded
in the interior of the bilayer
-polar regions of the protein protrude from both
sides of the bilayer
MEMBRANE PROTEIN
Integral proteins possess at least one
transmembrane domain
-Region of the protein containing hydrophobic
amino acids
-Spans the lipid bilayer
MEMBRANE PROTEINS
• Extensive nonpolar regions within a
transmembrane protein can create a pore
through the membrane.
• - sheets in the protein secondary structure
form a cylinder called a -barrel
• - -barrel interior is polar and allows water and
small polar molecules to pass through the
membrane
Cells are sugar coated
• Sugar moieties of glycolipid protruding
outward in plasma membrane.
• No sugar on inside
Permeability coefficient of membrane
• Water, urea, glycerol, indole, tryptophan, glucose,
O2,CO2 & N can easily pass
• Na, K Cl lag behind
Lipid raft, caveolae, & tight junctions
PASSIVE TRANSPORT
ACTIVE TRANSPORT