CellBiologyLec3Spring2020DrMIKotbEl Sayed
CellBiologyLec3Spring2020DrMIKotbEl Sayed
CellBiologyLec3Spring2020DrMIKotbEl Sayed
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Objectives CELL MEMB.
Remove cell wall under appropriate conditions and cell lives. However
Deductions:
Because of their molecular structure,
phospholipids can form membranes.
3-Observation:
Phospholipid content of membranes
isolated from red blood cells is just
enough to cover the cells with two layers.
Membranes isolated from red blood cells
contain proteins as well as lipids
Deductions:
Cell membranes are phospholipid
bilayer, two molecules thick.
There is protein in biological
membranes.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Components of Cell Membrane CELL MEMB.
The major components of all cellular membranes are lipids and proteins.
Lower concentration of carbohydrates are present.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Danielli and Davson Model CELL MEMB.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Problems with the Danielli and Davson CELL MEMB.
B. Peripheral (extrinsic).
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
The Fluid Mosaic Model CELL MEMB.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Lipids CELL MEMB.
They are the most abundant type of macromolecule present (40% and
80%).
Provide both the basic structure and the framework of the membrane and
regulate its function.
Three types of lipids are found in cell membranes: phospholipids,
cholesterol, and glycolipids.
1. Phospholipids:
The most abundant of the membrane lipids.
They are polar, ionic compounds that are amphipathic in nature. That is,
each has a hydrophilic head, which is the phosphate group plus whatever
alcohol is attached to it (for example, serine, ethanolamine, and choline)
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and a long, hydrophobic tail containing fatty acids.
Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Lipids CELL MEMB.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Lipids CELL MEMB.
2. Cholesterol
An amphipathic molecule, cholesterol contains a
polar hydroxyl group as well as a hydrophobic steroid
ring and attached hydrocarbon.
It is dispersed throughout cell membranes,
intercalating between phospholipids. Its polar
hydroxyl group is near the polar head groups of the
phospholipids while the steroid ring and hydrocarbon
tails of cholesterol are oriented parallel to those of
the phospholipids.
It fits into the spaces created by the kinks of the
unsaturated fatty acid tails, decreasing the ability of
the fatty acids to undergo motion and therefore
causing stiffening and strengthening of the
14membrane.
Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Lipids CELL MEMB.
3. Glycolipids:
Lipids with attached carbohydrate (sugars), glycolipids are found in
cell membranes in lower concentration than phospholipids and
cholesterol.
The carbohydrate portion is always oriented toward the outside of the
cell, projecting into the environment.
Glycolipids help to form the carbohydrate coat observed on cells and
are involved in cell-to-cell interactions.
They are a source of blood group antigens and can act as receptors for
toxins including those from cholera and tetanus.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Proteins CELL MEMB.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Association Proteins CELL MEMB.
1- Integral proteins:
They are inserted into membrane, so their hydrophobic regions are
surrounded by hydrocarbon portions of phospholipids.They may be:
Unilateral, reaching only part way across the membrane.
Transmembrane, with hydrophobic midsections between hydrophilic
ends exposed on both sides.
These proteins are oriented with their hydrophilic portions in contact
with the aqueous exterior environment and with the cytosol and their
hydrophobic portions in contact with the fatty acid tails of the
phospholipids.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Proteins CELL MEMB.
1- Integral proteins:
Integral proteins are difficult to remove from membrane.
2- Peripheral proteins:
They are not embedded but attached to the membrane's surface.
3. Lipid-anchored proteins:
They are attached covalently to a portion of a lipid without entering the
core portion of the bilayer of the membrane.
Both transmembrane and lipid-anchored proteins are integral membrane
proteins since they can only be removed from a membrane by disrupting
the entire membrane structure.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Proteins Functions CELL MEMB.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Functions of Membrane Proteins CELL MEMB.
1-Transport:
(a) A protein that spans the membrane may provide a
hydrophilic channel across the membrane that is selective for
a solute.
(b) Other transport proteins shuttle a substance from one
side to the other by changing shape. Some of these proteins
hydrolyze ATP as an energy source to actively pump
substances across the membrane.
2-Enzymatic activity:
A peripheral protein built into the membrane may be an
enzyme with its active site exposed to substances in the
adjacent solution. In some cases, several enzymes in a
membrane are organized as a team that carries out sequential
steps of a metabolic pathway.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Functions of Membrane Proteins CELL MEMB.
3-Signal transduction:
A membrane Lipid-anchored protein may have a binding
site with a specific shape that fits the shape of a chemical
messenger, such as a hormone. The external messenger
(signal) may cause a conformational change in the protein
(receptor) that relays the message to the inside of the
cell.
They include the G proteins, which are named for their
ability to bind to guanosine triphosphate (GTP) and
participate in cell signaling in response to certain
hormones.
4- Cell-cell recognition:
Some glycoproteins serve as identification tags that are
5- Intercellular joining:
Membrane proteins of adjacent cells may hook
together in various kinds of junctions, such as gap
junctions or tight junctions.
6- Attachment to the cytoskeleton and
extracellular Matrix (ECM):
Microfilaments or other elements of the cytoskeleton
may be bonded to peripheral membrane proteins, a
function that helps maintain cell shape and stabilizes
the location of certain membrane proteins.
Proteins that adhere to the ECM can coordinate
extracellular and intracellular changes.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Carbohydrates & Cell-Cell Recognition CELL MEMB.
basis for sorting of an animal embryo's cells into tissues and organs and the
rejection of foreign cells by the immune system.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membrane Carbohydrates & Cell-Cell Recognition CELL MEMB.
Cell-Cell Recognition:
The way cells recognize other cells is probably by keying on cell markers
found on the external surface of the cell membrane. Because of their
diversity and location, membrane carbohydrates are good candidates.
The blood grouping A, B, AB and O are based on oligosaccharides found
on the RBC’s membrane.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
The Membrane Fluidity CELL MEMB.
Lipid Movements:
Temperature and lipid composition determine fluidity of the membrane.
-At low temperature, membrane is less fluid and because the phospholipids
are more closely packed.
• Membranes rich in unsaturated fatty
acids are;
more fluid that those dominated by
saturated.
fatty acids because the kinks in the
unsaturated.
fatty acid tails prevent tight
packing.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
The Membrane Fluidity CELL MEMB.
Lipid Movements:
Steroid cholesterol which is wedged
between phospholipids also effects
membrane fluidity.
At warm temperature, it makes membrane
less fluid by restraining the movement of
phospholipid.
At low temperature, the membrane
remains fluid because cholesterol hinders
the close packing of the phospholipids.
Membrane proteins drift more slowly than
lipids
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membranes are Asymmetrical CELL MEMB.
Cholesterol can readily flip-flop or move from one leaflet to the other and is
distributed on both sides of the membrane bilayer.
The different types of phospholipids are distributed asymmetrically in the 2
phospholipid layers: PC is usually in the outer layer and PS, PI and PE are in
the inner bilayer.
The asymmetry is maintained by a "lipid pump" which requires ATP
energy.
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membrane proteins attach to the cytoskeleton.
Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
Membranes are Asymmetrical CELL MEMB.
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
New membrane synthesis CELL MEMB.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowTOC&rid=mcb.TOC
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Dr. Mohamed I. Kotb – Associate Professor of Pharmaceutical Biochemistry and Molecular Biology
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