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Lecture 5
Cell Biology ٢٢٢
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The plasma membrane
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Each cell must interact with its environment
in a number of ways.
Each cell needs to obtain oxygen and other
nutrients (carbohydrates, amino acids, lipid
molecules, mineral ions, etc.) from the
environment, maintain water balance with its
surroundings, and remove waste materials
from the cell.
The plasma membrane separates a cell
from its environment.
Cell Biology ٢٢٢
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The plasma membrane
Cell Biology ٢٢٢
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The plasma membrane functions
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The phospholipid bilayer provides the cell with a structure that
separates the outside from the inside of the cell.
The integrity of the membrane is necessary for life functions.
Because of the nature of the phospholipid, many molecules
cannot move across the membrane without help.
Maintains the cell's environment by regulating materials that
enter or leave the cell.
The plasma membrane is differentially, or selectively,
permeable. Some materials enter and leave easily through
the membrane, some with the assistance of membrane
molecules, and some prohibited. Provides mechanisms for
cell-to-cell communication.
Provides mechanisms for a cell to recognize "self" versus "nonself" (foreign materials), important to the immune system,
development and defense of the organism through genetically
unique cell recognition markers
Cell Biology ٢٢٢
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The Fluid Mosaic Membrane Structure
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The structure and function of a membrane depends of its
molecular composition.
The foundation of the membrane is its phospholipid bilayer,
with a number of associated proteins.
Membranes also contain carbohydrates (glycoproteins and
proteoglycans) and glycolipids.
The resultant membrane structure (proteins scattered
throughout the fluid phospholipid layers) resembles a mosaic,
hence the name "fluid mosaic membrane".
Membrane molecules are manufactured in the endoplasmic
reticulum and distributed by Golgi vesicles.
The orientation of membranes is determined at the
manufacturing site. Molecules on the inside of the ER and Golgi
vesicles become exterior membrane molecules.
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Membrane Phospholipids
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Phospholipids have both hydrophilic (polar) and hydrophobic (non
polar) regions (in other words, they are amphipathic).
The fatty acid "tails" of the two phospholipid layers are oriented
towards each other so that the hydrophilic "heads", which contain
the "charged" phosphate portion, face out to the environment as
well as into the cytoplasm of the cell's interior, where they can
form hydrogen bonds with surrounding water molecules.
The phospholipid molecules of a membrane provide for its
physical integrity.
Exterior
Cytoplasm
Phospholipid Bilayer
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Phospholipid Movements
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A membrane is held together, for the most part, by hydrophobic
interactions within the phospholipid bilayer.
Because individual phospholipid molecules are not bonded to each
other, a membrane is flexible, or "fluid", particularly to lateral
movement of the fatty acids.
Phospholipid molecules easily move along the plane of the
membrane; reversing exterior – interior position (or flip-flopping) is
less common.
Unsaturated/Saturated
Cell Biology ٢٢٢
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Phospholipid Movements
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Cholesterol, found in membranes of many animal cells,
reduces fluid movement of the phospholipids, helping to
maintain membrane integrity.
The saturation of fatty acids affects membrane fluidity –
the more saturated, the less movement.
Membranes will also solidify as temperature decreases,
reducing function.
The saturation of fatty acids will affect the temperature
at which the membrane "solidifies" (just as it does with
fats and oils).
Cell Biology ٢٢٢
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Membrane Proteins
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Interspersed throughout a membrane's phospholipid
layer are a number of amphipathic proteins.
The hydrophobic regions of the proteins are within the
fatty acid regions of the phospholipids and hydrophilic
regions of the proteins are at the interior and exterior
aqueous interfaces of the membrane.
This orientation is important to how the membrane
proteins function.
The membrane is also associated with a network of
supporting cytoskeletal filaments, some of which help
shape the cell and some help anchor proteins within
the membrane.
Cell Biology ٢٢٢
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Protein Mobility
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Many proteins within the membrane are mobile; studies of fused
mouse and human cells show that proteins from the two cells are
intermixed within an hour of fusion
Membrane proteins are divided into two categories, integral and
peripheral,depending on their location
Cell Biology ٢٢٢
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Membrane proteins
Integral (Transmembrane) Proteins
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Proteins that go through the membrane are called integral or
transmembrane proteins.
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They have hydrophobic (non-polar amino acids with alpha helix
coiling) regions within the interior of the membrane and
hydrophilic regions at either membrane surface
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Peripheral Proteins
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Are attached to the surface of the membrane, often to the
exterior hydrophilic regions of the transmembrane proteins.
On the interior surface, peripheral proteins typically are held in
position by the cytoskeleton.
On the exterior, proteins may attach to the extracellular
matrix.
Peripheral proteins help give animal cell membranes strength.
The different proteins contribute to the "sidedness" of
membranes so that the interior and exterior sides of
membranes have different properties that affect membrane
function.
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Anchoring proteins
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Other proteins have non-polar α helix regions that fix the protein
into specific regions of the phospholipid bilayers. Such proteins are
called anchoring proteins.
The protein receptors at neuromuscular junctions on muscle cells
are anchored proteins.
Anchor proteins can attach to the fibrous network of the
cytoskeleton to give shape and strength to some cells.
Some membrane lipid regions, called lipid rafts, are also
specialized to help anchor proteins within a specific region of a
membrane.
Cell Biology ٢٢٢
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Membrane Protein Functions
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1-Transport Proteins
Transport Proteins are transmembrane proteins that serve as carriers for
specific substances that need to pass through the membrane by providing a
hydrophilic channel or pore.
Transport proteins have binding sites that attract specific molecules. Most
of our ions, amino acids, sugars and other small nutrient molecules are
moved through transport proteins.
When a molecule binds to the carrier protein, the protein shape changes
moving the substance through the membrane. This process may require
energy (ATP), and the ATP complex is then a part of the transport protein.
Cell Biology ٢٢٢
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Membrane Protein Functions
2- Enzymatic Proteins
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Many enzymes are embedded in membranes, which attract
reacting molecules to the membrane surface.
The active site of the enzyme will be oriented in the membrane
for the substrate to bind.
Enzymes needed for metabolic pathways can be aligned adjacent
to each other to act like an assembly line for the reactions,
minimizing the need for intermediates to diffuse through the
cytoplasm of the cell.
Cell Biology ٢٢٢
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Membrane Protein Functions
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3- Signal Transduction (Receptor) Proteins
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Signal transduction proteins have attachment sites for chemical
messengers, such as hormones.
The signal molecule, when it attaches to the receptor promotes a
conformational change that relays the message into the cell to
trigger some cell activity.
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Cell Biology ٢٢٢
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Membrane Protein Functions
4-Attachment Proteins
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Attachment proteins attach to the cytoskeleton or extracellular matrix to help
maintain cell shape (particularly for animal cells)
5-Recognition (Identity) Proteins
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Glycoproteins serve as surface receptors for cell recognition and identification. They
are important to the immune system.
6-Cell Adhesion (Intercellular Joining) Proteins
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Special membrane proteins are responsible for the cell junctions (tight junctions,
desmosomes and gap junctions.
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they permit cells to adhere to each other.
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Membrane Carbohydrates
Glycoprotein
complex with
long
Polysacchari
de
‫ﻣﻌﻘﺪ ﺟﻼﻳﻜﻮﺑﺮوﺗﻴﻦ‬
‫ﻣﻊ‬
‫ﺳﻠﺴﻠﺔ ﻃﻮﻳﻠﺔ ﻣﻦ‬
‫اﻟﺴﻜﺎآﺮ اﻟﻤﺘﻌﺪدة‬
Collagen fiber
‫ﻟﻴﻔﺔ آﻮﻻﺟﻴﻦ‬
Intgrine
‫اﻧﺘﻴﺠﺮﻳﻦ‬
Cell Biology ٢٢٢
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Glycoproteins and
glycolipids are also
important to membrane
structure and function.
Glycolipids function as
recognition signals for cellto-cell interactions.
Glycoproteins, with their
oligosaccharides portions,
are critical for a cell to be
recognized by other cells
and by protein molecules,
and for cell-to-cell adhesion
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