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8/8/2019 Lecture 5 Cell Biology
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8/8/2019 Lecture 5 Cell Biology
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Plasma membrane= Cell membrane
SYSTEMS
ORGANS
TISSUES
CELL
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IntroductionWhat dose the cell need to live?
Each cell needs to obtain:
oxygen
other nutrients (carbohydrates, amino acids,
lipid molecules, mineral ions, etc.) from theenvironment
maintain water balance with its surroundings
remove waste materials from the cell
The plasma membrane separates a cell from itsexternal environment
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Functions of plasma membrane
1.The phospholipid bilayer separates the outside from the insideof the cell.
2.Maintains the cell's environment by regulating materials thatenter or leave the cell.
3.The plasma membrane is differentially, or selectively,permeable. Some materials enter and leave easily through themembrane, some with the assistance of membrane molecules,and some prohibited. Provides mechanisms for cell-to-cellcommunication.
4.Provides mechanisms for a cell to recognize "self" versus "non-self" (foreign materials), important to the immune system
8/8/2019 Lecture 5 Cell Biology
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The Fluid Mosaic Membrane Structure
1. The structure and function of a membrane depends on its molecular
composition.
2. The membrane is formed from a phospholipid bilayer, with a
number of associated proteins.
3. Membranes also contain carbohydrates (glycoproteins and
proteoglycans) and glycolipids.
4. The resultant membrane structure (proteins scattered throughout the
fluid phospholipid layers) resembles a mosaic, hence the name "fluid
mosaic membrane".
5. Membrane molecules are manufactured in the endoplasmic reticulumand distributed by Golgi vesicles.
6. 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
1. Phospholipids have bothhydrophilic (polar) andhydrophobic (non polar) regions (in other words, they areamphipathic).
2. The fatty acid "tails" of the two phospholipid layers areoriented towards each other so that the hydrophilic"heads", which contain the "charged" phosphate portion,face out to the environment as well as into the cytoplasmof the cell's interior, where they can form hydrogen bondswith surrounding water molecules.
3. The phospholipid molecules of a membrane provide forits physical integrity.
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Phospholipid Movements 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 (orflip-flopping)
is less common.
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Phospholipid Movements
Factors that a
ffect phosphol
ipid
s movement:
(1) Cholesterol, found in membranes of many animal cells, reducesfluid movement of the phospholipids, helping to maintainmembrane integrity.
(2) The saturation offatty acids affects membrane fluidity the
more saturated, the less movement.(3) Membranes will also solidify as temperature decreases,
reducing function.
The saturation of fatty acids will affect the temperature at whichthe membrane "solidifies" (just as it does with fats and oils).
Unsaturated/Saturated
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Membrane Proteins1. Interspersed throughout a membrane's phospholipid layer are a
number of amphipathic proteins.
2. The hydrophobic regions of the proteins are within the fatty
acid regions of the phospholipids and hydrophilic regions are at
the interior and exterior aqueous interfaces of the membrane.
3. This orientation is important to the membrane proteins function.
4. 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.
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Protein Mobility
Many proteins within the membrane are mobile;studies of fused mouse and human cells show thatproteins from the two cells are intermixed within anhour of fusion
Membrane proteins are divided into two, dependingon their location
Integral
peripheral
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Membrane proteins
Integral (Transmembrane) Proteins1. Proteins that go through the membrane are called
integral or transmembrane proteins.
2. Theyhave hydrophobic (non-polar amino acids
with alpha helix coiling) regions within the interior
of the membrane and hydrophilic regions at either
membrane surface
hydrophilic regions
hydrophilic regions
hydrophobic regions
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Peripheral Proteins
Are attached to the surface of the
membrane, often to the exteriorhydrophilic regions of thetransmembrane proteins.
On the interiorsurface, peripheralproteins typically are held in
position by the cytoskeleton.
On the exterior, proteins may attachto the extracellular matrix.
Peripheral proteins help give
animal cell membranes strength. The different proteins contribute to
the "sidedness" of membranes sothat the interior and exterior sidesof membranes have different
properties that affect its function.
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Anchoring proteins Other proteins have non-polar helix regions that fix the protein
into specific regions of the phospholipid bilayers. Suchproteins 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.
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Membrane Protein Functions
1-Transport Proteins Transport Proteins are transmembrane proteins that serve as carriers
for specific substances that need to pass through the membrane byproviding a hydrophilic channel or pore.
Transport proteins have binding sites that attract specific molecules.M
ost of our ions, amino acids, sugars and other small nutrientmolecules are moved through transport proteins.
When a molecule binds to the carrier protein, the protein shapechanges moving the substance through the membrane. This processmay require energy (ATP), and the ATP complex is then a part of the
transport protein.
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Membrane Protein Functions
2- Enzymatic Proteins
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 forthe 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.
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Membrane Protein Functions
3- Signal Transduction (Receptor) Proteins Signal transduction proteins have attachment sites
forchemical messengers, such as hormones.
The signal molecule, when it attaches to thereceptor promotes a conformational change thattransmits the message into the cell to trigger somecell activity.
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Membrane Protein Functions
4-Attachment Proteins Attachment proteins attach to the cytoskeleton or extracellular
matrix to help maintain cell shape (particularly for animal cells)
5-Recognition (Identity) Proteins
Glycoproteins serve as surface receptors for cell recognition and
identification. They are important to the immune system.
6-Cell Adhesion (Intercellular Joining) Proteins
Special membrane proteins are responsible for the cell junctions(tight junctions, desmosomes and gap junctions.
theypermit cells to adhere to each other.
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Membrane Carbohydrates
Glycoproteins and glycolipids are also important tomembrane structure and function.
Glycolipids function as recognition signals for cell-to-cell
interactions.
Glycoproteins, with their oligosaccharides portions, are critical
for a cell to be recognized by other cells and by protein
molecules, and forcell-to-cell adhesion
Glycoprotein complexwith long Polysaccharide
Collagen fiber
Intgrine
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Function of Cell MembraneGood Luck
Dr Mona
8/8/2019 Lecture 5 Cell Biology
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The cell membrane consists of three classes of amphipathic lipids:
phospholipids, glycolipids, and cholesterols. The amount of each
depends upon th
e type of cell, but in th
e majority of casesphospholipids are the most abundant.[5] In RBC studies, 30% of
the plasma membrane is lipid.
The fatty chains in phospholipids and glycolipids usually contain
an even number of carbon atoms, typically between 16 and 20.
The 16- and 18-carbon fatty acids are the most common. Fattyacids may be saturated or unsaturated, with the configuration of
the double bonds nearly always cis. The length and the degree of
unsaturation of fatty acid chains have a profound effect on
membrane fluidity[6]
as unsaturated lipids create a kink,preventing the fatty acids from packing together as tightly, thus
decreasing the melting temperature (increasing the fluidity) of the
membrane.
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Most membrane proteins must be inserted in some
way into the membrane. For this to occur, an N-
terminus "signal sequence" of amino acids directs
proteins to the endoplasmic reticulum, which insertsthe proteins into a lipid bilayer. Once inserted, the
proteins are then transported to their final
destination in vesicles, where the vesicle fuses with
the target membrane.
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