Biomembrane Structure &
Function
J a v a d
F a s a
J a m s h i d i
U n i v e r s i t y
o f
M e d i c a l
S c i e n c e s
The Biomembrane
The plasma membrane defines the cell and separates the inside
from the outside.
These biomembranes all have the same basic architecture-a
phospholipid bilayer in which proteins are embedded
They can bend and flex in three dimensions while still maintaining
their integrity
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Fluid Mosaic Model
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The Bilayer Structure of Biomembranes
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The Faces of Cellular Membranes
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Image From: Lodish, Molecular Cell Biology 7e. 2013
The Faces of Cellular Membranes
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Image From: Lodish, Molecular Cell Biology 7e. 2013
Three Classes of Biomembrane Lipids
Differ in their chemical structures, abundance, and functions in the
membrane
Phosphoglycerides
Sphingolipids
Sterols
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Phosphoglycerides
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Phosphoglyceride Head Groups
glycerol 3-phosphate
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Sphingolipids
All are derived from sphingosine, an amino alcohol with a long
hydrocarbon chain, and contain a long-chain fatty acid attached in amide
linkage to the sphingosine amino group
In sphingomyelin, the most abundant sphingolipid, phosphocholine is
attached to the terminal hydroxyl group of sphingosine
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Sterols
The major sterols in animals (cholesterol), fungi (ergosterol), and plants
(stigmasterol)
Like other membrane lipids, sterols are amphipathic.
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Mobility of Lipids in Biomembranes
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Phase Transition
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Image From: Lodish, Molecular Cell Biology 7e. 2013
Lipid Composition is Different Between Membranes
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Lipid Composition is Different Between Membranes
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Lipid Composition in the Exoplasmic and Cytosolic leaflets
A characteristic of all biomembranes is an asymmetry in lipid
composition across the bilayer
Unlike particular phospholipids, cholesterol is relatively evenly
distributed in both leaflets of cellular membranes
How the asymmetric distribution of phospholipids in membrane
leaflets arises is still unclear.
When cells die, lipid asymmetry is no longer maintained
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lipid bilayer of human red blood cells
Yellow= PhosphatidylEthanolamine
Red= PhosphatidylCholine
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Green= PhosphatidylSerine
Brown= Sphingomyelin
Lipid Rafts
Because cholesterol and sphingomyelin are found in more ordered, less
fluid bilayers, they can form microdomains, termed lipid rafts
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Membrane Proteins
Membrane proteins are defined by their location within or
at the surface of a phospholipid bilayer
Proteins associated with a particular membrane are
responsible for its distinctive activities.
The kinds and amounts of proteins associated with
biomembranes vary depending on cell type and subcellular
location.
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Proteins Interaction with Membranes
Membrane proteins can be classified into three categories on the
basis of their position with respect to the membrane
Integral
Lipid-anchored
Peripheral
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Integral Membrane Proteins
Transmembrane proteins, span a phospholipid bilayer and
comprise three segments.
The cytosolic and exoplasmic domains have hydrophilic exterior
surfaces
The membrane-spanning segments usually contain many
hydrophobic amino acids
The membrane-spanning domains consist of one or more α helices
or of multiple β strands.
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Integral Membrane Proteins
(1) a single α helix
(2) multiple α helices,
(3) as a rolled-up β sheets(β barrel).
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Lipid-anchored Membrane Proteins
They are bound covalently to one or more lipid molecules.
The hydrophobic segment of the attached lipid is embedded in one
leaflet of the membrane and anchors the protein to the membrane.
The polypeptide chain itself dose not enter the phospholipid bilayer.
Examples include bacterial lipoproteins, G proteins and certain
kinases
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Lipid-anchored Membrane Proteins
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Lipid-anchored Membrane Proteins
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Peripheral Membrane Proteins
Do not directly contact the hydrophobic core of the phospholipid
bilayer
Instead they are bound to the membrane either indirectly by
interactions with integral or lipid-anchored membrane proteins or
directly by interactions with lipid head groups
Peripheral proteins can be bound to either the cytosolic or the
exoplasmic face of the plasma membrane
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Peripheral Membrane Proteins
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Asymmetrically Orientation of Proteins
Every type of transmembrane protein has a specific orientation,
known as its topology, with respect to the membrane faces
Transmembrane glycoproteins are always oriented so that all
carbohydrate chains are in the exoplasmic
Glycoproteins and glycolipids can interact with components of the
extracellular matrix as well as lectins (proteins that bind specific
sugars), growth factors, and antibodies.
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ABO Blood Groups
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Relative Permeability of a Pure Phospholipid Bilayer
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Membrane Transport Proteins
Movement of virtually all small molecules and ions across cell
membranes is mediated by membrane transport proteins
Each protein transports a particular class of molecule (such as ions,
sugars, or amino acids) and often only certain molecular species of
the class.
Membrane transport proteins usually found to be multipass
transmembrane proteins
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Two Main Classes of Membrane Transport Proteins
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Channels
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Transporters
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Types of Transport
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Types of Transport
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