Transcript Biomembrane Structure & Function

The
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 ,
O c t o b e r
2 0 1 5
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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
Image From: Lodish, Molecular Cell Biology 7e. 2013
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The Faces of Cellular Membranes
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Image From: Lodish, Molecular Cell Biology 7e. 2013
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The Faces of Cellular Membranes
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Image From: Lodish, Molecular Cell Biology 7e. 2013
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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
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
Image From: Lodish, Molecular Cell Biology 7e. 2013
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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
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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
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.
Membrane proteins are defined by their location within or at the
surface of a phospholipid bilayer
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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 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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Peripheral Membrane Proteins
Do not directly contact the hydrophobic core of the phospholipid bilayer
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
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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
Table From: Lodish, Molecular Cell Biology 7e. 2013
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What did you Learn?
Structure of Biomembrane
Lipid Components of Biomembrane
Different types of membrane Proteins
Various types of transport in Biomembranes
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