J a v a d J a m s h i d i

F a s a 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

Biomembrane Structure & Function

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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

The Biomembrane

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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

Image From: Lodish, Molecular Cell Biology 7e. 2013

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The Faces of Cellular Membranes

Image From: Lodish, Molecular Cell Biology 7e. 2013

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Differ in their chemical structures, abundance, and functions in the membrane

Phosphoglycerides

Sphingolipids

Sterols

Three Classes of Biomembrane Lipids

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Phosphoglycerides

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Phosphoglyceride Head Groups

glycerol 3-phosphate

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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

Sphingolipids

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The major sterols in animals (cholesterol), fungi (ergosterol), and plants (stigmasterol)

Like other membrane lipids, sterols are amphipathic.

Sterols

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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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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

Lipid Composition in the Exoplasmic and Cytosolic leaflets

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lipid bilayer of human red blood cells

Yellow= PhosphatidylEthanolamine Green= PhosphatidylSerine

Red= PhosphatidylCholine Brown= Sphingomyelin

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Because cholesterol and sphingomyelin are found in more ordered, less fluid bilayers, they can form microdomains, termed lipid rafts

Lipid Rafts

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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.

Membrane Proteins

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Membrane proteins can be classified into three categories on the basis of their position with respect to the membrane

Integral

Lipid-anchored

Peripheral

Proteins Interaction with Membranes

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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.

Integral Membrane Proteins

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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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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

Lipid-anchored Membrane Proteins

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Lipid-anchored Membrane Proteins

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Lipid-anchored Membrane Proteins

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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

Peripheral Membrane Proteins

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Peripheral Membrane Proteins

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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.

Asymmetrically Orientation of Proteins

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ABO Blood Groups

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Relative Permeability of a Pure Phospholipid Bilayer

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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

Membrane Transport 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