Javad Jamshidi

Fasa University of Medical Sciences, October 2015

The

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

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

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Image From: Lodish, Molecular Cell Biology 7e. 20135

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Image From: Lodish, Molecular Cell Biology 7e. 20136

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

Phosphoglycerides

Sphingolipids

Sterols

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glycerol 3-phosphate

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

Like other membrane lipids, sterols are amphipathic.

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Image From: Lodish, Molecular Cell Biology 7e. 201313

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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When cells die, lipid asymmetry is no longer maintained

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Yellow= PhosphatidylEthanolamine Green= PhosphatidylSerine

Red= PhosphatidylCholine Brown= Sphingomyelin17

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

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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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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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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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(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 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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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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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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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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Table From: Lodish, Molecular Cell Biology 7e. 201335

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Structure of Biomembrane

Lipid Components of Biomembrane

Different types of membrane Proteins

Various types of transport in Biomembranes

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