Biological Membranes

Topics: Protein, Cell membrane, Lipid bilayer, Molecule / Pages: 4 (939 words) / Published: Oct 14th, 2012
Biological Membranes

Lipid Membranes
• Receptors, detecting the signals from outside: Light Odorant Taste Chemicals A Hormones Neurotransmitters Drugs • Channels, gates and pumps • Electric/chemical potential Neurophysiology Energy • Energy transduction: Photosynthesis Oxidative phosphorylation

• • • • • •

Structure Function Composition Physicochemical properties Self-assembly Molecular models

highly selective permeability barrier

Internal membranes for organelles

Bilayer Permeability
• Low permeability to charged and polar substances • Water is an exception: small size, lack of charge, and its high concentration • Shedding solvation shells for ions is very unlikely
• • • • • • • • • • •

Common Features of Biological Membranes
Sheet-like structure TWO-molecule thick (60-100Å) Lipids, Proteins, and carbohydrates Lipids form the barrier. Proteins mediate distinct functions. Non-covalent assemblies (self-assembly, protein-lipid interaction) Asymmetric (always) Fluid structures: 2-dimensional solution of oriented lipids and proteins Electrically polarized (inside negative ~-60mV) Spontaneously forming in water Protein/lipid ratio = 1/4 – 4/1 Carbohydrate moieties are always outside the cell

Protein/Lipid ratio
• Pure lipid: insulation (neuronal cells) • Other membranes: on average 50% • Energy transduction membranes (75%) Internal membranes of mitocondria and chloroplast Purple membrane of halobacteria • Different functions = different protein composition

Protein / Lipid Composition

Light harvesting complex of purple bacteria

Protein / Lipid Composition

General features of Lipids
• Small molecules • Amphipathic (amphiphilic) Hydrophobic/hydrophilic moieties • Spontaneously form vesicles, micelles, and bilayers in aqueous solution

The purple membrane of halobacteria

Micelle / Bilayer
• • • • • • Fatty acids (one tail) Phospholipids (two tails) Micelle max 20 nm Bilayer up to millimeters Self-assembly process Hydrophobic

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