Biological Membranes

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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 interaction is the driving force (also in protein folding and in DNA stacking)

Vesicles
• Phospholipids • Sonicating (~50nm) • Evaporation (1 micron) • Experimental tool for studying membrane proteins • Clinical use (drug delivery)

• Extensive; tendency to close on themselves; self-sealing (a hole is unfavorable)

hydrophobic

Phospholipids
hydrophilic

Structure of fatty acids
cis

not conjugated

No. of carbons No. of unsaturated bonds

18:2

hydrophobic

hydrophilic

ω

3

2

1
O

O

22:0 16:0

O

O

18:0

22:6ω3 18:1
cis-∆9-octadecenoate
ω all cis-∆4,∆7,∆10,∆13,∆16,∆19-docosahexaenoate

18:1

Polar head groups
POPC

Diffusion in Membrane
Einstein relation for diffusion:

6 Dt = ri (t ) − ri (0)
ri (t ) − ri (0) = S

2

6 Dt = S 2 4 Dt = S
2

in a 3 dimensional space in 2 dimensions (in-plane diffusion in a membrane) in 1 dimension

2 Dt = S 2

Lipid Diffusion in Membrane
Dlip = 10-8 cm2.s-1 Dwat = 2.5 x 10-5 cm2.s-1

Fluid Mosaic Model of Membrane

D = 1 µm2.s-1 50 Å in ~ 2.5 x 10-5 s ~9 orders of magnitude difference

Once in several hours! (104 s)

Lateral Diffusion Allowed

Flip-flap Forbidden

Ensuring the conservation of membrane asymmetric structure

Importance of Asymmetry
Cytoplasmic side Outside leaflet of disc

Highly asymmetric and inhomogeneous lipid composition of membrane Polyunsaturated lipids

22:6ω3: 47% in membrane PC : PE : PS 45 : 42 : 14 Outside leaflet

Phosphatidylethanolamine Phosphatidylserine 22:6ω3

Extracellular side Intradiscal region
Apart from some passive transport mechanisms, all membrane proteins function in a directed way, and their correct insertion in the cell membrane is essential for their biological function.

Phosphatidylcholine inside leaflet

Fluorescence recovery after photobleaching (FRAP)

Lipid Diffusion in Membrane
• FRAP - fluorescent recovery after photobleaching (Albert’s movie)

Fluid disordered state...
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