Cell Membrane
Variation of biomembranes in different cells
The faces of cellular membranes Cytosolic face Exoplasmic face
Phospholipid bilayers The phospholipid bilayer, the basic structural unit of all biomembranes, is a two-dimensional lipid sheet with hydrophilic faces and a hydrophobic core, which is impermeable to water-soluble molecules and ions.
Properties: Semi-permeable barrier Stable structure maintained by van der Vaals and hydrophobic interaction.
Lipid Protein Carbohydrate
The primary lipid components of biomembranes are phosphoglycerides, sphingolipids, and steroids. 磷酸甘油脂 glycerol
鞘磷脂 sphingosine
A phospholipid is a lipid that has a positively charged head that is linked by a phosphate group to the fatty acid tails. A glycolipid has a head consisting of an oligosaccharide, linked to a fatty acid tail. A sterol is a compound containing a planar steroid ring.
General features of lipids
Amphipathic structures have two surfaces, one hydrophilic and one hydrophobic. Lipids are amphipathic. Some protein regions may form amphipathic helices, with one charged face and one neutral face.
Different cellular membranes vary in lipid composition .
Phospholipids and sphingolipids are asymmetrically distributed in the two leaflets of the bilayer, whereas cholesterol is fairly evenly distributed in both leaflets.
Asymmetry of lipids Phosphatidylserine (PS) only locates on the cytosolic face and glycolipids (oligosaccharide) on the exoplasmic face.
Fluidity of Biomembrane Most lipids and many proteins are laterally mobile in biomembranes.
Fluorescence recovery after photobleaching (FRAP) experiments.
Natural biomembranes generally have a fluidlike consistency Natural biomembranes generally have a fluidlike consistency. In general, membrane fluidity is decreased by sphingolipids and cholesterol and increased by phosphoglycerides.
Lipid rafts PM PC Placental alkaline phosphatase (yellow peaks) Atomic force microscopy reveals sphyingomyelin rafts (orange) protruding from a dioleoylphosphatidylcholine background (black) in a mica-supported lipid bilayer. Placental alkaline phosphatase (yellow peaks), a glycosylphosphatidylinositolanchored protein, is shown to be almost exclusively raft associated.
Lipid rafts are microdomains containing cholesterol, sphingolipids, and certain membrane proteins that form in the plane of the bilayer. These aggregates are sites for signaling across the plasma membrane.
Protein Composition On the basis of the nature of the membrane–protein interactions. Integral protein Lipid-anchored protein Peripheral protein
Carbohydrates O-link, Ser N-link, Asp
Common features of biomembrane Selectively permeable, fluidity and asymmetry.