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
sphingosine 鞘磷脂
?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.