Cell Membrane

Variation of biomembranes in different cells

The faces of cellular membranes

• Cytosolic face• Exoplasmic fac

e

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. In general, membrane fluidity is decreased by sphingolipids and cholesterol and increased by phosphoglycerides.

Lipid rafts

PC

PM

Placental alkaline phosphatase (yellow peaks)

• 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.