Lipid bilayer

Each cell in the human body is composed of a unit membrane which separates the cytoplasm from the extracellular environment. The most important components of this membrane are phospholipids, cholesterol, proteins and chains of olgiosaccharides. The plasmalemma ranges from 7.5 to 10 nm in thickness and appears as a trilamellar structure in electron microscopes after fixation in osmium tetroxide. Because all membranes of cells and cell organelles have this appearance, the 3-layered structure was designated the unit membrane, which is a phospholipid bilayer.

Molecular structure of a phospholipid

The most common phospholipid is lecithin (phosphatidylcholine) that consists of a non-polar (water-repelling) tail build by two long-chain fatty acids linked to a charged polar (water-attracting) head. Within the hydrophilic portion the molecule of glycerol is esterified with two fatty acid molecules (hydrophobic portion) and with one phosphate molecule which is again esterified with the aminoalcohol choline. The hydrophobic portion is made up by one saturated fatty acid (palmitic acid) and one unsaturated fatty acid (oleic acid).

Arrangements of phospholipids in water

Monolayer

Phospholipid monolayers can be arranged either as a single surface film (e.g. in air and water or air and oil) or as micelles. A typical micelle is spherical and forms an aggregate with the hydrophilic heads in contact with water, sequestering the hydrophobic single-tail regions in the micelle centre. This phase is caused by the packing behavior of single-tail lipids in a bilayer.

Bilayer

The hydrophilic / lipophobic polar heads of the phospholipids are directed toward the surface of the membrane, in direct contact with water. The hydrophobic / lipophilic nonpolar fatty acid chains are faced toward each other, away from water. Between the phospholipids van der Waals forces dominate which are relatively weak interactions bteween molecules based on diploes and intermolecular forces. Cholesterol molecules are interspersed throughout the lipid bilayer often at a ratio of 1:1, affecting packing and fluidity of fatty acid chains in means of restricting their movements.

The most stable option of a lipid bilayer is gained under the following circumstances:

• outer leaf of the membrane: phosphatidylcholie, sphingomyelins, many molecules of cholesterol

• inner leaf of the membrane: phosphatidylserine, phosphidylethanolamine, cephalin

The inner and outer leaf molecules can form lipid rafts (lateral diffusion). A flip-flop is a swap of a phospholipid with the aid of flipases (enzymes).

Liposom / Vesicle

Lateral diffusion, fluidity and fluid mosaic model

The lateral diffusion adds an almost fluid character to the membrane. This fluidity is influenced by several factors:

1. The higher the surrounding temperature, the highter is the level of fluidity. Below a certain ambient temperature the membrane exist in a vicous-crystalline shape.
2. The degree of saturation and the length of the fatty acids have an impact on the level of lateral diffusion. Lonf chains develop more van der Waals forces, the cohesion becomes more stabile and the fluifity increases. Unsaturated fatty acids
3. Cholesterol acts as a "fluidity buffer"

Other components

The membrane not only contains (phospho)lipids but also proteins. Both proteins and lipids may have externally exposed olgiosaccharide chains and are then designated as glycolipids and glycoproteins.

Glycocalyx (carbohydrate)

- olgiosaccharide / sugar chains attached to phospholipids (glycolipids) and proteins (glycoporteins) and of cell-secreted glycoproteins and proteoglycans - 20 nm thick - is species-specific as well as cell-specific, thus facilitating cell-cell recognition - important for absorption and uptake on many molecules

Lipids (cholesterol, glycolipids)

- cholesterol molecules are interspersed throughout lipid bilayer often at a ratio of 1:1, affecting packing and fluidity of fatty acid chains (restricting their movements) *lipid rafts - lipid composition of each half of bilayer is different, e.g. in RBCs phosphatidylcholine and sphingomyelins is more abundant in outer half, whereas phosphatidylserine and phosphidylethanolamine is more concentrated in inner half - some of the lipids are glycolipids possessing olgiosacc. chains that extend outward from the surface and thus contribute to lipid asymmetry

Proteins

- fluid mosaic model emphasizes that a membrane consisting of phospholipid bilayer also contains proteins inserted in it or bound to cytoplasmatic surface (peripheral proteins) and that many of these move within the fluid lipid phase

- as with lipids, distribution of membrane proteins is different in the two surfaces of cell membranes, therefore, all membranes in cell are asymmetric

- integration: hydrophobic amino acids of integral membrane proteins present on outer region of the proteins interact with the hydrophobic fatty acid portions of the membrane - mosaic disposition of membrane proteins and fluid nature of lipid layer = fluid mosaic model for membrane structure:

- some membrane proteins are not bound rigidly in place and are able to move within the plane of the cell membrane, but (unlike lipids!) most of membrane proteins are restricted in their lateral diffusion by attachment to cytoskeletal components - in most epithelial cells, tight junctions (Zonula occludens: occludines and claudines) also restrict lateral diffusion of unattached transmembrane proteins and outer layer lipids to specific membrane domains

Membrane splitting and cyrofracture

Membrane splitting occurs along the line of fatty acid tails of the phospholipids, because only weak hydrophobic interactions bind the halves of the membrane along this line. When cells are frozen and fractured (cyrofracture), the lipid bilayer is often cleaved along the hydrophobic center. Electron microscopy of cyrofrature preparation replicas is a useful method of studying membranous structures. Most of the protruding membrane particles seen are proteins that remain attached to half of membrane adjacent to cytoplasm (= P or protoplasmic face). Fewer particles are found attached to the outer leaf of membrane (= E or extracellular face). For every protein particle that bulges on one surface, a corresponding depression appears in the opposite surface

References

Mescher, Antony (2010): Junqueira's Basic Histology. Text and Atlas. 12th edition. ISBN: 978-007-127190-5

Notes (2014): Biophysics. prof. RNDr. Evžen Amler, CSc. 2nd faculty of medicine, Charles University, Prague. Czech Republic.

Cytology I Lecture, Prof. Vajner