Lipid bilayer: Difference between revisions
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=== Bilayer === | === Bilayer === | ||
[[File:0303 Lipid Bilayer With Various Components.jpg|thumb|348x348px]] | [[File:0303 Lipid Bilayer With Various Components.jpg|thumb|348x348px]] | ||
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 | 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. | ||
Liposom / Vesicle | |||
== Lipid composition and other components (cholesterol, glycocalix, proteins) == | |||
The lipid composition of each half of the bilayer is different. In the '''outer leaf''' of the membrane '''phosphatidylcholie, sphingomyelins and many molecules of cholesterol '''can be abundant (espacially in red blood cells), whereas '''phosphatidylserine, phosphidylethanolamine and cephalin''' can be more concentrated in the '''inner half'''. | |||
'''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 membrane not only contains lipids (phospholipids and cholesterol) but also '''proteins''' that are immersed in the bilayer. Both lipids and proteins may have externally exposed olgiosaccharide chains and are then designated as '''glycolipids and glycoproteins''' leading to asymmetry of membranes. This so-called '''glycocalix''' can be 20 nm thick and is species-specific as well as cell-specific, thus facilitating cell-cell recognition and substance uptake. | |||
== Lateral diffusion, fluidity and fluid mosaic model == | == 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: | The '''lateral diffusion''' adds an almost '''fluid character''' to the membrane. This fluidity is influenced by several factors: | ||
# The higher the surrounding temperature, the | # The higher the surrounding temperature, the higher is the level of fluidity. Below a certain ambient temperature, the membrane exists in a vicous-crystalline form. | ||
# The degree of saturation and the length of the fatty acids have an impact on the level of lateral diffusion. | # The degree of saturation and the length of the fatty acids have an impact on the level of lateral diffusion. Long chains develop more van der Waals forces, the cohesion becomes more stabile and the fluifity decreases. Unsaturated fatty acids accumulate "bends" because of their cis-double bonds. Those irregularities lead to lesser cohesion, lesser van der Waal's interactions and to an increase of fluidity. | ||
# Cholesterol acts as a "fluidity buffer" | # Cholesterol acts as a "fluidity buffer" at higher and lower temperatures and prevents the collapse of the plasma membrane during thermical pressures. Cholesterol-rich micro domains in the bilayer are called lipid rafts. | ||
A flip-flop is a swap of a phospholipid with the aid of flipases (enzymes). | |||
- | |||
- | |||
The 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. Hydrophobic amino acids of integral membrane proteins present on outer region of the proteins interact with the hydrophobic fatty acid portions of the membrane. The mosaic disposition of membrane proteins and fluid nature of lipid layer is called the 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 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 | 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 | ||
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== References == | == References == | ||
Mescher, Antony (2010): Junqueira's Basic Histology. Text and Atlas. 12th edition. ISBN: 978-007-127190-5 | Mescher, Antony (2010): Junqueira's Basic Histology. Text and Atlas. 12th edition. ISBN: 978-007-127190-5 | ||
Huss, Sebastian (2012): Biologie Band 1. MEDI-LEARN Skriptenreihe. 5. Aufl. | |||
Notes (2014): Biophysics. prof. RNDr. Evžen Amler, CSc. 2nd faculty of medicine, Charles University, Prague. Czech Republic. | Notes (2014): Biophysics. prof. RNDr. Evžen Amler, CSc. 2nd faculty of medicine, Charles University, Prague. Czech Republic. | ||
Cytology I Lecture, Prof. Vajner | Cytology I Lecture, Prof. Vajner | ||
Revision as of 22:34, 1 December 2014
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.
Liposom / Vesicle
Lipid composition and other components (cholesterol, glycocalix, proteins)
The lipid composition of each half of the bilayer is different. In the outer leaf of the membrane phosphatidylcholie, sphingomyelins and many molecules of cholesterol can be abundant (espacially in red blood cells), whereas phosphatidylserine, phosphidylethanolamine and cephalin can be more concentrated in the inner half.
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 membrane not only contains lipids (phospholipids and cholesterol) but also proteins that are immersed in the bilayer. Both lipids and proteins may have externally exposed olgiosaccharide chains and are then designated as glycolipids and glycoproteins leading to asymmetry of membranes. This so-called glycocalix can be 20 nm thick and is species-specific as well as cell-specific, thus facilitating cell-cell recognition and substance uptake.
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:
- The higher the surrounding temperature, the higher is the level of fluidity. Below a certain ambient temperature, the membrane exists in a vicous-crystalline form.
- The degree of saturation and the length of the fatty acids have an impact on the level of lateral diffusion. Long chains develop more van der Waals forces, the cohesion becomes more stabile and the fluifity decreases. Unsaturated fatty acids accumulate "bends" because of their cis-double bonds. Those irregularities lead to lesser cohesion, lesser van der Waal's interactions and to an increase of fluidity.
- Cholesterol acts as a "fluidity buffer" at higher and lower temperatures and prevents the collapse of the plasma membrane during thermical pressures. Cholesterol-rich micro domains in the bilayer are called lipid rafts.
A flip-flop is a swap of a phospholipid with the aid of flipases (enzymes).
The 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. Hydrophobic amino acids of integral membrane proteins present on outer region of the proteins interact with the hydrophobic fatty acid portions of the membrane. The mosaic disposition of membrane proteins and fluid nature of lipid layer is called the 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
Huss, Sebastian (2012): Biologie Band 1. MEDI-LEARN Skriptenreihe. 5. Aufl.
Notes (2014): Biophysics. prof. RNDr. Evžen Amler, CSc. 2nd faculty of medicine, Charles University, Prague. Czech Republic.
Cytology I Lecture, Prof. Vajner
