Book : Cell Biology
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Posted by: CHELSEA
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Lipid Bilayers

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The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all living organisms and many viruses are made of a lipid bilayer, as are the membranes surrounding the cell nucleus and other sub-cellular structures. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role because, even though they are only a few nanometers in width, they are impermeable to most water-soluble (hydrophilic) molecules. Bilayers are particularly impermeable to ions, which allow cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps.

Biological bilayers are usually composed of amphiphilic phospholipids that have a hydrophilic phosphate head and a hydrophobic tail consisting of two fatty acid chains. Phospholipids with certain head groups can alter the surface chemistry of a bilayer and can, for example, serve as signals as well as "anchors" for other molecules in the membranes of cells. Just like the heads, the tails of lipids can also affect membrane properties, for instance by determining the phase of the bilayer. The bilayer can adopt a solid gel phase state at lower temperatures but undergo phase transition to a fluid state at higher temperatures, and the chemical properties of the lipids' tails influence at which temperature this happens. The packing of lipids within the bilayer also affects its mechanical properties, including its resistance to stretching and bending. Many of these properties have been studied with the use of artificial "model" bilayers produced in a lab. Vesicles made by model bilayers have also been used clinically to deliver drugs.

Biological membranes typically include several types of molecules other than phospholipids. A particularly important example in animal cells is cholesterol, which helps strengthen the bilayer and decrease its permeability. Cholesterol also helps regulate the activity of certain integral membrane proteins. Integral membrane proteins function when incorporated into a lipid bilayer, and they are held tightly to lipid bilayer with the help of an annular lipid shell. Because bilayers define the boundaries of the cell and its compartments, these membrane proteins are involved in many intra- and inter-cellular signaling processes. Certain kinds of membrane proteins are involved in the process of fusing two bilayers together. This fusion allows the joining of two distinct structures as in the fertilization of an egg by sperm or the entry of a virus into a cell. Because lipid bilayers are quite fragile and invisible in a traditional microscope, they are a challenge to study. Experiments on bilayers often require advanced techniques like electron microscopy and atomic force microscopy.

he structure of the lipid bilayer explains its function as a barrier. Lipids are fats, like oil, that are insoluble in water because of its long hydrophobic tails. The hydrophobic interactions among several phospholipids and glycolipids, a certain structure called lipid bilayer or bimolecular sheet is favored. Phospholipids and glycolipids have both hydrophilic and hydrophobic moieties (amphiphilic or amphipathic). Thus, when several phospholipids or glycolipids come together in an aqueous solution, the hydrophobic tails interact with each other to form a hydrophobic center, while the hydrophilic heads interact with each other by forming a hydrophilic coating on each side of the bilayer point radically towards the polar solvent.

This lipid bilayer formation is spontaneous since the hydrophobic interactions are energetically favorable to the structure. The lipid bilayer is a noncovalent assembly. The proteins and lipid molecules are held together by noncovalent interactions such as Van der Waals forces (which holds the hydrophobic tails together) and hydrogen bonding (which binds the hydrophilic heads with water), which help to stabilize the lipid bilayer structure.

Lipid bilayer membranes are asymmetric, which means the outside face of membrane is always different from the inner face of the membrane. Because of these interactions, lipid bilayer inherits unique properties. Lipid bilayers have "extensive" properties - they can enclose and form compartments. Lastly, they can also recover quickly if there is a hole in the lipid bilayer due to energetic reasons. However, phospholipids and glycolipids do not form micelles like fatty acids do because phospholipids and glycolipids have two hydrocarbon chains and they are too bulky to orient themselves into a sphere like a micelle. Additional properties of the lipid bilayer membrane include that they are: sheet-like, formed by lipids and proteins (sometimes carbohydrates), are amphiphatic, possess some noncovalent parts, are asymmetric, fluid, and are electrically polarized. Using fluid mosaic models, it can be seen that the bilayer undergoes rapid lateral diffusion, but flip-flop or transverse diffusion proceeds very slowly. There is also a hydrophobic transmembrane alpha helix that passes through the membrane, with the amine component on the extracellular side and the carboxy group on the cytoplasmic side.

The most important property of the lipid bilayer is that it is a highly impermeable structure. Impermeable simply means that it does not allow molecules to freely pass across it. Only water and gases can easily pass through the bilayer. This property means that large molecules and small polar molecules cannot cross the bilayer, and thus the cell membrane, without the assistance of other structures. This property of the lipid bilayer balance water and other organic molecules from influx/exflux through the cell and environment.

Another important property of the lipid bilayer is its fluidity. The lipid bilayer contains lipid molecules, and, it also contains proteins. The bilayer's fluidity allows these structures mobility within the lipid bilayer. This fluidity is biologically important, influencing membrane transport.

The function of the lipid bilayer membrane is mediated by the specific protein that is embedded in it. The proteins of the lipid bilayer function as pumps, channels, energy transducers, receptors, and enzymes.


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