The lipid bilayer (phospholipid bilayer/cell membrane) is a structural component of the cell that isolates the cell components (organelles, cytoplasm) from the extracellular environment.
Functions of the lipid bilayer
The lipid bilayer is made up of many phospholipids that align together. Each phospholipid is made up of a hydrophilic head and a hydrophobic tail (figure 1).
The most common phospholipid is phosphatidylcholine which contains a choline molecule bound to phosphate and glycerol (figure 1). The hydrophilic head is polar allowing it to form hydrogen bonds with water molecules, whereas the tail region - made from two hydrocarbon chains - is non-polar or hydrophobic.
It is this combination of both a hydrophobic and hydrophilic region (amphiphilic) that gives phospholipids such an important function within the cell. When placed in water, the phospholipid molecules naturally align into a bilayer (figure 2), allowing the hydrophobic tails to avoid water whilst the hydrophilic heads form hydrogen bonds with water molecules.
Interestingly, the lipid bilayer will form a closed sphere (liposome) to completely exclude water from the hydophobic tail.
The lipid-bilayer isolates the internal components of the cell from the extracellular environment. Whilst lipid-soluble molecules can cross the membrane directly through the bilayer, this is not so for large molecules such as glucose, water and other polar molecules. So how then do these enter the cell?
Transmembrane proteins are anchored throughout the entire bilayer and allow non-polar molecules to cross the membrane, facillitating the passing of these molecules into or out of the cell. These proteins not only allow essential components into the cell, they also offer an opportunity for the cell to control what can and cannot enter.
But how does the cell do this? The cell can tightly control what can and cannot enter the cell by altering the expression of genes encoding transmembrane proteins. For example, when the cell is low on glucose, genes that code for glucose transporters will be switched on. These newly transcribed proteins will translocate in the membrane where they can facilitate increased glucose uptake.
What else is found In the membrane?
Along with the various kinds of transporter proteins, other molecules are found within the lipid bilayer that are of functional importance to the cell and include:
Many of the molecules associated with the lipid bilayer help link the extracellular environement with the intracellular environment.
Singer and Nicholson (1972) were the first to describe the lipid bilayer with the the fluid mosaic model. But what does this actually mean?
What this means is that the individual phospholipids are not fixed and will move around within the bilayer, much like the movement of molecules in liquids. One way to imagine this is by thinking of a glass of water. Whilst the consistency remains constant, the molecules within the glass are free to move.
The term 'mosaic' refers to the proteins and other molecules found within the membrane that create a mosaic-iike appearence.
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