What is the lipid bilayer?
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
- Maintain an internal environment: the external cellular environment is often different to the cellular environment (ion, protein concentrations etc). The cell can control its environment by controling gene expression of transmembrane proteins (see 'crossing the membrane')
- Allows the cell to control what enters/exits: transporter proteins and channels enable the cell to selectively uptake molecules that it requires whilst excluding those that may be harmful.
- Protection: the lipid bilayer creates an envelope - housing the cellular components and offering some protection. Some cells, such as plant cells, have a cell wall which offers further protection.
- Contains important components involved in cell recognition, communication, signalling etc: the lipid bilayer is the boundary between the internal and external environment and must mediate communication between the two.
- Gives the cell shape (anchors with the cytoskeleton): in some cells the shape is important. One example is the red blood cell. Its concave shape increases its surface area allowing it to bind oxygen much more efficiently.
Structure 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.
Crossing the membrane
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:
- Cholesterol - provides support and reduces permeability.
- Other proteins:
- Glycoproteins, which have roles in cell-cell signalling, can act as receptors for hormones and other molecules.
- Antigens, which act as cell recognition molecules. Antigens have major roles in the immune system and help the body in identifying foreign cells.
Many of the molecules associated with the lipid bilayer help link the extracellular environement with the intracellular environment.
Fluid mosaic model
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.
- Essential Cell Biology
- Singer SG, Nichols,GL: The fluid mosaic model of the structure of cell membranes. Science 1972, 175(4023):720-31