Understanding Biomembranes: Structure, Function, and Properties
Understanding Biomembranes
A necessary condition for life is the existence of a barrier separating cell contents from the environment, since the cells must retain an internal environment appropriate to carry out the numerous chemical reactions necessary to sustain life. Biomembranes are the place where many chemical reactions take place. They regulate the entry and exit of materials from the cell. Some membranes are selectively permeable barriers that regulate the flow of molecules inwards and outwards, therefore allowing communication between the internal and external environment of the cell. They are part of a system of transfer and energy storage and are involved in the transmission of signals (hormones).
Structure, Properties, and Functions of Biomembranes
The basic structural components of cell membranes are molecules called lipids (phospholipids, mainly). The membranes are composed of a lipid bilayer which includes many large globular proteins. At least some lipid molecules and protein molecules can move laterally within the bilayer, thereby forming different patterns that vary from time to time. Therefore, this widely accepted model of membrane structure is known as the fluid mosaic model.
Lipid Component
Two types of membrane lipids are found: phospholipids and cholesterol. The phospholipids are the most abundant component of the membranes, forming the lipid bilayer. Cholesterol in biological membranes is present in animals but is absent from plants. It is not a bilayer, but is in relatively high concentrations in certain animal membranes.
Physical Properties of Lipid Bilayer
Fluidity is a key property. Although the structure of the lipid bilayer is stable by itself, the molecules of phospholipids and cholesterol have great freedom of movement within the plane of the membrane, so both sides spread quickly. The degree of fluidity of the membrane depends on lipid composition and temperature. In addition to the lateral displacement of the components of the membrane and bending of the fatty acid chains, there is a third type of movement that is restricted: the transbilayer diffusion or flip-flop, which is the movement of a lipid from one side of the bilayer to another. Another important feature is that they tend to resist the formation of free ends, as a result of which they also tend to be impermeable and, in many circumstances, to round up and form vesicles spontaneously closed. The lipid bilayer also possesses the property to merge with other bilayers in good condition, which is an important cellular phenomenon. When a vesicle fuses with another membrane, both membrane phospholipids and its compartments become continuous, allowing the transfer of materials to another compartment (endocytosis and exocytosis.)
Protein Component
The lipids provide the biological membranes with the structural basis, while proteins are required for functionality, e.g., transport of molecules and signal transduction through the lipid bilayer. Membrane proteins vary according to the feature of the cell (or the organelle in the case of intracellular membranes) and also from one place to another in the same membrane. The membrane proteins are grouped into two main classes depending on how close is their relationship with the lipid bilayer: integral proteins and peripheral proteins.