MEMBRANE TRANSPORT 

The transport of materials across the plasma membrane of cells is a fundamental requirement for the normal function of virtually every physiological system. For example, the ability of the digestive tract to absorb nutrients, the capacity of the kidneys to maintain water balance and the electrical events that enable the nervous system to function, all require very precise regulation of the movement of materials across the plasma membrane of cells.

The plasma membrane acts as an effective barrier between the intracellular fluid (ICF) and extracellular fluid (ECF) compartments. The image opposite shows a transmission electron micrograph of two adjacent cells with the plasma membrane separating the intracellular compartments of each cell from the extracellular space .

The plasma membrane of each cell consists  of a lipid bilayer made up a double layer of phospholipid molecules arranged parallel. Each phospholipid molecule is made up of a phosphate-containing (hydrophilic) head and a pair of fatty acid (hydrophobic) tails. The hydrophobic nature of the fatty acid tails means that the lipid bilayer acts as a barrier that prevents the movement of water-soluble substances across the plasma membane.

However the movement of substances across the plasma membrane is enabled by the presence of membrane proteins some of which can act as channels , receptors or pumps and thus provide a means of moving materials across the lipid bilayer in a highly regulated fashion.

Bearing in mind that both the ICF and ECF are essentially aqueous solutions (i.e. a complex mixture of solutes dissolved in water) we will restrict ourselves to considering membrane transport of the solutes at this point in time and return to the movement of water (osmosis) at a later date.

Although there are a variety of ways that substances can move across the plasma membrane these can be conveniently divided into two major types: passive processes which don't require the use of metabolic energy and active processes, which do. In subsequent modules in this lesson we will consider the features of these two types of membrane transport in more detail.