Transporter types

Protein-free lipid bilayers are relatively impermeable to most molecules, except small hydrophobic molecules. Polar, bulky or charged molecules require transporter proteins to cross the plasma membrane. Membrane transporters are proteins that span the plasma membrane and can be broadly categorized into aquaporins, channel proteins, carrier proteins and pumps (table 1). Each transport protein is highly specific and transports only one class of cargo molecules, such as ions or sugars, and often only one specific molecule within that class.

Table 1: Membrane transport proteins