Membrane transport

Due to the structure of the cell membrane, only small hydrophobic molecules can easily diffuse across the membrane (Figure 1).

Figure 1. Relative permeability of the plasma membrane

All other types of molecules that need to either enter or leave the cell, such as nutrients or waste products, need specialized proteins to cross the membrane. This transport may be passive (facilitated diffusion - does not require energy) or active (requires energy). When there is a difference in concentration and/or charge between both sides of the membrane, molecules will pass through a channel or carrier protein until the concentration and/or charge is balanced on both sides. The gradient of concentration and charge is known as the electrochemical gradient. Transport of molecules moving down the electrochemical gradient, from high to low concentration, is therefore known as passive transport or facilitated diffusion. Transporters that transport molecules against the electrochemical gradient, however, need energy. This is known as active transport. Primary active transport uses energy directly (from ATP or light), while secondary active transport uses an electrochemical gradient to drive transport. Secondary active transport does not require energy directly but relies on the electrochemical gradient established by primary active transport.

In addition to transporter-mediated transport across the membrane, sections of membrane can form an enclosed structure known as a vesicle. This allows the cell to secrete molecules into the extracellular environment (exocytosis) or take molecules up from the extracellular environment (endocytosis).