Only certain molecules can pass through cell membranes by simple diffusion. These are gases (such as O2 and CO2), lipid-soluble molecules (like vitamins A and E), and some small uncharged polar molecules (like water, urea, ethanol, or glycerol). On the other hand, charged molecules (i.e., ions), as well as large uncharged polar molecules (such as glucose or sucrose), require specialized membrane transport systems in order to permeate cell membranes.
There are several reasons why every cell needs to control the passage of different substances across its membranes:
- to obtain and keep the nutrients while excreting waste products,
- to control cell volume,
- to generate ionic gradients and maintain membrane potential, and
- to maintain intracellular homeostasis suitable for enzyme activity.
Therefore, every cell possesses a variety of sophisticated membrane transport systems allowing for control of the intracellular environment.
All cellular membrane transport systems could be divided into two major groups: Passive and Active. The difference between these two groups is obvious: while passive transport does not require cellular energy, active transport does.
Passive transport mediates the movement of molecules down their concentration or electrochemical gradients. Passive transport doesn’t require external energy input because it is driven by the existing gradient for a given molecule. Consequently, if no gradient exists, no passive transport occurs. The simplest form of passive membrane transport is diffusion (which, in case of water, is called osmosis). Simple diffusion through the lipid membrane occurs without the involvement of any membrane protein and is determined only by the concentration of molecules outside/inside the cell. This is fundamentally different from the facilitated diffusion – another form of passive transport – which is mediated by the integral membrane proteins – channels and carriers (or transporters). Channel proteins form the pores across the cell membranes, allowing for the simple diffusion of specific molecules. The examples of channel proteins include porins, gap junctions and ion channels. Carrier proteins, in turn, bind certain molecules on the one side of the membrane and release them on the other, by changing their shape (for example, glucose transporter GLUT).
Unlike passive transport, active transport mediates the movement of molecules against their concentration or electrochemical gradients. Therefore active transport requires a metabolic energy input (often in the form of ATP) in order to occur. No energy – no active transport. There are two types of active transport: primary active transport and secondary active transport. Primary active transport is mediated by pumps – ATPase proteins that use energy derived from ATP hydrolysis in order to transport molecules against their electrochemical gradients. Examples of pumps include Na+/K+ ATPase, Ca2+ ATPase, H+ ATPase, ABC transporters etc. In its turn, secondary active transport, which is also called coupled transport or cotransport, utilizes electrochemical ionic gradients created by primary active transport in order to move other ions or molecules against their gradients. Thus, secondary active transport is not directly using ATP as an energy source, but instead operates by coupling the flow of certain ions down their electrochemical gradients with the transport of other molecules against their electrochemical gradients. When the transport of both molecules occurs in the same direction – it is called symport, otherwise – antiport. Examples of secondary active transporters include Na+/H+ exchanger, Na+/glucose cotransporter, Na+/Ca2+ exchanger etc.
Discover membrane transport systems in the infographic below.