Transport of Solutes Across Membranes

Abstract

Cells and their environment are separated by the plasma membrane, which is permeable only for small hydrophobic molecules. Larger or hydrophilic molecules (or even ions) require protein transporters to get across. These can work as

Primary active transporters :

that hydrolyse energy-rich molecules to pump ions or molecules actively across a membrane against a concentration gradient. Substrate is usually ATP, but some transporters use other energy-rich molecules such as phospho-enolpyruvate, PP _i, or GTP. There are three main groups of ion pumps:

F-type :

occur in the mitochondrial and plastid membrane; they work as ATP-synthases that convert the chemiosmotic energy of an ion gradient into chemical energy of a phosphodiester bond. Archaea have a related ATP synthase, called A-type. Proton pumps related to F-type occur in vacuoles and other organelles (“V-type”); they are responsible for the low pH inside these compartments. Transported ion is usually H ⁺, but Na ⁺ may also be used.

P-type :

energise the plasma membrane by pumping ions against a concentration gradient. Bacteria, yeasts, and plants use a H ⁺-ATPase, and animals a Na ⁺ /K ⁺-ATPase. The $\upgamma$-phosphate of ATP is transferred onto an Asp residue of the enzyme, forming an acylphosphate. This is then hydrolysed by water.

ABC-type :

use the energy of ATP hydrolysis to pump nutrients into, or waste products out of a cell. Some members of the family have lost the ability to pump substrates actively; they act as regulated channels.

Secondary active transporters :

use the ion gradient produced by the pumps to transport molecules across the membrane against a concentration gradient. Ions and substrate can go in the same (symport) or in opposite direction (antiport).

Passive channels or pores :

that facilitate the diffusion of molecules down a concentration gradient, but unlike passive diffusion transport are selective and saturable.