Movement of Solute Molecules Across The Cell Membrane

Movement of solute molecules across the cell membrane into regions of higher concentrations or against a concentration gradient, with the use of metabolic energy input is known as active transport. Binding protein transport systems or ATP-binding cassette transporters (ABC transporters) is a good example of active transport which are active in bacteria, archaea and eukaryotes.These transporters are an example of ATP-dependent pumps. ABC transporters are ubiquitous membrane-bound proteins. These pumps can transport substrates in or out of cells. These binding proteins, bind the molecule to be transported and then interact with the membrane transport proteins to move the solute molecule inside the cell. E. coli transports different types of sugars (arabinose, maltose, galactose, and ribose) and amino acids (glutamate, histidine, leucine) by this mechanism.

Proton gradients are also used by bacteria, produced during electron transport to initiate and control active transport. The lactose permease of E.coli carries a lactose molecule inward as a proton simultaneously enters the cell. Such linked transport of two molecules in the same direction is called Symport. E.coli also uses proton symport to take up amino acids and organic acids like succinate and malate.A proton gradient also can power active transport indirectly, often through the formation of a sodium ion gradient. In E. coli, sodium transport system pumps sodium outward in response to the inward movement of protons. Linked movement in which the transported molecules move in opposite directions is called Anitport. The sodium gradient generated by this proton anitport system then drives the uptake of sugars and amino acids. E.coli has at least transport systems for the sugar galactose.

Group translocation is a process in which a molecule is transported into the cell while being chemically altered. For example, Phosphoenolpyruvate: Sugar phosphotransferease system (PTS). It transports a variety of sugars while phosphorylating them using phosphoenolpyruvate (PEP) as the phosphate donor. PEP + Sugar (outside) ? Pyruvate + Sugar-P (inside) In E. coli and Salmonella typhimurium, it involves two enzymes and a low molecular weight heat stable protein (HPr). HPr and enzyme I (EI) are cytoplasmic. Enzyme II (EII) is more variable in structure and often consist of three subunits. EIIA is cytoplasmic and soluble. EIIB also is hydrophilic but frequently attached to EIIC, a hydrophobic protein that is embedded in the membrane.

A high energy phosphate is transferred from PEP to enzyme II (EII) with the aid of enzyme I (EI) and HPr. Then a sugar molecule is phosphorylated as it is carried across the membrane by enzyme II (EII). Enzyme II (EII) transports only specific sugars and varies with PTS, whereas enzyme I (EI) and HPr are common to all PTS’s.PTS’s are widely distributed in prokaryotes. Aerobic bacteria lack PTS’s. Genera Escherichia, Salmonella, Staphylococcus and other facultative anaerobic bacteria have phosphotransferase systems; some obligate anaerobic bacteria (Clostridium) also have PTS’s. Many carbohydrates are transported by these systems. E. coli takes up glucose, fructose, mannitol, sucrose, N-acetylglucosamine, cellobiose and other carbohydrates by group translocation.