Transport



Cell membrane:

Transport: Since most prokarytotes live in low solute environments (natural environments), cell must rely on active transport mechanism to concentrate molecules with cells. Solute transfer across the cytoplasmic membrane is mediated by specific membrane proteins called transporter proteins, transport systems, carriers, or by analogy to enzymes, permeases (because of their kinetic properties, show saturation kinetics; Fig 4.10). Most transporters, whether of prokaryotic or eukaryotic origin, seem to have a common structural and functional design.

Mechanisms of solute movement across membranes:

I. Energy-independent mechanisms:

A. Simple/Passive diffusion- is an energy independent process, no membrane protein is involved; movement of a solute is from a region of higher concentration to a lower concentration; can never have a higher concentration of solute inside cell than outside. Few molecules enter by this process (H2O, O2, CO2, NH3, weak acids and bases, and certain hydrophobic molecules).

B. Facilitated Diffusion- is an energy independent process, dependent on stereospecific transmembrane proteins. Rate of solute permeability increases with the concentration gradient more rapidly, and at a lower concentration of the solute molecule, than in passive diffusion. Facilitated diffusion is an important transport mechanism in eukaryotes, but not a frequently exploited mechanism in bacteria. What molecule(s) can be transported in bacteria by this mechanism and what are aquaporins?

II. Energy-dependent Mechanisms: Fig. 4.11

A. Active Transport- a process that can move a solute from a low concentration environment to a high concentration environment (i.e., against its concentration gradient). To accomplish this requires an expenditure of energy and specific membrane proteins. There are two broad categories of active transport systems found in Bacteria, Archaea, and Eukarya.

I. Ion driven transport systems - The movement of molecules across the membrane at the expense of a previously established ion gradient (H+, K+ or Na+ gradient). The three transport strategies are uniport, symport, and antiport (see Fig. 4.12). What property to membrane transporters share

II. ATP Binding dependent systems- depends on the energy from ATP hydrolysis to mediate the movement of solutes across the membrane (see Fig. 4.15); high affinity of solute transport comes from the affinity of the binding protein for the specific substrate. Binding protein-dependent transport systems belong to a large group of transporters known as the ABC (ATP-binding cassette) superfamily of transport proteins. ABC transports found in prokaryotes and eukaryotesare, related to each other in sequence and structural organization. Give an example of a molecule that is transported by this mechanism.

B. Group Translocation- a process that can move solutes from a low to a high concentration environment, requires a number of proteins (cytoplamic and membrane), chemical energy in the form of phosphoenolpyruvate (PEP) is reqired, and molecule is modified (phosphorylated by PEP) as it moves across membrane. The best studied group translocation system is the phosphotransferase system (PTS; see Fig. 4.14) by which certain sugars are transported into cells.

revised 9/25/09