Entry of nutrition

Entry of nutrition :

It is a cytoplasm membrane that allows the passage of certain small molecules and actively concentrated

Others within this cell.

Passive diffusion :

Except for water and some lipid solubles molecules, few compound can pass through the cytoplasm (a lipid protein , semi-permeable cell membrane) by simple or passive diffusion. In this process, solute molecules across the membrane as a result of a difference in concentration of the molecule across the membrane . the difference in concentration (higher outside the membrane than inside) governs the rate of inward flow of the solute molecule with time, this concentration gradient diminishes until equilibrium is reached . in passive diffusion no substance in the membrane interacts specifically with the salute molecule.

Facilitated diffusion:

Another mechanism by which substances cross the semi-permeable cell membrane is facilitated diffusion . this process is similar to passive diffusion in that the salute molecule also flow from a higher to lower concentration. But it is different from passive diffusion because it involves a specific protein carrier molecule(called a porter or permease) located in the cytoplasmic membrane. The carrier molecule combines reversibly with the salute molecule, and the carrier salutes complex moves between an outer and inner surface of the membrane, releasing one salute molecule on the inner surface and returning to bind a new one on the outer surface. The entry of glycerol into the bacterial cell is by facilitating diffusion . although this mechanism of transport is common in the eukaryotic cell (eg. Sugars entry them in this way), it is relatively rare in the procratic cell . neither of the two mechanisms passive diffusion or facilitate diffusion required metabolic energy . solid can be concentrated within the cell several thousand times greater than outside the cell . These two mechanisms are group translocation and active transport .

Group translocation:

In group translocation , the solid is altered chemically during transport . the best-studied group translocation system is the phosphoenol pyruvate-dependent sugar phosphotransferase system. It is widely distributed in many bacteria genera and mediates the translocation of many sugar and sugar derivative . This salute enters the cell as sugar phosphates and is accumulated in the cell in this form. Phosphotransferase system (PTS) sugar uptake and phosphorylation required the participation of several soluble and membrane bonded enzymes . these proteins catalyze the transfer of the phosphoryl group of phosphoenolpyruvate to the sugar molecule. The products formed are therefore sugar phosphate and pyruvate . the overall reaction requires Mg²⁺ .

Specifically , a relatively heat stable carrier protein (HPr) is activated first by transfer of phosphate group. From the high energy compound phosphoenolpyruvate inside the cell .

PEP+HPr pyruvate + phospho-HPr

Enzyme I and HPr are soluble proteins and are non-specific components of the process at the same time , the sugar combines with enzyme II at the outer membrane surface and is transported to the inner membrane surface . enzyme II is specific for a particular sugar and is an integral component of the cytoplasm membrane. Here it combines with the phosphate group carried by the activated HPr. The sugar phosphate is released by enzyme II and enters the cell. This is illustrated by the reaction soon below. Some investigators have reported a peripheral membrane enzyme III that mediates between enzyme II and phospho-HPr in translocation the sugar.

Sugar+_phospho-HPr sugar-phosphate +HPr

Active transport:

It is the mechanism by which almost all solutes including sugars , amino, acids , peptides, nucleosides and ion are taken off by cells active transport involves 3 steps:

1. Binding of the solute to a receptor site of a membrane-bound carrier protein .
2. Translocation of solid carrier complexes across the membrane .
3. Coupling of translocation to energy-yielding reactions to lower the affinity of carrier protein for the solute at the inner membrane surface so that the carrier protein will release solute to the cell interior.

In this process, the energy released during the flow of electron through electron transport chain or splitting of a phosphate group ATP derives protein out of the cell . this generates the difference in pH value and electric potential between the inside and outside the cell or across the membrane . this proton gives rise to proton motive force which can be used to pump the solute into the cell . Many active transport system of gram-positive bacteria are associated with binding proteins in the periplasmic space. These binding proteins have very high affinities nutritious, including amino acids , sugars, and inorganic ions. Over 100 different binding proteins have been isolated and characterized . they are essential for the active transport of their specific substrate. However , they are not porters since they are located in the periplasmic space rather than in the cell membrane itself . but binding proteins functions in conjunction with porters in the active transport of specific nutrients.

• Proton motive force:

This mechanism is based on the unequal distribution of charges outside and inside the cell. The molecule transfers from a zone of higher charge concentration to a zone of lower charge concentration. In proton motive force, bacteria lets proton (H⁺) outside the cell , this increases the concentration of positive charges outside . In order to compensate the unequal distribution of charges , the bacteria uptake positively charge ions such as Ca⁺⁺.Mg⁺⁺,k⁺,Na⁺,etc.

• Sodium-potassium pump:

In this mechanism , bacteria allows 3 ions outside the cell and only take 2 potassium ions. This helps to build up the higher positive charge concentration outside . so a bacterial cell can always take essential positive charge such as Ca⁺⁺,Na⁺, Mg⁺⁺,etc. to reach an equilibrium .

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.