Membrane Transport - Active Transport & Passive Transport accross the cell membrane


  When the organisms are allowed to grow in a nutrient environment, the concentration of solutes in the cell is usually much higher than the concentration of solutes in the extracellular environment. 

 Moreover, the composition of Intracellular solute concentration is much different than that of extracellular envtironment. This difference in the solute concentration as well as Its composition, both in intracellular and extracellular environment, is mainly due to the role cell membrane that surrounds the cell.

The selective permeabllity of the cell membrane is responsible for the transport of molecules across the cell membrane, both in and out of the cell. This transport mechanism, therefore, plays a vital role in the uptake of nutrients by the cell and helps in matntaining cell's metabolism and growth.

Membrane Transport - Active and Passive Transpprt

A variety of mechanisms operate to allow entry of nutrients in the cell. They Include:
1. Passive Transport
2. Active transport

1). Passive Transport

The Passive Transport is of two types
   a). Passive Diffusion
   b). Facilitate Diffusion

a). Passive Diffusion

This is the simplest method of transport of molecules across the cell membrane. It involves passive diffusion of small molecules through the cell membrane in favor of concentration gradient.
   A solute or substance passes through cell membrane from a high concentration to a low concentration environment till the equilibrium is attained. This phenomenon does not involve utilization of extra energy. Very small molecules such as water and gases like O₂ and N₂ are transported by this mechanism.

b). Facilitated diffusion

Certain substance, otherwise, impermeable to cell membrane, can be transported into the cell by a special class of membrane proteins. These proteins are called permeases or carrier proteins. They are located in the periplasmic region of cell or may be embedded in cell membrane.
  The activity of these permeases allows diffusion or entry of molecules into the cell, without any expenditure of energy.

Mechanism and characteristics of facilitated diffusion :
  The permeases or carrier proteins participating in the membrane transport are enzymatic in nature. They are able to bind to the substrate to be transported.
  The resulting carrier-substrate complex then undergoes a conformational change, such that the substrate now gets transported in to cytoplasm. Such a transport of molecules by role of permeases is known as facilitated difusion.

  A model explaining facilitated diffusion has been shown in figure.


This facilitates the diffusion of the substance through the cell membrane. The uptake by carrier proteins

Follows Michaelis-Menten kinetics. This mode of transport is further characterized as under. 

 i). Transport occurs in the favor of concentration gradient.
ii). It possesses a considerable degree of specificity.
iii). It is often inducible i.e. cells will produce permeases in the instances, where the environment possesses the substrate to be transported.

2). Active Transport

  The transport mechanism which operates at the expense of biochemical energy is in general known as active transport.
   This mode of transport specific and can occur even against the concentration gradient. i.e. the substances may be allowed to accumulate within the cell that can be several thousand times greater than that in the environment. The metabolic energy required to drive active transport mechanism may be obtained as proton motive force or ATP.

Four different patterns of active transport mechanisms have been recognized in microorganisms. They are as under.
a). Role of binding proteins
b). Secondary active transport
c). Phosphate bond linked
     transport
d). Group translocation

Role of binding proteins in active transport :

  A variety of binding proteins have been recognized to occur in cell membrane of bacteria. They bind with the substrate to be transported in a highly specific manner and allow their entry Into the cell or their accumulation in the periplasmic region.
   Such accumulation requires conformational change in structure of the binding proteins such that they are carried and released inside the cytoplasm or periplasm from the outer environment. This conformational change occurs at the expense of energy. This energy may be derived from ATP or directly by utilization of proton motive force.

• The binding proteins play two roles in transport.

1.  By binding to substrate, they increase the effective concentration of the substrate in the periplasm so that carrier proteins can transport them more favorably inside the cytoplasm.
2. They may Interact directly with carrier proteins and stimulate their transport activity.

• Two types of binding proteins are found among bacteria:
1. Shock sensitive, which are easily dissociated by osmotic shock from the cells. These binding proteins use ATP for their activity.
2. Shock intensitive which remain firmly bounded to the cell membrane and are not released upon osmotic shock. These binding proteins use proton motive force directly.

Secondary active transport :
   Usually proton motive force is established by transport of electrons through electron transport chain, located in cell membrane. This proton motive force is called primary proton motive force. It may also be generated by passage of charged lons across the membrane. Such proton motive force is called secondary proton motive force.
  Transport of molecules by the utilization of such secondary proton motive force is called secondary active transport.
There are three types of secondary active transport mechanisms: symport, antiport and uniport.

Symport
Simultaneous transport of two molecules by a same carrier in one direction is called symport.
   Hence entry of one substance facilitates entry jo other substance simultaneously. Usually transport of an anion is associated with simultaneous transport of a cation in the same direction.

Antiport
  Simultaneous transport of two molecules by a same carrier In opposite directions is called antiport.
  Here entry of one substance causes simultaneous exit of other substance from the cell. In this case, usually, export of an anion is associated with entry of a cation and vice versa.

Uniport
The simultaneous transport of two substances by separate carrier is called uniport.


  Here a substance (usually cation) enters the cell through one carrier. Simultaneously another substance, usually anion leaves the cell through other carrier.

Phosphate bond linked active transport :

  Transport of certain substrates requires availability of free energy released from hydrolysis of energy rich phosphor ester bonds. i.e. they need phosphate bond energy for transport. These transport mechanisms are not capable of utilization of energy from proton motive force.

Group translocation :

  This methods of transport  allows entry of molecules inside the cell without any modification. However transport mechanisms do operate in cell that primarily cause chemical modification of the substances and then they are
permitted to enter the cell.
   Such modifications in the substances are carried out at the expense of metabolic energy and transfer of a particular functional group to the substance. Therefore this mode of transport is called group translocation.

Mechanism of group translocation
A specific group translocating machinery is involved in the group translocation. These transfer systems are as under.
  a). Phosphotransferase system for transport of sugars.
  b). AcylCoA synthetase system for transport of fatty acids.
  c). Phosphoribosyl transferase system for transport of purine and pyrimidine bases.

Phosphotransferase system
  Phosphotransferase system (PTS) is the most thoroughly studied system of group translocation in bacteria. It involves transport of sugars into the cell. The system involves phosphorylation of sugar by transferring phosphate group from PEP (Phospho enol pyruvate). The phosphorylated sugar is then able to enter the cell. PTS consists of 4 proteins: HPr, Enzyme I, Enzyme Il and Enzyme III as shown in the figure.


  As shown in the figure, the components of  phosphotransferase system are arranged in specific orientation. Ell is embedded in CM. Energy rich phosphate group is transported to Ell from PEP via role of El, HPr and EIII as shown in the figure. 

  Finally, the phosphate group is transferred to the sugar molecule to be transported. Enzyme II acts as a carrier protein for sugars and permits their entry Into the cell. The PTS is mediating specific sugar transport, where role of enzyme III and enzyme ll is specific for the sugar to be transported, whereas enzyme l and HPr act in nonspecific manner for all PTS.

Acyl CoA synthetase system
   This system works for transport of fatty acids into the cells. The fatty acids are initially converted into fatty acyl CoA by transfer of CoA group from acetyl CoA and then the fatty acyl CoA is permitted to enter the cell.

Fatty acid + Acetyl CoA ➞ Fatty acyl CoA + Acetate

Phosphoribosyl transferase
  This enzyme plays role in transport of purine and pyrimidine bases. These bases are impermeable in cell membrane. However, phosphoribosyl transferase transfers of ribosyl moiety to the purine and pyrimidine bases and convert them into corresponding nucleoside monophosphates, which then enter the cell.

Purine or Pyrimidine base + Phosphoribosyl pyrophosphate (PRPP)  ➞  nucleoside mono phosphate + P-P