Cells is a Miniature Chemical Factory

Due  to its incessant metabolic processes, a living   cell earns the  attribute  of a  miniature chemical factory   metabolites and  free oxygen  are its  most  essential raw materials  life is its  ultimate  product and  Co₂ H₂O and nitrogenous  wastes are  its  main by products.

Passive and Active  Transport : If  transportation  across the  plasma  membrane  by any  of the above  pathways id  due to normal kinetic  motion  of molecules  from  the  region  of their higher concentration  to the region  of their lower  concentration  it is  called passive transport or diffusion. Diffusion  through  interstices of lipids  bilayer  or through  watery  channels  of channel  proteins is called    simple  diffusion,  diffusion  by way  of carrier  proteins is, on  the other  hand called  facilitated  diffusion, Certain  substances are taken  in or extruded forcibly against concentrations  gradients i.e. from  the region  of lower  to the region of   higher  concentrations .The  process always  occur  through  the carrier proteins  and a lot  of energy  of a ATP  is  spent  on it. It is therefore ,called  active transport.

By  maintaining fixed  concentration  gradients  in the cytoplasm and  ECF, cells  render the  passive  transport  of certain solute molecules unidirectional .For  instance. O₂  and useful  metabolites.  Such  as glucose  and most  of the  amino  acids  are used up in metabolism  as soon  as these enter into a cells Hence  their passage throu7gh the plasma membrane   is always towards the cytoplasm from  ECF. Similarly  metabolic  by products continuously  form  in the cell Hence  there is  a continuous  traffic of these products  from  the cell into the ECF.

Sodium potassium pump  This  is the most important  example  of carrier mediated active  transport  system of cells ,Normally, the  plasma membrane  is  far less permeable to  sodium  ions Na⁺ than  to potassium  K⁺  ions . K^{+ i}ons  are required in the cells  for the  activity  of many  enzyme , and protein  synthesis .Hence  cells in general maintain a high internal concentration of k⁺  ions ,but  a low  concentration  of Na+  ions . Conversely ,  the ECF  always  has a high  concentration    Na ⁺  and  a low  concentration  of k⁺  ions. Due to their  concentration  gradients k⁺ ions  keep  escaping  out of cells  and Na+  ions  keep entering into them .cells  therefore  have to forcibly  extrude Na ⁺  and take in K⁺  against their  concentration  gradients through carrier proteins by  spending about  10% to40% of their total  energy  output. This is  called  sodium  potassium  pump. The  carrier protein  of plasma  membrane to operate this pump   is an enzyme  called Na⁺ K⁺  ATP ase .This  enzyme  molecule has three receptor   sites on its  inner  surface  for binding three  sodium  ions  and two receptor sites onits outer  surfade  for binding  two potassium  ions. As  sodium  and potassium ions  bind  at the respective  sites with this  molecule  an ATP  molecule gets   degraded  to ADP  releasing  energy which   triggers a conformational change in the molecule. As  a result of this released into the ECF   and  potassium  ions  into  the cytoplasm. The  precise mechanism  of this  conformational change  in the Na⁺ k⁺ ATP ase  molecule  in not  yet known.

One of the  most important  of Na⁺ K⁺  pump is to control  the volume  of the  cells. Due  to the  presence of a large number of nondiffusible    protein  molecules  and other  organic compounds  in the  cytoplasm water  always  tends to enter into the  cells from the ECF by   osmosis. This  might  cause the cells to swell  and ultimately burst. By pumping  out three Na⁺  k⁺  ions  from the  cells for every  two k⁺  ions pumped  into the cells the Na +  K+  pump  maintains  a continual net  loss of ions  by the cells  and resultant reverse  tendency  of water  out of the cells.

Another  important  of the  Na+ k+  pump  is that it creates an  electric  potential  across  the cell membrane ,i.e.  it is electrogenic. Continual  net  loss  of one positive  ion in each revolution  of the pump  maintains  positivity  on the  outside  and negativity  on the   inside  of plasma membrane.