Transport by membrane proteins

 


Effect of Membrane Lipids On Passive Transport:

There is a difference between diffusion Across biological membranes and artificial membranes, such as cellophane. Lipid solubility of diffusible molecules modifies the rate of penetration into cells. Generally, the more soluble a substance in lipids, the  more rapidly it passes into living cells. On the other hand, most nonlipid substances do not rapidly pass through biomembranes.

II .Biological Process

The biological processes which move the materials across the membranes in the cell include 1. transport by membrane proteins, and 2. internalization of macromolecules, fluid droplets and large particles.

1. Transport by Membrane Proteins. 

Many molecules rapidly pass through the biomembranes even though they are not very soluble in lipids. These include various small ions, glucose and  amino acids. They  are transported by membrane transport proteins. The transport proteins extend fully through the lipid  bilayer. Each transport protein is specific for only or a few chemically similar substances. On one surface of the membrane,  the carrier protein has a blinding site for the substrate to be moved from the medium through the membrane. Transport by membrane proteins is brought about in three important ways: passive transport by channel proteins, transport by carrier proteins and active transport by carrier proteins. 

(i) Passive Transport by Channel Proteins. 

The transport proteins of biomembranes form polar or charged channels extending through the lipid bilayer. These channels allow molecules that are soluble in water to pass through the membrane by simple diffusion, avoiding the  lipid bilayer's  hydrophobic interior. Passive transport via channels Carrie's materials only down concentration gradient. 

         The channels are of two types : some remain open all the time, whereas others open and close as if they have " gates". The  "gated channels" allow certain substances to pass only when their " gates" are open. Thus, the membrane's permeability changes from time to time, depending on whether the " gates" are open or closed. The channels open and close perhaps by changes in the form of the channel proteins response to some signal, electrical or chemical. If stumulus is chemical, it is a substance other than the one to be transported. For instance, neurotransmitter molecules stimulate a nerve cell to open gated channels that permit entry of sodium ions into the cell. Opening and closing of channel proteins enables the cell membrane's to respond to stimuli. It plays an important role in the functioning of nerves  and muscles.


(ii) Passive Transport by Carrier Proteins. 

Passive transport by carrier proteins is also called  facilitied transport, or facilitied diffusion. The carrier proteins are known as permeases.They transport materials down the concentration gradient. They do not use cellular energy. Instead,  facilitated transport occurs with the energy provided by favourable concentration gradient. A carrier protein combines with a specific substance to be transported and moves it from one side of the membrane to another through a channel formed in it. In liver and red blood cells, facilitied transport moves glucose across the cell membrane by specific carrier protein molecule in both directions, depending upon whether glucose concentrations is higher inside or outside the membrane. 




 

   
  Despite the help of protein molecules, the above two processes of transport across biomembranes are still regarded Passive because they move solute molecules down the concentration gradient. 

(iii) Active Transport by Carrier Proteins. 

Active transport moves substances against the concentration or electrochemical gradient. This " uphill" transport involves work and requires energy. Energy is usually provided by ATP or by concentration gradient of  ions.The carrier protein had s binding site for  ATP  in addition to the binding site for the substrate.  As  an ATP molecule bonds to the carrier protein, it is hydrolyzed to ADP. The energy so set free brings the substrate binding site of the carrier protein to the exoplasmic surface of the membrane. The substrate present in the medium joins the binding site of the carrier protein, forming careier__ substrate complex. The substrate __ bound carrier protein undergoes informational change and carrier the substrate through a channel in to the cytoplasmic side of the membrane. Here, the form of the binding site changes, and the substrate is released. Now the carrier molecules regains its original form and is ready to restart the process. 




Active Transport, a Vital Process. 

The active transport is a vital process. This is substantiated by the following ___

 (i) If the cells are deprived of oxygen, active transport halts because ATP production is stopped .This shows the need for ATP in active transport. 

(ii) Active transport is inhibited by poisons ( cyanide).

 (iii) Active transport is also inhibited by materials structurally similar to the solutes.

( iv) Active transport slow down with decrease in temperature. 

( v) Active transport is far more rapid than diffusion. 

( vi) Active transport can carry materials against their concentration gradient. 

( vii) It transports materials on selective basis.

Utility of Active Transport. 

Active transport is essential for  the life of the cells. It is useful as shown below__ 

(i) Absorptive cells of the small intestine actively transport glucose, amino acids  and minerals liberated by digestion of food from the intestinal cavity in the blood stream. This occurs even if the blood stream may contain many more molecules of these materials than are found in the intestine. 

    Large molecules of substances, such as sugars and amino acids, are usually changed during transit across the cell membrane. Glucose, for instance, is phosphorylated during absorption, getting phosphate group from  ATP and forming glucose phosphate. 

   ( ii)  Active transport plays a double role in the intestine. As the salts are passed from the Chyle into the cells by active transport, the water concentration of the chyle increases. This enables the cell to take up additional water molecules by osmosis. 


  (iii) The cells that line the tubules of the kidneys actively take up ions and glucose from the forming urine.

(iv) Bacteria, many protozoans and tapeworms absorb nutrients through the body surface by active transport. 

(v) Active transport of sodium and Potassium ions plays a role in generating nerve impulse and causing muscle contraction.

( vi) Sea birds excrete excess salt by active transport in their nasal salt gland.

( vii) The active transport brings about rapid and selective uptake of nutrients by cells.

(viii)  Active transport maintains ionic and water balance between cells and tissue fluid.

(ix) Active transport maintains the testing potential of the cell membrane. Resting potential is the difference in electrical charge across the ce membrane. The inside of the membrane is negatively charged compared to the outside.Most of the negative charge inside the cell is contributed by proteins and other organic molecules too large to escape through the membrane. Most of the positive charge outside the membrane results from Na+ ions turned out of the cell by the sodium_ potassium pump.

Active transport is far more rapid than diffusion and, unlike diffusion, it occurs in one direction. 

Resistance to Drugs in Cancer Cells. 

Some cancer cells are resistant to certain drugs. They can actively more the drugs out of the cell by using carrier proteins and energy. 

Active Transport Systems in Cells.


There are many active transport systems in cells. Two of these:  sodium _ potassium exchange pump and calcium pump,are described here.




(a) Sodium __ Potassium Exchange  Pump.

Many cells , such as nerve and muscle cells, have sodium,__ potassium exchange pump, often called sodium pump,  in the plasma membrane. This pump transfers Na+and K+ions against their concentration gradient. The process uses energy from ATP with the help of an  enzyme aosodium_ and _ potassium dependent adenosine triphosphatase present in the membrane. For each molecule of ATP used, three Na+ ions are pumped out and two K+ ions are pumped in simultaneously. This enables the cell to maintain much higher concentration of Na+ outside the cell than inside, making the outside of the cell membrane positively charged with respect to the inside. This creates and maintains the testing potential of the cell membrane. 


     

Importance. 

The sodium _ potassium exchange pump is very important for cells. About one _ third 
of our entire energy is used to power this pump.The Na+ __ K+ exchange pump plays a role 

( i) in maintaining a positive charge on the outside of the membrane and negative charge on the inside ; 

(ii) in nerve impulse conduction, 

(iii)in muscle contraction, 

(iv) in urine formation in kidney tubules, 

(v) in salt excretion in marine birds, and 

( vi) in controlling water contents of the cells.


    The pump adjusts the concentrations of sodium and potassium ions inside and outside the cell by active transport, and water then move passively through the plasma membrane by osmosis.

          Marine birds, such as seagulls and penguins, take sea water. They have nasal salt gland to eliminate excess sodium chloride. Sodium ions ( Na+) are actively expelled by sodium pump present in the plasma membrane of the gland cells. The chloride ions( CI-) pass out passively. The nasal secretion of these birds eliminates NaCI solution about 1.5 to 3 times more concentrated than the blood.



(b) Calcium Pulmp.

This pump keeps calcium concentration in cells at a low level. Red blood cells have calcium pump in the plasma membrane. It transport calcium ions out of the cells .Muscle cells have calcium pump in the membranes of sarcoplasmic reticulum. It transport ions from the cytosol into the SR, which stores these ions. Released of calcium ions from the SR into the cytosol brings about concentration, and their transport into SR causes relaxation. 

Energy for Pumps.

The active transport pumps described above use energy from ATP. Some other active transport mechanisms use energy from the movement of ions down a steep gradient across the membrane. 

Electrogenic Pump. 

A transport protein which generates voltage ( membrane potential) across a biomembrane is called an electrogenic pump. The sodium __ potassium pump is the major electrogenic pump of animal cells. The main electrogenic pump of bacteria, fungi and plants is a proton pump.This pump actively transport hydrogen ions( protons) out of the cell. This transfers positive charge from the cytoplasm to the extracellular fluid.

       By generating voltage across biomembranes, electrogenic pumps store energy which can be used for cellular work.The sodium - potassium pump builds a steep gradient of Na+ ions across the membrane of intestinal cells .The Na+ gradient then powers the active transport of glucose and amino acids into the cells. 

Ka+__ H+ exchange pump is present in the guard cells of stomata. 




































































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