Movement of molecule in and out of the cell: Na and K pump
The unequal distribution of ion in and outside the cell results in the establishment of sodium and potassium pump.
Structure of Na and K pump
Sodium and potassium pump have a particular structure that mainly is made of one alpha subunit and one beta subunit. Among the two, alpha unit binds to both sodium and potassium. Alpha subunit have some specialized amino acid that assist in this binding and is therefore responsible for some of the enzyme function whereas the beta subunit is involved in steering of alpha beta complex. Along with this beta subunit have one more function that restricts the release of potassium ion on the other side of the membrane at the time of conformational change .A difference between the two units is that alpha subunit is found on two parts i.e. the cytoplasm portion and the intracellular matrix whereas beta unit is present only in the extracellular portion. One more unit called gamma subunit is also present that acts only in tissue specific regions. All this together makes up the sodium potassium pump that helps in exchange of ions.
The exchange of sodium and potassium takes place via different methods that employs energy driven process.
Active transport: The extracellular fluid is rich in sodium ion whereas the inner of the cell membrane is rich in potassium ion. This creates difference in the ion gradient in and out of the cell. Thus equilibrium is rarely established in these areas. Since the plasma membrane is highly permeable to potassium ion, therefore there are more K+ leakage channels than sodium channels and this is the reason that the number of potassium ion that diffuse out of the cell, down their concentration gradient is more than those of the sodium ion. Likewise, the portion that is towards the cytosol have high affinity for Na+ ion and the side that is towards the extracellular matrix have more affinity for K+ ion. Therefore, the channels and pumps present there help in the transport of these ions in the opposite direction against their concentration gradient. This complete process requires energy from ATP hydrolysis. Firstly, the sodium ions binds at the high affinity sites inside the cell which creates hydrolysis of ATP and hence bring conformational change that exposes the binding site to the outside of the cell. This reduces its affinity for the same and releases Na+ outside the cell. Likewise the similar process goes on with potassium, where K+ sites with high affinity are exposed to the outside of cell that in turn binds potassium ion. Then the binding creates conformational change along with hydrolysis of ATP and hence release of potassium is done on the other side. This way active transport of sodium and potassium is done across the membrane with exchange of three Na+ for two K+ ion.
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Function of sodium potassium pump
Sodium potassium pump have different function that include generation resting potential, transport of molecules, helps in generation and transmission of signal, control in the activity of the brain, cell volume etc.
1. Transport of molecules: Along with export of sodium, many other membrane transport protein also imports many useful molecules that is used for the proper functioning of the cell.
2. Signal transduction: The sodium- potassium pump helps in regulation of protein tyrosine interaction that is mediated by protein- protein interaction.
3. Control of cell volume: Sodium potassium pump helps to maintain the right concentration of ion in the cell that avoids any kind of swelling in the cell.
4. It also acts as a computation element for the brain.
5. Most importantly, it generates resting potential within and outside the cell.
Results of failure of sodium potassium pump
Since sodium potassium is a complete cycle that when function properly keeps the cell active and in good condition, but on the other hand if this pump fails then the sodium ions gets accumulated along with the water that results in swelling of cell along with different diseases like increase in blood pressure, hypertension and many more of the heart and respiratory diseases.
This way proper functioning of the pump is very much necessary to maintain distance with the diseases and live a healthy life.
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