Cellular Biology

Overview of Active Transport across the Cellular Membrane



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Active transport across the cellular membrane refers to a process in which ions and some molecules are actively transported from one side of the cell to the other side of the cell membrane.  This process is referred to as active because it transports molecules and electrolytes against their concentration gradient. 


This process differs than diffusion which occurs from the higher concentration to the lower concentration side.  The active transport is required by many molecules and ions due to the unique structure of the cellular membrane which is not freely permeable to every compound except those compounds which are lipid soluble. 


The most important and the widely investigated active transport is the Na+/ K+ ions pump.  This ion pump is of extraordinary importance due to its role in building electric potential across the cellular membrane.  This is especially important in nerve cells in which case the communication between the cells is done via an action potential that spreads electric signals from one nerve cell to another. 


In these ion pumps the potassium ions are pumped to the interior of the cell while the sodium ions are pumped to the exterior of the cell.  Therefore a nonequilibrium state occurs in which potassium ions concentration inside the cell is much higher that that outside the cell.  On the other hand, sodium ions concentration inside the cell is much lower than that outside the cell. 


This imbalance in the ionic concentration across the cellular membrane is responsible for the creation of a negative electric potantial inside the cell on the membrane surface.  This type of pump requires energy in order to work that is supplied from the ATP molecules.  It also requires enzymes that catalyze the process and which are called ATPases.


The second type of active transport and which I discuss here is the H+ / K+ pump.  This type of active transport occurs in certain cells in the body such as cells of the stomach.  It usually pumps potassium inside the cell in exchange for protons which are pumped outside the cell. 


In the case of hyperkalemia in which case the concentration of potassium ions in the extracellular space is above that its normal value,  this pump comes into action in which the excess amount of potassium outside the cell is pumped to the interior of the cell in exchange for hydrogen ions which are transported to the exterior of the cell. 


As a result a state of acidosis occurs develops while correcting the hyperkalemia to its normal value.  In the case of metabolic acidosis this pump is also functional by pumping hydrogen ions to the inner side of the cell where they are buffered there by the phosphate ions.  In exchange potassium is pumped outside the cell.  Thus causing a state of hyperkalemia.


Another active transport across the cellular membrane is the calcium ions pump.  This type of pump requires ATPase and energy in the form of ATP to work adequately.  It functions by pumping the calcium ions ouside the cell.  It exists in certain types of cells in the body.  In muscle cells there is another type of calcium pumping from the cytoplasm to the sarcoplasmic reticulum which is an organ in muscle cells which stores calcium supply. 

Calcium ions are required for the contraction of muscle tissue.  They are functional in their free state.  There is also a bound state of calcium in the cell that is bound to phosphate and oxalate ions.


The last type of ion channels that is discussed here is the cotransport or symporter and antiporter pumps.  In this case of pumping the pump does not use ATPase but instead it relies on the concentration gradient of sodium ions that it can flow along its concentration gradient and the released energy is used to pump amino acids and glucose for example.  In this case the transport can be symport of antiport.  In addition to type of pumps antiport occurs also with Na+/Ca++ pumps in which case calcium is pumped outside the cell in exchange for sodium which is pumped to the interior of the cell.   

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