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Sodium pump, Calcium pump, Uniport, Symport and Antiport



For instance, suppose the sugar glucose is more concentrated inside of a cell than outside. If the cell needs more sugar in to meet its metabolic needs, how can it get that sugar in?
Here, the cell can't import glucose for free using diffusion, because the natural tendency of the glucose will be to diffuse out rather than flowing in. Instead, the cell must bring in more glucose molecules via active transport. In active transport, unlike passive transport, the cell expends energy (for example, in the form of ATP) to move a substance against its concentration gradient.

The salient features of active transport are: 

  • This form of transport requires energy. About 40% of the total energy expenditure in a cell is used for the active transport system.
  • The active transport is unidirectional. 
  • It requires specialized integral proteins called transporters. 
  • The transport system is saturated at higher concentrations of solutes. e. The transporters are susceptible to inhibition by specific organic or inorganic compounds.
It is the best example for active transport. Cell has low intracellular sodium; but concentration of potassium inside the cell is very high.

 

Sodium Pump

It is the best example for active transport. Cell has low intracellular sodium; but concentration of potassium inside the cell is very high. This is maintained by the sodium–potassium activated ATPase, generally called as sodium pump. The ATPase is an integral protein of the membrane. It has binding sites for ATP and sodium on the inner side and the potassium binding site is located outside the membrane. It is made up of two pairs of unequal subunits α2 β2. Both subunits of the pump (alpha and beta) span the whole thickness of membrane.

Calcium Pump

An ATP dependent calcium pump also functions to regulate muscle contraction. A specialized membrane system called sarcoplasmic reticulum is found in skeletal muscles which regulates the Ca++ concentration around muscle fibers. In resting muscle the concentration of Ca++ around muscle fibers is low. But stimulation by a nerve impulse results in a sudden release of large amounts of Ca++. This would trigger muscle contraction. The function of calcium pump is to remove cytosolic calcium and maintain lowcytosolic concentration, so that muscle can receive the next signal. For each ATP hydrolysed, 2Ca++ ions are transported.



Uniport, Symport and Antiport

Transport systems are classified as uniport, symport and antiport systems.
Uniport system carries single solute across the membrane, e.g. glucose transporter in most of the cells. Calcium pump is another example.
If the transfer of one molecule depends on simultaneous or sequential transfer of another molecule, it is called co-transport system. The active transport may be coupled with energy indirectly. Here, movement of the substanceagainst a concentration gradient is coupled with movement of a second substance down the concentration gradient; the second molecule being already concentrated within the cell by an energy requiring process.

The co-transport system may either be a symport or an antiport. In symport, the transporter carries two solutes in the same direction across the membrane, e.g. sodium dependent glucose transporter. Phlorhizin, an inhibitor of sodium-dependent co-transport of glucose, especially in the proximal convoluted tubules of kidney, produces renal damage and results in renal glycosuria. Amino acid transport is another example for symport.

The antiport system carries two solutes or ions in opposite direction, e.g. sodium pump or chloride-bicarbonate exchange in RBC.

Secretory Vesicles and Exocytosis




Under appropriate stimuli, the secretory vesicles or vacuoles move towards and fuse with the plasma membrane. This movement is created by cytoplasmic contractile elements; the microtubule system. The inner membrane of the vesicle fuses with outer plasma membrane, while cytoplasmic side of vesicle fuses with cytoplasmic side of plasma membrane. Thus the contents of vesicles are externalized.
Sodium pump, Calcium pump, Uniport, Symport and Antiport Sodium pump, Calcium pump, Uniport, Symport and Antiport Reviewed by gafacom on September 07, 2019 Rating: 5

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