Transport of Ions and Molecules Through The Cell Membrane

Explains the mechanism of ions and molecules transport through the cell membrane.

A) Structure of the cell membrane

B) Diffusion, it’s kinds

  • Is spontaneous movement of ions or molecule according to concentration current through lipid layer plus channel
  • There are 2 types of diffusion :
    1. Simple Diffusion
      • Movement of substance through membrane without energy
    2. Facilitated diffusion
      • Movement of ions through membrane with help of carries without energy

C) Osmosis

  • Is movment of water molecule from high concentration of water to low concentration of water though semi-permeable membrane

D) Primary active transport

  • Movement of molecules or ion against concentration gradient or electrical gradient with energy
  • There are few type of primary active transport
    • Primary active transport of calcium
    • Primary active transport of hydrogen ions
    • Sodium-potassium pump
      • A transport process that pumps sodium ions outwards through the cell membrane and at the same time, pumps potassium ions from the outside to the inside.
      • The pump present in all cells of the body and it is responsible for maintaining the sodium and potassium concentration differences across the cell membrane as well as for establishing a negative electrical potential inside the cells.
      • It has 3 receptor sites for binding sodium ions on the portion of the protein that protrudes to the interior of the cell
      • it has 2 receptor sites for potassium ions on the outside
      • The inside portion of this protein adjacent or near to the sodium has ATPase activity
      • When 3 sodium ions bind on the inside of the carrier protein, the ATPase function of the protein become activated.
      • The importance of this pump is to control the volume of the cells. Without this, most of the cells of the body would swell until they burst.

E) Secondary active transport

  • Movement of molecule according to concentration gradient with help of energy being stored in primary transport
  • There are 2 types of secondary active transport “:
    • Co-transport
      • diffusion energy of sodium literally can pull other substances along with the sodium through the cell membrane
      • Exp : Glucose and amino acid along with sodium ion.
      • Glucose and many amino acids are transported into most cells against large concentration gradients. The mechanism of this is entirely by co-transport mechanism
      • Sodium-glucose co-transport mechanism
      • The transport carrier protein has 2 binding sites of its exterior side, one for sodium and one for glucose
      • When the concentration of sodium ions is very high on the outside and very low inside, which provides energy for the transport.
      • The transport protein is that a conformational change to allow sodium movement to the interior will not occur until a glucose molecule also attaches.
      • But when they both become attached, the conformational change takes place automatically and the sodium and glucose are transported to the inside of the cell at the same time.
      • sodium co-transport
      • of the amino acids occurs in the manner as for glucose, except that it uses a different set of transport proteins.
      • It occurs especially through the epithelial cells of the intestinal tract and the renal tubules of the kidneys to promote absorption of the substances it into the blood
      • other co-transport of chloride ions, iodine ions, iron ions and urate ions
    • Counter-transport
      • sodium ion attempt to diffuse to the interior of the cell because of their large concentration gradient
      • Exp : sodium-calcium counter-transport and sodium-hydrogen counter-transport
      • calcium counter transport occurs through all or almost all the cell membrane, with sodium ions moving to the interior and Ca2+ to exterior, both bound to the same transport protein in a counter-transport mode. This is in addition to primary active transport of calcium that occurs in some cells.
      • Sodium-hydrogen counter-transport occurs in several tissues, especially important example is in the proximal tubules of the kidneys, where sodium ions move from the lumen of the tubule to the interior of the tubular cell, while hydrogen ions are counter-transported into the lumen.
      • This mechanism can transport extremely large number of H+.  
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