VPB 112: General Veterinary Biochemistry (1+1)

ACTIVE TRANSPORT

• In active transport, the cell uses energy to transport the substances. This process requires transport proteins.
• The solutes are usually transported from an area of lesser solute concentration to greater solute concentration. Hence, this process is an  Uphill Process.
• Primary active transport
  • It is powered by direct source of energy. The transport of sodium and potassium by sodium/potassium ATPase in the plasma membrane is an example. ATP provides energy for the transport. The concentration of sodium in the out side of the cell is higher and that in the interior of the cell is lower. This enzyme is responsible for about one third of the basal energy requirement of the human.
  • Osmotic regulation (Acid – Base balance).
• Secondary active transport
  • It is driven by the concentration gradient. for example sodium moves into the cell where its concentration is lower, which facilitates the transport of amino acids or glucose into the cells.
• Endocytosis and Exocytosis
  • Cells need to import and export larger molecules that could not be transported via pores, channels or transport proteins. In eukaryotes, proteins and some other large substances are transported into and out of the cell by endocytosis and exocytosis respectively. In both cases transport is by the formation of specialized type of lipid vesicles.
• Endocytosis
  • It is the process by which macro molecules are engulfed by plasma membrane and brought into the cell within lipid vesicles. The receptor mediated endocytosis begins with the binding of macromolecules to specific receptor proteins in the plasma membrane of the cell. The membrane then invaginates forming a vesicle that contains the bound molecules. It fuses with the lysosome, which contains the hydrolytic enzymes. Inside of lysosome the endocytosed material and the receptor may be degraded.
  • There are two types of endocytosis
    • Phagocytosis
    • Pinocytosis
  • Phagocytosis ( cell eating) Phagocytosis involves the ingestion of large particles such as virus, bacteria, cells, or debris. Phagocytosis occurs only in specialized cells such as macrophages and granulocytes
  • If the contents are liquid and smaller (protein) in size then the process is called Pinocytosis (cell drinking).
• Exocytosis
  • It is similar to endocytosis but the direction of transport is opposite. During exocytosis the materials to be secreted from the cell are enclosed in a vesicle the vesicles then fuse with the plasma membrane releasing the vesicles contents into the extracellular space. The zymogens of digestive enzymes are exported from the pancreatic cell in this manner.

• Group Translocation
  • Group translocation involves not only transport of substance across the membrane but also chemical modification of the substance during the process.
  • The substance is bound more tightly by the transporter prior to its modification. After modification, it is bound less tightly and is readily released.
  • The gama glutamyl cycle that transport amino acids into cells is an example.
• Transduction of extracellular signals
  • The transmission of chemical messages from hormones into cells is known as signal transduction. There are four methods of signal transduction
    • Production of cAMP by adenylate cyclase
    • Poduction of inositol phoshate and diacylglycerol from phosphatidyl inositol bisphosphate
    • Ativation of tyrosine kinase
    • Tansport to the cell nucleus.