Osmoregulation & Excretion (HL) (HL IB Biology)

• The kidney has two roles in the body of mammals
  • Excretion
  • Osmoregulation

Excretion

• Excretion is the process by which toxic waste products of metabolism are removed from the body
• The kidneys are involved with the excretion of nitrogenous waste
  • Nitrogenous waste comes from the breakdown of excess dietary amino acids and nucleic acids
• The waste is first converted into ammonia
• Ammonia is highly toxic; it cannot be stored in the body and must therefore be removed quickly from the body
• Some organisms convert highly toxic ammonia into less toxic urea; urea can remain in the body at low concentrations, but needs to be excreted before it builds up to a harmful level
• Organisms that excrete urea need to dilute it with water to form urine before it is excreted
• Urine is produced in the kidneys

Osmoregulation

• Living organisms need to maintain a safe balance of water and solutes in their cells; this is the osmotic concentration of the cells
• Failure to maintain this balance will mean that an organism's cells could either take on water and burst, or lose water and shrink due to the effects of osmosis
  • Cells with a lower water potential than their surrounding environment will gain water by osmosis and the resulting internal pressure increase could cause the cell to burst
    • Note that plant cells are protected from bursting by their strong cell walls
  • Cells with a higher water potential than their surrounding environment will lose water by osmosis and the resulting drop in internal pressure will cause the cell to shrink
• The units for osmotic concentration are osmoles per litre (osmol L⁻¹)