Urea as a Waste Product (Edexcel International A Level Biology)

Formation of urea

• The body cannot store excess protein or amino acids
• Liver cells, or hepatocytes, are responsible for removing the amino group from excess amino acids in a process called deamination
• During deamination the amino group (-NH₂) of an amino acid is removed, together with an extra hydrogen atom
• These combine to form ammonia (NH₃)

amino acids ammonia + keto acid 

• • The remaining keto acid may enter the Krebs cycle to be respired, be converted to glucose, or converted to glycogen / fat for storage
  • This means that the amino acids within the protein will not be wasted but can function as a useful source of energy
• Due to its toxicity ammonia is quickly converted into less toxic urea
  • This happens in a series of steps known as the ornithine cycle, which can be summarised as

ammonia + carbon dioxide urea + water 

• Urea forms part of urine and can be excreted by the kidneys
• Urea is filtered out of the bloodstream into the Bowman's capsule of the nephron by the process of ultrafiltration

During the process of deamination the amino group (-NH₂) is removed from the amino acid and converted into ammonia (NH₃)

Ultrafiltration

• Within the Bowman’s capsule of each kidney nephron is a structure known as the glomerulus; these two structures together carry out the process of ultrafiltration
• The blood in the glomerulus is at high pressure
  • The afferent arteriole that enters the glomerulus is wider than the efferent arteriole that leaves it, increasing the blood pressure as the blood flows through the glomerulus
• This high pressure forces small molecules in the blood out of the capillaries of the glomerulus and into the Bowman’s capsule
• The resulting fluid in the Bowman's capsule is called the glomerular filtrate
• Large molecules such as proteins remain in the blood and do not pass into the filtrate

During the process of ultrafiltration small molecules are forced out of the capillaries into the Bowman's capsule

• The structures within the glomerulus and Bowman's capsule are especially well adapted for ultrafiltration
• The blood in the glomerular capillaries is separated from the lumen of the Bowman’s capsule by two cell layers with a basement membrane in between them
  • The first cell layer is the endothelium of the capillary; gaps between the cells allow fluid to pass through
  • The next layer is the mesh-like basement membrane
  • The second cell layer is the epithelium of the Bowman’s capsule; gaps between the cells allow the passage of small molecules
• As blood passes through the glomerular capillaries the gaps between the cells and the mesh-like basement membrane allow substances dissolved in the blood plasma to pass into the Bowman’s capsule
  • The substances that pass into the Bowman’s capsule make up the glomerular filtrate
  • The main substances that form the glomerular filtrate are
    • Amino acids
    • Water
    • Glucose
    • Urea
    • Salts (Na⁺ and Cl^- ions)
• Red and white blood cells and platelets remain in the blood as they are too large to pass between the cells
• The basement membrane stops large protein molecules from getting through