15.4 - Excretion

Question 1

What is the difference between excretion and egestion?

Answer 1

Egestion: The removal of undigested food from the body

Excretion: Discharge of metabolic waste

Question 2

Give examples of metabolic waste products excreted

Answer 2

• CO₂ – cellular respiration waste product – excreted from lungs
• Bile pigments – formed from breakdown of Hb in old red blood cells in the liver. Excreted in bile from the liver > small intestine via the gall bladder and bile duct
• Nitrogenous waste products (urea) – formed from breakdown of excess aas in the liver. All mammals produce urea as nitrogenous waste
  
  • Fish produce ammonia as nitrogenous waste
  • Insects produce uric acid

Question 3

What are features of the liver?

Answer 3

• Grows quickly, when damages it regenerates very fast
• Liver has a rich blood supply
  
  • Supplied by the hepatic artery (oxygenated; supplies hepatocytes with oxygen) and the hepatic portal vein – partially deoxygenated - (which carries blood with digested food products from the intestines to the liver; 75% of the blood comes from the HPVein)
• Blood returns to the heart via the hepatic vein

Question 4

What is the structure of a liver?

Answer 4

Hepatocytes (liver cells): Large nuclei; Prominent Golgi Apparatus; Many mitochondria (indicating high metabolic activity)

Sinusoid – Spaces where blood from the hepatic artery and hepatic portal vein is mixed. It is surrounded by hepatocytes

Kupffer Cells – Resident macrophage – helping stop disease such as liver cirrhosis

Hepatocyctes//Bile – Hepatocytes secrete bile from the breakdown of red blood cells into spaces called canaliculi (bile canaliculi) – bile then drains from here to the bile ducts and then finally the gall bladder

Question 5

What is the function of the liver?

Answer 5

Carbohydrate metabolism

Hepatocytes are involved in homeostatic control of glucose. When blood glucose levels rise, hepatocytes are stimulated to convert glucose to glycogen. When blood sugar levels fall, the opposite happens using glucagon hormone

Question 6

What occurs during deamination and transamination?

Answer 6

Liver supports protein metabolism, where hepatocytes synthesis most of the plasma proteins

Transamination – conversion of one aas to another, the diet does not contain all the essential amino acids, transamination solves this

Deamination – the removal of an amine (-NH₂) from a molecule

• The body is unable to store proteins or amino acids.
• The liver initially deaminates amino acids, converts to ammonia first (very toxic), then urea.
• Urea is toxic in high conc. Urea is excreted to the kidney for excretion.
• Remainder of the aas is fed into cellular resp of converted to lipids for storage.

Question 7

What occurs during the ornithine cycle?

Answer 7

Where ammonia becomes urea

Question 8

What occurs detoxification?

Answer 8

• The liver is where detoxification of toxic substances from metabolic waste occurs.
• E.g. the breakdown of hydrogen peroxide. Hepatocytes contain the catalase enzyme.
• Hepatocytes contain alcohol dehydrogenase which detoxifies ethanol to ethanal.
• Ethanal is converted to ethanoate which is used to build up fatty acids to be used in cellular respiration.