A2 OCR Biology - Excretion



Excretion = the removal of metabolic waste from the body

Metabolic Waste = consists of waste substances that may be toxic or are produced in excess by the reactions inside cells

Two main substances secreted:

  • Carbon Dioxide (waste product from respiration)
  • Urea (produced in the liver from excess amino acids from ornithine cycle)
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Carbon Dioxide - 3 Main Effects

1. majority carried in blood as hydrogencarbonate ions (synoptic to AS) - this creates hydrogen ions. This occurs in the RBC under the influence of carbonic anhydrase (enzyme). Hydrogen ions combine with haemoglobin, so have to compete with oxygen - too much cardon dioxide reduces oxygen transport

2. carbon dioxide combines directly with haemoglobin to form carbaminohaemoglobin - this molecule has lower affinity for oxygen than normal haemoglobin

3. excess carbon dioxide can casue respiratory acidosis. Carbon dioxide dissolves in the blood plasma and once dissolved it can combine with water to produce carbonic acid which dissociates to release hydrogen ions

Hydrogen ions low pH - blood more acidic. Proteins in blood act as buffer to resist the change in pH. If change is small, pH detected my medulla oblongata which causes increased breathing rate to remove carbon dioxide quicker. Big change (drops below 7.35) = difficult breathing, rapid heart rate and changes in blood pressure ---> respiratory acidosis 

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Urea - Why is it Removed?

  • body cannot store proteins/amino acids (but do contain same amount of energy as carbohydrates)
  • excess amino acids are transported to liver (so they are not wasted)
  • amino group (which can be toxic) is removed by deamination:

amino acid + oxygen --> keto acid + ammonia

  • keto acid put straight into Krebs cycle of respiration or converted to carbohydrate/fat for storage
  • ammonia is highly soluble and toxic so needs to be converted into urea (less soluble and toxic):

ammonia + carbon dioxide --> urea + water

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Structure of the Liver

  • liver cells = hepatocytes
  • oxygenated blood supplied by the hepatic artery
  • blood rich in products of digestion (but not oxygen) supplied by hepatic portal vein
  • blood leaves liver via hepatic vein - rejoins with vena cava
  • bile duct - carries bile (secretion from liver) fomr liver to gall bladder - stored here until needed to aid he digestion of fats in small intestines
  • sinusoid = special chamber where blood from hepatic portal vein and hepatic artery mix
  • bile is released into the bile canaliculi which join together to form the bile duct
  • Kupffer cells = specialised macrophage - move about in sinusoid and involved in the breakdown and recycling of old RBC. One product of the breakdown of haemoglobin is bilirubin - excreted as part of the bile and in faeces - makes poo brown
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Arrangement of Liver Cells


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Liver Cells - Suited to their Function?

Hepatocytes appear to be relatively unspecialised for function:

  • simple cuboidal shape
  • many microvilli on surface (increases SA for absorption of molecules)
  • cytoplasm must be dense and is specialised in the amounts of certain organelles that it contains eg mitochondria, ribosomes
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