Excretion

Question 0

What are the two main substances that need to be excreted?

Answer 0

• carbon dioxide

* urea (nitrogenous compounds)

Question 1

What is excretion?

Answer 1

• it is 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

Question 2

Why must carbon dioxide be removed from the body?

Answer 2

• excess carbon dioxide is toxic and must be removed from the body, it has 3 main effects

- carbon dioxide can combine directly with haemoglobin to form carbaminohaemoglobin which has a lower affinity for oxygen than normal haemoglobin 
- majority of carbon dioxide is carried in the blood as hydrogen carbonate ions
       - -> co2 from respiring tissues diffuses into red blood cells and is converted to carbonic acid (catalysed by the enzyme carbonic anhydrase) 
       - -> the carbonic acid splits up to give hydrogen ions and hydrogen carbonate ions 
       - -> H+ ions then associate with haemoglobin to form haemoglobinic acid 
       - -> the H+ ions compete for space on the haemoglobin, if there is too much carbon dioxide in the blood it can reduce oxygen transport 
 - excess carbon dioxide can also cause respiratory acidosis 
       - -> the carbon dioxide dissolves in the blood plasma, once dissolved it can combine with water to produce carbonic acid 
      - -> the carbonic acid dissociates to release hydrogen ions which makes the blood more acidic 
      - -> this can slow the breathing rate and could lead to fainting and dizziness

Question 3

Why must nitrogenous compounds be released from the body?

Answer 3

• the body cannot store proteins or amino acids but amino acids contain almost as much energy as carbohydrates
• as a result they are transported to the liver and the potentially toxic amino acid group is removed (deamination)
• the amino group initially forms the very soluble and highly toxic compound ammonia which is then converted to a less soluble and less toxic compound called urea which can be transported to the kidneys for excretion
• the remaining keto acid can be used directly in respiration to release its energy or it may be converted to a carbohydrate or fat for storage