Final answer:
Unconjugated (indirect) bilirubin is 2. not water soluble. It binds to albumin in the blood and is transported to the liver where it is converted into water soluble bilirubin diglucuronide, which can then be excreted from the body.
Step-by-step explanation:
No, unconjugated (indirect) bilirubin is not water soluble. It becomes water soluble only after it undergoes a process in the liver where it gets conjugated with glucuronic acid, forming bilirubin mono and di-glucuronide. This process is carried out by the enzyme UDP glucuronyl transferase.
Unconjugated bilirubin must be transported in the body via the plasma bound to albumin to form an albumin-bilirubin complex due to its water insolubility. Once it reaches the liver, it is then taken up and conjugated to form bilirubin diglucuronide, which is indeed water soluble and can be easily excreted out in bile into the intestine. Upon reaching the intestine, most of it is transformed into stercobilinogen and then stercobilin, which colors the feces brown, with a small fraction reabsorbed and excreted in the urine as urobilin.