The conjugation of bilirubin to glucuronic acid in the liver -

Unconjugated bilirubin (hydrophobic) is converted to conjugated bilirubin (hydrophilic) by the process of conjugation with UDP-glucuronic acid in the liver. As conjugated bilirubin is hydrophilic (not lipophilic) it cannot cross the plasma membrane easily (lipophilic or lipid soluble substances cross plasma membrane easily). Thus, unconjugated bilirubin (lipophilic) can cross plasma membrane easily but not conjugated bilirubin (hydrophilic). In neonatal jaundice, conjugation process is immature.