What is the pentose phosphate pathway and what are its main functions?

The pentose phosphate pathway (PPP) branches from glycolysis at glucose-6-phosphate. The oxidative phase produces NADPH (for biosynthesis and antioxidant defense) and ribulose-5-phosphate, releasing CO2. The non-oxidative phase interconverts sugars, producing ribose-5-phosphate for nucleotide synthesis and glycolytic intermediates. The pathway is regulated by NADP+ availability - when NADPH is needed, the oxidative phase predominates; when ribose is needed more than NADPH, sugars can be converted via the non-oxidative phase. The PPP is important in rapidly dividing cells (for nucleotides), red blood cells (for NADPH to maintain reduced glutathione), and lipogenic tissues (NADPH for fatty acid synthesis).