Final answer:
The Proximal Convoluted Tubule (PCT) is responsible for enhanced absorption and secretion in the renal tubule system due to its brush border formed by microvilli, which significantly increases the surface area.
Step-by-step explanation:
The portion of the renal tubule system responsible for increasing the surface area for enhanced absorption and secretion is the Proximal Convoluted Tubule (PCT).
In the PCT, simple cuboidal cells with prominent microvilli form a brush border.
This adaptation maximizes the tubule's surface area, allowing for efficient absorption and secretion of solutes such as Na+, Cl-, glucose, and other substances.
A high number of mitochondria within these cells supplies the ATP needed for active transport processes, critical to the PCT's function.