The proximal convoluted tubule is part of the proximal tubule, with basically no difference between them except for shape. They are a part of the nephron in the kidneys of mammals.

The proximal tubule is the first part of the neprhon that ultrafiltrate comes into contact with once it leaves Bowman's Space. This part of the nephron serves to reabsorb a lot of the solutes in the ultrafiltrate. It contains, among other things, sodium/glucose transporters, which mediate glucose reabsorption. These require energy to operate, and therefore may be considered ATPases. In diabetes mellitus, there is too much glucose entering the nephron for these exchangers to handle, and some is excreted in the urine.

The proximal tubule reabsorbs a lot of water and some other solutes, especially sodium. It is also the major site of solute secretion. Not much homeostatic control is exerted on this portion of the tubule, as much of the fine-tuning is done in the distal tubule.

These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)