Final answer:
The cells of the proximal convoluted tubule contain abundant mitochondria to support the active transport necessary for the reabsorption of substances from the filtrate.
Step-by-step explanation:
The cells of the proximal convoluted tubule contain abundant mitochondria because of the importance of active transport to reabsorption. Within the proximal convoluted tubule, simple cuboidal cells with prominent microvilli cover the luminal surface, significantly increasing the surface area for the reabsorption and secretion of substances such as sodium ions (Na+), chloride ions (Cl-), and glucose. This reabsorption is a key function of the proximal convoluted tubule, which is the most active part of the nephron in terms of reabsorption and secretion. Since active transport is energy-intensive, these cells contain a high concentration of mitochondria to meet the energy requirements in the form of ATP (adenosine triphosphate).