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Active transport can be seen in the kidneys, at the reabsorption stage in the nephrons. Along the nephron, a large network of capillaries surround the tubules that carry the waste. Substances that the body needs from the waste that can be re-used are reabsorbed into the blood stream.

Mitochondria are the powerhouse of the cell, responsible for generating energy in the form of ATP (adenosine triphosphate) through a process called cellular respiration. The presence of a large number of mitochondria in the tubule cells of nephrons suggests that these cells require a significant amount of energy to perform their function. In the case of nephrons, the primary function is to filter waste products and excess substances from the blood to form urine.

The process of active transport requires the expenditure of energy to move substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This is in contrast to passive transport, which occurs spontaneously and does not require energy input.

The presence of a large number of mitochondria in the tubule cells of nephrons indicates that these cells are actively involved in transport processes. Let's explore why active transport is the correct answer:

1. Reabsorption of Nutrients: One of the main functions of the nephron is to reabsorb essential nutrients, such as glucose and amino acids, from the filtrate back into the bloodstream. This process requires active transport to move these substances against their concentration gradient.

2. Reabsorption of Ions: The nephron also plays a crucial role in maintaining the body's electrolyte balance. Active transport is necessary to reabsorb ions, such as sodium, potassium, and chloride, from the filtrate back into the bloodstream.

3. Secretion of Waste Products: In addition to reabsorption, the nephron is responsible for secreting waste products, such as urea and uric acid, from the bloodstream into the filtrate. This process also requires active transport to move these substances against their concentration gradient.

4. pH Regulation: The nephron helps regulate the pH balance of the body by reabsorbing bicarbonate ions and secreting hydrogen ions as needed. Active transport is involved in these processes to maintain the proper acid-base balance.

In summary, the presence of a large number of mitochondria in the tubule cells of nephrons suggests that these cells require a significant amount of energy to perform their active transport functions. This includes reabsorption of nutrients and ions, secretion of waste products, and pH regulation. Therefore, the nephron is involved in the process of active transport.