Amino acid absorption in the small intestine is an essential biological process for providing the body with the building blocks of proteins necessary for growth, repair, and metabolic functions. The absorption process involves specialized co-transport systems that rely heavily on minerals to drive the active transport of amino acids across the intestinal epithelial cells.

How Amino Acids Are Absorbed in the Intestine

Most amino acids are absorbed via secondary active transport, which couples the movement of amino acids with ions such as sodium (Na+). This co-transport allows amino acids to be absorbed efficiently even when their concentration inside the cells is higher than in the intestinal lumen.

The Role of Sodium in Amino Acid Co-transport

• Sodium Gradient:
  Sodium ions are abundant in the intestinal lumen and their electrochemical gradient is maintained by the Na-K ATPase pump on the basolateral membrane of the epithelial cells. This pump expels sodium out of the cell in exchange for potassium, keeping intracellular sodium levels low.

• Na-dependent Amino Acid Transporters:
  Amino acid transporters on the apical (luminal) membrane use the inward gradient of sodium to transport amino acids into the enterocyte against their concentration gradient. This symport process brings amino acids and sodium ions into the cell simultaneously.

• Energy Utilization:
  The primary energy source is ATP used by the Na-K pump to maintain the sodium gradient, while the amino acids “hitch a ride” with sodium ions through specific transporter proteins.

Role of Other Minerals (Potassium and Calcium)

• Potassium (K):
  Potassium is mainly involved in maintaining cell membrane potential and is transported by the Na-K pump but does not directly drive amino acid co-transport.

• Calcium (Ca):
  Calcium absorption occurs via distinct mechanisms not directly related to amino acid transport. It involves both active and passive absorption pathways but does not contribute to the cotransport of amino acids.

Summary Table: Minerals in Amino Acid Absorption

Conclusion

The mineral that plays the most important role in the co-transport of amino acids across the intestinal epithelium is sodium (Na). Through sodium-dependent symporters, amino acids are efficiently absorbed into the intestinal cells against their concentration gradients, making this mineral crucial for proper nutrient uptake.

Therefore, the correct answer is:
(3) Na