Question

According to an accepted model of plasma membrane, transport during 'symport' or 'co-transport' of sugar or amino acids through cell membrane is when

• A. Na${}^{+}$ ions move in both directions irrespective of concentration gradient.
• B. Na${}^{+}$ ions moves in the direction of its concentration gradient.
• C. Na${}^{+}$ ions moves against the direction of its concentration gradients.
• D. No movement of Na${}^{+}$ ions is involved.

Answer 1

The correct option is B Na${}^{+}$ ions moves in the direction of its concentration gradient.

The movement of glucose and amino acids across the membrane of epithelial cells against a concentration gradient occurs by co-transport with sodium ions. The Na${}^{+}$concentration is kept low within the cells by the action of primary active transport system located in the basal and lateral plasma membrane that pumps Na${}^{+}$ ions out of the cell against the concentration gradient. The tendency for sodium ions to diffuse inward across the plasma membrane down their concentration gradient is 'tapped' by the cells to drive the co-transport of glucose and amino acids into the cell against the concentration gradient. Thus, the correct answer is option B.