Question

at the resting membrane potential, a steep concentration gradient for sodium exists between the exterior and interior of cells due the action of the na⁺/k⁺ atpase. this electrochemical gradient is utilized as an energy source to transport a variety of other ions and molecules across the membrane through secondary active transport. some common secondary active transport proteins include: the sodium - phosphate cotransporter, the sodium - iodide cotransporter, the sodium - chloride cotransporter, the sodium - calcium antiporter, and the sodium - glucose cotransporter. each secondary active transport protein has its own unique ion/substrate coupling ratio, or the number of ions moving down their electrochemical gradients compared to the number of molecules actively transported against their gradients. for example, the sodium - chloride cotransporter has a coupling ratio of 1:1 with each sodium ion driving the movement of one chloride ion. the sodium - iodide symporter moves two sodium ions coupled with each iodide ion. the sodium - glucose symporter also displays a 2:1 coupling ratio. the sodium - phophate cotransporter has a 3:1 coupling ratio, moving three sodium ions for every inorganic phosphate. the presence of these cotransporters in a variety of cells and tissues is critical for the maintenance of homeostasis. 1. the sodium - iodide symporter plays a role in the accumulation of iodide in the thyroid gland. here, one iodide gets converted to one iodine, which is utilized for the formation of either of the two types of thyroid hormones, t₃ and t₄. t₃ and t₄ are named after the number of iodines found in each of these hormones. to produce a single molecule of t₃, a total of _ sodium ions must move down their concentration gradients by secondary active transport. the movement of iodide ions occurs in the _ direction as sodium ions. o three; opposite o three; same o four; same o two; same o six; opposite o two; opposite o four; opposite o six; same 2. ouabain is a poison that binds to the na⁺/k⁺ pump in the cell membrane and inhibits its action. the expected result of this poison on secondary active transport is

Explanation:

1. The sodium - iodide symporter has a 2:1 coupling ratio (2 sodium ions per iodide ion). T3 has 3 iodines. So for 3 iodides to enter the cell to form T3, 6 sodium ions must move down their concentration gradients as they are coupled with iodide ions in the same direction (symporter).
2. The Na⁺/K⁺ pump creates the electrochemical gradient for sodium which is used as an energy source for secondary active transport. If ouabain inhibits the Na⁺/K⁺ pump, the sodium electrochemical gradient will not be maintained, and secondary active transport which depends on this gradient will be impaired.

Answer:

1. six; same
2. Impairment of secondary active transport as the sodium electrochemical gradient cannot be maintained due to inhibition of the Na⁺/K⁺ pump.