You are examining the data from two time points taken in an experiment. At time 1 there was an extracellular concentration of Na of 115 mM outside the cell and 8mM inside the cell. An amino acid, lycine, is at 20mM outside and 40mM inside. At the second time measured Na is 100mM outside the cell and 23 mM inside the cell. Lycine is 15 mM outside the cell and 45mM inside. What is the best explanation

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Explanation:

The best explanation would be that both Na and the amino acid lycine are simultaneously diffusing (in bound form) from the outside to the inside of the cell.  

Considering the Na alone, one would see that the concentration outside the cell decreased in equal amount (15 mM) as the increase in the intracellular concentration of the same molecule.

Also considering the amino acid alone, the extracellular concentration increased by the same magnitude (5 mM) as the decrease in the intracellular concentration of the same molecule.

Hence, it could only be that both molecules bind together and diffused from the region of higher concentration (outside of the cell) to the region of lower concentration (inside).