When neurons "fire," the first thing that happens is that sodium ions enter the cell. this does not require an input of energy. what can you infer from this?

Answer;

One can infer that membrane transport proteins are required in order for neurons to fire.

Explanation;

Transport proteins are proteins that transport substances across biological membranes. Transport proteins are found within the membrane itself, where they form a channel, or a carrying mechanism, to allow their substrate to pass from one side to the other.

-Transport proteins transport materials such as; ions such as sodium and potassium; sugars such as glucose; proteins and messenger molecules; and many more. In neurons, they  play a fundamental role in the functioning of nerve cells. These transporters, a third class of membrane transport proteins, move a wide variety of ions and molecules across cell membranes.