what cell structure is largely responsible for controlling the entry and exit of substances into or out of the cell?

plasma membrane

Explanation:

The plasma membrane is also known as the cell membrane. It is the boundary between the internal components of a cell and its external surroundings. It controls the entry and exit of substances into or out of the cell and thus, facilitates the exchange of substances between the cells and their external environment.

he is very short, and he can only reach halfway up on the elevator buttons. but on a rainy day, he has his umbrella with him, and he uses it to poke the button at the top.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.