Fill in the blanks.
12. the basic units of structure and function in living things
13. a( a group of similar cells that perform a specific function in an organism.
14. nerve cells that carry response information to muscles and organs.
15. carbon dioxide are outputs of cellular respiration.
16. essential to living things because many of the chemical reactions that occur in living things require it.
17. scientists did not discover the cell until was invented because cells are too small to see without one.