fill in the blanks.

1. _________are the basic units of structure and function in living things
   
2. A(n)__________is a group of similar cells that perform a specific function in an organism.
   
3. __________are nerve cells that carry response information to muscles and organs.
   
4. Carbon dioxide and________ are outputs of cellular respiration.
   
5. __________is essential to living things because many of the chemical reactions that occur in living things require it.
   
6. scientists did not discover the cell until the__________ was invented because cells are too small to see without one.