When the two molecules in reaction b are joined together what other two molecules are produced?

The synthesis of glutamine from glutamic acid requires the production of an activated ... If proteins A and B have complementary surfaces, they may interact to form the Energy cannot be created or destroyed, but it can be converted into other ... which are often employed to join two molecules together in  reactions.

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.