Between the four different types of cells you observed, (plant, animal, protist, bacteria) what do these structures all have in common?

The plant, animal, protist and bacterial cells have many organelles which are common to all these cells.

Cytoplasm: Cytoplasm is the fluid in which all the organelles of a cell float. Cell membrane: Cell membrane can be described as a membrane which separates a cell from its outer environment.Ribosomes: Ribosomes are the sites where the synthesis of proteins takes place. Genetic material: All these cells have genetic material present in them.

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.