Groups of cells that are similar in structure and perform a common or related function form a(n)

tissue

Explanation:

All living organisms are made up of one or more cells. For example, unicellular organisms, such as amoebas, are made up of only one cell. Multicellular organisms, such as human beings, are made up of many cells. And cells in complex multicellular organisms  are organized into tissues,  which refers to similar groups of cells that work together on a specific task.

when a reversible reaction is occurring in both directions at the same rate; the products and reactants do not change in concentration. a chemical reaction where the compound is broken into its constituent parts.