Endosymbiotic theory – The evolutionary theory that certain eukaryotic organelles originated as separate prokaryotic organisms which were taken inside the cell as endosymbionts. Cellular respiration – The metabolic reactions and processes that take place in a cell or across the cell membrane to convert biochemical energy from fuel molecules into adenosine triphosphate (ATP) and then release the cell's waste products. Lipid bilayer – A membrane composed of two layers of lipid molecules (usually phospholipids).