electron microscope: A high-powered, expensive device that uses beams of electrons to bring the finest details of cells into focus.

endocrine system: A system of glands that secrete different types of hormones that help regulate organisms.

endoplasmic reticulum: The ER is a series of canals that connects the nucleus of animal cells to the cytoplasm outside those cells.

equilibrium: The state of a chemical reaction in which the amounts on each side of the reaction have stabilized.

eukaryotes: Organisms — including plants and animals, as well as fungi, protozoa, and most algae — with cells that contain a nucleus and chromosomes.