Science, Tech, Math › Science Biology Prefixes and Suffixes: Ex- or Exo- Share Flipboard Email Print Kaori Kurita / Getty Images Science Biology Basics Cell Biology Genetics Organisms Anatomy Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate By Regina Bailey Biology Expert B.A., Biology, Emory University A.S., Nursing, Chattahoochee Technical College Regina Bailey is a board-certified registered nurse, science writer and educator. Her work has been featured in "Kaplan AP Biology" and "The Internet for Cellular and Molecular Biologists." our editorial process Regina Bailey Updated February 21, 2019 The prefix (ex- or exo-) means out of, away from, outer, external, outside, or exterior. It is derived from the Greek exo meaning "out of" or external. Words Beginning With: (Ex- or Exo-) Excoriation (ex-coriation): An excoriation is a scratch or abrasion on the outer layer or surface of the skin. Some individuals suffer from excoriation disorder, a type of obsessive-compulsive disorder, in which they persistently pick at or scratch their skin causing sores. Exergonic (ex-ergonic): This term describes a biochemical process that involves the release of energy into the surroundings. These types of reactions happen spontaneously. Cellular respiration is an example of an exergonic reaction that happens within our cells. Exfoliation (ex-foliation): Exfoliation is the process of shedding cells or scales from the outer tissue surface. Exobiology (exo-biology): The study of and search for life in the universe outside of Earth is known as exobiology. Exocarp (exo-carp): The outermost layer of the wall of a ripened fruit is the exocarp. This outer protective layer can be a hard shell (coconut), a peel (orange), or skin (peach). Exocrine (exo-crine): The term exocrine refers to the secretion of a substance externally. It also refers to glands that secrete hormones through ducts that lead to epithelium rather than directly into the blood. Examples include sweat and salivary glands. Exocytosis (exo-cytosis): Exocytosis is a process by which substances are exported from a cell. The substance is contained within a vesicle that fuses with the outer cell membrane. The substance is thereby exported to the exterior of the cell. Hormones and proteins are secreted in this manner. Exoderm (exo-derm): The exoderm is the outer germ layer of a developing embryo, which forms skin and nervous tissue. Exogamy (exo-gamy): Exogamy is the union of gametes from organisms that are not closely related, as in cross pollination. It also means to marry outside of ones culture or social unit. Exogen (exo-gen): An exogen is a flowering plant that grows by increasing layers on its exterior tissue. Exons (ex-on): Exons are sections of DNA that code for the messenger RNA (mRNA) molecule produced during protein synthesis. During DNA transcription, a copy of the DNA message is created in the form of mRNA with both coding sections (exons) and non-coding sections (introns). The final mRNA product is generated when non-coding regions are spliced from the molecule and exons are joined together. Exonuclease (exo-nuclease): An exonuclease is an enzyme that digests DNA and RNA by cutting out a single nucleotide at a time from the end of the molecules. This enzyme is important for DNA repair and genetic recombination. Exophoria (exo-phoria): Exophoria is the tendency for one or both eyes to move outward. It is a type of eye misalignment or strabismus that can cause double vision, eye strain, blurred vision, and headaches. Exophthalmos (ex-ophthalmos): An abnormal outward bulging of the eyeballs is called exophthalmos. It is commonly associated with an overactive thyroid gland and Graves' disease. Exoskeleton (exo-skeleton): An exoskeleton is the hard outer structure that provides support or protection for an organism; outer shell. Arthropods (including insects and spiders) as well as other invertebrate animals have exoskeletons. Exosmosis (ex-osmosis): Exosmosis is a type of osmosis where fluid moves from the inside of a cell, across a semi-permeable membrane, to an external medium. The fluid moves from an area of high solute concentration to an area of lower solute concentration. Exospore (exo-spore): The outer layer of an algal or fungal spore is called the exospore. This term also refers to a spore that is separated from the spore-bearing apparatus (sporophore) of fungi. Exostosis (ex-ostosis): An exostosis is a common type of benign tumor that extends from the exterior surface of a bone. These outgrowths may occur on any bone and are called osteochondromas when they are covered with cartilage. Exotoxin (exo-toxin): An exotoxin is a poisonous substance produced by some bacteria that is excreted into their surrounding environment. Exotoxins cause serious damage to host cells and can cause disease in humans. Bacteria that produce exotoxins include Corynebacterium diphtheriae (diphtheria), Clostridium tetani (tetanus), Enterotoxigenic E. coll (severe diarrhea), and Staphylococcus aureus (toxic shock syndrome). Exothermic (exo-thermic): This term describes a type of chemical reaction in which heat is released. Examples of exothermic reactions include fuel combustion and burning.