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Toxin Definition

A toxin is a chemical substance which damages an organism. A toxin may be as simple as an ion or atom which negatively interferes with a cell. A toxin can also be in the form of complex molecules such as the proteins found in snake venom. Still other atoms and chemicals emit radiation, which has toxic effects on an organism. The effects of toxins vary widely in different organisms, and with different toxins. The end result of the strongest toxins is death, due to the damage they cause across the different cells of an organism. Different toxins act in different ways to affect the cells they damage.

In the study of Toxicology, the central motto is that, “all substances are toxic, it is only the dose which matters.” This fact can be demonstrated with water and oxygen. Both substances are normally good and we consider them beneficial for all forms of life. If your body holds too much water, your individual cells will not be able to operate efficiently and your body will slowly “drown”, even if you aren’t submerged in the water. And oxygen, the live giving gas, can be fatal at certain pressures.

Effects of a Toxin

The effects of a toxin are entirely determined by the biochemical reactions which take place when a potential toxin is introduced into an organism. Toxicologists must also take into consideration the environment in which the organisms lives. As mentioned before, things like pressure, heat, and metabolic rate can drastically change the effects of a toxin. Further, not all organisms react to toxins in the same way. Each organisms, even within a species, is essentially a unique biochemical factory. Some organisms are better equipped to handle certain toxins than others.

The specific effects of a toxin are determined by how it interacts with the cells of the organisms. Some toxins work by disrupting ion channels within the cells, while others can destroy the cell membrane or mutate the DNA. All of these conditions will eventually lead to the organism dying if the toxin is not removed. Organisms use their immune systems to target and remove protein-based toxins, while they rely on the filtration of their blood to remove ions and other free radicals. The damage done by a toxin is determined by its structure, atomically.

Types of Toxins

A toxin can come in many different shapes and sizes. It can be as simple as a charged particle, running rampant through the system affecting other reactions, to specific proteins that target the nervous system of a prey animal. Because “toxin” is such a broad category, it is impossible to define their size and shape. Toxins produced by animals are typically used to subdue prey or defend against attack. As such, they have evolved to specifically effect certain animals.

Snake venom, for instance, is a form of biological toxin which is created from a mixture of different proteins. These proteins attack the cells of organisms in different ways. Some snake venoms have evolved to destroy tissue, and they cause massive internal bleeding. Venom from a different species of snake may affect the ion-channels of the nerve cells, causing them to remain open. This essentially paralyses prey, and they cannot use their muscles. Still other venoms attack the muscle cells directly, causing them to convulse continuously. All of these toxins affect the prey in different ways, and not all of them are deadly to all animals.

For instance, many pesticides are designed to kill insects, but not to harm other organisms. These typically work by targeting a portion of the insect anatomy that other organisms do not have. Many pesticides are generally safe to use, and there are even some very natural pesticides which are toxin to insects but not to other organisms. However, some of these toxins have unknown effects on other organisms which can cause be very damaging. For instance, the pesticide DDT was invented for use against insects on crops. The molecule was found to be safe for other organisms and was put into widespread use. It wasn’t until decades later that environmental scientists found that the toxin had been slowly weakening the shells of birds at the top of the food chain. DDT was responsible for a massive loss of raptors across the nation, including the Bald Eagle.

In general, there are 3 main types of toxin. A toxin can be produced by an organism, making it a biological toxin. It may be a single atom or complex molecule produced in nature or in a laboratory, making it a chemical toxin. Lastly, radiation is a special form of toxin which is emitted from radioactive molecules in the environment. Like other toxins, radioactivity disrupts the processes of cells and can lead to death. The many millions of different toxins are classified and categorized differently by different branches of science, but in general they are classified towards their toxicity to humans.

Toxin vs Toxicant

Some areas of science prefer to define toxin as any harmful substance of purely biological origin. Anything produced artificially they refer to as a toxicant. However, other fields of science refer to pesticides like DDT as environmental toxins, and don’t use the term “toxicant” to define the artificial toxins. Due to the nature of the field of Toxicology, and how it sees any substance as a toxin once it becomes harmful, this article refers to both natural and artificial substances as “toxins”.


1. Which of the following is a toxin?
A. Water
B. Poison from a caterpillar
C. Nitrogen
D. All of the above

Answer to Question #1
D is correct. All of the above can be considered a toxin. Poison is an obvious toxin, because in most organisms it creates damage at low levels. The other two substances can also be toxic, at high concentrations. Scuba divers experience the toxicity of nitrogen as they dive too deep. The gas interferes with neurons at pressure, causing the brain to enter a state of euphoria and eventually lose consciousness.

2. Which of the following is the MOST toxic, to a human?
A. Water
B. Poison from a caterpillar
C. Nitrogen

Answer to Question #2
B is correct. In normal amounts, nitrogen and water will not damage a human. We simply breathe the nitrogen in, drink water, and both are expelled and regulated within our body. The caterpillar poison, on the other hand, has evolved to defend the caterpillar against threats like us, and will likely irritate or damage our bodies, making it more toxic to humans.

3. The study of toxins found in fungi is called Mycotoxicology. Fungi produce many chemicals, some of which are toxic to humans, and some which aren’t. What is the purpose of these toxins, in the fungi?
A. Defense against us
B. It is part of the biochemistry
C. No one knows

Answer to Question #3
C is correct. While the substances are slightly toxic to humans, many of them have shown to have beneficial health effects. Some mushrooms are even grown for food, and contain little to no toxins. While it is likely that they are simply a part of the mushroom’s chemistry, communities have been using them for thousands of years as food, medicine, and even as poison. Mycotoxicologist try to answer this question more thoroughly.


  • American Chemical Society. (2018, February 5). Toxicology. Retrieved from
  • Nelson, D. L., & Cox, M. M. (2008). Principles of Biochemistry. New York: W.H. Freeman and Company.
  • Rothman, K. J., Greenland, S., & Lash, L. T. (2008). Modern Epidemiology. Philadelphia: Lippincott Williams & Wilkins.

Cite This Article

MLAAPAChicago Editors. "Toxin." Biology Dictionary,, 01 Apr. 2018, Editors. (2018, April 01). Toxin. Retrieved from Editors. "Toxin." Biology Dictionary., April 01, 2018.

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