What On Earth Is Enantiostasis? | Top Marks QA #1


Enantiostasis is a bit of a weird topic in the HSC. I think a lot of students don’t fully understand it. A lot of students find it confusing. So this video is going to go through what it is, why organisms use it and few examples as well.

So first we’ll look at some definitions. The definition of Enantiostasis is “the maintenance of physiological function despite a changing external environment.” I think the easiest way to understand it is to compare to Homeostasis which is a concept that I think everybody understands quite well in HSC Biology. Homeostasis is “the maintenance of a relatively stable internal environment despite changing external environment.”

Now we know that Homeostasis is performed so that organisms can maintain their physiological function. They want to keep their internal environment stable so that they continue to function, and that’s the easiest way for them to do it. But Enantiostasis is different. They want to maintain their physiological function but they don’t do so by cutting out the changes and just maintaining their internal environment. Instead, they let their internal environment change and they just operate and function well at a big range of different environments

Now, why would we use that? Why do some organisms use Enantiostasis and some use Homeostasis? Well, the reason is that usually Osmoconformers, which are animals that perform Enantiostasis, live in environments that change massively even over the course of a single day. A good example of that is an estuary: where river meets the ocean. This has a complete flux from salt water to fresh water as the tide goes in and out, and that massive change over the course of the day is often just too difficult to resist with Homeostasis. So instead, certain organisms in this environment decide that they’ll let their salt concentration change but just function all the way across the spectrum. An example of that is mussels. They perform an Enantiostasis, they are Osmoconformers. Their internal salt concentration will match the external but they will still function across the entire spectrum. In contrast, an example of osmoregulator which performs Homeostasis is the blue crab and they are going to fight the changes. They’re going to keep the same salt concentration inside throughout the entire day and that’s their way of maintaining function. As I say some organisms do, some organisms don’t but the reason is typically that it’s just too difficult to resist the changes.

Now, specifically to the HSC, we rarely get more than a two or three mark question on this. What you need to know is the definitions and certainly some examples of organisms which will perform one or the other, and also the description of why we might need this – particularly pertaining to estuarine environments as that’s the example of what we’re given. Of course, the reason is that we have the massive change in salt across the day. I hope you found this video useful and if you have any further questions about this or other topics you’d like me to cover, I’d love if you left a comment below. Thanks.