Eli5: what makes fungi so fundamentally different from plants and animals, they get their own category?

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I mean, the animal part seems intuitive, however I’d like to hear the whole story…

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They aren’t plants. That’s the difference. They’re their own thing, having split off from other eukaryotes before even the “kingdom” part of evolutionary taxonomy was founded.

They don’t make their own food, they don’t undergo photosynthesis, and they don’t have the same cellular structure as plants.

You already know the difference between them and animals, what’s wrong with seeing them as not plants?

They have a fundamentally different structure to their cell walls based on chitin instead of cellulose.

They do not have any capability of generating chemical energy from sunlight via chloroplasts. So, they are consumers rather than producers.

It mostly comes down to the structure of their cells. (A cell is “the basic building blocks of life”, you are comprised of several animal cells, plants are made of plant cells, bacteria are themselves a single cell, etc)

Animal cells don’t have parts to make energy (food) from light. Plant cells can, and also have a very rigid border that keeps them upright.

Fungal cells look like plants, but many species don’t have cells that make food from light, and are generally more similar, structurally, to animal cells. But nobody would call a fungus an animal, they’re different enough to warrant their own kingdom.

That may be an easy line to draw, but for other cases it may not be so. You may be thinking that everything I’m saying is all well and good, but there must be some creatures that are so similar or so unique that they deserve a class of themselves.

We’re pretty decided on the different kingdoms, but taxonomists regularly disagree with each other on most other things.

I’m going to assume you’ve heard of the “tree of life” – the idea that all life has evolved from a common set of ancestors; and has diverged over time.

Animals, plants, and fungi are all “eucaryotes” – life made up of complex cells with a separate nucleus; but they’re only three branches of the eucaryotic life, and there’s a lot of other forms that are in-between them, evolutionary speaking.

Fungi are actually more closely related to animals than they are to plants – though current estimates are that the most recent common ancestor between animals and fungi lives between 1.1 and 1.5 billion years ago. In contrast, fungi and animals share a common ancestor with plants closer to 1.6 billion years ago.

>the animal part seems intuitive

What do you mean by that? That it makes sense that fungi are separate from animals but not plants?

Because fungi are actually *closer* to animals than they are to plants.

There’s a lot of reason that fungi are their own thing, but the tl;dr is that they share a few traits with plants and a bunch with animals. Here’s some of them:

First off, their cells have cell *walls* like plants have instead of cell *membranes* like animals do. Walls are hard structures and membranes are soft. But that’s basically where the differences with animals end.

Like animals, fungi secrete digestive enzymes and can not photosynthesize. Fungi get their energy from eating other stuff. Just like animals. The exact mechanics of it are of course different than most of us animals. But plants just…don’t do that. Even carnivorous plants tend to get some or most of their energy from photosynthesis.

And then at a molecular level, they are a lot like us. When a plant stores energy it does it in the form of starch. Fungi and animals store it as something called *glycogen.*

And remember those cell walls I mentioned before? In fungi, those are made of something called *chitin* which is the same thing that bug shells are made of. Animals create chitin all the time. But plant cell walls are made of *cellulose*, something almost no animal creates.