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Comment by mmooss | Hacker News Reader

> Comb jellies are ancient marine predators whose hairlike extensions known as cilia refract light as they swim, giving them an iridescent shimmer. They are the largest animals known to use cilia for movement, ...

I've long wondered: What is the difference between cilia and flagella? I've found no authoritative source that differentiates them or that says they are the same (except one paper from decades ago that strongly implies they are the same). It seems like something insiders know implicitly and never bother explaining.

Because a clear distinction is lacking, I don't trust non-expert sources like Wikipedia to be consistent, tor to not confuse them or miss any nuance.

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> What is the difference between cilia and flagella?

Broadly and generally:

• cilia come in large groups, in rows or in sheets; flagella come singly or in pairs (occasionally more)

• cilia are short, and they beat backwards and forwards in a coordinated way, like a Mexican wave; flagellae are long, and in prokaryotes, they spin around their axis.

(Eukaryote flagella -- e.g. spermatozoan "tails" -- are a different structure that just resembles the ones in prokaryotes. Prokaryotes are bacteria and archaea -- no cell nucleus. Eukaryotas have cell nuclei and other organelles, such as mitochondria and chloroplasts. Their flagellae bend and flap, they don't spin.)

Take 1 cell. Can you count the moving hair things? Then they're flagella. Is the beastie covered in lots of them, countless, like fur? Then they're cilia.

Thank you very much.

> in prokaryotes, they spin around their axis.

I read earlier this year about Berg's and Samuel et al.'s (and maybe others') amazing flagellar motor discoveries. I didn't realize they were only prokaryotes; the motor seems like a sophisticated mechanism.

Well prokaryotes have been around a lot longer, so they’ve had a lot more evolutionary time to find perfect proteins for their perfect little uses.

However, we have ATP Synthase. That’s also an ion driven molecular rotary engine. Of course, we use it to convert ionic gradients to energy (ATP).

Both are exceptionally efficient molecules for their tasks.

True! (This stuff is since my BSc and I need to do more reading, but it isn't my job, so there's no time.)

But the thing is that, AIUI, ATP-synthase is in the mitochondria and chloroplasts, so from an evolutionary perspective it is in prokaryotes... just symbiotic ones...

They are us, though. There’s no eukaryotes without mitochondria at all. We are part prokaryote in every cell, apart from the equal number of fully prokaryotic cells in our various microbiomes. Hence the term holobiont for animals like us. We’re pretty much a walking ecosystem.

In some ways, it’s wrong to imagine the branches of life are actually branches. Loki archea and alphaproteobacter together gave us the last eukaryotic common ancestor, and then bacteria continued to live around and in eukaryotes.

That said, mitochondria can’t be called prokaryotes within us. They’ve given up too many genes, and can’t survive on their own.

Of course, the rest of us can’t survive without them either, so it’s a complete mutual situation.

Indeed so. Nicely put!

> prokaryotes have been around a lot longer, so they’ve had a lot more evolutionary time to find perfect proteins for their perfect little uses.

I'm not sure that adds up: It's not that much longer - 500 million years out of ~3.5 billion?

And the evidence shows that Eukaryotes have evolved much faster and further - just look at the Eukaryota domain. And IIRC part of the reason is advantages such as 'sexual' [0] reproduction - rather than just splitting and (cloning?) DNA, combining chromosomes from two parents.

[0] If that's the correct term.

A billion to a billion and a half, maybe two billion if it turns out prokaryotes first appeared as far back as 4 billion years ago (jury is out on this too).

But the first complex multicellular life only appears about 1.6 BYA, and sexual reproduction and cell differentiation only about 1.2 BYA.

And no, I wouldn’t say eukaryotes have evolved faster or further, just differently. It’s questionable they’d be able to do any of this without the mitochondrial and chloroplast endosymbiotic events, and microbiomes have existed since the beginning as well. Framing it as a contest misses the point, in the end.

> What is the difference between cilia and flagella?

There are clear structural differences at microscopic level. they are fueled differently. In short, cilia are like eyebrow hairs, short, weak and simple. They appear in groups of tens or thousands, while flagella are like head hairs, unique or double, rarely more than three, and much longer, thicker and more potent, so faster. Structurally more complex also.

They move in a different way also. Lets say that cilia move like a pendulum and flagella moves like the ribbons in rhythmic Gymnastics

Thanks!

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