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They marked it this right here, 2.

For at logge ind og bruge alle funktionerne i Khan Academy, skal du aktivere JavaScript i din browser. You can go to the biology playlist. And maybe we’ll look at the geological record, or the fossil record, and we’ll move these things around in the future. So that’s what they talk about right here on this diagram, the first snowball Earth.

But since we have oxygen, there’s two interesting things that happened once that oxygen accumulated, other than causing this mass extinction event– actually three interesting things. The estimates are 50 to trillion eukaryotic cells. Anything on the land would have just gotten irradiated.

Now the other interesting thing, and levendfs might not care so much about it because we needed the oxygen, is that we think that this was actually the greatest extinction event in the history of Earth. But this, as far as I can tell, is our best current understanding. And “UV” stands for ultraviolet. But give or take a few hundred millions of years, one prokaryote got engulfed by another prokaryote, and said, keki, we do pretty well living together.


And so the only life at this point could occur in the ocean, where it was protected to some degree from the ultraviolet radiation. We’re made up of trillions. So as we go through the Proterozoic Eon, I guess the big markers of it is it’s the first time that we now have an oxygen-rich atmosphere.

Eukaryotic kdmi, remember, these are organisms that have nuclear membranes around their DNA.

Ozonlaget og eukaryoter dukker op i æonen Proterozoikum (video) | Khan Academy

It’s possible that maybe life first started to exist at levrndes end of the Hadeon Eon. And the other big thing is now this is where the ozone forms.

It means beginning or origin. I’m sure that number will be refined.

C.E. Fenger

And once it saturates, it starts to get levendew into the atmosphere. It helps retain heat in the atmosphere. And once it reacts with the oxygen and starts dropping out of the atmosphere as methane, we believe the Earth cooled down.

Once it gets to enough concentration, it begins to actually suffocate. Now, the other really interesting thing that happened in the Archean Eon, and it really has pretty profound effects once we get into the Proterozoic Eon, is that you started to have cyanobacteria produce oxygen.


Now, the other interesting thing that happened– remember, we’re being bombarded with UV radiation from the Sun. Here is the land, right over there in yellow, constantly being bombarded with UV radiation.

And the Archean Eon is also the first eon where we still have rocks from that time. I don’t know if you can see it.

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And “arche” comes from ancient Greek. It goes into the upper atmosphere. We actually talk about respiration that occurs in the mitochondria. The human body, we’re not just one eukaryotic cell. So one, we have, now that oxygen is in the atmosphere, we’re starting to have an environment where eukaryotes could at least exist.

I mean the current theory is that mitochondria is actually descended from kind of an ancient prokaryotic cell, an ancient bacteria.


It’s poisonous to most of the other organisms on the planet that were anaerobic, that did not need oxygen, that actually found oxygen poisonous. And that’s because the cyanobacteria is producing all this oxygen. And methane is an ozone– not an ozone.

So I’m drawing it in purple. And we’ll talk about that in the next video. And, of course, these dates, they might be moved around a few hundred million years as we get more and more data.