Overview of Archaea, Protista, and Bacteria | Cells | MCAT | Khan Academy
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Overview of Archaea, Protista, and Bacteria | Cells | MCAT | Khan Academy


– In this video, we’re going
to talk about the mostly unicellular organisms:
protists, archaea, and bacteria, which together make up most
of the living things on Earth. So, remember that all living things come from a common ancestor. So, I drew it this way because
one of the most important evolutionary differences
is between the kingdoms of eukaryotes and prokaryotes. And you can notice that the
protists are eukaryotes, while the archaea and
bacteria are prokaryotes. And since “karyote” means
nucleus, and “pro” means before, it means that prokaryotes
came before the nucleus, so they don’t have one. Whereas eukaryotes have a nucleus. So, we’re going to talk
about Archaea first because they’re the oldest ones, and you can remember
they’re the oldest ones because its name sounds like archaic, which just means really old. They were pretty much the first organisms to appear on Earth, that
means they’re used to really extreme environments, because Earth, when it was first starting, barely had any oxygen in the atmosphere, and there were really weird temperatures, and weather conditions. And they were probably also
exposed to a lot of UV light because the atmosphere was still forming. So, you can imagine that
they’re extremophiles, that means that they like
extreme environments. And there are three types;
there are thermophiles, organisms that like really
extreme temperatures. So here I’ve drawn a sun to
represent extreme temperatures. And there’s also halophiles, which like extremely salty environments, kind of represented here
by the puddle of NaCl, and methanogens, which make methane gas, which is also a swamp gas, or a particular type of human gas. And in order to live in
these extreme environments, archaea actually have
very different cell walls and cell membranes than
either bacteria or protists. And for our purposes,
it’s not too important to know what about those
membranes or walls are different, it’s just enough to know
that they are different. So that’s Archaea, now
let’s talk about Protista. Protista are kind of the grab
bag for unicellular organisms and some multicellular ones. It basically is any kind of eukaryote that’s not a plant, a
fungus, or an animal. In fact, really the only thing
that they have in common, is that they pretty much all live in moist or aquatic enviornments. And because they’re part of a grab bag, they have been more easily categorized into photosynthesizing protists, which are related to plants,
and these we call Algae, and non-photosynthesizing protists, which are related to fungi and animals. And these are just
simply called fugus-like, whereas the animal ones you
might have heard of as Protozoa, and this includes amoebas, while the fungal group
includes slime molds. So as you can imagine,
these eukaryotic protists have evolved these unique
ways of getting to nutrients, like photosynthesis, and
they also have evolved unique movement structures,
such as cilia, flagella, and of course the amoebas have evolved their amoeba-like movements. And now we’re going to
talk about Bacteria. Compared to Protista or Archaea, Bacteria can be found
in diverse environments. And what do I mean by
diverse environments? Well, all around us and even inside of us, so that is anywhere from the
computer you’re sitting at, to a nearby lake or ocean, and even inside of us, inside of our gut. And because of this, they
both could help and hurt us. So, we need to get a really
good understanding of bacteria. So let’s focus on talking about bacteria. Here is a general bacterial structure, and we’re gonna go over
each of these parts, which are color-coded, step by step. First, let’s go over
the outside structures of the bacterium. First, there is a capsule,
and in some bacteria, this is a slime layer instead, and the difference
between the two of these is that slime layers, in parentheses, can be washed off, whereas capsules can’t. So, underneath this capsule or
slime layer is the cell wall, and we’re not going to
go into this right now, but this is different for gram-positive or gram-negative bacteria. Underneath that cell wall
is the plasma membrane, and that’s something you’re familiar with because it’s just a lipid bilayer. And now there are only two things left on the outside of the cell that
we haven’t talked about yet. One of them is the prokaryotic flagella. Many bacteria use flagella for movement, and the reason that I
specified prokaryotic flagella is because they are different
from eukaryotic flagella, and this is because it is
made of the protein flagellin. So, just remember that protein’s name. Bacteria need this flagella to get to nutrients via chemotaxis, and chemotaxis is basically
you’re just sensing chemicals, and moving towards or away from it. And you can remember that
because “chemo” is for chemical and “taxi” is like taxiing or moving. And the last external structure, which are all of these
little yellow projections, is the fimbriae, also called a pili. And they’re in parentheses
because not all cells have them. Now, let’s talk about
what’s inside of a bacteria. So, of course it has a
cytoplasm and some ribosomes, but you should notice
that there is no nucleus. It has a nucleoid area for its chromosome, so notice that that’s not a nucleus. The chromosome is not membrane-bound, and is just sitting
inside of the cytoplasm. Also, keep in mind that,
unlike eukaryotic organisms, bacterial chromosomes are made of circular, double-stranded DNA. And some bacteria also have plasmids, drawn here in a darker blue, which are basically extra pieces of DNA. We’re not gonna go into
too much detail now, but these plasmids give important genetic advantages to a bacteria. Now, you might notice that there is one thing left that we didn’t label, and those are those purple circles. They’re inclusion bodies, and
all they do is store stuff for the bacteria, like nutrients. And this is actually super important because prokaryotes have no
membrane-bound organelles. And because they have no
membrane-bound organelles, like mitochondria to make ATP, they actually have to get their nutrients straight from the environment and store them in these inclusion bodies. So, that brings us back to the flagella. In order to get to those nutrients, they use chemotaxis to both get to nutrients or avoid toxins. And now, let’s zoom out to get a picture of
what we’ve talked about. We talked about the differences between Protista, Archaea, and Bacteria, and took a good look at
general bacterial structure.

About James Carlton

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12 thoughts on “Overview of Archaea, Protista, and Bacteria | Cells | MCAT | Khan Academy

  1. I believe that Eukarya, Archaea and Bacteria are a set of Domain and not Kingdoms. I'm taking the class, but I believe that's it.

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