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Types of Organisms, Cell Composition, excerpt 1 | MIT 7.01SC Fundamentals of Biology

October 7, 2019


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[email protected] PROFESSOR:So lecture
one, introduction. When people teach biology at
different universities, they teach many different things. You could focus on
the ecosystem. You could focus on memorizing
different phyla. You could focus on
many things. Biology is huge and diverse. We have a certain point of
view in the way we teach 7.012, and really a point of
view in the way that MIT organizes its biology. And so I want to point out the
levels of organization of life and indicate where we will be. You could decide to study
the entire biosphere. By the biosphere, I
mean everything. All the world’s ecosystems
working together to create a happy, healthy living planet. There are, oh I don’t know,
something like 10 to the sixth eukaryotic species on this
planet, more than a million species of things you can see
with your eye, and there are 10 to the God knows how many
species of microbes. And in fact, it’s even hard to
define species for microbes. So it’s a fantastic diversity. And if we think about examples,
the number of examples of biospheres that
we are aware of is one. Currently, it’s the earth. There might be other biospheres,
but we’re not aware of them. Alright, that’s one level
of organization. We will have virtually nothing
to say about that level of organization other than
that we’re fond of it. Next, you can study
an ecosystem. Now what do we mean
by ecosystem? That’s some interacting
community of organisms, like a forest, let’s say. You’ve got trees, and you’ve
got, oh I don’t know, mice and deer. And you’ve got fungi growing
down there on the bottom, and maybe you have, I don’t know,
some MIT student walking through the forest or
something like that. That’s all part of
the ecosystem. Alright, now the next level down
you could think about is whole organisms. So here we’re talking about one
individual, an individual in a species. And one can study how does
that individual work, the levels of physiology within
an organism, the overall integration of signals, the
distribution of oxygen across the body, things like that. This is a popular
topic to study. We’ll have a little bit to say
about physiology, but that won’t be our major focus, that
whole organismal level of physiology. Example of an organism, say a
human, in particular, let’s say an MIT student. OK. Next up, we could study
an individual organ. So by an organ, I’m thinking
about some part of the organism that’s a group of
tissues that are organized together to carry
out a function. So it’s a group of tissues that
are organized to carry out a function. And we have the eye, the
eye on that human. So we’ve got these
muscles here. We’ve got some photoreceptors
there on the back, and they send back nerve terminals
going there. All these tissues working
together to provide vision. OK. Going down one level below
that we have tissues. These are more homogeneous
groups of cells. So we could have,
say, the retina. Then those tissues are
made up of cells. So cells of course are bounded
by a membrane. They have nuclei, at least
in eukaryotes. So let’s say within the retina,
we could pick a photoreceptor cell. So the MIT student is walking
through the forest and with her eye is getting light
on the retina, which is triggering photoreceptor
cells. They each have a distinctive
shape, and that shape– there’s our nucleus. The light comes and impinges
there, sends a signal out the other end. OK. That’s a cell. Then we get down
to organelles. These are distinct components
within a cell. For example, this photoreceptor
is using up a lot of energy. It has mitochondria. These mitochondria in here
are involved in producing efficiently ATP. And then finally, we
have molecules. A wide variety of different
molecules in the cell: sugars and DNA’s and– but I’ll say ATP
will be our example here. Molecules. So if you think about it, we
have sets of levels of organization, all the way up
there from biospheres to ecosystems to organisms to
organs to tissues to cells to organelles to molecules. And biology thinks about all of
those different levels, and in principle, they
all interact. Of course, it’s somewhat hard
to keep in mind, say, the connection between the entire
biosphere and a ATP at any given point. It’s a bunch of levels
going up and down. At MIT, we tend to focus more
on the bottom of the picture than the top of the picture,
and that’s what this class will focus on. You run the risk that you’ll
forget there is a top of the picture, that there are
ecosystems, there are organisms, physiology
and all that. Oh well. What can I say? Why do we focus the bottom
of the picture? Why do we focus so much on, say,
the molecules of life? You’re going to get a lot of
molecules of life in the beginning of this course. We’re going to focus on– we’ll think about organelles
and cells. You’ll hear less about tissues,
less about organs, less about organisms
as a whole. And the reason is life is most
universal down at its bottom. That is what’s shared
across all life. Rather than, say, focus on
the glorious, wonderful differences between all the
different phyla and learning all sorts of genuses and of
all sorts of interesting– we want to focus on that which
is most universal. Now, there are some
differences. We will look differently at
microbe, prokaryotes versus eukaryotes and all. But for the most part, what
we’re trying to do is ask what are universal principles that
apply across all organisms? What are the ways organisms
are built up? Commonalities, like the genetic
code, that applies pretty broadly across all
organisms, commonalities of signalling systems, regulatory
systems. Things like that are what we’re
most interested in, because once you learn that,
it’s much easier to layer on top of it the differences, the
unique differences, that makes a fruit fly a fruit fly
or a tree a tree. So anyway, that’s our
perspective on it, and I hope you like it. But it’s the only prospective
we’ve got, so there you go. Next, a few dates
to keep in mind. Important dates in the
history of life. Again to provide perspective,
we’re not going to focus on lots of them, but a few
that you should know. About 4.0 billion years ago– BYA means billion years ago– the earth cooled. Before that, it was a
hot, molten mass. There until it cooled, there was
no real prospect of life. One of the things I find
absolutely wonderful is that by 3.7 billion years ago, there
was clear evidence of the first life. Whatever it was, it didn’t
take that all long to get life going. Somehow, it can’t be that hard,
although I got to say we don’t quite understand
all of it. Increasingly, people are looking
back to how early life can form out of prebiotic
materials. And these things were
prokaryotic organisms, like bacteria, and they played a
very important role, for example, in changing the entire
atmosphere of the earth to contain oxygen, which made
possible other things. So– sorry, in shaping the content
of oxygen in the atmosphere. So 1.5 billion years ago, one
has a totally amazing invention, which is the
first nucleated cells. These bacteria lack nuclei. Here we have nucleated
cells, eukaryotes. Major event, and it occurred
because of a fusion of two of these prokaryotic-type cells. One takes up residence in the
other, and there is a symbiosis between two
cells and that gave rise to your nucleus. The one that lives inside the
other eventually discarded most of its functions there. Sorry– yes, that gave rise to
a nucleated cell. It’s a fusion between two
different prokaryotic organisms that had different
properties, one of which is the nucleated cell, the other
of which you see in the mitochondria, for example. There’s still a genome in the
mitochondria, for example. And so we can recognize that
there was this prokaryotic fusion event. Notice it took 2 billion years,
more than two billion years, to make eukaryotes and
only 300 million years to make the first life. I find that quite remarkable. What does that tell us? That it was that much harder
to make a eukaryotic cell. Maybe. Or maybe it tells us about
the power of competition. Once you have some life– that the first prokaryotic
cells– it’s so much harder to get anything else that can come
in and compete with it, because it has to be better. Maybe the first life didn’t have
to compete with anything, and it was easy. And then in an innovation like
the eukaryotic cell, well, maybe that was a lot harder to
accomplish, because it had to be better than the prokaryotic
cells it was competing with. I’m not sure, but I find that
difference in time to be kind of a remarkably interesting
thing. That that was short, and
that took a lot longer. 0.5 billion years ago, roughly
speaking, what you have are multicellular organisms
with body plans. Up to this point, you have
single-celled organisms. Now you get multicellular
organisms with body plans. And another remarkable thing,
once body plans, multicellular body plans, are invented, they
explode very quickly. Somehow it was hard to invent
multicellularity, to get cells to work together and be an
organism, but once it was invented, it was not hard to
diversify it into zillions of different body plans
and forms. That tells us something. That as soon as we see
multicellularity, we start seeing huge numbers
of different body plans pretty quickly. Very interesting. A few other important dates. Two other dates. 0.005 billion years
ago are humans. Just to put yourself in your
place on this scale. And 0.00000015 billion years
ago, MIT was founded. Just to put this
in prospective. Alright. Couple of other things
you should know. The types of cells I’ve
been referring to. There’s really two fundamental
types of cells. And I’ve been using
the word already. Let me just get it out there. Prokaryotes. Eukaryotes. Karyo refers to nucleus. Eu- means true. Pro- means primitive, before. So eukaryotic cells– you’re, by the way,
a eukaryote. Right? Just in case you haven’t read
the owner’s manual, you’re eukaryotes. –have true nuclei. Inside the nucleus, there’s your
DNA, which is organized better than I just drew it
there, because, in fact, it’s very, very long and has
to be packed up well. It’ll have organelles
like mitochondria. It could have chloroplasts. It could have other
organelles. It’ll have other things
we’ll talk about– Golgi apparatuses
and endoplasmic reticula and all that. Bacteria don’t have
a true nucleus. It’s not that they’re DNA isn’t
somehow organized, but it doesn’t have a true nucleus
with the wall around it. There’s no wall around that
nucleus like that. They often have interesting
cell walls. A striking thing, although I’ve
drawn them as the same size, these are much smaller. These are 1 to 2 microns. These, 10 to 40 microns
in diameter. And so that means that if we
talk about volume, we cube it. So the volumes of these guys
are something like 1,000 to 60,000 times bigger. Eukaryotic cells are large
compared to prokaryotic cells. Now, remember these are the
guys who were early. These guys come along later. And there are important
differences that we’ll talk about in eukaryotic and
prokaryotic cells. You’re a eukaryote, all animals,
all plants are eukaryotes. Fungi, such as yeast,
et cetera.

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