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The Main Components

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Imagining Other Earths

Princeton University

4.8 (275 ratings) | 33K Students Enrolled

Are we alone? This course introduces core concepts in astronomy, biology, and planetary science that enable the student to speculate scientifically about this profound question and invent their own solar systems. All the features of this course are available for free. It does not offer a certificate upon completion.

Reviews

4.8 (275 ratings)

5 stars
231 ratings
4 stars
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FZ

Mar 23, 2017

Really enjoyed the course. It gave a very comprehensive introduction to Astrobiology and I enjoyed being pushed to write a science fictional short story at the end.

PT

Nov 10, 2016

This is simply fantastic! Such a charismatic lecturer leading us so professionally through heavy subjects in a light manner! Looking forward to more courses!

From the lesson

What is Life?

This lecture introduces different approaches to defining life, a necessary step if we are going to search for life. We then introduce the basic building blocks of biochemistry. One potential way of detecting life is through its byproducts like Methane. We discuss efforts to discuss life on Mars.

Introduction11:09

The Main Components10:17

How Do We Know it When We See it?7:59

Taught By

David Spergel
Charles Young Professor of Astronomy on the Class of 1897 Foundation and Chair

Transcript

Life on Earth seems to have a number of universal properties.

One of those properties is living things tend

to make complex structures out of simple building blocks.

In many ways life is like a child who has a set of Legos

or K'nex or Lincoln Logs or whatever favorite building toy you like, and

takes these Legos and makes complex structures.

This is a city constructed out of Legos in Legoland.

And here is a complex machine made simply out of K'nex, or made

complex made by taking many, many connects and connecting them in novel ways.

So the fundamental building block in either

of these cases is rather simple, but they

are assembled in ways to make complex, rich systems.

And that's how life works.

Life starts with a couple different types of basic building blocks.

You could think of one as Legos, the next

as K'nex, the next is your next favorite children's toy.

We'll look at lipids, proteins, nucleic acids, and carbohydrates,

all as examples of different building blocks that life uses to assemble cells.

And the goal of this class is just to introduce some of the basic concepts here.

So we'll just touch on this briefly and return

back to some of these concepts in later lectures.

So let me just remind you, for those of you who are

not familiar with this, what things some of these building blocks are.

Let's start with lipids.

Lipids are,

examples are fats and oils. Lipids tend to have the property

that one end of the lipid is hydrophilic. It

likes being in water. The other end is hydrophobic.

It doesn't like interacting with water. So if I take a set of lipids,

put it in water, one end of the lipid will go towards water, the other end

will be away from water, and I'll end up constructing a structure

like this where with this relatively simple structure I can build a membrane.

So this might be a good way to build a wall.

Lipids are also good ways to store energy. And humans,

many creatures, store our energy in the form of fats made up of lipids.

Proteins are another important tool in assembling the elements of life.

The proteins on, that are used to, that are

assembled are made up of 20 amino acids Proteins serve

as, often as enzymes, that's one of the things they

do in the body, they speed and control the reaction rates.

They also do things like providing structures.

And our cells assemble quite complicated proteins

out of these simple amino acid building blocks.

And one of the striking things about life on this planet is whether you're looking

at, you know, a simple bacteria, archaea,

a fungus, a dog, or a cow,

they're all going to make the same, use the same

Legos to build the proteins that they need to function.

Its one of the many ways in which we know that life

is all connected, that has all evolved from a common ancestor, because

they're all using the same set of amino acids, often in the

same way across quite a wide range of species to assemble proteins

that carry out similar functions.

Carbohydrates are another thing that we use to assemble complex structure.

The building blocks of complex carbohydrates, polysaccharides are sugars.

And here's an example of a set of sugars

that are called pentoses because they have five carbons

in a chain and by bonding in different orders,

these are two structures with the same chemical properties.

We can build up quite complicated polysaccharides.

And finally, we turn to nucleic acids and life takes

these, what are called nucleotides. A nucleotide is made of one of those five

carbon sugars, one or more phosphates, and nitrogen containing

base to assemble a nucleotide, a relatively

simple building block.

And these nucleotides are what, are assembled

to make large structures, like DNA and RNA.

And the DNA and RNA are the basis of our

genetic code, they serve as what stores and transmits genetic information.

And then code the information needed for construction of proteins.

So one of the universal

properties of life is that it assembles complex structures from

these simple ingredients. And these complex

structures are actually made from some of the most common

things in the universe. You'll notice that the simple building

blocks, the sugars, the nucleotides, the lipids are all assembled from

hydrogen, carbon, oxygen,

nitrogen, silicon, sometimes sulfur or iron.

If you look at this plot, this plot shows the relative

abundance of these elements in the universe versus atomic number.

And you'll notice that the most familiar things, things that life

are made of, are things at the top of the list.

These are the common things. And you can see listed here, you know,

basically the ten most common things in our universe.

And that's what life is made of.

This is why many of us suspect that the ability that life

could evolve somewhere else is quite reasonable,

because life is made up of common things.

Things like gold and platinum and zirconium, or

even things like lithium and beryllium, are pretty rare.

And, but life doesn't

use them that much.

Life usually uses as the key ingredients, relatively simple components.

One of the things we'll talk about later

in the course when we start talking about stars

is one of the great successes of twentieth

century astronomy is the ability to explain this chart.

The ability to explain why the universe is made

mostly of hydrogen and helium, and why carbon and

oxygen and nitrogen are common as products of stellar evolution.

And gold is hard to make.

It takes a very special environment to make gold, and

that's why it's a rare and valuable quantity that, on Earth.

So, now let's turn to the things that we assemble from these ingredients.

And broadly speaking we can divide life, or cells

into two types, prokaryotes, this are simple cells, these are things like

bacteria and archaea, we've got a cell wall, no

nucleus and relatively few components. This

is a relatively small cell and this represents the earliest form of

life. Eukaryotes have more complex structure.

They have a well-defined nucleus that contains the genetic material.

They also have mitochondria and other more complex structure.

And animals, plants, fungi, as well as some simple

things like protozoa are made of a eukaryotic life.

And you'll notice, to go back to some

of the things we talked about earlier, both prokaryotic

life and eukaryotic life have a well defined

cell boundary and all the interesting stuff inside.

There's a well-defined inside and outside to all living things.

Now, I want to end this lecture on a warning about how to do science,

and I want to turn to actually one of my first experiences in the lab.

And this was as a high school student, where I had

just gone through my first few

lectures and learned about prokaryotes and eukaryotes.

And one of our assignments was to learn about all the parts of the cells.

We were then given a slide and told to identify all

the parts of all, everything we could see in that slide.

And having studied all the parts of the cell, and not knowing what I

had in front of me, I looked at this potato starch and tried to

identify all the cell parts in the potato starch, convinced that features

like this were nuclei.

And much to my embarrassment, after I handed in my lab

report and got it back, I discovered I was just making stuff up.

And this to me was an important lesson and something that all

scientists have to be mindful of when they look at their data.

If you have a preconceived notion of what to expect, and what I

expected was my high school biology teacher would hand us cells to classify,

you'll find in your data what you expect to see.

So let me end on this, and then have you turn to the questions

and go back over some of the basic properties of the building blocks of life.

Thanks.

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