1.6. ASTROBIO – The Structure of Life: Cells

By | July 6, 2014
1.6. ASTROBIO - The Structure of Life: Cells

[BLANK_AUDIO] We've looked at the structure of life, The basic building blocks from which it's
constructed. Now, let's go a little higher up the
hierarchy and Look at the structures that are formed
form these molecules. So, here are our friends again, these
creatures That we're familiar with of the large
scale. What happens if we take them and we look
at them under the microscope? Well, one of the first thing that we
notice is that they're made up of cells. And this is an image of some skin cells, Essentially packages of reactions and
molecules that form cellular structures. Cellular structures can be something of a
few microns Size, a few thousandths of a millimeter in
diameter. And cells are quite complex, if you break
them apart you'll find Many functions within a cell that are
responsible for allowing life to Do its various chemical reactions. But in essence, there are three major
features of cells that We're going to look at here that are
important from an Astrobiological point of view, in the
sense that these are the Things that define how these cells
function, how they make up life. First of all, there's a membrane, a Membrane to enclose all those chemical
reactions. If you didn't have a membrane you would
simply dissipate into The environment. All of those macro-molecules, or those
large molecules that We saw earlier, were just dispersed into
the environment. So cells essentially have a membrane to
enclose the biochemical reactions. They also need an information storage
system. We've seen that's the deoxyribonucleic
acid, DNA. In order to encode the instructions, that
the Organism needs to keep up its cells to
grow Cells, and to reproduce. And that information storage system also Transmits information from one generation
to another.

So cells must have within them a coding
structure, an information storage system. And they must also have a system for
gaining energy. Where are they going to get the energy to
do all those chemical Reactions and building up those building
blocks that we saw earlier, and how Are those cells going to reproduce? Well, they need energy to do that, so
within Each cell there must be an apparatus to
make energy. So a membrane, an information storage
system, and an Energy producing system are basic to all
cells on Earth. If you look at a mammal cell, you can see
here, it's very, very complex. It includes all sorts of organelles. These are other small structures inside
the cell, that do Other things, important for communication, For example, between different mammal
cells. And for encoding other instructions that
allow the cell To move around and do other complex
patterns of behavior. But even if you look at a complex cell
like this from a mammal, You'll find that it still has those three
basic units, those three Basic ingredients that it needs in order
to be able to function. A membrane to enclose everything, the
information storage System in DNA and a method for gathering
energy. Let's have a look at those three
structures and think about Them in more detail, and why they're
important for making life work. First of all, let's look at The membrane. The cell membrane is made up of compounds
called lipids. We saw earlier how these are building Blocks, structures, complex molecules that
make up life. Lipids are another type of complex
molecule. They're very ingenious, because some of
these lipids have, as you can see here, a Head that's attracted to water and a long
tail that doesn't like water and that tail Wants to get away from water. If you take these molecules and you put
them in Water and you mix them up, those tails,
spontaneously, are

Attracted towards each other because
they're trying to get away From the water, but the heads are
attracted towards the water. And as you can see here, what you get Formed is essentially what's called a
bi-lipid membrane layer. It's a layer of these molecules that
spontaneously forms a membrane in water. And that layer of molecules can form
vesicles, spherical Structures, or near-spherical structures,
that can enclose material inside. So a cell membrane is made up of These phospholipids, these lipid molecules
that spontaneously arrange In liquid water, automatically they
arrange together to Form these, these vesicles, these
membranous structures that can Enclose things within them. Once we've formed a membrane in this way,
we then have to form An information storage system, and this is
made up of DNA, deoxyribonucleic acid. Let's have a little bit of a look at this
molecule and see how it works, see how It functions in its role both as an
information Storage system, but also in transmitting
generation met-, in transmitting Information from one generation to
another. DNA has a backbone, as you can see in this
double Helix, and between this double helix are
what we call base pairs. And these base pairs are made up of Four different molecules: adenine that
we'll call A, Thymine that we'll call T, cytosine that
we call C, and guanine that we call G. These are the four letters of an alphabet
if you like, a code, That instructs the cell on how to make
particular molecules. For example, in one cell if we were To string those particular molecules
together, it might Read something like AATCGCACG, and that
might mean, Build this particular molecule responsible
for eye color. Another piece of DNA might have a Different sequence of these letters,
GGCAGAT, and maybe That means construct a molecule that's
responsible for helping To build bones, and so on, and so forth. By stringing these four letters together

In different combinations, we can create
codes Of long strings of letters that tell the
cell to do particular things. It's a really ingenious code. And particular codes, discrete codes are
called Genes, and genes are a set of instructions That tell the cell to produce a particular
protein or a particular molecule. So DNA is a molecule in which these four
smaller molecules, A, T, C and G, are strung together in a long
code, and Provide the information storage for that
cell, to, in order to be Able to do its housekeeping functions and
make new types of molecules. So you might then wonder, well okay, That explains how DNA acts as an
information storage system. We can see how this molecule can use these
four letters to create long ingenious Codes for producing particular molecules,
but how Does it replicate from one generation to
another? Well, DNA has another very intriguing and
ingenious trick up its sleeve. These four letters that we've just spoken
about, A, T, G, and C, Can only bind to each other in particular
combinations. A can only bind to T and C can only bind
to G. And they bind together to form these base
pairs And you can see a diagram here of base
pairs, Of a long string of these letters on the
left Hand side and the complementary bases on
the other side. So where we've got a C, attached to it, we
have a G. And where there's a T, there's an A, And vice versa. And so throughout the center of DNA there
are these base pairs of TA, and G and C, bound together through
the center of the molecule. Let's have a look at what happens if we
pull that base, Those base pairs apart, if we split the
DNA double helix in two. Well, if we split the double helix in two,
we end up with two single strands. And along one strand, we've got that code
that we just Talked about. And on the other strand we've got the

Code, those letters that previously bound
to the other strand. Well, we've just said that G can only bind
to C. So, if there's a G in that single strand
of DNA, the Cell knows that it has to bind a C back
onto that G. And next to a T it has to bind an A, and
so on and so forth. So if we split the two single strands
apart, we can Immediately synthesize a complementary
strand on both of Those new strands that are now single
strands. And that way we can generate two new
double Helixes, which you can see here, depicted
in this diagram. So the DNA double helix is split in two.
We've ended up with two single strands and Then the DNA knows from that code which of
those four letters it Must bind to the letters already there in
the single strand. And once it's done that, it can
re-synthesize two strands of DNA. And this is the way in which DNA
replicates information from one generation To another, faithfully replicating
information from One cell to another as it divides. And this is how cells divide. And here's a diagram of a bacterium
dividing, and The DNA is splitting in two, the double Helix is splitting into single strands
that are Being used to resynthesize double strands,
and as A result two cells can form from one. This is the basis of DNA replication and
cell division. It's what allows our cells to grow, and
it's what allows organisms to Grow from puppies to large dogs, as we saw
earlier in the course. So we can see now very simply how we can Enclose information in a membrane and make
a cell, how We can use DNA, a very ingenious molecule,
to instruct The cell to produce new molecules, and how
it can divide. What about energy? All cells need energy to grow and to
reproduce, and To get that energy to divide DNA in the
first place. All cells contain systems of gathering
energy and there are

Different types of energy systems. Here are just three of them to give you
some idea. You and I, we're called heterotrophs, and
that means we get our carbon from organic Carbon like steak or other types of Meat, organic molecules, organic molecules
made of carbon. There are other types of organisms in the
biosphere, which we'll Come across throughout this course, that
use different methods of getting energy. For example, the phototrophs, like trees,
that Gather their energy from sunlight,
photosynthetic organisms. These are called phototrophs, literally,
eating sunlight. And then there are organisms called
chemolithotrophs, which Literally means chemical rock eater or
chemical element eater. These are creatures that can get their
energy by eating rocks. And these are confined to the
micro-organisms. These are literally bacteria, for example,
that Get their energy by consuming iron in
rocks. And these turn out to be very interesting
for astrobiology, because the Chemolithotrophs live in very extreme
environments, Like volcanic environments and deep in the Crust of the Earth where there's no
organic material to feed them like You and I need, and there's no sunlight,
so they can't do photosynthesis. Instead they live off the energy from
rocks. These chemolithotrophs are the Sort of organisms we might look for deep
in The crust of other planetary bodies such
as Mars. So these are just three examples of energy
gathering. Photosynthesis from light, heterotrophy by
eating organic molecules, including you And I, and chemolithotrophs,
microorganisms That get their energy from rocks. This is a diagram of the energy gathering
apparatus in cells. We don't need to worry about this, but I
wanted to Show you it, because you can see it's
actually very, very complex. The biochemical machinery to gather energy
from the Environment has an enormous of molecules
involved in it.

But at the end of the day if you Strip away all that complex biochemistry,
it's very, very simple. It's all about extracting energy from
atoms and molecules in The environment to provide the energy for
cell replication and Cell growth. And you can understand that getting energy
from Rocks might be, biochemically, a complex
thing for A cell to do, but at the end of the day,
it's using some pretty simple materials. The elements within rocks that you and I
might find outside in our garden. So life looks very diverse, and it looks
very unrelated. And when you look at it in a large scale,
particularly The sorts of organisms that you can see
with the naked eye, Such as these ones here, it may look Like that diversity makes life very
complex to understand. And, indeed, life is very complex. But at its core, it's very, very simple. And what we've seen in this lecture is
that the Cellular structures from which life is
made, the cellular structures Built up from those building blocks that
we saw in An earlier lecture, are actually common to
all life on earth. Energy gathering, Cellular membranes to enclose the
biochemical reactions, the biology of Life and an information storage system,
DNA, used by life On Earth to transmit ene-, to transmit
information from one Generation to another, and to encode
instructions in the cell. What have we learned then in this lecture? We've learned that life is made of cells.
We've learned that cells have some Basic features: a membrane, information
storage system, And the ability to gather energy,
although, depending Upon the type of cell, it may Also have many other organelles and
complex structures. But these are the three basic building
blocks of cells. And we've learned that all life on Earth
has these characteristics. In other words, all life on Earth has some
very simple

Systems from which the cells are made, and
which have Been the case since the origin of life on
the Earth.