- [Instructor] We are now going to discuss
molecular variation in cells.
You are probably familiar with the idea
that you have a variation
of genetic makeups in a population.
But even within an organism you have
variation in the types of molecules
that an organism can produce
and when they produce them.
So, for example, we know that we all have DNA,
all organisms, living organisms
that we know about, they have DNA.
I'll just do this as a quick drawing of DNA.
We know that we have genes in our DNA
that code eventually, they go from DNA
to messenger RNA, and then they go to the ribosomes
to be translated into proteins.
And these proteins are a major way
of expressing what is encoded in our DNA.
Now, it turns out that our DNA
will encode for not only multiple proteins
but multiple types of the same protein,
and it can encode for some of these proteins
more under certain circumstances
and other proteins more in other circumstances
based on environmental factors.
Those environmental factors might influence
what part of the DNA is being transcribed
into mRNA, which then is translated
into proteins at different times.
And there's several
very interesting examples of this.
It turns out that hemoglobin,
which you might recognize
as the protein complex that binds
to oxygen in our red blood cells,
that the type of predominant hemoglobin changes
from when we are inside our mothers' wombs
to when we become independent beings.
So, this right over here is a picture
of a hemoglobin molecule.
You see here four heme groups
that each bind two oxygen,
and when you're a fetus the primary type
of hemoglobin is hemoglobin F,
and then once we come out of our mothers' wombs
the hemoglobin F stops getting produced
and we go to hemoglobin A.
Now you might say, well, why do we have
this variation in the type of hemoglobin?
And the answer is, that those are
two different environments.
When a fetus is in the mother's womb,
it's not directly breathing.
It's getting its oxygen from the mother's blood.
The mother's blood does not mix
directly with the baby's blood,
but there's a boundary where you have
the mother's blood here, and I'll say
this is the baby's blood right over here,
and you have the gas exchange of the oxygen
going through that boundary,
and then of course the release
of the carbon dioxide going the other way.
And this environment
where the baby's red blood cells
have to bind to the oxygen
is a relatively low oxygen environment
compared to, say, our lungs,
because it has oxygenated and deoxygenated blood
mixing in that same place,
and it does not have direct access
to, say, the lungs.
And so in this low oxygen environment,
the hemoglobin molecules have to be
really really really good at binding to oxygen.
And we can see that from this diagram
right over here, where the horizontal axis
is the partial pressure of oxygen,
and the vertical axis is how saturated
with oxygen these different
hemoglobin molecules can become.
And you can see that the fetal hemoglobin,
which is depicted by this blue curve,
it gets 50% saturated at a lower partial pressure
of oxygen than the adult hemoglobin.
So one way to think about it,
it is stickier, it binds with that oxygen.
It can pull that oxygen out of the blood
far better, which makes sense
for the environment that the fetus is in.
But once it comes out of the mother's womb,
it doesn't need that stickiness
and there're some drawbacks of that stickiness as well
because it makes it hard for that oxygen
to go into as many of the body's tissues,
and so that's why you have this transition
from hemoglobin F to hemoglobin A.
It's not just hemoglobin where we see
this molecular variation.
Plants and other organisms that conduct photosynthesis
contain multiple types of chlorophyll.
Remember chlorophyll is a very important
molecule in capturing light energy
which can then be used to help synthesize
carbohydrates in things like plants.
And here we see how two different chlorophyll molecules,
both that would be found in plants,
how well they absorb light of different frequencies.
So you can see chlorophyll a is really good
at absorbing the violet bordering on blue light,
while chlorophyll b is better
at the blue-green type of light.
And then you have another peak here
where chlorophyll b is better at absorbing
an orangish red, while chlorophyll a
is better at absorbing, I guess you could say,
a red bordering on infrared wavelength.
And the reason why it is valuable
is that the light that the plant gets,
especially at different times of day,
at different times of year,
is going to have different wavelengths,
and so this just lets the plant
capture more energy that it can use
in photosynthesis.
And these were just two examples
of molecular variation.
In our cellular membranes,
there's multiple types of phospholipids
that are forming the phospholipid bilayer,
and those multiple types are,
they have different levels of how fluid
they are at different temperatures.
And there're animal studies that show
that the variations change
depending on the conditions.
For example, a cold-blooded animal
might have more of the fluid phospholipids
when it is very cold so that the membranes
don't become overly rigid.
But I will leave you there.
This is just to appreciate this idea
that we have all sorts of molecular variation
inside organism cells, and it allows
those organisms to better adapt
to their environment or different stages
of their development.