Remember in the last lecture
how I said that the cell is the basic
unit of life? That's because the cell is
literally the smallest thing that
performs
all the functions an organism needs to
have to be considered alive.
A single-celled organism uses energy,
maintains homeostasis,
reproduces, responds, and has adaptations
that are the result
of millions of years of evolution.
However,
you probably know that cells are still
made of stuff.
That stuff is matter: Anything that
occupies space and has
mass. The cell is the smallest unit that
could be considered alive,
but it's made up of plenty of things
that by themselves are not considered
alive.
That would be the cell's organelles like
the Golgi apparatus
or mitochondria or the DNA that's in the
nucleus.
So anything we study in biology - plants,
animals, bacteria,
single-celled organisms - they're alive
and have at least one cell.
In multicellular organisms like you
there's cellular hierarchy and
organization as cells of similar types
form tissues,
tissues of similar functions form organs,
and organs work together in organ
systems like your respiratory system.
However, those cells still have plenty of
non-living components like their
membranes, their organelles, and even the
water they contain in the cytoplasm.
That's all made up of matter. The food
and minerals that living organisms need
to survive
aren't living, but they're also made up
of matter. My point is that everything
that occupies space
and has mass is made up of matter, both
living organisms and
obviously non-living things like rocks
are made up of matter.
Matter can be organized into elements -
basic substances that can't be broken
down by natural means.
You're probably familiar with some
elements like Oxygen, Carbon and, Hydrogen.
These are actually the three most
abundant elements that make up your body.
You're probably pretty familiar with
calcium as well, although it only makes
up about 1.5 percent of your body.
You've probably heard you need to eat
more calcium for healthy bones at some
point in your life,
although someone probably exaggerated
the amounts you need to eat.
Still, you need to consume calcium in
some form
like in dairy because it's a requirement
to build and maintain
healthy bones. All of the elements in
your body
have come from the food you've consumed
or the air
you've breathed. Generally speaking,
elements are rarely found on their own
in nature.
Usually elements join together with
other elements to form compounds.
Water, with its chemical structure of H2O,
isn't an element but a compound of two
elements:
Hydrogen and Oxygen. When writing about
the elements it's normal to symbolize
them with a one or two letter symbol.
Carbon is usually signified as the
letter C, whereas
Calcium is standardized as two letters Ca
where the second letter is always in
the lower case.
When chemical reactions are written, we
normally don't
write out the whole element, but we use
the shorthand
symbols as i've done here to show the
bonding of sodium which is symbolized by
Na
and chlorine which is symbolized by Cl
to form salt or sodium chloride
which is written out as NaCl.
The combination of those two elements to
form
a compound. Humans have recognized that
items made of different elements behave
differently
for thousands of years. We knew that gold
symbolized as Au
would melt at a different temperature
than silver symbolized as
Ag, for example. But it wasn't until the
1800s and early 1900s that we began to
figure out why
elements behave differently. Each element
has its own atom and the different
structures of those atoms determine the
characteristics of the element.
if you held a chunk of pure 100% gold in
your hand
and start dividing it into half over and
over again
you'd eventually reach a point where all
that was left are identical
gold atoms. The same would happen with a
chunk of pure silver.
Gold atoms are physically different than
silver atoms.
And the specific structure of gold atoms,
silver atoms,
carbon atoms, or any other element's atoms
determine the properties of that element
and how it interacts with other elements.
By the way, it's a good time to mention
that any drawing I make here of atoms
or their parts isn't a truly accurate
representation.
Same goes for any image you see in your
textbook - it's pretty much
physically impossible to draw an atom
accurately, but to help you learn and get
ideas across
these representations basically work. In
reality,
electrons, protons, and neutrons are not
the same size,
and electrons don't neatly orbit in
circles.
There's some other stuff ,but yeah just
know these are representations.
Let's take a look at the basic parts of
an atom,  the subatomic particles:
The protons, the neutrons, and the
electrons.
At the center of the atom you'll find
the nucleus of the atom,
not to be confused with the nucleus of a
cell. Sometimes in biology,
terms get reused in a way that
frustrates students, but hey, i'm not the
one who made these up.
In the nucleus of the atom, you'll find
two of the three
subatomic particles - the protons and
neutrons.
Protons are subatomic particles that
carry positive charge,
so i add a little plus sign when i draw
them. Neutrons
are subatomic particles that carry no
charge, so i draw them with an equal sig.n
The protons and neutrons are generally
held together by
atomic forces to make the nucleus, which
is where almost
all of the density of the atom is
located.
Now we have some important definitions
that we use to determine some basic
characteristics of different elements
and their atoms. the atomic number of an element
determines the number of protons an
atom of that element has carbon has an
atomic number of six
so it always has six protons you can
never change the number of protons
or the atomic number of an element. if
you do
you no longer have that original element.
it's probably best to make a study note
in your brain
that says the atomic number tells you
the number of protons.
carbon will always have six protons you
can't have a carbon with seven protons
oxygen has an atomic number of eight so
oxygen will always
have eight protons. now we get to our
second
important number the atomic mass. if you
add up the two subatomic particles that
are in the nucleus,
the protons and the neutrons, you get the
atomic mass of an atom
so atomic mass and atomic number refer
to different things,
keep them separate when studying
outside the nucleus you have the third
subatomic particle these are the
electrons
you might be wondering why do we add
together the protons
and the neutrons to get the atomic mass
but not the electrons.
doesn't the electron contribute to the
mass of an atom?
well technically it does but the thing
about electrons is that they are so much
smaller than the protons and neutrons
that they don't really contribute to the
atomic mass of an atom.
they're that tiny. like i said the
drawings i'm making are useful
but totally inaccurate in this regard. in
no way are electrons
anywhere near the size of the protons
and neutrons.
additionally an important thing
electrons carry
negative charge, that's why i draw them
with a little minus sign.
while you can't change the number of
protons in the atom of a specific
element
and still have that same element you can
change the number of electrons
and neutrons and still have an atom of
the same
element. carbon always has to have six
protons
but you can change the number of
electrons and neutrons
and still have a carbon atom. the first
of these variations will make
is to the number of neutrons. elements
can have atoms with varying numbers of
neutrons
and these different forms are known as
isotopes. now is probably a good time to
mention the periodic table this massive
chart that organizes all known elements
by their properties
i'm not going to draw it here but i will
link a picture in the description below.
the only reason i bring it up now is
because carbon's entry on the periodic
table relates to this conversation on
isotopes
in the middle of the periodic box entry
you have the symbol C. at the top you
have the atomic number which for carbon
is 6.
that's the number of protons in any
isotope of carbon.
note that it's a whole number, no weird
6.0002
or something like that. at the bottom
you'll notice a number that's not
a whole number - that's the atomic mass of
carbon and it's a weighted average
of the masses of all the isotopes in
carbon.
different isotopes of an element are not
equally distributed in the universe.
because 99 percent of carbon atoms are
carbon-12
isotopes, that atomic mass is very
heavily weighted towards 12.
however the contributions of even small
amounts of carbon-13 and carbon-14
are making the atomic mass slightly
above 12.
if i ask you what is the atomic mass of
carbon-13
the answer is 13. but if i ask you what
is the atomic mass of carbon
the answer isn't 12. you're accounting
for all of the carbon isotopes in
existence.
so it's slightly above 12 at 12.0107
as a hint when you see carbon hyphen 12
or
c hyphen 12 that hyphen to the left of
the number is telling you
you're looking at a specific isotope of
that element.
that number also tells you the atomic
mass of that isotope.
so if i were to ask you how many
neutrons are there in carbon-13
you should know that the atomic mass 13
minus the atomic number
(6 for carbon) no matter what isotope
tells you that carbon-13
has 7 neutrons some isotopes are
energetically unstable
sometimes because their nuclei are too
heavy or there's a large difference
between the number of neutrons and
protons.
that's more chemistry stuff than i
needed to know for this class
but as a result that energetically
unstable isotope decays
and will give off particles and energy
until it reaches a more stable
configuration
this is known as nuclear radiation. you
might think that all
nuclear radiation is medically harmful
but that's not true.
you're actually being bathed in nuclear
radiation right now, it's just at levels
that probably won't
impact you. here's a fun example of the
usefulness of that nuclear radiation:
plants absorb carbon from carbon dioxide
a very small fraction of CO2 has
radioactive C-14
as the carbon atom. plants will use that
carbon to grow more of itself so you end
up with a plant that contains C-14.
that plant could get eaten by a deer and
the radioactive C-14 is recycled into
deer tissues like bones. we know that the
half-life of Carbon-14 is roughly 5730
years
which means that it takes that long for
half of your C-14
in a dead organism to disappear. so if
you look at the amount of C-14 in a
fossil deer
bone and compare it to the amount found
in a freshly dead deer
you can approximately estimate the age
of a fossil
using C-14 radiometric dating.
to summarize so far the number of
protons in an atom of an element
never changes: Carbon's atomic number is
six.
it will always have six protons if that
changes,
you no longer have a carbon atom. you can
vary the number of neutrons in an atom
and that gives you different isotopes.
the sum of the protons
plus the neutrons gives you the atomic
mass of an atom.
the atomic mass of an element is the
weighted average of the atomic masses of
its different isotopes.
now we have our last subatomic particle
to discuss in detail
and that is going to be the electrons.
electrons have negative charge, they are
tiny,
and they are located in energy shells.
these shells are filled with electrons
and are filled according to certain
rules.
the first shell can only fit two
electrons
and the second shell can only fit eight
electrons.
if an atom has six electrons, two of
those
will go into the first shell and then
the remaining four will go into the
second shell.
the second shell can only fit eight
electrons, so if an
atom has ten electrons, two go in the
first shell
and then it completely fills its second
shell with the remaining eight.
if an atom has eleven electrons, two go
in the first shell,
eight go in the second shell and then
one goes into the third shell.
there are certainly more complicated
electron shell filling rules
but for the purposes of general biology,
"two in the first shell and eight
in the second shell" will get us where we
need to be.
for the elements we'll be using in this
class, we're also going to go with
another generalization
known as the octet rule. if an atom has
more than two electrons,
and fills more than the first shell it,
is most stable
when its valence shell has eight
electrons despite whether it actually
has
more or less than eight electrons in its
valence shell.
the valence shell by the way is the
outermost electron shell.
so like i said again this is a
generalization and if you happen to be
taking
intro chemistry at the same time as this
course, there are a lot more complicated
rules and you should default to your
chemistry professor.
but for the biology class, octet rule is
fine to use.
we can see the octet rule in practice
with two atoms
first starting with sodium, symbolized by
the symbol
Na. sodium has an atomic number of 11,
which means it has 11 protons. you'll
also have the same number of electrons
as protons starting out
so also 11. fun fact the atomic number
not only tells you the number of protons
it also tells you the number of
electrons in an atom that has
no charge. two go in the first shell,
eight
completely fill the second shell, and
then the valence shell has this one
awkward
leftover electron. now we can look at the
situation in chlorine.
chlorine has an atomic number of 17,
which means it has 17 protons and also
17 electrons. two are going to go in the
first shell,
eight are going to go in the second,
shell, and the third shell is only going
to have
seven. the most logical thing to happen
is for
sodium to donate its one awkward one
to chlorine to help chlorine complete
its valence shell.
once sodium gets rid of that one awkward
electron, that
third ring poofs away and sodium has
fulfilled the octet rule.
however when you do this and you get rid
of an electron
or you accept an electron, you create an
atom that has
charge and this is known as an ion.
in sodium's case, although sodium got rid
of an electron,
it still has its 11 protons that are
standard.
it now has 11 protons and 10 electrons.
this means that it has more positively
charged particles
so now it overall has more positive
charge.
we say that you now have a sodium ion
and technically since you have a
positive sodium
ion you have a cation. when we have a
positive ion by the way you use a plus
sign.
chlorine has the opposite problem.
chlorine has now
accepted an electron. it satisfied its
octet rule
but now it has more electrons than it
does
protons this means that it has more
negatively charged particles.
this means that you are now having an
ion that is negatively charged
symbolized with a minus sign.
we call this an anion. so you have
created these ions,
and this is all due to the fact that
electrons are going to
travel to fulfill the octet rule to get
to that stable number of eight.
in the next video i'll show you what
happens
to these ions and how they might bind
together,
and we'll talk a little bit about
chemical bonding which further goes into
detail about the travels
of electrons
