Water's unique properties, stemming from its polarity and ability to form hydrogen bonds, make it essential for life by enabling cohesion, adhesion, thermal regulation, and acting as a universal solvent.
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in the last section we talked about how
carbon and macromolecules are chemically
important for life but there is another
molecule that is equally important and
deserves its own video and that molecule
is water wherever we see water we see
life and we know as humans we need to
constantly consume water to replenish
the supply that is lost through routine
functions like breathing and excretion
what makes water so special which is
similar to why carbon is so special is
its properties let's take some time to
describe a few first and foremost water
molecules are polar this means that
there is an unequal distribution of
charges throughout the molecule causing
one side of the water molecule to be
partially negative and the other side to
be partially positive the electrons
shared between the bonds of the hydrogen
and oxygen are pulled closer to the
oxygen because it is more
electronegative and because the general
shape of a water molecule is uneven the
pull of these electrons causes charges
to form this does not happen with evenly
distributed bonds like we see in methane
where the poles of electrons are evenly
distributed and it does not create polar
ends an easy way to tell if a molecule
is polar or not is to ask yourself can i
draw a straight line through this
molecule and have all positive charges
on one side and all negative charges on
the other if you can do that the
molecule or at least that part of the
molecule should be polar illustrating
that here we can see that the line can
be drawn through the water molecule to
separate the charges but there is
nowhere i can accomplish that with the
methane molecule which means that it is
non-polar okay back to water this
polarity is important because within the
world of physics and chemistry opposite
charges are attracted to each other so
when multiple water molecules get
together the negatively charged side of
the molecule with the oxygen is
attracted to the positively charged
hydrogen end of another molecule this
forms a weak but very relevant bond
called a hydrogen bond the hydrogen is
important here as our hydrogen atom at
this point is a positively charged
proton if this attraction takes place
between this proton and another atom
that is negatively charged we can
classify it as a hydrogen bond another
important property of water is cohesion
which describes the ability of water
molecules to attract and stick to other
water molecules as we can see in this
image this is mainly due to the polarity
of each water molecule and the formation
of hydrogen bonds these negative and
positive charges keep the water
molecules together making them more
difficult to pull apart compared to
molecules of some other liquids cohesion
plays an important role in biological
systems another
similar important property of water is
adhesion which describes how water
molecules can be attracted to and stick
to other surfaces just like cohesion
adhesion can be explained by the
structure of water molecules and their
polarity and hydrogen bonds surfaces
that are polar or possess charged
molecules can attract water molecules
causing them to stick
glass for example is made out of
molecules that have charged ends these
negatively charged portions of the
molecules attract the positively charged
hydrogen ends of the water molecules
this attraction often makes the water
stick to the glass causing a meniscus to
form within a graduated cylinder this
same adhesive property helps pull water
molecules up the stem of a plant which
is referred to as capillary action
adhesion therefore is a property that
in addition to cohesion and adhesion the
polarity and hydrogen bonds found
between water molecules exhibit unique
thermal properties and when i say
thermal here i am referring to heat and
the ability for water to change states
of matter based on the amount of thermal
or heat energy that is within the system
when looking at water specifically we
find that it does a good job of
retaining heat and compared to some
other molecules it takes a decent amount
of energy to change the temperature
enough to cause a state change this is
mainly due to the weak hydrogen bonds
that exist between charges of different
water molecules these bonds keep the
molecules glued together and
collectively take a lot of work to break
and change the water let's say from a
liquid to a gas this means it has a
higher melting and boiling point
especially when compared to other
molecules that have weak interactions
like methane this is important for the
body because there are a large amount of
reactions taking place within the
solution inside of and between cells and
some of these reactions release heat if
water was not able to handle all of this
energy it could change states which
could be very bad to have the water
boiling inside of our body and our cells
but because of those hydrogen bonds and
high boiling point the water can handle
all of the reactions just fine in
addition we also can use water as a
coolant for the body in the process of
sweating if our body gets too hot we
secrete a water solution to sit on the
surface of our skin we know that the
hydrogen bonds between water molecules
need to be broken to change it from a
liquid to a gas and this is accomplished
from the heat radiating off the skin it
can break the bonds over time by
transferring the heat to the water which
in turn has the heat energy leaving the
skin and cooling the body down the last
main property that water has is the
ability to dissolve particles within it
in a liquid state we call this the
solvent property as water is the main
substance that charged particles can
dissolve in this works just like all
other properties due to the fact that
water is polar and has hydrogen bonds
take a salt crystal for example when you
shake some salt into a cup of water and
swirl it around you will notice that it
disappears this isn't a magic trick and
the atoms that were in the salt cube are
not actually gone they were just
separated by the water and spread out
evenly into tiny pieces that you can no
longer see with the naked eye this works
because the salt itself is made out of
sodium and chlorine and when these ionic
bonds are broken apart they separate
into two charged ions sodium has a
positive charge and chlorine has a
negative charge as per our charge
pairing rule the positively charged
sodium ions will be attracted to the
partial negative charge of the water
molecule near the oxygen end and the
negatively charged chlorine atoms
properly called chloride will be
attracted to the partial positive charge
of the hydrogen end of the water
molecule this makes the water the
solvent and the sodium and chlorine the
solute the substance that is dissolved
in the water this separation of the salt
crystal will continue to happen until
each atom is separated assuming there is
enough water molecules to do the job but
this does not only happen with salt it
happens with many other ions within many
different biological systems it is for
this reason we call water a universal
solvent lastly we can relate all of the
information we have learned about water
back to the idea of atoms and molecules
being hydrophilic or hydrophobic if
something is hydrophilic it means that
it is attracted to water and again the
reason for this is because it is an ion
or a molecule that is polar the
attraction of these charges will drive
either the negative or positive end of
the molecule to the opposite charge
within the water molecule if a substance
is hydrophobic it means that it is not
attracted to and actually repels water
not wanting to mix with it this happens
when there is no polarity or charge
present for the charged ends of the
water molecule to be attracted to we
talked about this earlier with
phospholipids which are structures that
make up the cell membrane the head of
the phospholipid has a variable group
that is polar meaning the water
molecules will be attracted to it and
the tails of the phospholipid are
nonpolar for this reason the tails of
multiple phospholipids will end up
sitting next to other tails all because
water will be more attracted to the
polar head and not mix well with the
nonpolar tails but the cell membrane is
not the only place we see this happen if
we take a look at the content within
human blood we see some interesting
interactions or lack of interactions
between the molecules found within the
bloodstream blood plasma is a liquid
part of blood excluding the red blood
cells and one of the primary components
of blood plasma is water your blood is
used as a highway for your body to move
many different nutrients but how this
happens solely depends on the nutrient
and whether it is hydrophilic or
hydrophobic hydrophilic components like
salt glucose oxygen and ionized amino
acids can easily move on their own
directly through the plasma because they
play nice with water based on their
polarity other hydrophobic molecules
like fats and cholesterol are not
soluble in water just like those
phospholipid tails we talked about and
need to form larger complexes with
hydrophilic components to move through
the blood plasma we'll touch on this a
bit more later [Music]
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