0:05 [Applause] [Music]
0:21 [Music]
0:23 in the last section we talked about how
0:26 carbon and macromolecules are chemically
0:28 important for life but there is another
0:30 molecule that is equally important and
0:33 deserves its own video and that molecule
0:36 is water wherever we see water we see
0:39 life and we know as humans we need to
0:41 constantly consume water to replenish
0:44 the supply that is lost through routine
0:46 functions like breathing and excretion
0:49 what makes water so special which is
0:51 similar to why carbon is so special is
0:54 its properties let's take some time to
0:56 describe a few first and foremost water
0:58 molecules are polar this means that
1:00 there is an unequal distribution of
1:03 charges throughout the molecule causing
1:05 one side of the water molecule to be
1:07 partially negative and the other side to
1:09 be partially positive the electrons
1:11 shared between the bonds of the hydrogen
1:14 and oxygen are pulled closer to the
1:16 oxygen because it is more
1:18 electronegative and because the general
1:21 shape of a water molecule is uneven the
1:24 pull of these electrons causes charges
1:26 to form this does not happen with evenly
1:29 distributed bonds like we see in methane
1:31 where the poles of electrons are evenly
1:33 distributed and it does not create polar
1:36 ends an easy way to tell if a molecule
1:38 is polar or not is to ask yourself can i
1:40 draw a straight line through this
1:43 molecule and have all positive charges
1:46 on one side and all negative charges on
1:48 the other if you can do that the
1:50 molecule or at least that part of the
1:52 molecule should be polar illustrating
1:54 that here we can see that the line can
1:56 be drawn through the water molecule to
1:58 separate the charges but there is
2:00 nowhere i can accomplish that with the
2:03 methane molecule which means that it is
2:05 non-polar okay back to water this
2:08 polarity is important because within the
2:10 world of physics and chemistry opposite
2:12 charges are attracted to each other so
2:14 when multiple water molecules get
2:16 together the negatively charged side of
2:19 the molecule with the oxygen is
2:21 attracted to the positively charged
2:24 hydrogen end of another molecule this
2:26 forms a weak but very relevant bond
2:29 called a hydrogen bond the hydrogen is
2:32 important here as our hydrogen atom at
2:34 this point is a positively charged
2:36 proton if this attraction takes place
2:39 between this proton and another atom
2:41 that is negatively charged we can
2:43 classify it as a hydrogen bond another
2:46 important property of water is cohesion
2:48 which describes the ability of water
2:51 molecules to attract and stick to other
2:53 water molecules as we can see in this
2:56 image this is mainly due to the polarity
2:58 of each water molecule and the formation
3:01 of hydrogen bonds these negative and
3:03 positive charges keep the water
3:05 molecules together making them more
3:08 difficult to pull apart compared to
3:11 molecules of some other liquids cohesion
3:13 plays an important role in biological
3:15 systems another
3:17 similar important property of water is
3:19 adhesion which describes how water
3:22 molecules can be attracted to and stick
3:25 to other surfaces just like cohesion
3:27 adhesion can be explained by the
3:29 structure of water molecules and their
3:32 polarity and hydrogen bonds surfaces
3:34 that are polar or possess charged
3:37 molecules can attract water molecules
3:39 causing them to stick
3:41 glass for example is made out of
3:43 molecules that have charged ends these
3:45 negatively charged portions of the
3:48 molecules attract the positively charged
3:50 hydrogen ends of the water molecules
3:52 this attraction often makes the water
3:54 stick to the glass causing a meniscus to
3:57 form within a graduated cylinder this
3:59 same adhesive property helps pull water
4:01 molecules up the stem of a plant which
4:04 is referred to as capillary action
4:06 adhesion therefore is a property that
4:21 in addition to cohesion and adhesion the
4:23 polarity and hydrogen bonds found
4:26 between water molecules exhibit unique
4:28 thermal properties and when i say
4:30 thermal here i am referring to heat and
4:32 the ability for water to change states
4:35 of matter based on the amount of thermal
4:38 or heat energy that is within the system
4:40 when looking at water specifically we
4:42 find that it does a good job of
4:44 retaining heat and compared to some
4:47 other molecules it takes a decent amount
4:49 of energy to change the temperature
4:51 enough to cause a state change this is
4:53 mainly due to the weak hydrogen bonds
4:56 that exist between charges of different
4:58 water molecules these bonds keep the
5:00 molecules glued together and
5:02 collectively take a lot of work to break
5:04 and change the water let's say from a
5:07 liquid to a gas this means it has a
5:09 higher melting and boiling point
5:11 especially when compared to other
5:13 molecules that have weak interactions
5:15 like methane this is important for the
5:16 body because there are a large amount of
5:18 reactions taking place within the
5:22 solution inside of and between cells and
5:24 some of these reactions release heat if
5:26 water was not able to handle all of this
5:28 energy it could change states which
5:30 could be very bad to have the water
5:33 boiling inside of our body and our cells
5:35 but because of those hydrogen bonds and
5:38 high boiling point the water can handle
5:39 all of the reactions just fine in
5:42 addition we also can use water as a
5:44 coolant for the body in the process of
5:46 sweating if our body gets too hot we
5:48 secrete a water solution to sit on the
5:50 surface of our skin we know that the
5:52 hydrogen bonds between water molecules
5:54 need to be broken to change it from a
5:57 liquid to a gas and this is accomplished
5:59 from the heat radiating off the skin it
6:01 can break the bonds over time by
6:03 transferring the heat to the water which
6:06 in turn has the heat energy leaving the
6:08 skin and cooling the body down the last
6:10 main property that water has is the
6:13 ability to dissolve particles within it
6:14 in a liquid state we call this the
6:17 solvent property as water is the main
6:19 substance that charged particles can
6:22 dissolve in this works just like all
6:23 other properties due to the fact that
6:26 water is polar and has hydrogen bonds
6:28 take a salt crystal for example when you
6:30 shake some salt into a cup of water and
6:32 swirl it around you will notice that it
6:35 disappears this isn't a magic trick and
6:37 the atoms that were in the salt cube are
6:39 not actually gone they were just
6:41 separated by the water and spread out
6:44 evenly into tiny pieces that you can no
6:46 longer see with the naked eye this works
6:48 because the salt itself is made out of
6:51 sodium and chlorine and when these ionic
6:53 bonds are broken apart they separate
6:56 into two charged ions sodium has a
6:58 positive charge and chlorine has a
7:00 negative charge as per our charge
7:03 pairing rule the positively charged
7:05 sodium ions will be attracted to the
7:06 partial negative charge of the water
7:09 molecule near the oxygen end and the
7:11 negatively charged chlorine atoms
7:13 properly called chloride will be
7:16 attracted to the partial positive charge
7:18 of the hydrogen end of the water
7:20 molecule this makes the water the
7:22 solvent and the sodium and chlorine the
7:25 solute the substance that is dissolved
7:27 in the water this separation of the salt
7:29 crystal will continue to happen until
7:32 each atom is separated assuming there is
7:34 enough water molecules to do the job but
7:36 this does not only happen with salt it
7:39 happens with many other ions within many
7:41 different biological systems it is for
7:44 this reason we call water a universal
7:47 solvent lastly we can relate all of the
7:49 information we have learned about water
7:51 back to the idea of atoms and molecules
7:54 being hydrophilic or hydrophobic if
7:56 something is hydrophilic it means that
7:58 it is attracted to water and again the
8:01 reason for this is because it is an ion
8:03 or a molecule that is polar the
8:05 attraction of these charges will drive
8:07 either the negative or positive end of
8:09 the molecule to the opposite charge
8:11 within the water molecule if a substance
8:13 is hydrophobic it means that it is not
8:17 attracted to and actually repels water
8:19 not wanting to mix with it this happens
8:21 when there is no polarity or charge
8:22 present for the charged ends of the
8:24 water molecule to be attracted to we
8:26 talked about this earlier with
8:27 phospholipids which are structures that
8:29 make up the cell membrane the head of
8:31 the phospholipid has a variable group
8:32 that is polar meaning the water
8:35 molecules will be attracted to it and
8:37 the tails of the phospholipid are
8:39 nonpolar for this reason the tails of
8:41 multiple phospholipids will end up
8:44 sitting next to other tails all because
8:45 water will be more attracted to the
8:48 polar head and not mix well with the
8:50 nonpolar tails but the cell membrane is
8:52 not the only place we see this happen if
8:54 we take a look at the content within
8:56 human blood we see some interesting
8:58 interactions or lack of interactions
9:00 between the molecules found within the
9:02 bloodstream blood plasma is a liquid
9:04 part of blood excluding the red blood
9:06 cells and one of the primary components
9:08 of blood plasma is water your blood is
9:11 used as a highway for your body to move
9:12 many different nutrients but how this
9:15 happens solely depends on the nutrient
9:17 and whether it is hydrophilic or
9:19 hydrophobic hydrophilic components like
9:22 salt glucose oxygen and ionized amino
9:25 acids can easily move on their own
9:27 directly through the plasma because they
9:29 play nice with water based on their
9:31 polarity other hydrophobic molecules
9:34 like fats and cholesterol are not
9:35 soluble in water just like those
9:37 phospholipid tails we talked about and
9:39 need to form larger complexes with
9:42 hydrophilic components to move through
9:43 the blood plasma we'll touch on this a
9:45 bit more later [Music]
10:16 [Music] you
