Importance of water for life | Chemistry of life | AP Biology | Khan Academy

Importance of water for life | Chemistry of life | AP Biology | Khan Academy

October 8, 2019 15 By Ewald Bahringer


– [Instructor] When we
look out into the cosmos for alien life, many folks
look for signs of water on moons or planets. And that’s because life as we know it is dependent on water. And to understand that, we just have to take a closer look at some of the properties of water. So what you see here are
some molecules of water. This might be a review for you. Every water molecule has one oxygen atom. And it is bonded to two hydrogens. So that is a hydrogen, and
that is a hydrogen as well. And the nature of that bond, it is a covalent bond, which means that the
oxygen shares electrons with each of the hydrogen atoms. But oxygen is more electronegative, and that’s just a fancy way of saying that even though those
electrons are shared, they’re going to be spending more time around the oxygen than
around the hydrogens. One way to think about it is oxygen likes to hog electrons
more than hydrogen does. And since the electrons
will spend more time around the oxygen than
around the hydrogen, and because it’s a bent molecule with the hydrogens on
one side of the molecule, what happens is the side
where the oxygen is, where the electrons spend more time, that gets a partially negative charge. So this is the lowercase
Greek letter delta. That just means partially negative charge. And then the sides
where the hydrogens are, those acquire a partial positive charge. And so what you see here is that a water molecule is
not charged in aggregate. But either side has a partial charge, so it is a polar molecule. And so you can imagine when you put a bunch of water molecules together what might happen? Well, the partially positive side of one water molecule
where the hydrogens are would be attracted to the
partially negative side of another water molecule. And so they would be attracted, and this is known as a hydrogen, hydrogen, hydrogen bond. And I could keep drawing that. This is going to be
partially positive here. This is going to be partially negative. They will attract. This oxygen end is going to be attracted to that hydrogen end. This oxygen end is going to be attracted to the that hydrogen end as well. And so it’s this hydrogen bonding that gives water a lot of the properties that make it special
that, as far as we know, for harboring life or for even
allowing life to be possible. Life as we understand it
needs a fluid environment. Things move around and
bump into each other. And it’s these hydrogen bonds, when the temperature and
conditions are appropriate, that allow water to be in that liquid form where they’re strong enough so that the water stays together, but they’re weak enough so that they allow the water molecules to
flow past each other. And not only does it provide
a good fluid environment, it’s a very good solvent. Water is often known as the universal, universal solvent, but it’s
worth putting a disclaimer here. Even though people say it
is a universal solvent, that does not mean that
it dissolves everything. Water does dissolve more
things in its liquid state than anything else we know about. But there are many molecules
that it cannot dissolve well. The things that it does dissolve well are polar molecules or
things that have a charge. For example, when sodium
chloride dissolves in water, a sodium ion is positive, so that is positively charged. And so you can imagine
it might be attracted to the side of the water molecules where the oxygen is. But it dissolves well. But things that don’t have charge don’t tend to dissolve well in water. But even the property that there’s certain things
that it does not dissolve is also good for life. Later on in biology we’re going to study phospholipid bilayers where
you have these molecules where one end is hydrophilic, which means it’s attracted
to water molecules. And then the other ends are hydrophobic, which means they’re not
attracted to water molecules. And many evolutionary biologists believe that this property of having one side that’s hydrophilic and one
side that’s hydrophobic would have allowed these molecules to start collecting into membranes, eventually forming these
spherical membranes which could be the containers
for early cellular life. Now, another property of water which makes it very suitable for life is it’s high heat capacity. Sometimes you’ll hear people say it has a high specific heat. The specific heat is the
amount of energy needed to raise one gram of water
by one degree Celsius. And you might say why
does that matter for life? Well, many life forms can only operate within a certain range of temperatures. And so if it was really easy
to raise the temperature of water really high or very
low temperatures very fast, well, that would make it much harder for life to operate within water, or even life to be made up of water. A related idea to this is that water also has a
high heat of vaporization. We talk more about this
in detail in other videos, but this is talking about how much energy does it take for water to
go from its liquid form to its gas form. And this has proven
valuable in many life forms for a form of cooling
where the vaporization of water, evaporative cooling, can take heat away from an organism so that it doesn’t overheat. Other properties that
are important about water include cohesion and adhesion. Cohesion is the property
of water molecules that is attracted to
other water molecules. And you saw it here
with the hydrogen bonds. But then when you look at a macroscale, you’ll see things like
water droplets form. You’ve all seen water
droplets, or dew droplets. These droplets couldn’t form if not for the cohesion of water. And even one drop can be an environment in which thousands of
microorganisms can live. Adhesion is the property of water where it can adhere to other things. You might have seen this
in a glass test tube where it looks like the water is kind of crawling up the top of the sides. And that’s because some of the polarity of the glass molecules of the test tube. But this property,
along with the cohesion, is what allows water
to transport nutrients, say, from the roots of a tree all the way to the top of a tree. These properties are also an action in our own blood vessels, when you get to the really small blood
vessels, the capillaries. And that is called capillaries ’cause you have capillary action of water, which is due to its
cohesion and its adhesion. A last property of water, and this is not an exhaustive list, is that it is less dense as a solid. So another way to think about it is ice, which is solid water, is less, less dense than liquid water. Now, you might be thinking
why does that matter for life? Well, imagine the environments where we think life first arose. If you imagine some type of a pond, and this is the cross-section of it, if ice was more dense than liquid water, and for many substances that is the case, a solid form tends to be more dense, then what would happen? If it’s cold up here in the air, say, in the winter, then
this part would freeze. But then as it got more dense it would sink to the
bottom right over there. Then the next surface water would freeze and sink to the bottom. And then over time, the
entire lake or the entire pond would freeze over, and
life would not be able to live in that pond. Because when water freezes, it breaks membrane-bound
structures as we know it. And so that would not
be suitable for life. But because ice is less dense than water, what typically happens is
just that top layer freezes. And then it’ll freeze down as things get colder and colder. But you have an entire environment where life can continue to thrive even when the air is much colder than what is suitable for life. And because of water’s high specific heat, that temperature variation in that water is going to be much less than the temperature variation
outside of the water, either in the air or on the land. So this is just an introduction, but hopefully it makes you appreciate water a little more. And remember, and I’ve
said this in other videos, we are mostly water. One way to think about it is that each of us is made
up of trillions of cells which are primarily made up of water and exist in a water-based environment. They coordinate with each other and eventually have emergent complexity that thinks that it is a
sentient being like each of us.