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

When we look out into the cosmos for alien life, many folks look for signs of water on moons or planets. That's because life, as we know it, is dependent on water. 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 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. The nature of that bond 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. 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 lower case Greek letter delta that just means partially negative charge. Then the sides where the hydrogens are, those require a partial positive charge. 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.

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 bond. 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 is going to be attracted to 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.

Not only does it provide a good fluid environment, it's a very good solvent. Water is often known as the universal solvent, but it's worth putting a disclaimer here. Even though people will 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. You could 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.

Even the property that there are 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. 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 its 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. You might say, "Why does that matter for life?" Well, many life forms can only operate within a certain range of temperatures.

So if it was really easy to raise the temperature of water really high or very low temperatures very fast, that would make it much harder for life to operate within water or even for 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.

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 when you look at a macro scale, 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 of some of the polarity of the glass molecules of the test tube.

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 in action in our own blood vessels. When you get to the really small blood vessels, the capillaries, and that is called capillaries because 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 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, the solid form tends to be more dense.

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 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. 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.