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  • Water is essential for Life.

  • Here are the numbers:

  • We can go weeks without food, but well die in a few days if we don’t have water.

  • A living cell is about 75-95% water, depending on the organism.

  • We humans are about 60% water - and there are some organisms that are

  • as much as 90% water!

  • Water covers about 75% of the Earth’s surface.

  • Good thing for us, then, that water is so well-suited to support life.

  • What is it about water that makes it so special?

  • How does water support life?

  • Water has many special properties that make it thesolvent of life.”

  • Chief among these properties is the extensive hydrogen bonding between water molecules that

  • make water an extremely cohesive liquid.

  • In other words, the water molecules stick together.

  • Let’s take a closer look at what hydrogen bonding in water looks like, diagrammatically.

  • First, remember for individual water molecules, they are held together by polar covalent bonds.

  • Oxygen atoms are more electronegative than hydrogen atoms, so in a water molecule, electrons

  • spend more time around the oxygen atom than around the hydrogen atoms.

  • As a result, the oxygen atom has a partial negative charge, here shown with a lowercase

  • greek delta minus sign.

  • Because the electrons are spending less time around the hydrogens, those atoms have partial

  • positive charges, shown as delta plus.

  • Hydrogen bonding is a chemical behavior that emerges

  • when you have more than one water molecule.

  • The partial positive charge on the hydrogen of one water molecule is attracted to the

  • partial negative charge on an oxygen atom in a second water molecule.

  • This is shown as a dotted line, to let you know this is a weak bond, weaker than a covalent

  • bond which is shown as a straight UNBROKEN line.

  • Imagine hydrogen bonds kind of blinking on and off, like christmas lights.

  • At any one time, a water molecule might be hydrogen bonded to one, two, three, or even

  • four other water molecules.

  • This interconnectedness of water molecules results in a very significant emergent property

  • - water is cohesive.

  • How do we observe water being cohesive?

  • Think about how water beads up on a surface.

  • Because water is so interconnected by hydrogen bonds, the sides of a drop of water pull together,

  • forming a rounded shape.

  • Compare this with a less cohesive liquid, like ethanol or isopropyl alcohol.

  • You can see how those less cohesive fluids flow more rapidly and they don’t bead up.

  • That’s all very interesting, but what does that have to do with how water supports life?

  • Because of water’s cohesive nature, it forms a kind of a skin on its surface.

  • We say water has high surface tension.

  • It can actually support weight on its surface, because the molecules are so interconnected.

  • This creates a new place for life.

  • Have you ever seen a water stick insect, or a Jesus Lizard - they can run across water

  • without sinking?

  • If water were not so cohesive, those ecological niches would not exist.

  • The Jesus lizard wouldn’t be able to escape from a land-based predator.

  • A closely related property is water’s ability to adhere to surfaces,

  • again, by forming hydrogen bonds.

  • Adhesion allows water to crawl along surfaces, for instance, how water grabs onto the walls

  • of xylem tubes in plants.

  • So here’s an example of another emergent property - that is, you don’t see it from

  • one water molecule, but you start to see it when they act in concert - the cohesive nature

  • of water and its adhesion to the sides of tubes allows water to be pulled up through

  • plants in a process called transpiration.

  • The combination of these two properties allows water to reach every part of a plant.

  • That means plants can grow a lot taller - Anything taller than a few inches is due to these phenomena.

  • Think of giant redwoods that can grow hundreds of feet tall.

  • Life forms like these would not exist if not for these unusual properties of water.

  • Another emergent property due to water’s extensive hydrogen bonding is

  • its high Specific Heat.

  • That is, it’s hard to raise the temperature of water.

  • Increased temperature means increased kinetic energy.

  • Basically the molecules are vibrating faster at a higher temperature.

  • You can’t make water molecules vibrate faster unless you first break the hydrogen bonds

  • that are holding them together.

  • So it takes more heat energy to raise the temperature of water than you might think.

  • It takes 1 calorie of heat energy to raise the temperature of 1 gram of water

  • 1 degree Celsius.

  • That may not seem like a lot, when you say it that way, but it’s significantly higher

  • than many other liquids.

  • For instance, ethanol.

  • It takes only 0.59 calories to raise 1 gram of ethanol 1 degree Celsius.

  • So that might sound like an interesting difference in behavior of water in the lab, but what

  • about that makes it well suited for life?

  • This means that anything watery has a pretty stable temperature.

  • This is true whether were talking about a beaker of water in the lab, or an ocean,

  • or a swimming pool, or an organism that contains a lot of water - which we do.

  • Water on Earth, and in living things, helps moderate temperature changes.

  • It’s a lot harder to quickly heat up or cool down something that has

  • a high water content.

  • So animals can go out into the desert and not immediately turn 120 degrees Fahrenheit

  • (or 50 degrees Celsius)

  • We have a variety of mechanisms to help keep us cool in these circumstances, but one inherent

  • advantage is that we are made mostly of water, which resists temperature changes.

  • Like a temperature buffer.

  • A related idea is that water has a high heat of vaporization.

  • That is, it takes a lot of energy for water to evaporate - going from a liquid to a gas

  • (water vapor).

  • Again, this is because of water’s extensive hydrogen bonding.

  • Water has to reach a certain temperature before it turns to gas - 100 degrees Celsius.

  • The water molecules have to first be broken free of their hydrogen bonds holding them

  • together, before they can start vibrating faster and have a higher temperature.

  • Then eventually they will reach 100 degrees Celsius and evaporate out of the liquid phase

  • to the gaseous phase.

  • This emergent property of water also helps support life.

  • Many living things take advantage of evaporative cooling to maintain a constant temperature.

  • Dogs pant, we sweat - basically we allow a thin film of water to coat our skin, and when

  • that water reaches a high enough temperature, it evaporates.

  • When the water molecules evaporate, they take a lot of heat energy with them.

  • The hottest molecules leave.

  • So the average temperature of the molecules left behind is lower.

  • That’s evaporative cooling.

  • This is another survival strategy, another way organisms maintain the status quo.

  • We call this homeostasis - in this case, temperature homeostasis.

  • We can only survive if our bodies don’t get too hot or too cold.

  • All of our biochemical reactions that we need to do to stay alive have ideal temperatures

  • at which to run.

  • The fact that our biochemistry is water-based - that water is the solvent of life, and all

  • of our biological molecules are surrounded by water - that makes it easier for us to

  • keep these biochemical reactions happening at the ideal rates.

  • Of course, not all biological molecules are dissolved in water.

  • Some are ionic, or polar and hydrophilic, and so THEY dissolve in water, but some are

  • hydrophobic.

  • So this is one way you can classify biological molecules.

  • Are they hydrophilic or hydrophobic?

  • The hydrophilic molecules can dissolve in water.

  • They find themselves surrounded by a “Shell of Solvation.”

  • Here you can see how a positively charged species is surrounded by water in one orientation,

  • while a negatively charged species is surrounded by water oriented in the opposite way.

  • You can see the partial positives on hydrogen will be attracted to negative regions, and

  • repelled by positive regions.

  • Similarly, the partial negatives on oxygen will be attracted to positive regions, and

  • repelled by negative regions.

  • Hydrophobic molecules repel water.

  • These include lipids - things like fats, oils, waxes.

  • Hydrophobic molecules are especially good at making barriers, so we can have distinct

  • compartments in cells.

  • Well talk more about how cell membranes achieve this in another video.

  • There’s one last odd property of water that supports life that well talk about today.

  • That’s the fact that solid water - ice - is less dense than liquid water.

  • That’s not what you normally expect from materials.

  • Generally gases take up the most space.

  • Then when gas particles slow down, and temperature decreases, and the material becomes liquid,

  • the molecules are closer together.

  • So liquids are more dense than gases.

  • You would expect the next step, going from a liquid to a solid, the molecules are moving

  • even slower and so they would pack even closer together.

  • The solid form of a material is generally more dense than the liquid form.

  • But this is NOT the case for water.

  • The liquid form is more dense than the gaseous form.

  • Liquid water takes up less space than water vapor.

  • But this strange thing happens when the molecules cool down and slow down even more,

  • and water becomes ice.

  • Once again, we can blame water’s peculiar behavior on hydrogen bonding.

  • When water molecules cool down enough to start forming solids, the hydrogen bonds lock the

  • molecules into a very open lattice formation.

  • You can see there’s a lot of space between the water molecules in solid ice - more space

  • than there was in the liquid form that had fewer hydrogen bonds.

  • This is why solid ice floats to the top, on top of liquid water.

  • Now again, like the case of the high surface tension of water - one way this unusual property

  • of water supports life is that it provides additional habitats for living organisms.

  • Youve seen polar bears on ice floes.

  • Or penguins.

  • ...these are places these animals can meet up and do their business.

  • Eat, sleep, mate, all those important things.

  • Ice also acts as an insulator and protects life in small bodies of water like

  • ponds and lakes.

  • In the winter, the top layer of a lake freezes, but once there is a layer of ice on top, the

  • water underneath stays liquid.

  • The liquid water is protected from the freezing cold air and wind.

  • All the fish and other forms of life are protected underneath as well.

  • Imagine if ice were denser than liquid water - it would sink, and

  • crush the life underneath it.

  • More and more ice would form and sink to the bottom, and pretty soon the whole lake would