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We have a riddle for you.
Ok, not really a riddle.
We’re bad at riddles.
More like clues about our favorite molecule.
It’s polar.
It’s shaped like an outstretched V. Its properties are awesome.
We have a riddle for you. Ok, not really a riddle. We’re bad at riddles. More like
clues about our favorite molecule. It’s polar. It’s shaped like an outstretched
V. Its properties are awesome. Also, it makes up ¾ of the Earth’s surface. That one always
gives it away. Our favorite molecule is water.
And we’re not going to lecture you about how important water is and the fact that all
life as we know it could not survive without it, because yeah that’s true, but we really
want to talk about what makes water so unique for biology itself. What are these awesome
properties?
We mentioned the shape of water and the fact that it is polar. It has one very electronegative
oxygen that is always trying to keep the electrons closer to it than to the hydrogens it is bonded
to. This actually gives the oxygen a slightly negative charge---because of the electrons
that are spending more time next to it---and it gives the hydrogens a slightly positive
charge. Well that means that other water molecules have an easy time bonding together. Why? Because
the hydrogen of one water molecule with its slightly positive charge can bond to another
water molecule’s oxygen with a slightly negative charge. These bonds among the water
molecules are hydrogen bonds, and these very bonds are what allow water to do these things
that it can do…which we are about to talk about.
Have you ever looked at a really tall tree and wondered, “How does the water get all
the way up there?” I mean, it’s got to go against gravity. Gravity! Well in our plant
video, we talk about the xylem. It’s vessels within certain types of plants, like trees,
that transports water. But what’s really neat about water is that---it sticks to the
xylem walls in what is known as adhesion. This helps fight gravity. But water molecules,
with their hydrogen bonding, also bond to each other in something called cohesion. In
almost a “beads on a string” kind of fashion, as water molecules evaporate from a leaf---the
next water molecule in line is pulled upwards and so on.
Cohesion is also a reason that water striders, one of our favorite insects, can skate on
water. Cohesion contributes to the surface tension of water. Water actually has quite
a bit of surface tension compared to other liquids. And it’s not just water striders.
There are a lot of insects, spiders, and even larger animals like reptiles and birds that
can walk on water. So to the Google for that.
With water being polar, it also is a powerful solvent for other molecules. That means that
water can dissolve many other molecules especially polar molecules and ionic compounds. Why does
that matter? This is important because many of the processes occurring in organisms use
water as a solvent. Just consider the need for water for your kidneys in their filtering
properties and all different types of body fluids.
I’ll never forget when I was little, my father built us a pond. We had some goldfish
in there. We loved our pond. Well in West Texas where we’re from---it can freeze in
the winter. And one morning I went outside, terrified, to find that the top of my pond
had frozen. I thought my fish were goners. Only to find them swimming and doing their…fish
activities…under the ice layer. See most substances actually contract when they freeze
and become more dense. But water expands when it freezes and becomes less dense when frozen.
Resulting in floating ice where it can actually make an insulated surface layer that makes
a difference for many organisms. This is due to the hydrogen bonds. At freezing level,
the breaking and reforming of hydrogen bonds---which is usually happening often---is not happening
very much. The molecules are set into a lattice of hydrogen bonded molecules just far enough
apart that it is less dense in ice form than in water form. That is all going to be very
important for aquatic life.
Speaking of temperature, water resists changing its temperature. It has what is called a high
specific heat. Specific heat is a measurement of heat that needs to be absorbed or lost
for 1 g of a substance to change its temperature by 1C. That’s why, on the first day that
school is out in the summer, it may be super hot outside but the water can still be pretty
cold. It’s really good that water is like this for life---it is stabilizing for aquatic
environment temperatures. It also means that water can absorb a lot of heat in the summer
without reaching as high of temperatures itself---which is useful when the winter comes along. The
water can release heat as it cools in the winter.
Still on the topic of temperature, consider evaporation. Many animals rely on evaporation
to cool them. Think of water molecules. They are moving, but those that have more energy---are
“hotter” per se---are moving the fastest. They are more likely to make the phase change
to gas. As these molecules leave, their energy---their heat---is no longer on your skin. By the way,
all of this is not just animals too. Plants use evaporative cooling to aid them in hot
temperatures. Excessive high temperatures can be dangerous for many different processes
in both plants and animals---it can be damaging to the enzymes in many of those processes.
Well we went through a lot of features of water---definitely something to consider next
time you encounter it. Which based on how much we depend on it…will likely be soon.
Well that’s it for the Amoeba Sisters and we remind you to stay curious.