There has been a lot of talk lately about the fact that
water is life. That means a great many things to a great many people. To the
fishing communities here on the coast of Maine, water provides a living, a work
place, a source of income and food. On a hot summer day water provides relief,
a place to cool your body. For many people the water is life idea is a
spiritual endeavor; water represents the blood of the earth, traveling across
the body of the earth, bringing sustenance. Even those recreationalists less
spiritually aligned recognize water as a means of direct immersive contact with
the natural world. We all understand intuitively that water is life, but few of
us understand exactly why. But talk to a biologist, or a chemist, and you will
learn that “water is life” has literal concrete molecular meaning.
Water has a specific and intimate role in the most basic
biological reactions, playing a key part in both photosynthesis and aerobic
respiration. I’ve talked about
photosynthesis before on the show, but it is hard to overemphasize the
importance of this reaction to the fact that we are all here. It is the means
by which all of the energy our bodies use becomes accessible to us; our bodies
can’t use light energy, but we can use the chemical energy stored in organic
molecules created by plants and transferred up the food web. Virtually our
entire economy is based on photosynthesis*, and until a couple of hundred years
ago that was all “current photosynthesis”. The advent of the industrial age
brought a reliance on fossil fuels, but the “fossil” in fossil fuels is
photosynthesis. All the chemical energy stored in oil, gas and coal originated
exactly the same way as the energy in your bowl of Wheaties-through an
intricate set of biological reactions that move electrons up and down and from
atom to atom and result in a final product that has higher potential energy
than the original ingredients. Photosynthesis pushes energy up hill. We rely on
it utterly. And it relies on water.
The kind of photosynthesis we are talking about, oxygenic
photosynthesis is the kind you learned about in school, when you learned that
plants can take carbon dioxide and water in the presence of sunlight and make
glucose and oxygen. A specific group of bacteria, called cyanobacteria (also
known as blue green algae, but this is a misnomer as they are not algae) and
certain members of the eukaryotes, a group of organisms that includes us and
pretty much everything else you would picture as alive are the organisms that
can perform oxygenic photosynthesis. There are other forms of photosynthesis,
but they don’t make oxygen, and more importantly for the story today, they
don’t use water.
So what role does water play in this most fundamental
biological process? There are actually two key roles, Water goes into the
photosynthetic reaction and gets deconstructed for parts. The oxygen gas
generated as a byproduct of this reaction, the oxygen that changed the
composition of the atmosphere and enabled all of the life we see around us is
the O in H2O. Take apart a couple of water molecules and you have two oxygen
atoms, that bond to each other to create the diatomic oxygen gas we know, love
and depend on. But photosynthesis isn’t
trying to create oxygen, it is trying to turn electromagnetic energy into
storable chemical energy, and to do that it needs some electrons. Electrons are
the currency of non nuclear atomic energetics. The light energy that gets absorbed
by chlorophyll serves to excite electrons, and the net result of the first part
of photosynthesis is that a special chlorophyll molecule called the reaction
center loses some of these excited electrons as they are passed on to other
molecules in the photosystem. That is
its job, to absorb sunlight and pass that energy along to the rest of the
system in the form of electrons. To keep doing its job though, the reaction
center chlorophyll needs electrons to replace the ones it gave away. Where does
it get those replacement parts? You know it already—it gets them from water. That
is the reason water is required for photosynthesis, and therefore life—its
electrons.
Getting electrons from water isn’t easy to do. Oxygen is an
incredibly greedy atom and is loath to relinquish any electrons, chemists call
that being strongly electronegative. The only things that can get electrons
away from a water molecule are an even more strongly electronegative reaction
center chlorophyll molecule, and some very clever enzymes.
So there you have it, water is life, because water is the
source of the electrons that provide the medium by which the sun’s energy gets
turned into chemical energy, and as a handy byproduct also happens to provide
the world with the oxygen it needs. What we need that oxygen, and yet more
water for, we’ll talk about next week.
References:
As if often the case, any college level Biology text book
should cover this in sufficient detail. I use Freeman et al, Biological Science
6th ed. Pearson Higher Ed
