Saturday, December 24, 2016
Water is Life Part 1: Photosynthesis
Note: This program first aired December 24, 2016.
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.
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