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.

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 6^th ed. Pearson Higher Ed

Photosynthesis (the best thing ever)

Note: This program first aired on May 14, 2016.

Every day this time of year the world gets more and more green. Over night it seems like lawns spring up lushly, and the light green fuzz that covers the deciduous forest asserts itself more strongly every day. All of that green is a result of the growth of the photosynthetic structures of plants, structures that are replacing the ones that died last fall in preparation for the cold and dry winter season, or are growing anew from last year’s seeds. The primary photosynthetic structure for land plants is the leaf, a structure that has a lot of surface area relative to its volume. The surface area is important, as the leaf’s main job is to absorb solar radiation, so surface area equals absorption potential.

You all learned (I hope) in school that plants take carbon dioxide and water and sunlight and make sugar and oxygen gas*. The energy from the sunlight, electromagnetic radiation, gets transformed into chemical energy as stored in the bonds between the carbons and the hydrogens in the glucose molecule that is formed. Something we can’t store gets turned into something we can. It sounds quite simple, and if you look at them chemical equation for this reaction, it even looks quite simple. The reality though is far different, as my biology students this spring learned. Photosynthesis is amazing, and beautiful, and a process that virtually all life depends on, but it is not simple.

Nor should it be. Plants execute a very tricky procedure in fixing the carbon from carbon dioxide in to a glucose molecule. It isn’t easy to transform and store energy. Photosynthesis starts with light and that light energy gets transferred to electrons, electrons that are hanging out on chlorophyll molecules in the chloroplasts of plant leaves. Chlorophyll is a pigment that absorbs light, mostly red and blue light, because those wavelengths are especially good at activating chlorophyll’s electrons. Green light incidentally, is not good at all at exciting chlorophyll’s electrons, so it isn’t absorbed, it is reflected instead, which is why plants look green. Those electrons go through a series of steps in processes imaginatively called photosystems I and II. The light bumps them up to a high energy state, and over the course of photosystem I and II they bump back down, releasing that energy along the way. The end results of photosystems I and II are a whole bunch of temporary energy storage molecules called ATP, and a few electron acceptor molecules. ATP is adenosine triphosphate and is the go to molecule for temporary chemical energy in cells. The phosphate part is what makes it good for holding energy, but is also what makes it only able to do it temporarily, as the phosphate is very unstable. So ATP isn’t a viable long term storage solution, but cells make ATP to then use the energy stored in ATP to do other cellular business. Photosystems I and II are the light dependent parts of photosynthesis, and where the water (a reactant in the reaction) is used and the oxygen (a product of the reaction) gets formed.  The water gets split, and is where the electrons that get moved around ultimately come from, and the oxygen is a byproduct of splitting the water. Notice, no where in this part of photosynthesis have we talked about carbon or glucose. Carbon doesn’t have anything to do with this part.

The other nearly entirely separate part of photosynthesis is the light independent reaction—called the Calvin cycle. This is where all that ATP gets used, and those electron acceptor molecules from the photosystems give their electrons back up. Carbon dioxide gets incorporated into several different intermediate molecules with names like PGA and G3P and RuBP, and after 6 full turns of the Calvin cycle, you end up with enough carbons being fixed to make one glucose molecule. That requires a lot of electrons , and even more ATP. When you study the Calvin cycle you gain an appreciation for just how much energy really goes into making glucose, the energy molecule of life.  And note here, in the Calvin cycle, this is where the carbon comes into play. The carbon dioxide is a reactant in the chemical equation, and the glucose is the product linked to it. The water and oxygen gas that appear right next to the carbon and glucose on paper aren’t really connected to them at all in the plant cell, except for a few electrons.

And what that also means, if you think about it, is that the material of glucose, the carbon that makes up the main backbone of the glucose molecule comes only from the carbon dioxide the plant inhales from the atmosphere. That means that plants make food out of air. The material portion of that food, the actual atoms that make up the sugar we all eat, came out of thin air. And that is something I can’t get over, every time I think about how amazing plants are that is what I come back to, plants make food out of air (specifically the carbon dioxide gas) and oxygen out of water. I only wish I could make that much of a difference in the world.

*Here’s that reaction equation: 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂

References:

Look at any college level biology book and  you will find the basic mechanics of photosynthesis. I used Freeman’s Biological Science 5^th ed in my class this past year https://www.pearsonhighered.com/product/Freeman-Biological-Science-5th-Edition/9780321743671.html