- Published on April 15, 2010
I think about oxygen a lot more than I used to. This is because I am studying a chemical defense that turns ordinary oxygen, the same old stuff we obtain from breathing air – the same stuff that is absolutely necessary for most organisms to live – into a chemical weapon.
This weapon is referred to as an “oxidative burst,” and it is literally a burst of very toxic oxygen molecules that can be used to kill pathogens and prevent infection. I study the oxidative burst in seaweed, but this defense is especially fascinating because it is extremely widespread. A great deal of very distantly-related organisms use it in almost exactly the same way. In fact, your white blood cells employ it to protect you from diseases!
If we want to understand how oxygen – that seemingly benign gas that we breathe – can turn into a deadly weapon, we must look a little more closely at what most of us take for granted: oxygen itself.
Oxygen is vital to life in aerobic (or oxygen-requiring) organisms because we use oxygen molecules to generate energy from the food we eat through a process called cellular respiration.
But why are we aerobic? Why do we need oxygen in order to create energy? Why is oxygen in particular so central to this process?
Oxygen is so important to aerobic organisms because it has some very special characteristics; in other words, because it is a strange molecule.
For one, oxygen is a very strong oxidant. An oxidant is something that steals electrons from other atoms or molecules. Oxidants do this because they are very electronegative, which is to say they are electron hungry. Oxygen is the second most electron hungry element known to man (the first is fluorine).
Why does oxygen want electrons so badly? An atom of oxygen naturally has 8 electrons. Two of those are hugged very close to the atom’s nucleus and they orbit the nucleus in its first “atomic orbital,” or "shell.” Six electrons are hugged a little more loosely in the second atomic orbital. Since the second orbital has room for 8 electrons, oxygen has space for two more and it wants the complete set. So one atom of oxygen (see the picture – who knew that molecules were so incredibly good-looking?!) really wants 2 more electrons to join the two lonely unpaired electrons.
A hydrogen atom is made of one electron and one proton, and by taking 2 hydrogen atoms, an atom of oxygen can become truly happy because now it is surrounded by 8 electrons and it has become a molecule of water (H2O, see picture). Oxygen loves turning into water; with those two electrons, it feels complete. However, we don’t usually see just one atom of oxygen. Under atmospheric conditions, oxygen exists as a pair of oxygen atoms (it is really called dioxygen, see picture). Dioxygen ultimately wants 4 electrons (or 4 hydrogen atoms), which will produce 2 molecules of water (see picture).
Being very electronegative, or being a strong oxidant, isn’t what is strange about oxygen (many molecules are electronegative, like fluorine, chlorine, bromine, and nitrogen), but it is one reason we can use oxygen to generate energy from food.
When oxygen, or any strong oxidant, takes electrons from another molecule, it releases a LOT of energy. If you have had chemistry you might remember that this is called an exothermic reaction. This exothermic, or energy-releasing, reaction is literally combustion.
In order to burn anything, you need an oxidant, a fuel, and often a spark. It is strange to imagine, but giving electrons to oxygen is what makes fire. A campfire, for example, is the energy and leftover reaction products that are produced when an oxidant (oxygen) takes electrons from a fuel (wood, and sometimes lighter fluid to get it going!). So giving electrons to oxidants can be a big deal; it can make fire and even cause explosions.
Organisms have evolved a way to harness the energy that is released when electrons from fuel (molecules formed from the food we eat) are given to oxygen. This harnessed energy can then be used for moving around and thinking and everything else we need to do in order to stay alive. After taking those electrons and releasing the energy we need to live, oxygen turns into regular old water.
But if many other molecules are just about as electronegative or more than oxygen, why is oxygen the special molecule? Why can’t we use any of those other molecules instead?
The reason oxygen is so special is because even though it wants two electrons so badly, there is something about an oxygen molecule that under normal conditions prevents it from taking those electrons. In other words, there is something called a “kinetic barrier” to oxygen accepting an electron pair to obtain a complete shell.
When you think about it, it is obvious that there is a kinetic barrier to oxygen stealing electrons and releasing huge amounts of energy to become water. The air in our atmosphere is about 21% oxygen. If oxygen weren’t barred from taking electrons under normal circumstances, the very air around us would spontaneously blow up in a huge explosion! Organisms just can’t metabolically use a molecule that randomly explodes.
Another reason that oxygen is special is because after accepting 2 electrons, it simply becomes water, and water isn’t toxic to living things. The oxidation product of, say chlorine, however, is hydrochloric acid (HCl). Can you imagine an organism that could tolerate cells flooded with this corrosive acid? Maybe such organisms exist, but the vast majority of aerobic beings could not tolerate it. These sorts of corrosive acids are not good for our DNA, our proteins, or other biomolecules.
So the kinetic barrier to oxygen accepting an electron pair is what makes oxygen so special to aerobic organisms. It prevents oxygen from reacting explosively with everything around us, and it makes reactions with oxygen more controlled. Organisms have evolved ways to force oxygen into accepting electrons – a theoretical spark that causes the fire to burn, but it generally only burns an appropriate electron donor, and only when sparked.
So how is oxygen used in biowarfare as a true chemical weapon among seaweeds, spores, humans, and microbes??
Stay tuned to find out……!