Follow the money-to-be! Tracking OXIDATION NUMBERS in metabolism is like watching a poker player lose chips until it’s chipless and those chips getting cashed in for cellular energy money. Those chips are electrons, which are negatively-charged subatomic particles that atoms share in order to form bonds. In order to do this tracking we need to know how many chips each player owns. And when it comes to electron ownership, I like to think of dogs… Because life’s really just a bunch of dog-meet-dog interactions!
First an overview, then a little background on our molecular pound, then more on oxidation numbers. Molecules are made up of atoms & if you think of atoms as dog walkers (positive protons) trying to keep control over a bunch of dogs (negative electrons), bonds are like 2 atoms sharing custody of a pair of dogs. In a pure covalent bond there’s even joint custody but in a polar covalent bond one of the owners is better able to woo the dogs. And in an ionic “bond” one dog breaks leash with the original walker all together.
To find an oxidation number, you pretend like all the bonds are “ionic bonds” – meaning that there’s no joint custody allowed and, like how in the musical Annie where Annie’s showing the dog snatchers that Sandy’s hers, the dog has to choose who to stay with. Except with oxidation number it’s actually *you* doing the choosing because this is all a hypothetical thing. The molecules are still stuck together in reality, you’re just trying to see where the electrons are coming and going and where “weak spots” are.
So, a bit of an intro to the situation to help folks follow along. Molecules are made up of atoms and those atoms are held together because they’re sharing pairs of electrons which are really tiny little negatively charged things that whizz around the central part of the atom, the nucleus, which is home to positively charged tiny but not quite as tiny things called protons and some other non-charged things called neutrons.
I like to think of atoms as dog walkers (the protons) trying to keep control over a bunch of dogs (electrons). Especially the super-hyper dogs that are out on the edge (valence electrons). Those hyper dogs are the most likely to get distracted – they can get attracted to nearby dogs getting walked by other owners and drag their walker and the rest of the dogs with it. If the dogs fall in love, the atoms can end up sharing custody of the pair of dogs, forming a new covalent bond.
In a pure covalent bond, they share equally, but if one of the walkers is “dog whispery” (electronegative) it can woo the dog and convince it to “change loyalty” so that – were the walkers to divorce, the dog would stay with it. (both electrons in the shared pair would be “owned” by the more electronegative atom).
More on dog whisper power (ELECTRONEGATIVITY) here: http://bit.ly/2XBhNUQ
But basically it comes from having a strong whisper (lots of protons) (towards the right of the periodic table) & having the furthest dogs able to hear you (valence electrons nearer the nucleus)(towards top of the periodic table). In terms of “pulling strength” for some of the common ones we encounter in biochemistry: H < C < S < N < O. So hydrogen (H) has a loose leash whereas oxygen (O) keeps its electrons on a tight leash.
In a pure covalent bond, a dog goes in for a sniff, falls in love, and the dog walkers end up sharing equal “joint custody” of the pair of dogs they share -each walker holds a leash to each of the dogs and you’re just as likely to find either of the dogs closer to either of the walkers.
Such pure covalent bonds are only really found when you have identical atoms sharing (eg. O₂). The closer the electronegativities of the sharing walkers, the more even the sharing and the less “ionic character” the bond has. C & H have pretty similar electronegativities, so C wins it, but barely.
But oxygen is more electronegative than both of those, so if you have an O attached to a C or an H, you get something called a polar covalent bond. In a polar covalent bond, walkers share custody but not 50/50 because one of the atoms is more dog-whispery. It’s like both walkers have a leash on each dog in the pair but one of the leashes is looser so the dog can get closer to the other walker.
It’s like the oxygen dog walker meets a hydrogen dog walker, and the hydrogen dog starts sniffing one of the oxygen dogs, & the oxygen dog walker convinces it to stay. And it “brainwashes” the dog into switching its loyalty from the H to the O. So now the O basically “owns it” (what’s really going on is that the electrons are whizzing around & if you were to take a snapshot, the electrons from pair of shared electrons formed is more likely to be closer to the oxygen).
If the dog whispering power’s even greater, the dog can actually switch ownership, which we call an ionic bond. We talk about ionic “bonds” but really these are just really strong attractions – there’s no actual doggy-sharing. In an ionic bond, it’s like you have 2 dog walkers – one of them has a really strong dog whisperer (like Cl) and the other a weak one (like Na). So when the dog walkers get near each other, one of the dogs getting walked by the weaker one hears the stronger one professing its love. And so this dog decides to go check it out.
It drags its walker and the other dogs it’s walking with over with it, sniffs around, decides it likes the new walker better and “swaps ownership.” Its original walker feels the loss as losing a negative charge (thus becoming more positive as its # of electrons decreases compared to the # of protons available to neutralize it).
So now that old walker sticks around because of the opposite charge attraction – it sticks by to watch the dog it lost – not because it misses that particular dog, but because it wants to balance its charge. Since it no longer is connected to that dog by a “leash” (no true bonds, just strong attraction) it’s free to come and go if it finds a better offer (e.g. if you stick NaCl in water it’ll dissociate into Na⁺ & Cl⁻).
When we assign oxidation numbers we pretend that all the bonds – even the real covalent ones – are actually ionic ones and then we determine what charge each of the atoms would have – which depends on how many dogs it gets to keep and how many positive protons it has to neutralize the negative charge those dogs bring.
When thinking about which atoms “own” which electrons, I find the dog analogy useful, but at the bigger molecule scale of things it can be helpful to think in terms of poker chips.
When it comes to energy, cells operate kinda like a casino – instead of paying out cash, reactions transfer “poker chips” that can later get “turned in” for the real money.
The “poker chips” of chemical reactions are electrons – they can get passed from one atom to another (with the giver called the oxidant and the receiver called the reductant). And then those transferred electrons eventually get “cashed out” through a process called oxidative phosphorylation that uses them to power a pump to drive a proton gradient and create ATP, which is like cellular cash. More on ATP here: http://bit.ly/2CHvGWN
A common place to see redox in play is catabolism, the branch of metabolism where you break down big things into little things either for energy, or to make new big things (this making is the anabolism part of metabolism – think of anabolic steroids making big muscles (and big problems when abused).
You can use oxidation numbers to kinda keep a running tally of how many “poker chips” the molecule has. A good fuel comes in which lots but gradually they get transferred to other molecules and then finally “cashed in” for the big payoff in oxidative phosphorylation.
Some common fuels our cells use are carbohydrates (like the sugar glucose), proteins, and lipids (fats, oils, and waxes). They come in with different #s of poker chips that they gradually lose until they have no more to lose (they’re at their most oxidized – which, for a carbon, means being attached to 2 oxygens (CO₂). In that state, each of the Os forms a double bond with the C, so it owns 4 of the C’s electrons. And C only had 4 – so O owns them all. And if they were to divorce, the O would get to keep all the dogs.
Formally, we can write the oxidation state of the C to be 4-0 = +4
And the oxidation state of the Os to each be 6-(4 from double bond + 2 lone pairs) = 6-8 = -2
That may seem “formal” but those are NOT “formal charges” – they’re not even “real charges” – they’re hypothetical – no divorce has actually happened we’re just saying what the charges *would* be
“Real charges” are called formal charges – whereas the oxidation # is hypothetical. (i.e. oxidation # says – if the walkers were to divorce who would the dog want to stay with, whereas the formal charge says these dogs really are divorced and this is how many dogs it has compared to walkers).
To help distinguish between the 2, oxidation states are often given in Roman numerals. So we’d write the oxidation state of that C to be IV, and the O’s would be -II)
Since oxidation states aren’t real charges, the sum of the hypothetical charges should equal the formal charge (the real one). if our molecule doesn’t have a real charge, its “fake charges” should add up to 0. If you add up the oxidation states of all of the atoms in a molecule you should get the actual charge of the molecule. Let’s check: 4 + 2(-2) = 4-4 = 0. Yay!
Not all players come with the same amount of chips – the more reduced the player is the more electrons it has to lose (and energy we have to gain). Take a lipid (fat, oil, or wax) and compare it to a sugar. Even if they have the same number of C’s, most of the C’s in the lipid have lower oxidation #s because they’re linked to H’s which are really weak dog whisperers so if you force them to divorce the dogs stay with the C’s and this would give the C’s a negative charge.
So most of the Cs in a lipid have an oxidation # of -2 or -3. This is like having a lot of poker chips to cash in. What if you look at a sugar like glucose? Here a lot of the C’s have already “cashed in” some of their chips. You see a lot of alcohol (-OH) groups in place of just H’s. And when C’s and O’s share ) doesn’t share fair – so when they get divorced O keeps the dog. So the C’s have higher oxidation numbers (from -1 to +1 depending on what else they’re attached to), and therefore different food sources have different calorie counts.