So by this point, you’re probably aware that your body needs oxygen to survive, right?
In fact, every cell in your body needs that precious oxygen.
Those cells use the oxygen to produce energy in the form of ATP, or adenosine triphosphate,
a super super important molecule, sometimes even called “the molecular unit of currency”.
The cells use it to basically pay the molecules inside the cell to do their specific jobs.
It’s like one big factory with a bunch of workers that all have specific jobs needed
to run the factory, and they only take ATP as payment.
Now the mitochondrion of the cell takes in oxygen and makes ATP to pay the workers, through
a process called oxidative phosphorylation, the mitochondrion’s like the factory’s
payroll department, right?
When the cell doesn’t get enough oxygen, and so payroll can’t produce the ATP that
they need to pay the workers to do their jobs, the whole cellular factory can be damaged
or even die, and we call that process hypoxia, where hypo means “less than normal” and
oxia means “oxygenation”.
When the oxygen comes in, typically it goes straight to payroll, specifically to the inner
mitochondrial membrane where oxidative phosphorylation takes place.
Oxygen’s used in one of the last steps, and serves as an electron acceptor, and this
allows the process to finish and produce ATP.
So without oxygen, we can’t finish oxidative phosphorylation and produce ATP.
But why does the whole factory fall apart when payroll stops making ATP?
Why don’t they just pause for a bit?
Take a little break?
Well, when certain workers stop doing their jobs...things get a little out of hand.
One super important worker is the sodium potassium pump on the cell’s membrane, pretty much
like the bouncer that makes sure there isn’t too much sodium diffusing into the cell, basically
by pumping it back out every time it diffuses in and maintaining a concentration gradient,
this process also keeps too many water molecules from passively diffusing into the cell; think
of it like this: water molecules want to go every which way and are constantly moving
back and forth, inside and outside the cell, but the all these sodium ions on this side
tend to physically block more of them from leaving that side, so over time more water
molecules get retained, or almost trapped, on the side with more sodium—in short, the
more sodium molecules: the more water molecules.
But, our pump doesn’t do all this for free, and it needs ATP.
So without ATP, it stops pumping sodium back out, and sodium diffuses in...and keeps diffusing
in and the concentration gradient goes away, now with less sodium particles on the outside
blocking the water molecules from going into the cell, water follows sodium in, which causes
the cell to swell up.
When the cell swells up, a couple things happen.
First, usually you have these really tiny microvilli on the cell’s membrane, which
look sort of like little fingers that help increase the cell’s surface area and therefore
help the cell absorb more things, when the cell swells up and gets all bloated, the water
sort of fills these little fingers and reduces the surface area, which makes it harder to
absorb molecules since there’s less surface area, right?
Also, sort of along the same lines, the cell can bleb, or bulge outward from all this water,
this is a sign that the cell’s cytoskeleton or this structural framework is beginning
to fail, and is letting water slip through.
Finally, the rough endoplasmic reticulum, or the rough ER, also swells when the cell
swells.
And remember that the rough ER has all these little ribosomes on its outside, and these
are really important for the cell in making proteins, but when the rough ER swells, they
detach, and stop making proteins, so protein synthesis goes down.
All the ATP isn’t immediately lost though when you lose oxygen and oxidative phosphorylation
stops, luckily your cell can make ATP another way, called anaerobic glycolysis, anaerobic
meaning in the absence of oxygen.
This is like the backup ATP generator, which, isn’t nearly as efficient and only produces
a net of about 2 ATP molecules per glucose, whereas oxidative phosphorylation makes about
30-36…
So it helps a little, but what also happens is it produces the byproduct lactic acid,
which lowers the pH inside the cell.
This more acidic environment can denature or essentially destroy proteins and enzymes.
Now, up to this point, it’s not all bad, because one super important thing about these
processes that happen to the cell, is that they’re potentially reversible, meaning
that if we all the sudden get oxygen again and start making ATP, then these changes aren’t
necessarily permanent.
After enough time, though, irreversible damage can happen to the cell.
Kind of like the sodium potassium pump, there’s also a calcium pump that helps keep too much
calcium from getting in, and if that stops working, then calcium starts to build up,
which isn’t a great thing.
First, calcium can activate certain enzymes that you might not necessarily want to activate,
like proteases that can slice up proteins and damage the cell’s cytoskeleton, which
remember is the structural framework that keeps the cell together.
Also, endonucleases can be activated, which can cut up DNA, the cell’s genetic material.
And if we get back to thelactic acid, as more lactic acid builds up and the environment
gets more acidic, the lysosomal membrane can be damaged as well, which usually houses these
hydrolytic enzymes whose job is basically to grind up large molecules, and when they
get out, well, they’re also activated by calcium and then they just start cuttin’
everything in sight, and basically start digesting the cell from the inside.
Finally, the phospholipase enzyme, which basically splits phospholipids, gets activated...since
the cell’s membrane’s made of phospholipids, these can destroy the cell membrane, which
is probably the most important sign of irreversible damage.
When the membrane’s destroyed, those enzymes we just listed, along with others, can leak
out into the blood and continue wreaking havoc.
Finally, let’s jump back to calcium.
Enzyme activation isn’t the only effect calcium can have; calcium can get into the
mitochondria, causing a cascade the leads the mitochondrial membrane to be more permeable
to small molecules and so it lets a molecule that usually stays in the mitochondrial, cytochrome
c, to leak out into the cytosol.
This is a big big red flag to the cell that things have gone south, and is kind of analogous
to the self-destruct button,
and activates a process called apoptosis, or programmed cell death.
This is a bit like cellular suicide.
At this point, the cell’s not in good shape, right?
And all this happens eventually because of a lack of oxygen, or because of hypoxia.