Cut off the (oxygenated) blood flow long enough, and living cells will die. What does this mean? What happens in an oxygen-deprived cell such that restoring the supply of oxygenated blood will not cause the usual metabolic processes to resume where they previously left off?
Useless nitpick: there is no “blood flow” inside a cell.
There are other fluids that move around some…
When the chromosomes begin to denature, cell reproduction is impossible. At that point, the cell is doomed.
When the cell’s outer membrane bursts, and there is no longer an inside and an outside, the cell is toast.
On top of that, there are proteases that perform catabolic deconstruction of proteins. These will continue to function after cell death, resulting in degradation of protein structures, but nothing using the resulting free amino acids to build up new protein structures.
Chemically, there is nothing different between a living cell and one which is no longer receiving external metabolic support, and “dead” cells can be reinvigorated for some time after organism death, hence why many organs can successfully be removed, transported, and transplanted into another organism. But without regular metabolic support, the internal organelles and functions will break down and the cell will no longer be viable.
Stranger
Imagine you are a triage person in a military hospital.
After a battle, soldiers will arrive in three (actually four) states.
a) light injuries, can wait for treatment
b) serious injuries, will respond to emergency treatment
c) very serious injuries, too far gone for treatment
d) dead (therefore, left out of triage)
Cells might be considered the same way. For example, shortly after loss of blood flow to a piece of the brain, the most metabolically active and least hardy cells (eg neurons) will die fairly quickly. In the case of cells, lack of energy leads to loss of the ability to maintain the innumerable ongoing metabolic processes the keep the status quo (homeostasis). A cell moves along a pathway similar to a-b-c-d in the triage chart. There will be a time when it is a “walking dead man”, having sustained irreversible damage, but in the words of Monty Python, “not dead yet”. Eventually, they do die, helped along by inflammation, the body’s “clean up and repair” processes, and “programmed cellular death” (aptosis).
This happens after tumor irradiation as well. Sometimes there are cancer cells that remain, still “alive” in the sense that they can metabolize and persist, but unable to reproduce.
Similarly, what is the difference between a living person, one who has cardiac arrest but is eventually resuscitated, and one who has cardiac arrest but is not resuscitated.
Today we think of the middle category as not supremely special, but even 50 years ago this was considered fodder for real ethical pondering.
Yeah, you used to hear people say “I died, but they brought me back to life.” In other words, their heart stopped beating for a while, but then started again. But “No hearbeat=dead” is a silly definition.
I’m assuming a dead neuron for example, means no matter what resuscitating nutrients etc. you supply, it can no longer resume a significant fraction of its normal activity - passing electrica currents(?) along through the synapses. Similarly, dead muscle cells no longer respond to the neuron impulses that say “contract”. Cells accept new nutrients and oxygen, burn these “consumables” to do their job, and produce poisonous byproducts that the bloodstream eventually flushes away. With the breakdown of bodily functions, these will accumulate. You can, for example, keep much of the body alive even if the brain does not function, as long as new food comes in and the waste is flushed out the kidneys…
Getting as hijacky-as-possible here, but:
When my father in law was in a moribund coma in a hospital bed, a doctor we never saw before tried to explain to my near-hysterical, in a terror of denial, MIL–an illiterate, truly simple woman–in his opening words why he had come (for a DNR) and what it was: “if his heart stops, and he is dead, we make him alive.”
Her world collapsed; she thought all the smart men in white did was to make people alive.
A special place in hell for that bastard.
In a dead cell, all processes will eventually come to chemical equilibrium, unlike a living cell.
So are these destructive processes happening all the time, and the (re)constructive processes that cancel them out are the things that cease when nutrient/O2 influx stops?
You might imagine a business. There’s lots of stuff to do all the time (metabolic processes), and lots of people doing it (enzymes and cellular structures). This is just to keep things on an even keel.
When the flow of cash (nutrients/energy) slows or stops, various activities will slow or stop. Windows aren’t washed, building maintenance is limited to essential functions, and the workforce is downsized. Eventually, a business may decline so far, it may not recover (it can’t handle orders, its reputation is trashed, it can’t find or keep good workers and customers), even if it does come into some cash.
The analogy isn’t perfect. It helps if you imagine that the walls are ALWAYS crumbling and need CONSTANT maintenance. Also, cash is the most perfect nutrient, as it is very fungible. In real life, I doubt that any business could fail if it had access to unlimited cash, no matter how far down the spiral it had gone.
Blue Blustering Barnacle hinted at it, but I want to emphasize that the “destructive processes” are tightly controlled and limited to very specific structures. In particular a lot of breakdown happens in an organelle called the lysosome. It’s packed full of many different kinds of enzymes that can break down just about any sort of biological molecule. However, it also keeps these enzymes contained and separate from the rest of the cell, and in a healthy cell only breaks down molecules and structures that are specifically delivered to it. If the lysosome membrane is damaged, all of its enzymes are released into the cell and before long there’s nothing but a soup of broken down molecules.
This brought to mind a memory of once reading that cancer cells are immortal. I questioned the memory until I googled it.
Relatively immortal. There are tumor “stem cells” that don’t “age” like most other cell types. That is, they can reproduce indefinitely.
Most cancers don’t survive their hosts. (Exceptions include rate transmissible forms of cancer, such as that which afflicts Tasmanian Devils, and cancer cells grown in culture, such as HeLa.
Of course, even HeLa cells (probably) won’t outlive the sun, and they certainly won’t outlast the heat death of the universe. 
rare … its only possible in the devils because of inbreeding, seems that they wouldnt reject each others organs if they received organ transplants.
This doesn’t help anyone save an individual , as the cancer that is wiping them out is transferring during their social feeding activities, and it gets on their face first. Its then too complicated to do any sort of surgery to remove the cancer, let alone rebuild the face…
True, but there are no special “chemicals of life” that exist in a living cell and not a (recently) expired one. And even after death (the cessation of active anabolism), it will take a long time for the cell to actually come to equilibrium, hence why forensic pathologists can determine the time of death given the ambient temperature and condition of the corpse, although they are looking at macroscopic degradation.
Absolutely true. Life is a condition of non-equilibrium thermodynamics (NET) and this especially evident at the cellular level where an the organism is in constant but carefully balanced modification. It’s like a house whose owner is constantly renovating rooms while the house is still occupied. At the detail level, if is amazing that it all works because even a slight imbalance will result in the cell either dying (as they often do, but only when programmed or damaged) or growing out of control (cancer and other tumors). That these happen so rarely on average in the tens of trillions of cells of the human body is fundamentally amazing. Advocates of intelligent design would argue for intended design to achieve this balance, but in fact, no intelligent designer would make a system that requires such a fine balance. The reality is that this is just a state that evolution (at least on Earth) has made work by uncountable and progressive trial and error.
Stranger
If you’re just talking about cutting off oxygen to cells, they need energy to keep everything going, and oxygen is essential to producing energy. No oxygen, they start using anaerobic pathways to produce emergency energy; unfortunately these pathways produce lactic acid, nitric oxide, and free radicals, all of which cause massive damage to the inside of the cell, which the cell can survive if the cut-off is short-term.
At the same time, the pumps that keep the interior of the cell in a functional state by pumping electrolytes across the cell membrane can’t work anymore due to lack of energy. The interior of the cell only works if the pumps keep the potassium concentration high and the sodium concentration low. Once the pumps stop working, the potassium leaks out and the sodium comes in, causing many of the enzymes essential to life to denature and stop working. Once the sodium gets in, the water follows and the cell bursts. Once burst, the cell is irreversibly dead.
There are other kinds of cellular death, such as apoptosis.
Yeah, good point. My analogy several posts prior would have been better with a leaky ship, in constant need of bailing.
Collaps of the membrane potentials and of internal ATP levels. Cells use the sodium/potassium gradient across the cell membrane to transport nutrients across the cell membrane. The cell uses part its energy (ATP) to generate the membrane potential across the plasma membrane. Metabolic activity generates a membrane potential across the inner membrane of the mitochondria, its energy is used to replenish the ATP supply. Once the membrane potential across the plasma membrane collapses, the cell can no longer take up nutrients and replenish its energy.
Hijack: There are two known transsmissible tumors in animals, the ones of Tasmanian Devils, which is malignant, and a very common one found in dogs.
The dog tumor is a venereal disease, is mostly benign, and is curable. And has been around for quite some time.
/end of hijack