A solitary figure stalks a herd of grazing Thompson's gazelles. The wary animals suddenly wheel and gallop faster than quarter horses. Most predators would give up on seeing their prey turn into a billowing cloud of dust. Not this one. He becomes a spotted streak and overtakes one of the herd and brings it down within a few seconds.
In a few seconds, the cheetah has not only accelerated beyond the capacities of other mammals, it also took its metabolism and body temperature to the outermost limits of what it can survive-- 105 degrees Farenheit. This cheetah will remain very still for a while, recovering. Its kill is now easily stolen by other predators. In fact, kings once captured many cheetahs from the wild and used them to hunt, knowing that they could seize the prey from the normally savage and untameable cheetah while immobile.
Yet, 105 degrees farenheit is not the hottest that life can get.
Many organisms thrive at near-boiling temperatures at underseas vents, at geysers at Yellowstone, and in other places. What makes the difference? Why does the cheetah sizzle, while the archeobacteria or tubeworm does not?
In short, biochemistry. Proteins not only serve as structure, but also catalyst chemical reactions key for life. The hotter it gets, the less stable the protein is. Proteins can be destroyed by excess heat, as anybody who's ever cooked a steak knows. The protein structure is warped, the protein broken apart, and the amino acids reacted with other substances. The cheetah can take his metabolism to the maximum due to heat shock proteins, which are produced when temperatures shoot up, allowing the cell to help manage and repair itself and offset the impact of overheating.
Protein damage also can be achieved with acid or bases, of course, since proteins tend to rely on weak ionic attractions between their parts to hold their shape, and they work best in a very narrow pH zone.
Heat-loving archea have proteins very similar to ours, but reinforced with extra sulfur-bearing amino acids that bind to each other, lacing up the protein stiffly with disulfide bridges. Such straitlaced proteins are so stable they can withstand the energy of hotter fluids, but at "our" temperature zone, they might as well as be frozen for all the unbending they can perform to catalyze reactions.
However, these proteins are not necessarily damaged by cold. Neither are ours, unless we get down to freezing point, since ice expands in relation to water and can burst cells and rupture cell structure. However, life can be disrupted by cold since different proteins have different suspectibilities to cold, and it's possible for a cell to fall out of sync.
Most organisms, from plants to animals also have what are called cold-shock proteins (akin to heat shock proteins) that are released in response to temperature changes to help stablize the cell in response to cold, so the metabolism doesn't trip over suddenly inert enzymes.
This kind of explains why cells cannot be revived once dead. All cells have been descended from quadrillions of cell divisions since the first proto-cell managed to divide itself. The cells have changed, swapped substances, replaced their parts, but never once stopped and restarted again.
There is no "off switch" that does not lead to death, and there's also no "reboot button" to get things moving once again in the perfect synchrony of life.
Once you fall out of the zone of life, don't expect to get back in again.