Death is the end of life. You are going to die. As a concept, death is easier to define and apply to multicellular animals, since many plants and single-celled lifeforms are capable of dividing (mitosis), budding, or sprouting into what can become independent organisms which are genetic clones of the original critter.
 In humans
Death is a very ambiguous concept for humans, and it is not as much a single event, but like the beginning of life, is many events over time, and is best described as a process rather than a single point. For many reasons, it's still convenient for us to think of it as an event. Various conventions have been established to make a legal or medical decision regarding whether a person has "died". This is very important because doctors (and lawyers) need to know how to define death; when is it okay to stop trying to save someone's life, when can they be harvested for organ transplants, when is it safe to say that someone has killed or been killed, and so on. Thus, death is defined as a point where certain things stop happening.
 Clinical death
Before heart-lung machines were available, the persistence of a pulse was the main factor when considering if someone was alive or not. If someone's heart stops beating, then their cells cannot be supplied with oxygen or nutrients (carried by blood) and they are not going to get up and walk about. Neurones in the brain will begin to starve after ten seconds of oxygen deprivation, leading to irreversible
drain bramage brain damage and loss of consciousness. Phagocytes will no longer be able to clean up rogue enzymes and destroy decomposers like bacteria or fungi, so decay begins after minutes.
When the pulse stops, this is referred to as "clinical death". It is important to note that while clinical death often leads to brain-death, it is not the same thing. Stories of people "coming back from the dead" or "being dead for three minutes" are the result of people being classed as clinically dead but who haven't suffered irreversible brain failure. Tales of near-death experiences (NDEs) are always good for a book deal, or a stint on Coast to Coast AM, but the universality of the experiences across cultures suggests a biological explanation, perhaps the reaction of any human brain to mortal trauma.
 Brain death
More recently, brain-death has been considered to be a more relevant landmark. This is demonstrated by the presence or absence of certain brain waves[more detail please] that can be measured - although the lack of a pulse is a good indication that someone isn't going to make it, their consciousness may still be working normally, if only for a few minutes. Brain death occurs when the brain is starved of oxygen, usually after the heart stops beating. Unlike the heart, though, the brain cannot be restarted (see cryonics for some people who dispute this fact). This definition can prove very controversial in cases where it is the other way around. Consciousness can be lost perpetually, but the rest of the body can still go on due to unconscious processes. This was the case with Terri Schiavo, who was unfortunately tossed around like a political football by various political conservatives trying to score points with fundamentalist Christians.
 Cellular death
The whole idea of defining "death" becomes more complicated by the idea of cellular death. Even if someone's heart stops, and their brain ceases to function, the chemical and biological processes inside each cell will continue. They will continue respiring, producing proteins and replicating until they run out of energy in the form of food and oxygen. Cells are not intelligent or aware (even if the organism that they form is) so they don't know that they're part of someone who has died.
It is claimed that you could clone a pig from sausages if they're fresh enough, because many of the cells inside them are still alive (think about that next time you buy a fresh string from the butcher's). As cells live on, then, could a person be classed as still alive? Not necessarily, as a "human" (or arguably, any many-celled organism) is an emergent entity composed of these cells when they join together and organise into tissues and organs and function in a coordinated manner. The death of the person is when the cells are no longer able to act in such an organised fashion, due to the brain being unable to coordinate the entity's actions (brain death) or the inability to send nutrients where needed (clinical death). This leads to a issue with people who claim that life begins at a cellular stage as an argument against abortion or birth control; to avoid special pleading, they should also conclude that if life begins at this cellular stage, it would also end at the cellular stage (and it is impossible to keep tabs on every last macrophage that's now taking part in the decomposition). As far as most people are aware, no one says that "death" happens at cellular death.
 Chemical death
Although not a "death" in any way by any sensible definition, it is best to cover the very final step. This is because the form of a body will still exist after death and often beyond even cellular death (although it may not look too good at this point). At a chemical level, the previously living body stops resisting entropy and comes to equilibrium with the environment. Usually this is through being consumed by a variety of other (still-living) organisms in a process termed decomposition. Even the greatest hair-splitters in the world wouldn't want to push back the "point" of death further than this. It's not even clear how death could possibly be defined any point after everything that was once the body has been scrambled out of existence by decomposition and erosion.
It should be noted that, unless there is some active intervention by modern medical technology, the entire process described above is an inevitable consequence of clinical death. In other words, if your heart stops, then you will turn to mush.
Cryonics advocates define death as "information-theoretic death" - when the information that made up your mind quite definitely could not even hypothetically be recovered, even with miraculous technologies that have yet to be invented. This is why they freeze brains.
Immortality is the ability to never die. At this point, it is theoretical only, and any real immortal being would need to be able to deal with things like quantum tunneling, end-of-universe scenarios like the Heat Death or Big Crunch, proton decay, &c. Immortality is a very common theme in religion; most gods are believed immortal, and most religions promise eternal paradise or torment.
See our main article on this subject.
An afterlife is the imaginary concept that one's disembodied consciousness will magically leave the body post mortem. As modern neuroscience has shown, consciousness can be reduced to neuron interactions, so if they go, you go. Bitching about it isn't going to change anything.
One of the more immediately relevant obstacles to immortality is aging. Individual cells can only reproduce so many times before they accrue so much damage that it commits suicide via programmed cell death. This appeared in animals as a "safety mechanism" lest some of those mutations result in the cell becoming cancerous. As stated at the beginning of the article, single-celled creatures and plants often reproduce asexually, making "immortality" difficult to define for them: is a growth of aspen trees "immortal" if nothing happens to it?
Biological immortality is the ability to not be affected by ageing: one's likelihood of dying doesn't change no matter how old one is. Notice, though, this does not imply that that likelihood is zero: you can still get eaten, starved, asphyxiated, exploded, or otherwise killed. It just means that things won't break down as you age.
Biologically immortal creatures include:
- Hydrae are a genus of simple, fresh-water animals possessing radial symmetry. The fact that all cells continually divide allows defects and toxins to be diluted among daughter cells. It has been suggested because of this that hydrae do not undergo ageing, and as such are biologically immortal. The way hydrae keep their telomeres from shortening due to constant replication is as yet unknown.
- Turritopsis nutricula is a 1/5-inch-long species of hydrozoan (stinging animals that do not belong to any other category thereof) which uses a process called transdifferentiation to replenish cells after sexual reproduction. As such, the life cycle of the immortal jellyfish goes from egg to planula (larval stage of stinging animals) to polyp (sessile creature rooted to substrate) to medusa (jellyfish), then back to polyp, back to medusa, et cetera ad infinitum. This cycle can potentially repeat indefinitely, rendering it biologically immortal. Unfortunately, the immortal jellyfish is zooplankton ("plankton" simply referring to any marine life that can't move against the current) and as such gets eaten a lot. The species originated from the Caribbean Sea, but since become cosmopolitan.
- Both sexual and asexual planarian flatworms (class Turbellaria) appear to exhibit an ability to live indefinitely and have an "apparently limitless [telomere] regenerative capacity fueled by a population of highly proliferative adult stem cells." Plus, you can mince and slice a planarian pretty much however you want and it'll still grow back. Science!
 See also
- ↑ Predominant suppression of apoptosome by inhibitor of apoptosis protein in non-small cell lung cancer H460 cells: therapeutic effect of a novel polyarginine-conjugated Smac peptide
- ↑ Hydra senescence
- ↑ Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms
- ↑ Even to the point where it'll grow back from a piece that's 1/279th of the original.