Necrosis/Apoptosis

When we think of change in cities, we think usually of growth, of expansion, of gleaming new buildings that make our world a better place. We almost always focus on the new, while forgetting about what happens to the old. When parts of a city are changed, old structures are torn down to make room for better ones. What happens during this deconstruction is just as important as what happens in reconstruction. Like us, a city is a complex organism, and when a piece of a living creature is disposed of, there are consequences.
Building demolition is exactly like the death of cells in the body. In most living creatures, cells can die in two ways: apoptosis or necrosis. The first is what is known as programmed cell death. This means that the destruction of the cell is intentional and deliberately triggered by chemicals in the body. Apoptosis happens when a defect is detected in the genetic makeup of physical structure of the cell and it is killed off to prevent its "bad" genes from being passed on through mitosis. Apoptosis also involves the release of chemicals that tell neighboring cells to absorb the apoptotic bodies. It is very clean, like any good killer.
The other form is necrosis, and this is a bit nastier. For our more squeamish readers, I've done you the favor of not including a picture, but I suggest you Google it, then find a trash can. Necrosis is different from apoptosis because it is cell death triggered by an outside force, like a brown recluse bite. The dying cells do not send signals to neighboring cells, so they are not absorbed. Instead, they sit around and decay. And who loves decaying flesh? Bacteria, which as you know, is generally not good. Even the smallest area of necrosis can spread disease throughout the body, often leading to amputation or death. Even the smallest events of necrosis are massive disruptions.
Cities function in exactly the same ways. Like I've said over and over, cities are living organisms. Buildings are cells, streets are arteries, plumbing is the digestive tract, etc. (I could go on for days with these analogies). And just like our cells, buildings have to die, and it's the reason for their destruction and its aftermath that I bring up apoptosis and necrosis.
Again, we'll start with apoptosis (programmed cell death). In most cases, when a building is destroyed, it is done intentionally. This is often because there is something wrong with the building, but in most cases it is done purely for the sake of new construction. This process can either be a quick implosion or a slow dismantling, but both are carefully coordinated in the same way a body would plan a cell's destruction. Materials are then sold and used in other buildings, just as surrounding cells absorb a dead cell's nutrients. In this way, both buildings and cells that carry out apoptosis are actually beneficial to the organism around them. Below is an example of a building going through apoptosis.

This brings us again to necrosis (non-programmed cell death). In cities this happens when a building is destroyed by a natural disaster, fire, or events like terrorist attacks. Necrosis happens unexpectedly, which guarantees chaos and tragedy. They hurt both individual people and the city as a whole. Take the horrifying events of 9/11. A malicious outside force struck two of New York's "cells", but the destruction did not stop at just the Towers themselves. The effects of the Trade Center's necrosis spread, causing the tragic deaths of thousands of innocent people, the panic of a city, and damage to the surrounding area as well.
It is therefore one of architecture's and evolution's main goals to make it so destruction only occurs under safe and controlled circumstances, because often the fate of an entire city rests on the fate of one building. It is an architect's job to make sure that destruction leads to growth rather than death.