Theories of Aging: Rate-of-Living, Cellular & Programmed-Cell-Death

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  • 0:01 The Uncomfortable Topic
  • 1:11 Rate-of-Living
  • 2:36 Cellular
  • 4:24 Programmed-Cell-Death
  • 5:32 Lesson Summary
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Lesson Transcript
Instructor: Devin Kowalczyk

Devin has taught psychology and has a master's degree in clinical forensic psychology. He is working on his PhD.

This lesson focuses on various theories of how living cells age and die, resulting in the people with those cells aging and dying. We'll look at broad definitions, as well as some of the popular sub-theories.

The Uncomfortable Topic

Um, let's see. I'm supposed to tell a humorous anecdote or story about cell death which ultimately leads to people dying. Even if we put some kind of spin on it, the discomfort of the topic is still there. Sometimes what we have to talk about just isn't much fun. For instance, if we spun it and made a joke about the eventual return of dead cells as undead cells and the crave of human flesh, it still doesn't make it less uncomfortable.

When it comes to how cells grow up and then break down, there are two main theories: programmed and damage/error. Programmed theories state cells natural maturation follows a timetable with the death of the cell imminent. Damage/error theories focus on how cells accumulate damage to their structure or errors to their DNA or mitochondria, which results in cell death. There are several different overarching types and subtypes of each of these; however, we are going to focus on a three main areas: rate-of-living, cellular, and programmed-cell-death.


Rate-of-living is an assumption that every organism has a set metabolic potential. How long does a house fly live? Something like a few weeks (not 24 hours). But have you ever seen those little buggers move? Their little hands are really fast. They just move, move, move. This means their metabolic rate is extremely high, which results in a faster rate of living and, thus, a shorter lifespan.

We can keep this up for almost every animal. Mice live longer than flies, but shorter than monkeys. Monkeys live shorter lives than elephants. It's those dang humans, though, that live for a really long time even though they aren't very big.

This theory was originally proposed in the 1920s. Big animals lived longer than small animals, and machines that worked harder tended to break down sooner. However, clever experiments over the last century have cast some doubt on this theory. Experiments where animals had their metabolic rate artificially heightened did not die sooner. This means that, while we can say that in general larger animals live longer and the larger animals have a slower metabolism, this does not always directly translate into a longer or shorter lifespan.


Cellular theory is a collection of error/injury theories. We will discuss each one briefly, but focus on them as a more holistic grouping.

Wear and tear is an accumulation of errors and damage due to natural use. Think about a car. If you drive it, it will begin to break down from general wear and tear. The same thing happens to your cells; they are little machines that run and break down due to wear and tear.

Free radicals are a byproduct of metabolism that creates highly reactive and damaging proteins. This one is a little tricky to understand. Your cells produce proteins to build all the stuff inside of it, but sometimes the proteins don't form correctly. Think of a cell like a factory. In making certain things, the factory produces toxic waste. This toxic waste causes problems and starts to kill people off. Free radicals do the same thing, but to your cells. They can damage your DNA and cause cancer, or mangle parts of your cell and cause it to die.

Senescence focuses on programmed and predictable cell death due to age. A human cell can replicate itself about 40 to 60 times before the DNA or other parts of it becomes so damaged it can no longer function. This would mean that there is a programmed-cell-death and natural age limit to cells and, thus, people.

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