How our cells strategize to keep us alive

Umbrellayour cells, each cell consists of 100 trillion atoms made of particles from the Big Bang, filled with all sorts of structures. These include organelles—small factories like energy-producing mitochondria—and microscopic molecular machines like ATP synthase, whose rotors and shafts spin at full speed. 300 rpm to make ATP, the molecules that carry energy in our cells. The insides of our cells are also full of molecules that randomly collide with tremendous speed. For example, water molecules zigzag at a staggering speed of more than 1,000 miles per hour (even though they only move about 4 billionths of an inch before hitting another molecule). In addition to collisions, cells face a multitude of other threats from within and without. You might assume they would suffer the same fate as our cars and dishwashers and keep breaking down. But they don’t. Your body has an ingenious three-part strategy to keep you out of the scrap yard.

Physiologist Dan Kirschner told me that just thinking about everything that could happen in cells used to keep him awake at night. He was studying cell growth in a graduate course just as his wife was about to give birth. He is so overwhelmed by the chances of making a mistake that he fears that his daughter will be born with a neck like a giraffe.

She didn’t. Our cells have come up with some clever strategies to avoid a short life. The first is that their machines are surprisingly reliable. For example, ribosomes insert the wrong amino acids into a protein in the order of once 10 thousand times. Our DNA replication machines only make mistakes about one in a million to 10 million or so.

However, nothing is perfect. Sometimes, mistakes happen. collision collision, ultraviolet rayand dangerous molecules like Free radicals also cause damage. Ingeniously, our cells have several ways of responding to these threats. For one, they are full smart repair mechanism—the machines whose job it is to patrol to find bugs and fix them. Our cells feature error-checking molecular machines and autocorrect feedback loops to ensure unsurpassed fidelity.

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A 1954 newspaper story in Constitution of Atlanta suggests a second strategy that our cells have adopted to maintain life. “Bored with yourself? Tired of the same old frames and faces? Take another look then. In other words, you are constantly being reborn. Humanity, like the auto industry, changes its chassis radically every year.” The science behind this strange claim is the work of an innovative nuclear physicist named Paul Aebersold.

Aebersold began his career at the cyclotron in Berkeley’s Radiation Laboratory, which pioneered the production of radioisotopes. Later, at the Atomic Energy Commission, Aebersold oversaw the development of isotopes for medical use. At some point, he realized he could use his isotopes to find out how often we replace atoms in our bodies. All he had to do was irradiate a substance like table salt, ask an extremely easygoing subject to swallow it, and trace the salt’s path with a radiation tracking device like a Geiger counter. You can track radioactive atoms in quantities as small as “a billion billionth of an ounce,” Aebersold proudly told an TV interviewer. He found that we swap half of our carbon atoms every one to two months and we replace the whole 98% of all our atoms each year.

Wait, what? Is that even possible? Obviously it is. More than half of your body is water and we know that we are constantly replacing water. Another big percentage of you is protein, and as you may recall, Most proteins break down within hours or days. We’ve even disassembled and replaced ribosomes and large organelles like mitochondria, which are made up mostly of protein.

Aebersold discovered another strategy that allows our cells to last so long: our cells continuously replace their seemingly permanent structures and old molecular machines crumbling with new ones. The only thing they don’t replace are our giant chromosomes. Instead, we have machines running along them looking for problems and fixing them.

What if the damage to a cell is too great to repair? We also have a backup plan for that. We simply destroy the entire cell, cut it into recyclable units and create a new one. On average, you replace most of your cells once every 10 years, which equates to about 330 billion cells per day. Those who worked in the harshest conditions retired most often. Damage to many cells in your gut, exposure to strong acids, is so predictable that they commit suicide and are replaced every two to four days. You replace your skin cells, which are resistant to scratches and UV rays, every month or so. Your red blood cells, which beat continuously as they move through your blood, are replaced every 120 days. That means you have to make almost 3.5 million new red blood cells every second. Other cells, such as those in our bones, are less active, only about once every 10 years.

So, in addition to using reliable machinery, our cells have a three-pronged motto for survival: constant error checking, constant repair and constant replacement. In a way, your body is like a major highway in New York—always open and always under repair.

Adapted excerpt from What’s Gotten Into You: The story of the atoms in your body, from the Big Bang to last night’s dinner by Dan Levitt. Published by HarperCollins on January 24, 2023. Copyright © 2023 by Daniel Levitt. Copyright Registered.

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