Life Lines 15

AGING IS A MATTER OF HOW MANY MITOCHONDRIA YOU HAVE LEFT

Way back in high school you learned that the mitochondrion was “the powerhouse of the cell.” If you took a college biology course as an undergraduate you probably learned that the mitochondria in your cells are bacteria-like in size and have their own DNA and that their major function was to take small carbon-bearing molecules from your digested foods and burn them with the oxygen you breathe to produce chemically stored molecules, chiefly ATP. The mitochondria power the metabolic activities of the cell, tearing molecules apart and synthesizing more complex molecules from simpler ones. Each of your cells has about 1000 mitochondria and each mitochondrion has several dozen copies of its small circular DNA. There are about 60 genes in a mitochondrial chromosome. Most of those genes are involved in production of chemical energy and thus the “powerhouse” reputation that you may remember. If you multiply the number of your cells by the number of mitochondria per cell you get a staggering 100 quadrillion of them in your body (that’s a one followed by 17 zeroes). When they act collectively making ATP they produce heat. The warmth of your body reflects the activity of your mitochondria. That’s why you get hotter in the summer when you exercise and why you shiver and jump around in the cold to warm your body.

Unlike the genes in the nuclei of your cells that produce 99.99 percent of your body, the mitochondrial genes only function is this one essential function of producing the bulk of the energy you need to stay alive. They do so at a price. They lack repair enzymes so mutations accumulate faster in mitochondria than in your nuclear chromosomes. When you are young this is less of a problem because your cells are still dividing but when you are an adult most of your cells slow down and rarely divide. So the mutations accumulate and your cells do not replace worn out mitochondria. The business of making energy by burning small carbon-bearing molecules with oxygen also produces a lot of chemicals that damage DNA. Your mitochondria make hydrogen peroxide. Have you ever seen it bubble when you daub it on a sore? These reactive oxygens readily damage DNA. So here’s the problem. You begin to lose mitochondria as your cells age. That means less energy per cell. When the energy is too low the cell dies. So if I look at the 76 year old skin on the back of my hands I see wrinkles, as you do if you are also old like me, but because I am a biologist, I also see in my mind’s eye, the cells that have winked out on me like so many dead light bulbs over the years and those cells have not been replaced (because I am no longer a child or teenager). The dead cells in the lower layers of my skin reduce the mass of my hands and the covering skin wrinkles as it collapses into those numerous cavities. So now you know why you wrinkle when you get old. Dead mitochondria lead to dead cells that lead to wrinkles. But, alas, there is more. Because we get old eventually, our mitochondria are fewer in number per cell and thus we produce less heat for our body. So now you know why your grandparents need a warmer temperature in their apartments or homes and why they heap blankets on themselves or wear clothes around the house that younger people would take off or feel stifled. I think of my interview with Sir Julian Huxley in his home in London, and he sat with gloves on (the fingers cut off) and a shawl around his shoulders and a small blanket over his lap. His mitochondria were fading in his memory too and his wife had to keep correcting him. Those who long for extending their lives have a tough job when they begin to shift their attention from our nuclear genes and chromosomal telomeres and start looking at our disappearing