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Soundings: The weird world within our bodies

It takes energy to move a muscle or think a thought. Energy comes from oxidizing (‘burning’) food. But the energy released when a candy bar reacts with oxygen can’t be used directly.

Wood fuels a steam engine, but it’s not the fire that does the work. The fire’s heat boils water. It’s steam that pushes the pistons.

In nearly every cell in the body, food energy is harnessed to make a molecule called ATP. It’s ATP that then energizes muscles, brains and glands. Every day, our bodies manufacture half our body weight in ATP (and use it as quickly as it is made).



ATP is just one of the thousands of different types of molecules our cells create. And cells are not slouches; a single cell churns out thousands of molecules per second.

Molecules react when they draw close enough to trade atoms. One of the keys to high output is to quickly get molecules into intimate contact. Cells have pumps and ferries to move some molecules, but for most molecules, they must rely on simple diffusion – the same random process by which perfume drifts from a neck on one side of the room to a nose on the other.




This process might not seem very efficient, but our cells inhabit a different world than we do.

A housefly walks straight up a wall and, without missing a beat, continues, upside down, across the ceiling. The fly’s body is so light that gravity has little influence on it – much less influence than the electromagnetic forces that allow its feet to stick to the ceiling. (Human feet are electromagnetically sticky, too, but our bodies are so heavy that gravity is the more compelling of the two forces, and ceiling-walking is a very challenging skill.)

The smallest living cells are those of bacteria. Typical bacteria could be lined up 25,000 to the inch. Anything small enough to reside within the watery environment of the bacterium’s body experiences the force of gravity as not just puny, but completely irrelevant.

The more energy a molecule possesses, the faster it moves. Suppose a bacterium needs to combine some molecule in its body with, say, a molecule of vitamin C. Knowing the mass of a “C” molecule (about 88 times heavier than a molecule of hydrogen), and how warm it is (say, 37 degrees Celsius – body temperature), we can calculate the average speed at which a molecule of vitamin C moves from point to point in a bacterial cell: 450 miles an hour.

At that rate, a molecule of vitamin C could make the trip from one end of the bacterium to the other, and back again, more than a hundred million times a second.

But the cell is not empty; it’s chock full of other molecules, into which the vitamin C collides, again and again.

In his book “Bionanotechnology,” David Goodsell (Scripps Research Institute) notes that, given the tiny distances involved, simple diffusion within the cell ensures that, on average, every molecule that’s not tied down will collide with pretty much every other molecule in the cell – at least once every second.

Human cells, though larger than bacteria, are still in the ballpark when it comes to the efficacy of diffusion.

With molecules bouncing around at hundreds of miles an hour, the cells of our bodies can churn out thousands of new molecules every second.

Alan Stahler trained as a biologist and is an amateur astronomer. He teaches enrichment classes for children and adults at Sierra Friends Center. His science programs can be heard at noon on alternate Tuesdays on KVMR-FM (89.5). He will be speaking with David Goodsell on “Soundings” on Tuesday, March 9, at noon.


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