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What are cells made of?

Most creatures on Earth are just a single cell that includes bacteria, archaea and one-celled organisms called Protozoa
© Created for Mini Medical School by JJ Cohen 2021 Creative Commons Attribution-Noncommercial-Share Alike 3.0 License

All living things that we know on Earth are made up of cells. Most creatures are just a single cell that includes bacteria, archaea (very like bacteria in some ways and probably older), and the dizzying variety of one-celled organisms that used to be called ‘Protista’. However, the latter is now so complicated that the very good article ‘Protozoa’ in Wikipedia makes a heroic effort to sort it all out.

I once weighed lymphocytes as part of a project, and found that a billion (109) of these cells weighed a gram. This means that the blue whale, which is the largest animal that ever lived and can tip the scales at 160 tonnes (a tonne is 1000 kilograms, about 2,200 pounds), could be made up of 160,000,000,000,000,000 cells. Unless, being so huge, a whale is made up of huge cells? It turns out, no.

Cell dimensions

Cells have to have certain dimensions and these are dictated by their physical properties. A cell is a lot of complicated small stuff surrounded by a ‘lipid bilayer’ membrane — two sheets of fat molecules back-to-back so that their water-liking (hydrophilic) sides are in contact with the water inside and outside, while their water-hating (hydrophobic) backs are pressed together in the center of the sandwich.

Some smaller molecules can diffuse through the membrane, and others gain entrance or exit via specialised pores or pumps. The size of cells, then, is limited by how quickly something can get in or out of the cell. For example, human red blood cells are exactly the right size and shape (being flat discs they have a lot of available surface for their volume) to grab a full load of oxygen — and off-load carbon dioxide — in their brief passage (about 3 seconds) through the lungs.

Huge cells would only be able to exchange with the outer bit of their volume, which surely would not make for an efficient life. The ‘lot of complicated stuff’ inside also has precise requirements, so that when a protein is being made, for example, the right amino acids have time to diffuse to where they need to be. How could all that have come into being? It seems impossible that so many rules and pathways and structures have all come together to make cells that work so well.

Evolution of cells

The key fact to remember is this started about 3.5 billion years ago. That’s a lot of time for evolution; the last common ancestor of the human and the mouse lived a mere 50 million years ago. One of the longest-running discussions in science is: what came first?

Minerals that could assemble small molecules along their crystal structure?

A primitive RNA that could copy itself on the mineral surface?

And all the while, selection is happening so that if something better should pop up, it will have a better chance of staying in the game until it, too, is replaced.

Archaea and bacteria fuse

Eventually archaea and bacteria appear, and a pair of those fuse together to start the line of the true cells with nuclei (Eukaryotes). One of the pair donated an organised nucleus, and the other became mitochondria, so there was a control centre and a power house.

Cells began to aggregate and become organisms. Plants started it, and after millions of years of good weather made enough oxygen for us animals. All that was needed was the invention of sexual reproduction, which accelerated the rate of evolution, until… there we were.

Could this happen on another planet, whirling around a distant star? A lot of people think so. The great physicist Enrico Fermi asked, though: “If this is really happening, why have none of those creatures contacted us to join with them?” I’m pleased to say I recently have solved Fermi’s Paradox. I believe the Galactic Federation visited Earth last year, looked around carefully, watched some TV, read the papers, and said… maybe another time.

© Created for Mini Medical School by JJ Cohen 2021 Creative Commons Attribution-Noncommercial-Share Alike 3.0 License
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Mini Medical School: Introduction to Medical Science

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