The Living Cells

The Living Cells

The living cell is a regime as complex, and beautiful as the realm of the galaxies and the stars. The elaborate machinery of the cell has been painstakingly evolved over four billion years. Fragments of food are transmogrified into cellular machinery. Today’s white blood cell is yesterday’s creamed spinach. How does the cell do it? Inside is a labyrinthine and subtle architecture that maintains its own structure, transforms molecules, stores energy and prepares for self-replication.

If we could enter a cell, many of the molecular specks we would see, would be protein molecules, some in frenzied activity, others merely waiting.

The most important proteins are enzymes, molecules that control the cell’s chemical reactions. Enzymes are like assembly-like workers, each specializing in a particular molecular job: Step 4 in the construction of the nucleotide guanosine phosphate, say, or Step 11 in the dismantling of a molecule of sugar to extract energy, the currency that pays for getting the other cellular jobs done. But the enzymes do not run the show. They receive their instructions – and are in fact themselves constructed – on orders sent from those in charge. The boss molecules are the nucleic acids. They live sequestered in a forbidden city in the deep interior, in the nucleus of the cell.

An oak tree and I are made of the same stuff. If we go far enough back, we have a common ancestor. Image: © Elena

Human beings grew up in forests; we have a natural affinity for them. How lovely a tree is, straining toward the sky. Its leaves harvest sunlight to photosynthesize, so trees compete by shadowing their neighbors. If you look closely you can often see two trees pushing and shoving with languid grace. Trees are great and beautiful machines, powered by sunlight, taking in water from the ground and carbon dioxide from the air, converting these materials into food for their use and ours. The plant uses the carbohydrates it makes as an energy source to go about its planty business. And we animals, who are ultimately parasites on the plants, steal the carbohydrates so we can go about our business. In eating the plants we combine the carbohydrates with oxygen dissolved in our blood because of our penchant for breathing air, and so extract the energy that makes us go. In the process we exhale carbon dioxide, which the plants then recycle to make more carbohydrates. What a marvelous cooperative arrangement – plants and animals each inhaling the other’s exhalations, a kind of planet-wide mutual month-to-stoma resuscitation, the entire cycle powered by a star 150 million kilometers away.

There are tens of billions of known kinds of organic molecules. Yet only about fifty of them are used for the essential activities of life. The same patterns are employed over and over again, conservatively, ingeniously for different functions. And at the very heart of life on Earth – the proteins that control cell chemistry, and the nucleic acids that carry the hereditary instructions – we find these molecules to be essentially identical in all the plants and animals.