Chapter 2: Building Blocks
33 - The Cell
A cell is the fundamental living form. It’s here that molecules like DNA, chromosomes, and proteins all come together. To begin with, information inherited through genes is stored in the familiar double-helix DNA molecules. Those DNA strands are wrapped in protective protein to form a chromosome (we have forty-six of them, which together hold some three billion genes). In complex organisms such as plants and animals, this delicate system is further enclosed in a nucleus, essentially a separate room at the heart of the cell.
In this system, parts of our genetic information are copied as we need them, with the copies then being ferried out of the nucleus and into the larger inner world of the cell. There they are used to guide assembly of the many different kinds of proteins our bodies use. That process takes place on a minuscule scale—imagine the cell as a domed sports stadium and protein production as marching bands parading around the infield.
The messengers used to make the transfer are called RNA. They transcribe and transport the data, just as an athlete might scribble notes from a playbook to carry out on the field while leaving the book itself safely behind. At the end of that journey, the instructions an RNA molecule carries are fed like punch tape through a microscopic decoder—the ribosome—where they guide the assembly of amino acids into fresh proteins.
The twenty different amino acids are combined in a variety of ways by life forms to make the assorted proteins they need. Those proteins are crucial building blocks, used in nearly every function of the body. When we eat the bodies of other organisms, our digestive enzymes snip their proteins apart. Those disassembled parts—now once again loose amino acids—are transported to our cells. There, our ribosomes reassemble them to make new proteins tailored to our own special needs.
If there’s a cheerleader inside the cell, it’s ATP, adenosine triphosphate, the energy-bearing molecule made by mitochondria. ATP spurs activity with the help of an internal cell framework called a cytoskeleton. The old notion of the cell as a big sac with the nucleus and other parts sort of floating around inside is obsolete. The interiors of plant and animal cells are actually laced with a threadlike gossamer scaffolding. That scaffold serves a dual function: It anchors the cell’s organs—the organelles—such as the nucleus and the ribosomes. It’s also part of a transport system. ATP powers protein molecules that serve as cargo carriers, pushing them along the cytoskeleton’s rails.
On the outside surface of each cell membrane, on the roof of the stadium dome as it were, there are craterlike receptors into which lock only specific molecules that come floating down the bloodstream, such as hormones or specially flagged proteins. When that happens, they trigger a signaling cascade, like a section of fans in the bleachers doing “the wave.” The end result could be the start of transcription for a given protein, or a muscle might contract, or blood vessels might dilate to allow greater flow. Certain dilation receptors are activated by sex drugs like Viagra. They contain molecules focused—much like the front page of a supermarket tabloid—primarily on a single function of the body.
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