The fundamental unit of life is the cell–a contained self-replicating assembly. For many species, including all bacteria and archaea, the organism consists of a single cell. And for nearly all species, no matter how many cells an organism eventually contains (probably around 10 trillion in your case), it started life as a single cell (an egg, in your case). The details vary, but every cell on Earth is the same at heart–a DNA-based replicating machine built from just four macromolecules: nucleic acids, proteins, lipids and carbohydrates. In the environment, molecules interact rarely and randomly. Bringing them together enables the reproducible reactions required for life. So no matter what the first self-replicating molecules were (likely ribonucleic acid, or RNA), they were not a cell until they acquired a container.
How would you build a container for a cell? You would probably want a porous material that allowed you to sort specific molecules from the environment. Evolution agrees. All cells are enclosed by a selectively permeable membrane, made of phospholipids and proteins (Learn More ⇩), that allows them to separate their contents from the environment. The chemical properties of phospholipids make membranes impermeable to ions and large or hydrophilic molecules (but not to water). This property is a critical feature for the life of the cell (⇩).
With a membrane, your cell now has a clearly delineated exterior and interior. The interior is called the cytoplasm (“cell substance,” from the Latin for something molded, in this case by the membrane). Almost all archaea and many bacteria, like these Mycoplasma genitalium cells, are monoderms (“single skin”). This means that their cytoplasm is enclosed by a single membrane. At this resolution, the membrane looks like a single dark line, but remember that it is really a bilayer, as you will be able to see in some later examples. The cytoplasm contains the many macromolecules that carry out the various functions of the cell’s metabolism. The most prominent are the ribosomes, which produce new proteins (⇩).
Other structures you see in this cell function in motility and will be explained in Chapter 6. Remember that tomograms show cells in their entirety, so the example we choose to illustrate one feature will likely highlight others as well. For now, focus on the feature being discussed. Later, when you have learned about other features, you may want to use the Feature Index to find additional examples of them. To help orient you, feature labels in movies are color-coded according to the chapter in which they are discussed. Chapter colors appear in the Navigation Menu (top left).