2. What's a Cell?

Humans have known of the existence of cell-like structures since the 1600s when early microscopists used their instruments to look at plant and animal tissues. They found that the plant tissues were made up of many small compartments. (Animal cells are harder to visualize largely because they have no hard cell walls.) They called them “cells” but didn't conceptualize them as individual biological units.

It wasn’t until the mid-1800s that a coherent theory of the cell—not surprisingly called “cell theory”—was put forth. It has withstood the test of time. Cell theory has three claims. The second one provides the premise of this book. Let's take a look at all three.

A depiction of cells comprising a tissues

 1. All living things are made of cells. Biologically, every animal and plant is just an aggregation of cells.

2. Cells are the most basic units of life. Nothing smaller possesses the behaviors we associate with life.

3. New cells always come from prior cells. Cells beget more cells. There's no other source of cells.

Let’s consider these claims in more detail and begin the larger task of building our conception of, and intuitions abou,t human cells.

The first claim is that all living things are made of living cells. This implies that all one's actions are no more and no less than the sum of the coordinated actions of one's cells. “You” are reading this page. But, really, it’s your cells doing it: muscle and retinal cells in your eyes, nerve cells to transmit signals from the those cells to your brain, and more nerve cells in the brain to process the signals and produce a mental image of words on a page. The working assumption among scientists is that, biologically speaking, everything you do and everything you think is carried out by your cells.

If we are our cells, then how many cells are at work making you function as a human being? About 40 trillion. To get a sense of this, 40 trillion seconds comes out to 1.3 million years. Frankly, the number is inconceivable to the human mind.

What about other organisms? Let’s focus on heavily studied organisms--what biologists call "model organisms." The bacterium E. coli is a model organism. It's unicellular. How about the yeast S. cerevisiae? It's also a single cell. Scientists study single cell organisms for the same reason that we're focused on individual cells in this book: they are, relatively speaking, the simplest embodiments of life.

What about multicellular model organisms? The worm C. elegans has exactly 1,031 cells . The fruit fly D. melanogaster has between 50,000 and 100,000 cells. The lab mouse (M. musculus) has as many as 10 billion cells. And, again, humans have around 40 trillion cells. So, what we conceptualize as a living organism can range from one cell to trillions or even quadrillions of cells—that is, thousands of trillions of cells, as are roughly estimated for the Giant Sequoia (S. giganteum).

Not all cells are the same. The majority have specialized functions. I’ve mentioned a few: rods, cones, and neurons. And, of course, there are specialized cells in all your organs: liver, stomach, kidneys, lungs, skin, etc.

Given that most human cells are specialized, it would be fair to ask what kind of human cell we’ll be focused on in this book. In fact, we won’t be focused on a specific cell type, but on something like a generic, composite human cell.

We can get away with this in part because while specialized cells perform their own specific functions, many functions, termed "housekeeping functions," are common to all cells. All cells generate energy from nutrients, use genes to make RNAs, use RNA's to make proteins. And there are many other housekeeping functions. Our focus here will be a housekeeping function among cells that divide: the process by which those cells replicate their genome prior to cell division.

Having briefly looked at the number of cells in various organisms and the the fact that cells have both specialized and housekeeping functions, let's also say a few words about the size of our generic human cell. To get a sense, I’ll borrow a comparison example used in Dr. David Goodsell’s outstanding book, The Machinery of Life.

Goodsell estimates that a typical human cell is about 10 µm (micrometers) long and notes that that's about 1,000 times smaller than the length of the last joint in your finger. A 1,000-fold difference in size isn't difficult to visualize: the length of a grain of rice is about 1,000 times shorter than the width of a typical bedroom. In other words, we could line up 1,000 grains on the floor end to end from one wall to the other. Now imagine that bedroom filled with grains of rice. That will give you an idea of the billion or so cells that make up just your fingertip.

The second claim of cell theory is that the cell is the most basic unit of life. It is the smallest thing that displays the hallmark characteristics of life such as the ability to process food energy, to grow and develop, to respond to changes in the environment, to reproduce, and, from a broader species perspective, to adapt to an environment over the course of generations. Humans do all these things. Fruit flies, too. And individual cells—including those in your fingertip—do these things.

There is a thought-provoking corollary to the second claim of cell theory. If the cell is the smallest living thing, then, by definition, nothing contained in a cell is alive. Cells are the dividing line between the animate and the inanimate. Many different molecules will be introduced in this book. Molecules are contained in cells. They’re not alive. They’re just matter, or “stuff.” In fact, everything in the cell is stuff. But when that stuff is arranged in a certain way in an enclosed space, life happens.

How can it be that a cell is alive but nothing inside the cell is? Today many scientists would explain it by arguing that life is an “emergent property." This is a characteristic or a behavior of a system that arises from the interactions of the components of that system. An emergent property can't be understood by examining the system’s components in isolation. In the case of a cell, life emerges from the interactions of inanimate stuff contained in the cell.

A Swiss watch and a traffic jam illustrate two aspects of emergence. In a watch, none of the individual parts can keep time on their own. Nor can all the parts if they are randomly strewn into a box. Timekeeping arises only from a specific arrangement of, and interaction between specific gears, springs, etc. No one part and no other arrangement of parts produces the behavior.

A traffic jam shows the same principle in a more spontaneous form: no driver intends to create a traffic jam, and no central activity controls it, yet a stable large-scale pattern emerges from many individuals following simple driving rules.

These examples show how complex, system-level behavior (timekeeping and traffic jams) can arise from the interaction between and organization of components without being located in or directed by any single component.

I'll address the third claim of cell theory quickly. The claim is that every cell comes from a pre-existing cell. This occurs through a process of cell division where an initial "parent cell" makes an exact copy of its genome, builds up additional cell components and resources, and then divides in half to create two nearly identical cells called "daughter cells." By the way, no biological meaning is attached to this gender term. It's a historical convention.

So, all known cells come from preexisting cells, the first cell being the only exception. That first cell didn't appear fully formed, but rather emerged gradually from pre-cellular chemistry on the early Earth about four billion years ago. Once a membrane-bound organic system crossed the threshold into true cellular life, the continuity of cells began—and has never been broken since.

Cell division and, more specifically, the challenge of replicating the parent cell's genome prior to cell division are the targets of this book . So I won't say any more about cell division here.

In the next chapter we'll continue to establish your early conception of a cell by introducing the parts of a cell: organelles, macromolecules, smaller molecules (mainly metabolites), and ions. We need to lay this groundwork before we get into molecular mechanisms.