What's a Cell?

Humans have known of the existence of cells since the 1600s when scientists (then called “natural philosophers”) used simple microscopes to look at plant and animal tissues. They found the tissues were made up of many small compartments, and they called them “cells.” It wasn’t until the mid-1800s, though, that a coherent theory of the cell was put forth. That theory—“cell theory”—has withstood the test of time. Today it’s taught in every high school and college biology course.

A depiction of cells making up a tissue.

 Cell theory has three claims.

1. It claims that all living things are made of cells. Think of any animal or plant. It’s just an aggregation of cells.

2. It holds that the cell is the most basic unit of life. Nothing smaller possesses the characteristics or behaviors that we associate with living things. All cells do.

3. It asserts that new cells always come from prior cells. Cells beget more cells.

Let’s consider these claims in more detail.

Claim 1

The first, once again, is that all living things are made of living cells. This implies that all your actions are no more and no less than the sum of the coordinated actions of your cells. “You” are reading this page. But really, it’s your cells that are reading the page: muscle cells and retinal cells in your eyes, nerve cells to transmit electrochemical signals from the retinal cells to your brain, and still more nerve cells in the brain to produce a mental image of words on a page. Cells do everything! And you are nothing more than your cells.

How many cells are you? The short answer is “about 40 trillion.” But this brings up the more general question: How many cells are in different kinds of organisms? Let’s pick a few. How many cells are in a bacteria like E. coli? One. A bacterium is a single cell. It is also known as a prokaryotic cell, a distinction we’ll cover in the next chapter. A yeast such as S. cerevisiae is also a single cell. But unlike the bacterium, the yeast cell is a eukaryotic cell.

How about some multicellular organisms? We’ll focus on “model organisms”—those most heavily studied by scientists. The worm C. elegans is one. It has exactly 1,031 cells. Another, the fruit fly D. melanogaster, has between 50,000 and 100,000 cells. The lab mouse (M. musculus) has 3-4 billion cells. And, as I mentioned, humans have around 40 trillion cells. So, what we think of as organisms range from the single-celled to those having trillions or even several quadrillion cells—that is, thousands of trillions of cells like the Giant Sequoia (S. giganteum).

Let’s get back to humans. Humans are comprised of a lot of cells. But, of course, they are not all the same. Many have very specialized functions. I’ve already mentioned a few: retinal cells like rods (light detecting) and cones (color detecting), neurons (that carry electrochemical signals), and other neurons in the brain (that process the incoming signals and form the images of consciousness. There are also liver, stomach, kidney, lung, and skin cells, to name a few.

It would be reasonable to ask exactly what type of human cell we’ll be focusing on in this book. In fact, we won’t be focusing on a specific cell type, but rather on something like a generic human cell. Although specialized human cells perform different and specific functions, many activities are performed by all cells.

For example, all cells must generate energy from foodstuff, use genes to make proteins, replicate their genomes prior to cell division, etc. These and many other cell functions are termed “housekeeping functions” because every cell must perform them, regardless of the cell’s specialized function. So, on this website we’ll be focused on the some of the housekeeping functions performed by cells.

Finally, as we consider this first claim of cell theory, that all organisms are made of cells, we should also discuss the size of oure generic human cell. To get a rough sense of how small one is I’ll borrow an excerpt from Dr. David Goodsell’s outstanding book, The Machinery of Life:

“Cells are about 1000 times smaller in length than objects in our everyday world... Typical human cells are about 10 µm in length. This is roughly 1000 times smaller than the last joint in your finger. A 1000-fold difference in size is not difficult to visualize: a grain of rice is about 1000 times smaller in length than the room you’re sitting in. Imagine your room filled with grains of rice. That will give you an idea of the billion or so cells that make up your fingertip.”

The sizes of some other cell types, relative to our generic human cell, can be found in Table __.

Claim 2

The second claim of cell theory is that the cell is the basic unit of life: it is the smallest thing that displays the hallmark characteristics of life. The typical behaviors of living things include the ability to process food energy to fuel their life activities, to grow and to develop, to adapt to the environment over generations based on evolutionarily theory, to respond to changes in the environment, and to reproduce. Humans do all these things. Fruit flies do them, too. And individual cells—even those that are part of a larger organisms—do all 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 life and non-life, between the animate and the inanimate. Many different molecules will be discussed in this book. Molecules are contained in cells. They’re not alive. They’re just matter, or “stuff.” In fact, everything inside the cell is just inanimate stuff. But when the 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? Philosophers of science might explain it by arguing that life is an “emergent property.’ An emergent property is a characteristic or a behavior of a system that arises from the interactions of the components of the system. An emergent property cannot be understood by examining the system’s components in isolation. In the case of a cell, life (the emergent property) emerges from the interactions of inanimate stuff contained in the cell.

Consider a non-biological example of emergence. Take apart a Swiss watch and put all the parts in a small box. Positioned randomly in the box, the parts can’t tell time. But arranged in a specific way, the parts interact and a new property emerges: timekeeping. The same “stuff” that’s in the ticking watch is also in the silent box of random parts. The only difference is the arrangement. One arrangement produces timekeeping. Most of the others do not.

So, nothing inside a living cell is itself alive. In the next few chapters, we’ll be discussing (in order of decreasing size) cell organelles (or “small organs”) that perform key cell functions, large macromolecules (e.g., DNA, proteins, lipids and carbohydrates), smaller molecules (e.g. metabolites and the nucleotide and amino acid components of DNA and proteins, respectively), and chemicals and ions (e.g., carbon, nitrogen, hydrogen, and oxygen) that are the raw materials from which the smaller molecules are built. None of these cellular components is alive.

Claim 3

The third and last claim of cell theory is that all cells come from other cells. In other words, cells divide to generate more cells. I won’t say much more about this claim because cell division is the focus of this book: specifically, the challenge of replicating an entire DNA-based genome for a new daughter cell that results from cell division.

In my next blog post, I'll introduce the components of a cell: first organelles and then macromolecules, smaller molecules (mainly metabolites), and ions.