2. What's a Cell? (851)

To understand life, we need to start with the simplest living thing: a cell.

But that immediately raises a deeper question--what does it mean for something to be alive at all?

What counts as alive?

Animal and plant cells are the smallest things that we unreservedly consider to be alive--that is, that display the hallmark characteristics of life. Arguments can be made for and against viruses. We'll skip over that philosophical debate.

Cells have the ability to process food to generate energy, and to grow and develop. They also respond to changes in the environment, grow, reproduce, and, from a species perspective, evolve over the course of generations.

Humans, redwood trees, and elephants all display these hallmarks. Individual cells--including the cells in your fingertip--do these things, too.

Nothing in the cell is alive

There is a thought-provoking corollary to this claim. If a cell is the smallest living thing, then nothing contained inside a cell is alive. Cells represent the dividing line between the inanimate and the animate.

Many different molecules will be introduced in this book. Molecules are not alive. They’re just matter, or stuff.

And yet, when that matter is arranged in a certain way, life appears.

How can it be that a cell is alive but nothing inside the cell is? The answer lies in a concept called emergence.

Emergence: how life arises

Today many scientists would describe life as an emergent property: a behavior of a system that arises from interactions between the system's components but that can't be understood by examining those components in isolation.

Imagine an ant colony searching for food. Individual ants leave the nest and wander randomly. When one finds food, it carries it back to the nest and leaves behind a faint chemical trail.

Other ants come across the trail and follow it. If they also find food, they reinforce the trail on their return. Over time, trails that lead to food become stronger.

No ant understands the overall situation. No ant knows where the best food source is. No ant directs the others. And yet, the colony as a whole charts an efficient path between the nest and the food source.

Inside the cell, the same logic applies

Something similar happens inside a cell. Molecules don't search for their targets in any directed way. They diffuse, collide, bind, and separate, responding only to local conditions.

Over time, patterns of interaction emerge--pathways, structures, and processes--that appear coordinated and purposeful and that result in life. But, like the ant colony, there is no central control system.

This is what we mean by emergence: coordinated behavior arising from many simple interactions.

Given the number of components and the frequency of these interactions, these patterns are not rare--they are inevitable.

Your cells are you

This logic scales upward. An organism’s behavior emerges from the interactions among its cells.

You are reading this page. But, really, it’s your cells reading it: muscle and retinal cells in your eyes, nerve cells to transmit signals from the eyes to neurons in your brain, and more neurons 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 (as the first claim states) then how many cells are at work making you function as a human being? About 40 trillion.

What kind of cell?

In multicellular organisms, of course, not all cells are the same. Most have specialized functions. I’ve mentioned a few: rods, cones, neurons and muscle cells.

What kind of cell are we describing in this book? In fact, we won’t focus on a specific cell type, but rather on something like a generic human cell.

We can make this move because many cellular functions (so called housekeeping functions) are common to all cells. All cells generate energy from nutrients, use genes to make mRNAs, use mRNA's to make proteins, etc.

To refine our mental picture of the cell, it also helps to get a sense of scale.

How large are cells?

To get a sense of the size of our generic cell, I’ll borrow an example from David Goodsell’s book, The Machinery of Life. He estimates a typical human cell is about 1,000 times shorter than 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 dimension of a typical bedroom. 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 your fingertip.

What we’re left with at this point is a system made entirely of non-living stuff--molecules moving, colliding, and interacting--and yet somehow producing coordinated, life-like behavior.

To understand how this works, we need a way to picture the cell itself. One way to do that is to compare it to something more familiar: a factory.