Life's Ratchet: How Molecular Machines Extract Order from Chaos (3 page)

Let the adventure begin.

The human body is a machine that winds itself, a living picture of perpetual motion.

—J
ULIEN
O
FFRAY DE
L
A
M
ETTRIE

Come, said my soul, Such verses for my Body let us write, (for we are one).

—W
ALT
W
HITMAN
,
L
EAVES OF
G
RASS

L
IFE IS THE DANCE OF A BEE AND THE ROAR OF A LION. IT IS the tangle of a rain forest and the mortal battle between bacteria and host. Life is amoeba and elephant, evolution and extinction, and the power to transform a planet. The complexity and variety of life is staggering, but for physicists, life begins at a more basic level. All life started as a circle dance of molecules billions of years ago. The lion and the bee, the humble yeast and the mighty blue whale all share the same jittering molecules in their cells; we are all cousins.

But while life is based on molecules and energy, it seems to defy a purely physical explanation. When we look at a living being, we immediately recognize it as alive, as fundamentally different from a rock or a cloud. Yet, when we try to define life, we run into difficulties. There seems to be something indefinable, some special ingredient that separates inanimate matter from living flesh. When a loved one dies, we despair at not being
able to recreate life. It is as though a special ingredient, a “life force,” has left the body. Life seems forever beyond our powers and understanding.

And yet, we know that modern science has the power to manipulate life. From genetic engineering to brain imaging, science has penetrated living matter to its very core. The dichotomy between our everyday experience of the purposefulness and magic of life, and the fact that when we go looking for the magic ingredient, we only find matter and mechanism, has occupied human minds for thousands of years. It has led to a drawn-out battle between those who see purpose and those who see mechanism. In this battle, sometimes one side gained the upper hand, sometimes the other. Has the battle finally been decided? And if yes, who won?

If I had to vote for the most abused scientific terms,
energy
,
power
, and
force
would be on the short list. According to the motivational speaker Bob Proctor, human beings are an incredible source of power and could use the “power in their body” to illuminate “a whole city for nearly a week.”
^*

Since Proctor is so precise (“nearly a week”—why not a whole week?), it may be worth double-checking his calculations. It turns out that it is quite easy to calculate the power rating of a human being. Power, in physics, measures how
fast
energy is transformed from one form to another, and not the amount of available energy. Proctor is confusing power with energy. But I don’t want to quibble about that. Let’s pretend he means that the power rating of one human is equivalent to the hundreds of thousands of light bulbs that illuminate a city.

Humans transform energy from food into motion, heat, and thought. Energy is conserved. The energy we expend during a day comes from the food we eat. A typical energy intake from food is 2,500 food calories per day. One food calorie is equal to 4,184 joules of energy. A human consuming 2,500 food calories takes in approximately 10.5 million joules (2,500 calories × 4,148 joules) in energy from food a day. This sounds like a lot.
However, a day has 86,400 seconds, and therefore the
rate
at which our bodies transform this energy is 10.5 million joules divided by 86,400 seconds, or about 120 watts (where 1 watt = 1 joule per second). Far from illuminating a whole city, a human being has about the same power rating as
one
light bulb.

Humans talk, write, walk, and love using the same amount of energy per second as a light bulb, a device that does nothing but shine light and get hot. This amazing fact, far from denigrating humans, is a testament to how efficient a human body is. But even more importantly, it is a testament to the wondrous complexity of our bodies, which can do so much with so little.

Humans and other living beings are not sources of energy. We are consumers of energy, taking high-grade energy in the form of food and releasing it in the form of low-grade heat into the environment. When we stop eating, we starve and die. This simple truth is nothing new, yet books like Rhonda Byrne’s
The Secret
(which claims that “human beings manage their own magnetizing energy”) sell millions of copies, making us believe that there are untapped sources of energy within us.
^*
Why is this idea so persuasive? Where did this notion of life force or energy come from?

The idea that life is infused with special energies or forces is as ancient as humanity itself. When people today are attracted to books like
The Secret
, it may be because the idea of a life force is deeply engrained in our psyche. For at least a hundred thousand years, humans have tried to bridge the gulf between life and death by placing flowers, food, or tools in burials with their departed. For our ancestors, death was an unnatural state, as all of nature seemed to be ever changing, moving, and alive.

Nature’s powers of motion and change were associated with
anima
, the soul. Animism, the belief that all of nature was alive and governed by
spiritual forces, survived the centuries and was part of respectable European philosophizing well into the twentieth century.
^*
The use of “magic” crystals and magnets for healing is still part of some people’s beliefs today, as they believe that these items have special energies that affect life and health. In animism, not only animals, but also rocks, the wind, the river, were alive. In such a belief system, the concept of dying did not make much sense. The ancients believed that when a person died, he or she was not really dead, but instead the person’s spirit had moved somewhere else. It was important to supply the dead person with tools and gifts for this new existence. Our now familiar distinction between living beings and lifeless matter evolved much later. Once this happened, most things—rocks, water, air—were recognized as lifeless, and
living
became a mystery in need of explanation. Living matter was now seen as being substantially different from all other matter and had to be endowed with extraordinary forces or a soul. We call such a belief
vitalism
.

Vitalism began with the Greeks, most notably with the philosopher and scientist Aristotle (384–322 BCE). For Aristotle, life was different from inanimate matter because it had “soul”; it was “animated.” It is not a coincidence that we identify the word
animated
with being in motion. Purposeful motion, which includes locomotion, growth, and internal motion of the organism, was for Aristotle (and still is today) the most conspicuous attribute of life.

Aristotle spent a lot of time thinking about the soul, as recorded in his book
De Anima
. He identified several problems with defining soul: Is the soul a whole, or is it made of parts? Are there different types of souls for horses, dogs, and people? What is the soul’s relationship to the body? Aristotle realized there was a problem distinguishing the soul from the body: “Are all affectations of the complex of body and soul, or is there any among them peculiar to the soul itself ? . . . there seems to be no case in which the soul can act or be acted upon without involving the body.” But
despite raising this issue, Aristotle never addressed the question about the necessity of the soul. In fact, he would have found such a question absurd—for him and his fellow Greeks, the existence of a soul was self-evident: “Knowledge of the soul . . . contributes greatly to the advance of truth in general, and, above all, to our understanding of Nature, for the soul is . . . the principle of . . . life.”

FIGURE 1.1.
Democritus—typically shown as the bearded, laughing philosopher. In the history of science, he certainly had the last laugh. After all, he was (mostly) right about atoms.

When I learned about Greek philosophy in high school, I first noticed all the things the Greeks got wrong. To my teenage self, it seemed naive to think of earth or fire as elements. In truth, the Greeks made enormous progress, from their belief in Olympian gods Zeus and Hera to their model of nature using four elements. The Greeks were also the first people to base ideas on scientific observations. The philosopher Anaximenes of Miletus (585–529 BCE) determined that air was the fundamental element, because it could be rarefied or condensed. He based this idea on his observations of evaporation, condensation, drying, and wetting. Other observations that guided Greek philosophy included the growth of seeds (Anaximander), breathing (Anaximenes), fossils in rock (Xenophanes), the necessity of water for life and buoyancy (Thales), and the random motion of suspended dust (Democritus;
Figure 1.1
).

Aristotle was, without a doubt, the most prolific (and most scientific) of all Greek philosophers. In
De Anima
, he provided a comprehensive overview of what his predecessors thought of the mystery of the soul. Based on meager experimental evidence, ancient philosophers ventured
surprisingly close to modern ideas: According to Aristotle, the philosopher Democritus imagined the soul as a fire consisting of myriads of jostling particles, which Democritus called “atoms.” Democritus got the idea for the incessant motion of atoms from observing the random movements of dust grains in beams of sunlight. As we will see, the ceaseless motion of atoms and molecules plays a central role in our modern understanding of life—a motion we can rightly call a
molecular storm
.

Unfortunately, Aristotle found the ideas of Democritus and Pythagoras absurd. For him, movement is the result of thought and will, not the random motion of atoms: “Democritus says that . . . atoms . . . owing to their ceaseless movements draw the . . . body after them and so produce its movements. . . . we may object that it is not in this way that the soul appears to originate motion in animals—it is through intention or process of thinking
.
”