17
would grind to a halt. (Most life, anyway. Microbial “chemilithoauto-
18
trophs” feed off free energy locked up in mineral compounds. Even without
19
the sun, the Earth still wouldn’t be in perfect thermal equilibrium.)
20
But imagine that we were surrounded by the sun— the whole sky was
21
raining photons down on us as bright as the sun does now. The Earth would
22
rapidly equilibrate, but we would come to the high temperature of the sur-
23
face of the sun. There would be a lot more energy reaching Earth than there
24
is now, but the solar- temperature radiation would all be useless, disordered
25
energy. Life would be just as impossible under those conditions as it would
26
be without the sun at all.
27
What matters to life is that our environment here on Earth is very far
28
from equilibrium, and will be for billions of years. The sun is a hot spot in
29
a cold sky. Because of that, the energy we receive in the form of solar pho-
30
tons is almost entirely free energy, ready to be turned into useful work.
31
And that’s exactly what we do. We receive photons from the sun, pri-
32
marily in the visible- light part of the electromagnetic spectrum. We process
33
the energy, and then return it to the universe in the form of lower- energy
34
infrared photons. The entropy of a collection of photons is roughly equal to
35S
the total number of photons you have. For every one visible photon it re-
36N
ceives from the sun, the Earth radiates approximately twenty infrared
2 42
Big Picture - UK final proofs.indd 242
20/07/2016 10:02:48
l Ig h t A n d l I F E
photons back into space, with approximately one- twentieth of the energy
01
each. The Earth gives back the same amount of energy as it gets, but we
02
increase the entropy of the solar radiation by twenty times before returning
03
it to the universe.
04
The energy here on Earth is not exactly constant, of course. Since the
05
Industrial Revolution, we have been polluting the atmosphere with gases
06
that are opaque to infrared light, making it harder for energy to escape and
07
thereby heating the planet. But that’s another story.
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
S35
N36
2 43
Big Picture - UK final proofs.indd 243
20/07/2016 10:02:48
01
02
03
30
04
05
Funneling Energy
06
07
08
09
10
11
12
13
14
15
It’s worth seeing how all this grand physics theorizing plays out in bio-
logical practice.
The basic power battery of life here on Earth is a molecule called aden-
osine triphosphate, or ATP. We’re using “battery” in a broad sense, as some-
16
thing that stores free energy for later use. Think of ATP as a compressed
17
spring, ready to push apart when it is released and expend its energy doing
18
something (hopefully) useful. And useful it is: the free energy stored in
19
ATP is used for muscle contraction, transporting molecules and cells
20
through the body, synthesizing DNA and RNA and proteins, sending sig-
21
nals through nerve cells, and other vital biochemical functions. ATP plays
22
a crucial role in allowing an organism to move around and maintain itself,
23
as Schrödinger highlighted as the defining characteristic of life.
24
25
NH2
26
O
O
O
N
27
N
28
HO P O P O P O
N
N
29
OH
OH
OH
O
30
31
32
OH OH
33
The chemical structure of adenosine triphosphate, ATP. It includes atoms of hydrogen (H), 34
oxygen (O), phosphorus (P), nitrogen (N), and carbon. Following chemical tradition, the 35S
carbon atoms aren’t indicated explicitly, but are located at each unlabeled vertex or bend in 36N
the diagram.
2 4 4
Big Picture - UK final proofs.indd 244
20/07/2016 10:02:48
F u n n E l I n g E n E Rg y
The release of energy from ATP typically happens in the presence of
01
water (H O). One of the three phosphates— groups with one phosphorus
02
2
atom (P) surrounded by oxygen atoms (O), at the left of the diagram— splits
03
off from the ATP, leaving us with adenosine diphosphate (ADP). The phos-
04
phate then joins with a hydrogen atom from a nearby water molecule, leav-
05
ing the remaining OH to combine with the ADP.
06
The total energy of these final products is less than that of the original
07
ATP molecule; the process thus releases both free energy (to do some useful
08
biochemical work) and disordered energy (heat). Fortunately, ATP is a re-
09
chargeable battery; the body then uses an external source of energy, such as
10
sunlight or sugar, to convert the phosphate and ADP back into water and
11
ATP, which is then ready to be put to work once again.
12
13
ATP + water
useful
14
work
15
free
16
energy
17
disordered
18
energy
ADP + phosphate
19
20
Free energy from external sources (photosynthesis, sugars) is stored in ATP, so that
21
it can be converted to useful work where the body needs it. Such a process necessar-
ily produces disordered energy as well.
22
23
All of the energetic activity going on in your body uses up a tremendous
24
amount of ATP; a typical person churns through an amount of ATP equal<
br />
25
to about their body mass each day. When you flex your biceps to lift a bar-
26
bell or a glass of wine, the energy to contract your muscles comes from ATP
27
snapping apart, causing proteins to slide against one another in your muscle
28
fibers. The individual atoms making up the ATP aren’t used up; each mol-
29
ecule is simply broken apart and then reassembled, hundreds of times a day.
30
31
•
32
Where does the free energy come from to create all that ATP from the
33
lower- energy ADP? Ultimately it comes from the sun. The process of pho-
34
tosynthesis occurs when a molecule of chlorophyll in a plant or some mi-
S35
croorganism absorbs a photon of visible light, whose energy knocks loose
N36
2 45
Big Picture - UK final proofs.indd 245
20/07/2016 10:02:48
T H E B IG PIC T U R E
01
an electron. The energetic electron is shuttled across a membrane by a series
02
of molecules called an electron transport chain. As a result, there are more
03
electrons than protons on one side of the membrane, setting up an electrical
04
gradient, with a net negative charge on one side and a net positive charge on
05
the other.
06
This is the basic way life funnels energy: protons on one side of a mem-
07
brane push each other apart, with some escaping through an enzyme called
08
ATP synthase. The proton trying to escape winds up the synthase, provid-
09
ing it with energy that it uses to synthesize ATP from ADP, in a process
10
called chemiosmosis. Some of the energy, inevitably, becomes disordered,
11
and is released in the form of low- energy photons and thermal jiggling
12
(heat) of the surrounding atoms.
13
p+
14
photon
ADP +
ATP
15
phosphate
+ water
16
17
e–
18
19
photo-
ATP
20
system
synthase
21
membrane
p+
22
p+
p+
23
p+
p+
p+
proton
p+
24
p+
excess
p+
p+
25
How photosynthesis stores free energy from the sun in ATP. A photon hits a pho-
26
tosystem embedded in a biological membrane, causing an electron (e–) to be ejected.
This process leaves an excess of protons (p+) on the other side of the membrane.
27
Electrostatic repulsion pushes the protons away, until one escapes through an ATP
28
synthase enzyme. The ATP synthase uses energy from the proton to convert ADP
29
into ATP, which can then carry energy elsewhere.
30
31
You and I don’t personally photosynthesize. Our free energy doesn’t
32
come directly from the sun, but from glucose and other sugars, as well as
33
fatty acids. Tiny organelles called mitochondria, the powerhouse of the cell,
34
use the free energy locked in these molecules to convert ADP to ATP. But
35S
the free energy in those sugars and fatty acids that we eat ultimately came
36N
from the sun via photosynthesis.
2 46
Big Picture - UK final proofs.indd 246
20/07/2016 10:02:48
F u n n E l I n g E n E Rg y
The basic setup seems to be universal within life here on Earth. The
01
phrase proton- motive force has been coined to describe the powering of ATP
02
synthase by the protons flowing through it. The mechanism was discovered
03
by British biochemists Peter Mitchell and Jennifer Moyle in the 1960s.
04
Mitchell was an interesting character. Forced to resign his academic posi-
05
tion when the pressures of his job led to severe health problems, he eventu-
06
ally set up a private laboratory at a place called Glynn House. He was
07
awarded the Nobel Prize in Chemistry in 1978 for the idea that the proton-
08
motive force was responsible for ATP synthesis via chemiosmosis.
09
10
•
11
The cell is the basic unit of life: a collection of functional subunits, organ-
12
elles, suspended in a viscous fluid, all surrounded by a cellular membrane.
13
Immersed as we are in a technological society, we tend to think of cells as
14
tiny “machines.” But the differences between real biological systems and the
15
artificially constructed machines that we’re used to dealing with are as im-
16
portant as their similarities.
17
These differences stem in large part from the fact that machines are gen-
18
erally created for some particular purpose. Because of this origin, machines
19
tend to be just good enough for their designated purposes, and no better.
20
Design tends to be specific, and brittle. When something goes wrong— you
21
lose a tire on your car, or the battery dies on your phone— the machine
22
doesn’t work at all. Biological organisms, which have developed over the
23
years with no specific purpose in mind, tend to be more flexible, multipur-
24
pose, and self- repairing.
25
Cells don’t merely tolerate chaos; they harness it. They have little choice,
26
given the environment in which microbiology takes place.
27
Our human- scale world is relatively calm and predictable. Throw a ball
28
on a day with good weather, and you can estimate with some confidence
29
how far it will travel. Cells, by contrast, operate at the scale of nanometers,
30
billionths of a meter. Conditions in that world are dominated by random
31
motions and noise— what biophysicist Peter Hoffmann has dubbed a “mo-
32
lecular storm.” Just from ordinary thermal jiggling, molecules inside our
33
bodies bump into one another trillions of times a second, in a maelstrom
34
that puts ordinary storms to shame. Scaled up to human size, living in the
S35
equivalent of the cell’s molecular storm would be like trying to throw a ball
N36
2 47
Big Picture - UK final proofs.indd 247
20/07/2016 10:02:48
T H E B IG PIC T U R
E
01
that was constantly being bombarded by other balls, each of which carried
02
hundreds of millions of times the energy that your arm could impart.
03
It doesn’t seem like a hospitable environment for any microscopic sport-
04
ing events, or for the delicate operations that are part of the cellular ecosys-
05
tem. How do cells manage to do any kind of organized activity under such
06
conditions?
07
There is a great deal of energy in the maelstrom, but it is all disordered
08
energy; it isn’t directly useful for tasks like pulling a muscle or sending nu-
09
trients through the body. The ambient molecules are in a near- equilibrium
10
state, bouncing off one another randomly. But the cell can take advantage
11
of the low- entropy free energy bundled up in ATP— not only to perform
12
work directly, but to focus the disordered energy in the surrounding
13
medium.
14
Consider a ratchet— a gear whose teeth are slanted in one direction. Let
15
it be subject to random jiggling back and forth— Brownian forces, named
16
after botanist Robert Brown. It was he who, in the early nineteenth cen-
17
tury, noticed that small dust particles suspended in water tended to move
18
around in unpredictable ways, a phenomenon we now attribute to their
19
being constantly bombarded by individual atoms and molecules. A Brown-
20
ian ratchet, by itself, doesn’t tend to move one way or the other; it drifts
21
back and forth unpredictably.
22
But imagine that the teeth of our ratchet aren’t fixed, but are something
23
we could control from the outside. When the ratchet moves in the direction
24
we want it to, we make the angle low and easy to move across; when it
25
moves the other way, we increase the angle and make it harder. That would
26
allow us to convert the random, undirected Brownian motion into di-
27
rected, useful transport. Of course, it requires the intervention of some
28
external agent that is itself low- entropy, far from equilibrium.
29
This kind of Brownian ratchet is a simple model for many molecular
30
motors inside a living cell. There aren’t any external observers changing the
31
shapes of the molecules to fit specific purposes, but there is free energy car-
32
ried around by ATP. The ATP molecules can bind to the moving parts of
33
the cellular machinery, releasing their energy at just the right time to allow
34
fluctuations in one direction, while inhibiting them in the other. Getting
35S
work done at the nanoscale is all about harnessing the chaos around you.
The Big Picture Page 42