The answer lies with proteins. A specific kind of protein called an enzyme lowers the
activation energy of chemical reactions. Reactions that would normally take years happen
in seconds. Figure 7.1 shows how this effect alters chemical equilibrium. Consider a set
of chemicals that have two paths to interact as shown in figure 7.1. One reaction is a
side reaction that is undesirable. The other is desirable.
The enzyme lowers the activation energy for the desirable reaction
making it happen quickly. The undesirable reaction does not have a chance. Also notice
that the entropy of the universe is maximized by the undesirable reaction. Thus, from
thermodynamic considerations, one might think that the undesirable reaction is always
dominant, but because the laws of thermodynamics have no way to deal with time, this
observation is seldom true. The enzyme is not violating the second law by forcing the
reaction in the preferred direction. Its reaction is also spontaneous in that it also
increases the entropy of the universe. By making the desired reaction happen faster, the
enzyme does not give the undesirable reaction time to happen.
Figure 7.1: Enzymes and the Second Law
Figure 7.1 illustrates a powerful technique, but this trick
alone does not enable life. Enzymes have another much more impressive trick. They can
force chemicals reactions that should never take place to happen and happen quickly. They
do this by coupling a favorable chemical reaction to an unfavorable one.
Lowering the activation energy will not help a reaction go forward if
the entropy of the final state is lower than the initial. In figure 7.1, the final state
of the desired products is a higher entropy state than the initial state (in these
diagrams moving down hill represents increasing entropy). Thus, the chemical reaction is
spontaneous as it increases the entropy of the universe. Figure 7.2 considers the case in
which the opposite is true.
Figure 7.2: Enzymes Couple Reactions
In figure 7.2, a chemical reaction that increases the entropy of the
universe is coupled to one that decreases it. The favorable chemical reaction turns the
chemical, ATP into AMP. This reaction is represented by the falling weight. It is attached
by a rope to the unfavorable reaction. The rope and pulleys represent the enzyme.
Life requires both the techniques shown in 7.1 and 7.2 to maintain its
state so far from equilibrium, and the technique shown in 7.2 requires a continual input
of energy. This is why life requires food. Sugar is required by animals to create
chemicals like ATP. ATP is a very high energy chemical, and when it reacts with water to
form AMP, the entropy of the universe is greatly increased. This reaction is coupled by
enzymes to many unfavorable chemical reactions.
The two previous examples show how enzymes implement procedural
knowledge. The knowledge may also be conditional (figure 7.3).
Figure 7.3: Enzymes Can Make Decisions
Conditional knowledge is very powerful. It allows life to change when
the environment changes. Life does not have to think about how to do this. The decision
making is preprogramed just like a computer. Conditional knowledge is not limited to
enzymes. Often proteins interact with sections of DNA to promote or repress gene
expression. The effects of conditional knowledge can be seen in any higher form of life.
Conditional knowledge determines whether a cell becomes a skin, liver, muscle, lung,
kidney, or stomach cell.
The last trick used by life is the most subtle, but yet at the same
time the most powerful. Life uses teams of enzymes that work together to create desired
products, and in many cases, the enzymes that do this do not even need an energy source to
accomplish their goal, see figure 7.4.
Figure 7.4: Team of Enzymes Working Together
In figure 7.4, enzyme 1 lowers the barrier enabling A to form B, but
because the change in entropy is small, the reaction proceeds in both directions. The same
is true for enzyme 2, and product C, but these first two reactions are coupled to a third,
in which C changes into D. This reaction only proceeds in the forward direction because of
the large increase in entropy; as a result, the chemicals, A, B and C, will all eventually
be converted to D, and only D will exist, if the system is allowed to approach
equilibrium. Figures 7.1 to 7.4 illustrate how proteins (enzymes) implement the knowledge
contained in DNA.
To avoid violating the second law, life requires quite a bit of complexity.
Hundreds of enzymes work together with DNA to decide which chemical reactions take place
and then make sure that only the desired chemical reactions happen. Enzymes alter
equilibrium by speeding up desirable reaction pathways compared to undesirable ones, and
if they need to, they will couple unfavorable reactions to favorable ones forcing the
unfavorable reactions to happen and happen quickly, and all of this is subject to the
built in ability to make decisions as to which chemical reactions are appropriate at any
given time.
Hundreds of enzymes make it possible for life to exist in a state of
very low entropy. Most origin of life theories concentrate on a single self replicating
chemical. What is not clear is how such a chemical can self replicate without violating
the second law. Such a chemical does not have a hundred enzymes working in concert on its
behalf to help it self replicate. Before the second law was understood, many scientists
tried to build machines that would run forever (perpetual motion machines). They all
failed because the second law does not allow such machines to exist. A self replicating
molecule is very similar to a perpetual motion machine. If one day, a self replicating
molecule is created in a lab, it will replicate itself for a very short period of time and
then cease.
Perhaps the easiest way to envision life is as a flow of entropy. When
sunlight hits a black pavement, most of its energy is converted into heat. This process
greatly increases the entropy of the universe. When sunlight strikes a plant something
else happens. Plants know how to harness the energy in sunlight to do work. While much of
the sunlight is still converted to heat, some of it is used to make sugar in a process
called photosynthesis. Photosynthesis also increases the entropy of the universe. So the
second law does not prohibit the process. The entropy increase in photosynthesis is
certainly less than that if the sunlight had struck a black pavement, but both processes
are allowed because both create more entropy. Plants possess
molecular knowledge, and this knowledge enables them to harness the energy of the sun to
do work.
Consider a bush that is on fire. Fire is a chemical reaction in which
oxygen combines with complex organic chemicals to create carbon dioxide and water. The
heat released by fire increases the entropy of the universe, and it does so very quickly,
but something very different happens when a deer eats the bush. The leaves are eventually
converted into carbon dioxide and water, but the energy released by this process is
harnessed to perform work. It is used to create ATP. ATP is then used to drive many
unfavorable chemical reactions (figure 7.2). The deer does not have to think about how to
do this. The knowledge is built into the molecules that make up the deer.
next: Energy Sources
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