The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

An Introduction to Mitochondria
acid and alkali components are mostly not bound together. Thus, in a sense, it would be
inaccurate to speak of a solution of sodium acetate; more precise would be to speak of a
solution of both sodium hydroxide and acetic acid. What people have tended to do, therefore,
for reasons of brevity, is to treat the term “acetic acid” as synonymous (when discussing it in
the context of aqueous solution) with “acetate”. This applies to all organic acids; thus the
term “lactate” is identical in meaning to the term “lactic acid”, and so on. In this book, I will
normally use the “-ate” form in preference to the “-ic acid” form.
2.3.1.2. The Electron Reservoir
Biological reactions do not only use and/or produce free protons, however: they do the
same with electrons. Indeed, the availability or disposability of electrons is vital to many of
the reactions that will be discussed later on. Now, whereas protons are harmless when
discarded into the medium, electrons are potentially highly damaging; in particular, they
have a tendency to latch onto other molecules and make free radicals, as will be discussed in
detail in Chapter 3. Evolution has discovered a molecule which helps to avoid this problem,
by acting as a reversible electron carrier. This molecule is nicotinamide adenine dinucleotide,
or NAD. It is capable of carrying two electrons in a very easily reversible configuration,
involving the simultaneous carrying of just one proton. When not carrying the electrons it
exists as a cation, NAD + ; when carrying them it is uncharged (because of the proton) so is
NADH.
In fact, NAD is not the only molecule that performs this role. In a few of the reactions
that we will encounter in later sections, a closely related molecule, flavin adenine dinucleotide
or FAD, is used instead; it behaves in the same way except that it is always uncharged, since
when it is carrying two electrons it carries two protons (and is denoted FADH2). A third
very similar molecule, NADP, is used instead in some circumstances, but not in any of those
which will be discussed in this book.
2.3.1.3. ATP: The Energy Reservoir
Nutrients in the diet are many and varied, and processes requiring energy are just as
varied. Evolution might, in principle, have developed separate systems for pairing each
nutrient with each energy-requiring process, but that would have multiplied up to an
enormous number of systems. Instead it has settled on a division of the problem into two.
The processes that extract energy from nutrients all transfer it into the construction of just
one molecule: adenosine triphosphate, or ATP. The processes that require energy, similarly,
all derive it from the destruction of ATP. ATP thus constitutes an energy reservoir. It is also
often described as the “legal tender” of cellular chemistry, because the merit of this system,
compared to the alternative of a separate system for passing energy from each nutrient to
each process, is absolutely analogous to the merit that civilisation has discovered in replacing
barter with money.
The use of the words “construction” and “destruction” above is perhaps a slight
exaggeration. Energy-requiring processes only break ATP into two pieces, ADP (adenosine
diphosphate) and H2PO4 — (inorganic phosphate, usually abbreviated Pi). Thus, the effective
energy store in ATP is really only the energy in one chemical bond—or, more strictly, in the
maintenance of a higher than equilibrium concentration of ATP relative to ADP and Pi.
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