Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

HOW CELLS OBTAIN ENERGY FROM FOOD
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HEAD POLYMERIZATION (e.g., PROTEINS, FATTY ACIDS)
TAIL POLYMERIZATION (e.g., DNA, RNA, POLYSACCHARIDES)
6 +
7
1
+
7
6
each monomer carries a high-energy
bond that will be used for the
addition of the next monomer
7
each monomer carries
a high-energy bond
for its own addition
7
1
growing polymer, and it must therefore be regenerated each time that a monomer
is added. In this case, each monomer brings with it the reactive bond that will be
used in adding the next monomer in the series. In tail polymerization, the reactive
bond carried by each monomer is instead used immediately for its own addition
(Figure 2–44).
We shall see in later chapters that both of these types of polymerization are
used. The synthesis of polynucleotides and some simple polysaccharides occurs
by tail polymerization, for example, whereas the synthesis MBoC6 m2.68/2.44 of proteins occurs by a
head polymerization process.
Figure 2–44 The orientation of the
active intermediates in the repetitive
condensation reactions that form
biological polymers. The head growth of
polymers is compared with its alternative,
tail growth. As indicated, these two
mechanisms are used to produce different
types of biological macromolecules.
Summary
Living cells need to create and maintain order within themselves to survive and
grow. This is thermodynamically possible only because of a continual input of
energy, part of which must be released from the cells to their environment as heat
that disorders the surroundings. The only chemical reactions possible are those that
increase the total amount of disorder in the universe. The free-energy change for a
reaction, ∆G, measures this disorder, and it must be less than zero for a reaction
to proceed spontaneously. This ∆G depends both on the intrinsic properties of the
reacting molecules and their concentrations, and it can be calculated from these
concentrations if either the equilibrium constant (K) for the reaction or its standard
free-energy change, ∆G°, is known.
The energy needed for life comes ultimately from the electromagnetic radiation
of the sun, which drives the formation of organic molecules in photosynthetic organisms
such as green plants. Animals obtain their energy by eating organic molecules
and oxidizing them in a series of enzyme-catalyzed reactions that are coupled to the
formation of ATP—a common currency of energy in all cells.
To make possible the continual generation of order in cells, energetically favorable
reactions, such as the hydrolysis of ATP, are coupled to energetically unfavorable
reactions. In the biosynthesis of macromolecules, ATP is used to form reactive
phosphorylated intermediates. Because the energetically unfavorable reaction of
biosynthesis now becomes energetically favorable, ATP hydrolysis is said to drive
the reaction. Polymeric molecules such as proteins, nucleic acids, and polysaccharides
are assembled from small activated precursor molecules by repetitive condensation
reactions that are driven in this way. Other reactive molecules, called either
activated carriers or coenzymes, transfer other chemical groups in the course of
biosynthesis: NADPH transfers hydrogen as a proton plus two electrons (a hydride
ion), for example, whereas acetyl CoA transfers an acetyl group.
HOW CELLS OBTAIN ENERGY FROM FOOD
The constant supply of energy that cells need to generate and maintain the biological
order that keeps them alive comes from the chemical-bond energy in food
molecules.
The proteins, lipids, and polysaccharides that make up most of the food we eat
must be broken down into smaller molecules before our cells can use them—either