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

MBoC6 m2.32/2.11
50 Chapter 2: Cell Chemistry and Bioenergetics
Figure 2–10 The folding of proteins
and RNA molecules into a particularly
stable three-dimensional shape, or
conformation. If the noncovalent bonds
maintaining the stable conformation are
disrupted, the molecule becomes a flexible
chain that loses its biological activity.
many unstable
conformations
one stable folded
conformation
larger structures, thereby forming intricate machines with multiple moving parts
that perform such complex tasks as DNA replication and protein synthesis (Figure
2–11).
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Summary
Living organisms are autonomous, self-propagating chemical systems. They are
formed from a distinctive and restricted set of small carbon-based molecules that
are essentially the same for every living species. Each of these small molecules is
composed of a small set of atoms linked to each other in a precise configuration
through covalent bonds. The main categories are sugars, fatty acids, amino acids,
and nucleotides. Sugars are a primary source of chemical energy for cells and can be
incorporated into polysaccharides for energy storage. Fatty acids are also important
for energy storage, but their most critical function is in the formation of cell
membranes. Long chains of amino acids form the remarkably diverse and versatile
macromolecules known as proteins. Nucleotides play a central part in energy transfer,
while also serving as the subunits for the informational macromolecules, RNA
and DNA.
Most of the dry mass of a cell consists of macromolecules that have been produced
as linear polymers of amino acids (proteins) or nucleotides (DNA and RNA),
covalently linked to each other in an exact order. Most of the protein molecules
and many of the RNAs fold into a unique conformation that is determined by their
sequence of subunits. This folding process creates unique surfaces, and it depends
on a large set of weak attractions produced by noncovalent forces between atoms.
SUBUNITS
covalent bonds
MACROMOLECULES
noncovalent bonds
MACROMOLECULAR
ASSEMBLIES
e.g., sugars, amino acids,
and nucleotides
e.g., globular proteins
and RNA
30 nm
e.g., ribosome
Figure 2–11 Small molecules become covalently linked to form macromolecules, which in turn assemble through noncovalent interactions
to form large complexes. Small molecules, proteins, and a ribosome are drawn approximately to scale. Ribosomes are a central part of the
machinery that the cell uses to make proteins: each ribosome is formed as a complex of about 90 macromolecules (protein and RNA molecules).