FoundationalConceptsNeuroscience

molecule, created by all the electrical and geometric properties of the
constituent amino acids guiding the folding of the chain of amino
acids into a unique three-dimensional form.
Many functional proteins in living organisms are composed of a
complex of more than one polypeptide subunit, with each subunit
generally consisting of hundreds of amino acids. Such an arrange-
ment is termed the quaternary structure of the protein. For example,
hemoglobin—which binds and transports oxygen in our blood—is
composed of four polypeptide subunits. And ionotropic receptors for
the neurotransmitter GABA (see Chapter 6) are composed of five sub-
unit polypeptides.

Our third category of biological molecule is that of carbohydrates. The

name comes from a conjunction of carbon (carbo) and water (hydrate),

and carbohydrates are built from atoms of carbon, hydrogen, and

oxygen, covalently joined to form molecules. Some carbohydrates are

small molecules, such as the sugars glucose, fructose, and ribose.

HO HO O O

Oo OH wo oo os Vo

HO OH

HO OH Fructose HO OH

OH Ribose

Glucose

And some carbohydrates are enormous molecules, such as glycogen,
starches, and cellulose. Starches consist of polymers of hundreds or
thousands of glucose molecules linked by covalent bonds into very
long chains. As with the fats, substantial amounts of energy are stored
in chemical bonds joining the carbon, hydrogen, and oxygen atoms,
and carbohydrates serve as sources of fuel for living organisms.