Chemistry Reference
In-Depth Information
as the organism extracts energy from food, maintains its structures and
tissues, and repairs any damage. These reactions must be carefully or-
chestrated so that the products are present when needed, but wasteful
overabundances are avoided. Temperature is important—the human
body maintains a steady temperature of about 98.2°F (36.8°C)—but or-
ganisms can also vary the reaction rates by producing more or less of
the necessary enzymes.
As reactions proceed, reactants are chemically combined into com-
pounds, which then perform their functions. These compounds are even-
tually consumed in other reactions, and their fragments are used to gener-
ate new molecules or are excreted. The life cycle of chemicals in the body
is a critical aspect of biochemistry, though neurochemistry researchers
are hampered by a limited access to the brain. The following sidebar on
pages 82-83 describes a neurotransmitter's metabolic journey.
Because of the skull, researchers cannot easily sample the brain's
chemicals in most organisms. Another hindrance is the blood-brain
barrier, which the body uses to control the substances that enter and
exit the brain. Oxygen and glucose are welcome, but many other sub-
stances flowing in the blood are not. Small molecules can slip in and
out, but others, such as serotonin, are too big. (This is the reason why
neurons must manufacture serotonin—the blood contains this large
molecule, but it cannot get into the brain.)
The blood-brain barrier foils most efforts to use the blood to mea-
sure the brain's chemistry, but researchers can get around this obstacle
by sampling cerebrospinal fluid (CSF). CSF is the fluid that circulates in
the meninges of the brain and spinal cord and keeps the delicate tissues
from getting rattled around and damaged in their hard, bony container.
The brain makes CSF from blood, and certain metabolites get mixed in.
One of these metabolites is 5-hydroxyindole acetaldehyde (5-HIAA), a
major metabolite of serotonin. Researchers who carefully puncture the
meninges and extract a sample are rewarded with information concern-
ing serotonin levels in the person's brain, as described below.
With a limited ability to sample the brain's chemicals directly, re-
searchers have turned to other methods. As described in the sidebar on
pages 82-83, proteins such as enzymes and receptors play a vital role in
neurotransmitter function. Since these proteins are specific to a specific
neurotransmitter, they serve as markers for its use. By attaching various
molecules such as dyes, radioactive substances—which emit energetic
particles or radiation—or fluorescent molecules—which emit light—to
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