rectly to an atom other than oxygen.
Examples are hydrogen chloride
(HCl) and hydrogen sulphide (H 2 S).
Such compounds are sometimes
called hydracids. Compare oxoacid.binding siteAn area on the sur-
face of a molecule that combines
with another molecule. Binding sites
on enzymes can be *active sites or
*allosteric sites.bioaccumulationAn increase in
the concentration of chemicals, such
as pesticides, in organisms that live
in environments contaminated by a
wide variety of organic compounds.
These compounds are not usually de-
composed in the environment (i.e.
they are not biodegradable) or metab-
olized by the organisms, so that their
rate of absorption and storage is
greater than their rate of excretion.
The chemicals are normally stored in
fatty tissues. *DDT is known as a per-
sistent pesticide, as it is not easily
broken down and bioaccumulates
along food chains, so that increasing
concentrations occur in individual or-
ganisms at each trophic level.bioactivationA metabolic process
in which a product that is chemically
reactive is produced from a relatively
inactive precursor.biochemical fuel cellA system
that exploits biological reactions for
the conversion of biomass (chemical
energy) to electricity (electrical en-
ergy). One potential application is the
generation of electricity from indus-
trial waste and sewage. Methyl-
trophic organisms (i.e. organisms that
use methane or methanol as their
sole carbon sources) are being inves-
tigated for their potential use in bio-
chemical fuel cells.biochemical oxygen demand
(BOD)The amount of oxygen taken
up by microorganisms that decom-pose organic waste matter in water.
It is therefore used as a measure of
the amount of certain types of or-
ganic pollutant in water. BOD is cal-
culated by keeping a sample of water
containing a known amount of oxy-
gen forÜve days at 20°C. The oxygen
content is measured again after this
time. A high BOD indicates the pres-
ence of a large number of microor-
ganisms, which suggests a high level
of pollution.biochemistryThe study of the
chemistry of living organisms, espe-
cially the structure and function of
their chemical components (princi-
pally proteins, carbohydrates, lipids,
and nucleic acids). Biochemistry has
advanced rapidly with the develop-
ment, from the mid-20th century, of
such techniques as chromatography,
X-ray diffraction, radioisotopic la-
belling, and electron microscopy.
Using these techniques to separate
and analyse biologically important
molecules, the steps of the metabolic
pathways in which they are involved
(e.g. glycolysis) have been deter-
mined. This has provided some
knowledge of how organisms obtain
and store energy, how they manufac-
ture and degrade their biomolecules,
and how they sense and respond to
their environment. See Chronology.biodieselSee biofuel.bioelementAny chemical element
that is found in the molecules and
compounds that make up a living or-
ganism. In the human body the most
common bioelements (in decreasing
order of occurrence) are oxygen, car-
bon, hydrogen, nitrogen, calcium,
and phosphorus. Other bioelements
include sodium, potassium, magne-
sium, and copper. See essential el-
ement.
bioenergeticsThe study of the
Ûow and the transformations of en-binding site 68b