51
CHAPTER TWO
Chemical Basis of Life
2.1
INTRODUCTION
Chemistry
considers the composition of substances and how
they change. It is possible to study anatomy without much
reference to chemistry. However understanding the basics of
chemistry is essential for understanding physiology, because
body functions result from cellular functions that, in turn,
result from chemical changes. The human body consists of
chemicals, including salts, water, carbohydrates, lipids, pro-
teins, and nucleic acids. The food that we eat, liquids that
we drink, and medications that we take are chemicals.
As interest in the chemistry of living organisms grew
and knowledge of the subject expanded, a F eld of life sci-
ence called biological chemistry, or
biochemistry,
emerged.
Biochemistry has been important not only in helping explain
physiological processes but also in developing many new
drugs and methods for treating diseases.
PRACTICE
1
Why is a knowledge of chemistry essential to understanding
physiology?
2
What is biochemistry?
2.2
STRUCTURE OF MATTER
Matter
is anything that has weight and takes up space. This
includes all the solids, liquids, and gases in our surround-
ings as well as in our bodies. All matter consists of particles
organized in speciF
c ways.
Table 2.1
lists some particles of
matter and their characteristics.
Elements and Atoms
All matter is composed of fundamental substances called
elements
(el
e
˘-mentz). At present, 90 naturally occurring
BIOMARKERS HERALD DISEASE
breast cancer. Cholesterol tests assess lipid levels in blood serum, which may be
correlated to risk of cardiovascular disease. The prostate speciF
c antigen (PSA)
test measures the levels of a protein in the blood normally on cells of the pros-
tate. An elevated level can indicate increased risk of prostate cancer. Genetic
tests detect mutations in DNA that cause inherited disease or levels of RNA mol-
ecules characteristic of various disorders.
New biomarker tests evaluate levels of a number of chemicals in a body
±
uid sample, rather than one at a time. To assess exposure to tobacco smoke, for
example, a biomarker panel measures carbon monoxide and biochemicals that
the body produces (metabolites) as it breaks down several carcinogens in ciga-
rette smoke. A biomarker test that measures levels of 120 di²
erent biochemi-
cals in blood, including many immune system substances, hormones, growth
factors, clotting factors, and proteins associated with cancer cells, provides risk
estimates of cancer types.
Analysis of the human genome is allowing drug developers to assess the
increasing and decreasing levels of many biochemicals in body ±
uids, and their
research is providing information to develop new biomarker tests. However,
the utility of such tests is important to consider. ³or example, a biomarker test
would be useful if it led to earlier diagnosis or helped select a drug likely to help
without causing adverse e²
ects. It may be less useful for a patient who has a
disease not currently treatable.
I
n our body ±
uids lie chemical clues to our health. A
biomarker
is a sub-
stance in the body that indicates a disease process or exposure to a
toxin. Many medical tests that measure biomarkers are familiar, such
as that for serum cholesterol. Such tests may indicate an increased
risk of developing a particular disease or a presymptomatic stage, be
used as a basis for diagnosis, or re±
ect response to a treatment.
Not just any chemical in the body can form the basis of a useful test.
Biomarker tests should be simple and inexpensive and fulF
ll the following
criteria:
Sensitivity. The ability of a test to detect disease only when it is really
present. The more sensitive a test, the fewer false positives.
SpeciF
city. The test’s ability to exclude the disease in a patient who
does not have it. The more speciF
c a test, the fewer false negatives.
Reproducibility. Results mean the same thing in di²
erent patients.
Noninvasiveness: The test can be performed on an easily obtained
body ±
uid.
The four major types of chemicals in a human body—carbohydrates,
lipids, proteins, and nucleic acids —form the basis of many biomarker tests.
Elevated levels of certain carbohydrates in the blood are signs of certain
bacterial infections, and telltale carbohydrates in nipple ±
uid may indicate
TABLE
2.1
|
Some Particles of Matter
Name
Characteristic
Name
Characteristic
Atom
Smallest particle of an element that has the properties of that
element
Neutron (n
0
)
Particle with about the same weight as a
proton; uncharged
and thus electrically neutral; found within an atomic nucleus
Electron (e
)
Extremely small particle with almost no weight; carries a
negative electrical charge and is in constant motion around an
atomic nucleus
Ion
Particle that is electrically charged because it has gained or
lost one or more electrons
Proton (p
+
)
Relatively large atomic particle; carries a positive electrical
charge and is found within an atomic nucleus
Molecule
Particle formed by the chemical union of two or more atoms
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