C H A P T E R
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Chemical composition of living beings
Biogenic elements
constitutes approximately 21% of the total
Earth’s weight.
Carbon belongs to the same group in the periodic table as silicon and shares many of its properties. However, carbon can form more stable
chemical bonds long branched chains, double
and triple bonds, covalent bonds with different
atoms, and can adopt a variety of different spatial conformations. This gives carbon the unique
potential to generate a variety of chemical combinations that are essential for the makeup of the
molecules of living organisms.
The selection of the other elements that
accompany carbon as components of the living
matter depends on the size of these atoms and
their ability to share electrons in covalent bonds.
The smaller atomic size of these elements allows
them to establish stable bonds and stronger
molecular interactions.
Taking into consideration their relative
amounts, biogenic elements can be classified
into three main categories:
Life emerged on Earth many millions of years
after the planet was first formed. Only a few elements within the inorganic matter of the Earth’s
crust and atmosphere were selected as the building blocks of all living organisms. These basic
elements of life are called biogenic elements.
Mammals, animals of great complexity, are
composed of merely 20 elements, four of which
(oxygen, carbon, hydrogen, and nitrogen) are
the most abundant ones, comprising approximately 96% of the total body mass (Table 1.1).
All elements existing in the human body,
except for iodine (which has an atomic number
of 53), are placed within the first four periods of
the periodic table and possess atomic numbers
lower than 34. Among the four most abundant,
oxygen has the highest atomic number (8).
While oxygen is relatively common on Earth,
the other biogenic elements are less abundant,
suggesting that they have properties that gave
them a selective advantage in forming the basic
units of life. For example, carbon and not silicon
has been the element around which life developed, even though silicon is widespread and
Medical Biochemistry
https://doi.org/10.1016/B978-0-323-91599-1.00025-0
1. Primary elements. These include oxygen,
carbon, hydrogen, nitrogen, and the less
abundant, calcium and phosphorus.
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1. Chemical composition of living beings
TABLE 1.1 Elements of the human body and their
relative abundance.
Primary elements
Oxygen
65.0
Nitrogen
3.0
Carbon
18.5
Calcium
1.5
Hydrogen
10.0
Phosphorus
1.0
Secondary elements
Potassium
0.30
Chlorine
0.15
Sulfur
0.25
Magnesium
0.05
Sodium
0.20
Iron
0.005
Fluorine
0.001
Zinc
Traces
Cuprum
0.0002
Cobalt
Traces
Iodine
0.00004
Molybdenum
Traces
Manganese
0.00003
Selenium
Traces
Oligoelements
Values are expressed as a percent of total body mass.
Together, these six elements account for more
than 98% of the total body mass. Oxygen and
hydrogen form water, the most abundant
substance in the body. Carbon, oxygen,
hydrogen, nitrogen, and phosphorus form
part of a variety of essential organic
molecules. Calcium, in combination with
other substances, is mainly found in bone,
and in its ionic state, is involved in numerous
physiological processes.
2. Secondary elements. The elements in this group
include potassium, sulfur, sodium, chlorine,
magnesium, and iron. They are found in
much lower relative quantities than those of
the previous group and they exist as salts,
inorganic ions, and form part of different
organic molecules.
Na+ and Cl are the main extracellular
ions, while K+ is the main intracellular ion.
Mg2 + is necessary for many reactions carried
out by biological catalysts or enzymes. Iron is
an essential component of substances of high
biological importance, including hemoglobin.
Sulfur is present in almost all proteins and
other molecules of biological interest.
3. Trace Elements. These are also known as micro
constituents or oligoelements, because they
are present in the body in very small
quantities.
Iodine is a constituent of the thyroid hormone. The other oligoelements (Cu, Mn, Co,
Zn, Mo, and Se), while also scarce, are vital for
the normal function of the body and most are
essential factors for the proper activity of
enzymes.
Biological compounds
The elements mentioned earlier are found as
part of different inorganic or organic compounds. Among the inorganic compounds, water
is of exceptional importance, not only because it
exists in large quantity (comprising 65% of the
total body weight of an adult individual), but
also due to its numerous biological roles. Also,
of great significance are minerals, such as calcium phosphate, which constitute the nonsoluble components of bones and teeth. The
remaining inorganic components dissolved in
body fluids and in the cytoplasm of cells play
critical roles in many body functions as ions.
Within the organic compounds, Carbon is a
key component in most of the solid substances
of the body, forming part of molecules of high
biological relevance, such as proteins, carbohydrates, lipids, and nucleic acids. There are other
compounds, such as vitamins, hormones, and
pigments, that also have critical roles.
Table 1.2 provides a general view of the relative
chemical composition and approximate
amounts of inorganic and organic compounds
in some human tissues.
The chapters that follow will describe in more
detail the structure and properties of the main
substances that compose the human body.
Further reading
TABLE 1.2 Chemical composition of human tissues.
Muscle
Bone
Brain
Liver
Water
75.0
22.0
77.0
70.0
Carbohydrates
1.0
Scarce
0.1
5.0
Lipids
3.0
Scarce
12.0
9.0
Proteins
18.0
30.0
8.0
15.0
Other organic
substances
1.0
Scarce
1.5
1.0
Other inorganic
substances
1.0
45.0
1.0
Scarce
Values are expressed as a percent of total tissue mass.
Summary
Carbon has been the element around which
life developed. This is due to the capacity of carbon to form stable chemical bonds, long
branched chains, double and triple bonds, covalent bonds with different atoms, and a variety of
different spatial conformations.
Biogenic elements include a series of substances, which can be divided into the following
groups:
1. Primary elements include oxygen, carbon,
hydrogen, nitrogen, and the less abundant,
calcium and phosphorus. They comprise
98% of the total body mass and constitute all
the essential body molecules and water.
2. Secondary elements comprise potassium,
sulfur, sodium, chlorine, magnesium, and
iron. While in much lower amounts than
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primary elements, they exist as free ions or
form salts and form part of different body
molecules. Na+ and Cl are the main ions in
the extracellular space, while K+ is the main
intracellular ion. Mg2+ is indispensable for
many reactions catalyzed by enzymes. Iron is
an essential component of substances, such as
hemoglobin. Sulfur is present in almost all
proteins.
3. Trace elements, microconstituents, or
oligoelements, are present in the body in
scarce quantities. Iodine is present in the
thyroid hormone. Cu, Mn, Co, Zn, Mo, and Se
are essential for the normal function of
enzymes.
Biological compounds include inorganic and
organic
substances.
Among
inorganic
compounds is water, the solvent in body fluids
and tissues. It comprises 65% of the total body
weight of an adult individual. Nonsoluble
inorganic compounds have different roles, for
example, calcium phosphate is an essential component of bone. Organic biological compounds
include proteins, carbohydrates, lipids, and
nucleic acids. Others, such as vitamins, hormones, and pigments perform critical roles,
and all contain carbon as the key component.
Further reading
de Duve, C., 1995. The beginnings of life on Earth. Am. Sci.
83, 428–437.
Morowitz, H.J., 2002. The Emergence of Everything (How the
World Became Complex). Oxford University Press,
Oxford.