Concept 2.1: Matter consists of chemical elements in pure form and in combinations called compounds
Organisms are composed of matter, which is anything that takes up space and has mass. Matter exists in many forms. Rocks, metals, oils, gases, and living organisms are a few examples of what seems to be an endless assortment of matter.
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1. Elements and Compounds
Matter is made up of elements. An element is a substance that cannot be broken down to other substances by chemical reactions. Today, chemists recognize elements occurring in nature; gold, copper, carbon, and oxygen are examples. Each element has a symbol, usually the first letter or two of its name. Some symbols are derived from Latin or German; for instance, the symbol for sodium is Na, from the Latin word natrium.
A compound is a substance consisting of two or more different
elements combined in a fixed ratio. Table salt, for example, is sodium
chloride , a compound composed of the elements sodium and chlorine in
a ratio. Pure sodium is a metal, and pure chlorine is a poisonous gas.
When chemically combined, however, sodium and chlorine form an edible
compound. Water , another compound, consists of the elements hydrogen
and oxygen in a ratio. These are simple examples of organized matter
having emergent properties: A compound has characteristics different
from those of its elements (Figure 2.2).
Figure 2.2
The emergent properties of a compound.
The metal sodium
combines with the poisonous gas chlorine, forming the edible compound
sodium chloride, or table salt.
2. The Elements of Life
Of the natural elements, about are essential elements that an organism needs to live a healthy life and reproduce. The essential elements are similar among organisms, but there is some variation—for example, humans need elements, but plants need only.
Just four elements—oxygen (O), carbon (C), hydrogen (H), and
nitrogen (N)—make up approximately of living matter. Calcium (Ca),
phosphorus (P), potassium (K), sulfur (S), and a few other elements
account for most of the remaining or so of an organism’s mass. Trace
elements are required by an organism in only minute quantities. Some
trace elements, such as iron (Fe), are needed by all forms of life;
others are required only by certain species. For example, in
vertebrates (animals with backbones), the element iodine is an
essential ingredient of a hormone produced by the thyroid gland. A
daily intake of only of iodine is adequate for normal activity of the
human thyroid. An iodine deficiency in the diet causes the thyroid
gland to grow to abnormal size, a condition called goiter. Consuming
seafood or iodized salt reduces the incidence of goiter. Relative
amounts of all the elements in the human body are listed in Table
2.1.
Table 2.1
Elements in the Human Body
Some naturally occurring elements are toxic to organisms. In humans,
for instance, the element arsenic has been linked to numerous diseases
and can be lethal. In some areas of the world, arsenic occurs
naturally and can make its way into the groundwater. As a result of
using water from drilled wells in southern Asia, millions of people
have been inadvertently exposed to arsenic-laden water. Efforts are
under way to reduce arsenic levels in their water supply.
InterviewInterview with Kenneth Olden: Assessing susceptibility to
environmental toxins using genomics (see the interview before Chapter
2)
Case Study: Evolution of Tolerance to Toxic Elements
EVOLUTION Some species have become adapted to environments
containing elements that are usually toxic; an example is serpentine
plant communities. Serpentine is a jade-like mineral that contains
elevated concentrations of elements such as chromium, nickel, and
cobalt. Although most plants cannot survive in soil that forms from
serpentine rock, a small number of plant species have adaptations that
allow them to do so (Figure 2.3). Presumably, variants of ancestral,
nonserpentine species arose that could survive in serpentine soils,
and subsequent natural selection resulted in the distinctive array of
species we see in these areas today. Serpentine-adapted plants are of
great interest to researchers because studying them can teach us so
much about natural selection and evolutionary adaptations on a local
scale.
Figure 2.3
Serpentine plant community.
These plants are growing on serpentine soil, which contains elements that are usually toxic to plants. The insets show a close-up of serpentine rock and one of the plants, a Tiburon Mariposa lily (Calochortus tiburonensis). This specially adapted species is found only on this one hill in Tiburon, a peninsula that juts into San Francisco Bay.
Concept Check 2.1
MAKE CONNECTIONS Explain how table salt has
emergent properties. (See Concept 1.1.)
Is a trace
element an essential element? Explain.
WHAT IF? In
humans, iron is a trace element required for the proper functioning of
hemoglobin, the molecule that carries oxygen in red blood cells. What
might be the effects of an iron deficiency?
MAKE
CONNECTIONS Explain how natural selection might have played a role in
the evolution of species that are tolerant of serpentine soils.
(Review Concept 1.2.)
For suggested answers, see Appendix A.
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