Page 48 - Environment: The Science Behind the Stories

therefore lightest-weight) hydrocarbons (those consisting of
four or fewer carbon atoms) exist in a gaseous state at moder- Phosphate
ate temperatures and pressures. Larger (and therefore heavier) group
hydrocarbons are liquids, and those containing over 20 carbon
atoms are normally solids. Sugar Nitrogenous
Some hydrocarbons from petroleum are known to pose base
health hazards to wildlife and people. For example, some (a) DNA nucleotide
polycyclic aromatic hydrocarbons, or PAHs (Figure 2.7c),
can evaporate from spilled oil and gasoline and can mix with
water, putting the eggs and young of fish and other aquatic Nitrogenous
creatures at risk. PAHs also occur in particulate form in vari- base
ous combustion products, including cigarette smoke, wood
smoke, and charred meat. T
A
At the same time, some hydrocarbons from petroleum G
have become ubiquitous in our modern lifestyle because they A C Sugar-phosphate
are moldable into nearly any shape and resist chemical break- T backbone
down. While these plastics have many benefits for manufac-
tured goods, they can be a persistent source of pollution due to
their longevity in the environment (pp. 450–454). (b) DNA double helix
Figure 2.8 Nucleic acids encode genetic information in the
sequence of nucleotides, small molecules that pair together
Macromolecules are building blocks of life like rungs of a ladder. DNA includes four types of nucleotides (a),
each with a different nitrogenous base: adenine (A), guanine (G),
Just as carbon atoms in hydrocarbons may be strung together cytosine (C), and thymine ( T ). Adenine (A) pairs with thymine ( T ),
in chains, organic compounds sometimes combine to form and cytosine (C) pairs with guanine (G). In RNA, thymine is replaced
by uracil (U). DNA (b) twists into the shape of a double helix.
long chains of repeated molecules. These chains are called
polymers, and there are three types of polymers that are essen-
tial to life: proteins, nucleic acids, and carbohydrates. Along Lipids include fats and oils (for energy storage), phospho-
with lipids (which are not polymers), these types of molecules lipids (for cell membranes), waxes (for structure), and steroids
are referred to as macromolecules because of their large sizes. (for hormone production). Although chemically diverse, these
Proteins consist of long chains of organic molecules called compounds are grouped together because they do not dissolve
amino acids. The many types of proteins serve various func- in water.
tions. Some help produce tissues and provide structural sup- All of these macromolecules are found in cells, the most
port; for example, animals use proteins to generate skin, hair, basic unit of organismal organization. Organisms range in
muscles, and tendons. Some proteins help store energy, whereas complexity from single-celled bacteria to plants and animals CHAPTER 2 • E ART h’s Physi CAL
others transport substances. Some act in the immune system to that contain millions of cells. Cells vary greatly in size, shape,
defend the organism against foreign attackers. Still others are and function. Biologists classify organisms into two groups
hormones, molecules that act as chemical messengers within based on the structure of their cells. The cells of eukaryotes
an organism. Proteins can also serve as enzymes, molecules (plants, animals, fungi, and protists) contain a membrane-
that catalyze, or promote, certain chemical reactions. enclosed nucleus and various membrane-enclosed orga-
Nucleic acids direct the production of proteins. The two nelles that perform specific functions. Prokaryotes (bacteria
nucleic acids—deoxyribonucleic acid (DNA) and ribonucleic and archaea) are generally single-celled, and their cells lack
acid (RNA)—carry the hereditary information for organisms membrane-enclosed organelles and a nucleus.
and are responsible for passing traits from parents to offspring.
Nucleic acids are composed of a series of nucleotides, each of
which contains a sugar molecule, a phosphate group, and a Energy: An Introduction
nitrogenous base. DNA contains four types of nucleotides and
can be pictured as a ladder twisted into a spiral, giving the Creating and maintaining organized complexity—of a cell, an s ys TE m s: mATTER , E NER gy, AN d
molecule a shape called a double helix (Figure 2.8). Regions organism, or an ecological system—requires energy. Energy
of DNA coding for particular proteins that perform particular is needed to organize matter into complex forms, to build and
functions are called genes. maintain cellular structure, to govern species’ interactions,
Carbohydrates include simple sugars that are three to and to drive the geologic forces that shape our planet. Energy
seven carbon atoms long. Glucose (C H O ) fuels living cells is somehow involved in nearly every chemical, biological, and
6
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and serves as a building block for complex carbohydrates, such physical phenomenon.
as starch. Plants use starch to store energy, and animals eat But what, exactly, is energy? Energy is the capacity to
plants to acquire starch. Plants and animals also use complex change the position, physical composition, or temperature of gE o L ogy
carbohydrates to build structure. Insects and crustaceans form matter—in other words, a force that can accomplish work.
hard shells from the carbohydrate chitin. Cellulose, the most As we saw with Japan’s 2011 earthquake and tsunami, geo-
abundant organic compound on Earth, is a complex carbohy- logic events involve some of the most dramatic releases of
drate found in the cell walls of leaves, bark, stems, and roots. energy in nature. Energy is omnipresent in living things as 47

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