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H H H H single bond in hydrogen gas and the POO double bond in
O O OO oxygen gas are broken. At the same time, each oxygen atom
forms new covalent bonds with two hydrogen atoms, making
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2H 2 + O 2 2H O
2
Hydrogen gas Oxygen gas Water the products of the reaction two molecules of water.
In fact, this reaction is the origin of the word “hydrogen,”
which literally means “water former.” The reaction releases a
H H good deal of energy and is used in some rockets as a booster
H H O O O in satellite launches. Certain microorganisms also perform
H H
H H this reaction in a much smaller and more controlled manner.
O
These microorganisms benefit from the released energy, which
Reactants Products they use to perform some of their cellular biochemistry.
FIGURE 1.10 A chemical reaction In biological systems, chemical reactions provide a way to
build and break down molecules for use by the cell, as well as
During a chemical reaction, atoms retain their identity, but their
connections change as bonds are broken and new bonds are formed. to harness energy, which can be held in chemical bonds. We
In this reaction, a hydrogen molecule O(H H) reacts with one of the explore these topics in more detail in Unit 3.
O atoms of an oxygen molecule (O PO), exchanging bonding partners
and establishing OOH O H, also written as HO. Because this happens
2
twice, two HO molecules are formed. ✓ Concept Check
2
8. Describe the differences among covalent bonds,
FIGURE 1.10 shows an example of a chemical reaction. polar covalent bonds, nonpolar covalent bonds, and
In this case, two molecules of hydrogen gas (2H) and one ionic bonds.
2
molecule of oxygen gas (O ) react to form two molecules of 9. Use the polar property of water to describe the
2
process by which water (HO) dissolves sodium
water (2HO). In this reaction, the numbers of each type of chloride (NaCl). 2
2
atom are conserved, meaning that the number of atoms does 10. In the reaction 3H 2 + N 2 → 2NH , identify the
3
not change, but their arrangement in the reactants is different reactants and the products.
from the arrangement in the products. Specifically, the OH H

1.4 Carbon is the backbone of shows the relative abundance by mass of chemical elements
organic molecules present in human cells after all the water has been removed.
Just four elements— carbon (C), oxygen (O), hydrogen (H),
When you first learned to read, you probably began by and nitrogen (N)—account for approximately 90% of the
learning the shapes and sounds of the letters of the alphabet. total dry mass, and the most abundant element is carbon.
After that you learned letter combinations and then sim- While other types of cells may vary somewhat, virtually
ple words. Our introduction to the basic chemistry of life is all contain about the same ratios of these elements. Human
similar. Now that we’ve reviewed the basic nature of atoms life, and all life as we know it, is based on carbon. Carbon-
and molecules, we can turn to the chemistry of life. As we containing molecules play such an important role in living
mentioned earlier, the chemistry of life is based on carbon. organisms that they have a special name, as we saw earlier—
In this section, we will examine what makes carbon well they are called organic molecules. Carbon has the ability to
suited to its role as the chemical backbone of living things, combine with many other elements to form a wide variety
and introduce the four major types of organic molecules. of molecules, each specialized for the functions it carries out
in the cell. For example, carbon- based molecules make up
The Chemistry of Carbon the structure of cells, participate in and speed up chemical
Hydrogen and helium are by far the most abundant elements reactions, and store energy for use by the cell.
in the universe. In contrast, the solid Earth is dominated by Why has life evolved with carbon as its key element? Of
silicon, oxygen, aluminum, iron, and calcium. In other words, the elements commonly observed in cells, carbon is unique
Earth is not a typical sample of the universe. Similarly, the in its bonding capacities. Of a carbon atom’s six electrons,
cell is not a typical sample of the solid Earth. FIGURE 1.11 four are in its outermost shell and are available to form
36 UNIT 1 CHEMISTRY OF LIFE

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