section 2.7 The pH scale measures
the concentration of hydrogen ions in a
fluid. Acids release hydrogen ions (H^1 ), and
bases release hydroxide ions (OH^2 ) that can
combine with H^1.
At pH 7, the H^1 and OH^2 concentrations
in a solution are equal; this is a neutral pH. A
buffer system maintains pH values of blood, tissue fluids, and
the fluid inside cells. A salt is a compound that releases ions
other than H^1 and OH^2.section 2.8 Carbon atoms bonded
together are the backbone of organic
compounds. Functional groups help deter-
mine the chemical and physical properties
of many compounds.
Chemical reactions in cells build, change,
and break apart organic compounds. In
condensation reactions two molecules covalently bond into
a larger one. Cleavage reactions such as hydrolysis split a
molecule into two smaller ones. Other reactions transfer
functional groups or electrons from one molecule to another.
Enzymes speed all these reactions. A polymer is a molecule
built of three or more subunits; each subunit is called
a monomer.
Cells have pools of dissolved sugars, fatty acids, amino
acids, and nucleotides. These small organic compounds
are building blocks for the larger biological molecules—the
carbohydrates, lipids, proteins, and nucleic acids (Table 2.5).section 2.9 Cells use carbohydrates for
energy or to build cell parts. Monosaccharides,
or single sugar units, are the simplest ones.
Chains of sugars linked by covalent bonds
are oligosaccharides; common ones, such as
glucose, are disaccharides built of two sugar
units. Polysaccharides are longer chains that
store energy in the bonds between the sugar
units (Table 2.5).section 2.10 Cells use lipids for
energy, to build cell parts, and as signaling
molecules. The most important dietary fats
are triglycerides. Phospholipids and sterols
are building blocks of cell membranes.expLoreon yoUr oWn
sUmmary
section 2.1 An element is a funda-
mental substance that can’t be broken down
to other substances by ordinary means. The
four main elements in the body are oxygen,
carbon, hydrogen, and nitrogen.
An atom is the smallest unit that has the
properties of an element. Atoms are com-
posed of protons, neutrons, and electrons.
Atoms of an element may not all have the same number of neu-
trons. These variant forms are called isotopes. The number and
arrangement of an atom’s electrons determine how it interacts
with other atoms.
section 2.3 Electrons move in orbitals
within a series of shells around an atom’s
nucleus. An atom with one or more unfilled
orbitals in its outer shell is likely to take part
in chemical bonds.
A chemical bond is a link that forms between atoms when
their electrons interact. Bonds join atoms into molecules. A
chemical compound consists of atoms of two or more elements
in unchanging proportions. In a mixture, two or more kinds of
molecules mingle in variable proportions.
section 2.4 Atoms generally have no
net charge. An atom that gains or loses one
or more electrons becomes an ion with a
positive or negative charge.
In an ionic bond, positive and negative
ions stay together by the mutual attraction
of their opposite charges. In a covalent bond,
atoms share one or more electrons. A hydrogen bond is a weak
bond between polar molecules.
section 2.5 Water is vital for the physical
structure and chemical activities of cells.
Hydrogen bonds between its molecules give
water special properties, such as the ability
to resist temperature changes and to dissolve
other polar substances. A dissolved substance
is a solute. Polar molecules are hydrophilic
(attracted to water). Nonpolar substances, such
as oils, are hydrophobic (repelled by water).Section 2.7 discussed shifts in the pH of body fluids. A simple experiment shows how every-
day substances can be indicators of pH changes. All you need is some cranberry juice, a little household
ammonia, and some white vinegar.
Using a clear glass jar or bowl, start by mixing two tablespoons of cranberry juice with a tablespoon of
water to dilute it a bit. Use an eyedropper or the tip of a clean spoon to add a few drops of ammonia. Wait
a few moments and then add a few drops of vinegar. Fruits like cranberries and purple grapes get their
color from a phytochemical called anthocyanin, which is sensitive to pH shifts. How did adding ammonia
alter the pH and color of the juice? What was the effect of adding vinegar? Hint: Figure 2.12 shows where
ammonia and household vinegar rank on the pH scale.38 Chapter 2© Volosina/Shutterstock.comCopyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).