Biology - Campbell 9Th Edition - Chapters 2, 7, 4, And 5 (Selected Readings From Mit Opencourseware Class)

Front 1

Matter

Back 1

Organisms are composed of matter, which is defined as anything
that takes up space and has mass.* Matter exists in many
diverse forms. Rocks, metals, oils, gases, and humans are just a
few examples of what seems an endless assortment of matter.

Hint 1

*Sometimes we substitute the term weight for mass, although the two are not identical. Mass is the amount of matter in an object, whereas the weight of an object is how strongly that mass is pulled by gravity. The weight of an astronaut walking on the moon is approximately 1/6 the astronaut’s weight on Earth, but his or her mass is the same. However, as long as we are earthbound, the weight of an object is a measure of its mass; in everyday language, therefore,
we tend to use the terms interchangeably.

Front 2

Element

Back 2

An element is a substance that
cannot be broken down to other substances by chemical reactions.
Today, chemists recognize 92 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

Front 3

Compound

Back 3

A compound is a substance consisting of two or more different
elements combined in a fixed ratio. Table salt, for example,
is sodium chloride (NaCl), a compound composed of the
elements sodium (Na) and chlorine (Cl) in a 1:1 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 (H2O), another compound, consists
of the elements hydrogen (H) and oxygen (O) in a 2:1 ratio.
These are simple examples of organized matter having emergent
properties: A compound has characteristics different
from those of its elements (Figure 2.3).

Hint 3

Front 4

Essential Elements

Back 4

Of the 92 natural elements, about 20–25% 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
25 elements, but plants need only 17.
Just four elements—oxygen (O), carbon (C), hydrogen (H),
and nitrogen (N)—make up 96% of living matter. Calcium
(Ca), phosphorus (P), potassium (K), sulfur (S), and a few other
elements account for most of the remaining 4% of an organism’s
mass.

Hint 4

Front 5

Trace Elements

Back 5

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 (I) is an essential ingredient of a
hormone produced by the thyroid gland. A daily intake of
only 0.15 milligram (mg) 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. Where it is available, eating seafood or iodized
salt reduces the incidence of goiter. All the elements needed by
the human body are listed in Table 2.1.

Hint 5

Front 6

Mass of Proton and Neutron

Back 6

The neutron and proton are almost identical in mass, each
about 1.7  10^-24 gram (g)

Front 7

Dalton

Back 7

Thus, for atoms and subatomic particles (and for
molecules, too), we use a unit of measurement called the
dalton, in honor of John Dalton, the British scientist who
helped develop atomic theory around 1800. (The dalton is
the same as the atomic mass unit, or amu, a unit you may
have encountered elsewhere.) Neutrons and protons have
masses close to 1 dalton. Because the mass of an electron is
only about 1/2,000 that of a neutron or proton, we can ignore
electrons when computing the total mass of an atom.

Front 8

Atomic Number

Back 8

Atoms of the various elements differ in their number of subatomic
particles. All atoms of a particular element have the
same number of protons in their nuclei. This number of protons,
which is unique to that element, is called the atomic
number and is written as a subscript to the left of the symbol
for the element. The abbreviation 2He, for example, tells
us that an atom of the element helium has 2 protons in its
nucleus. Unless otherwise indicated, an atom is neutral in
electrical charge, which means that its protons must be balanced
by an equal number of electrons. Therefore, the
atomic number tells us the number of protons and also the
number of electrons in an electrically neutral atom.

Front 9

Energy & Potential Energy

Back 9

An atom’s electrons vary in the amount of energy they
possess. Energy is defined as the capacity to cause change—
for instance, by doing work. Potential energy is the energy
that matter possesses because of its location or structure. For
example, water in a reservoir on a hill has potential energy
because of its altitude. When the gates of the reservoir’s dam
are opened and the water runs downhill, the energy can be
used to do work, such as turning generators. Because energy
has been expended, the water has less energy at the bottom
of the hill than it did in the reservoir. Matter has a natural
tendency to move to the lowest possible state of potential energy;
in this example, the water runs downhill. To restore the
potential energy of a reservoir, work must be done to elevate
the water against gravity.

Hint 9

The electrons of an atom have potential energy because of
how they are arranged in relation to the nucleus. The negatively
charged electrons are attracted to the positively charged
nucleus. It takes work to move a given electron farther away
from the nucleus, so the more distant an electron is from the
nucleus, the greater its potential energy. Unlike the continuous
flow of water downhill, changes in the potential energy of
electrons can occur only in steps of fixed amounts. An electron
having a certain amount of energy is something like a
ball on a staircase (Figure 2.8a). The ball can have different
amounts of potential energy, depending on which step it is on, but it cannot spend much time between the steps. Similarly,
an electron’s potential energy is determined by its energy
level. An electron cannot exist between energy levels.

Front 10

Inert (elements)

Back 10

At the far right of the periodic table are helium,
neon, and argon, the only three elements shown in Figure 2.9
that have full valence shells. These elements are said to be
inert, meaning chemically unreactive. All the other atoms in
Figure 2.9 are chemically reactive because they have incomplete
valence shells.

Hint 10

Front 11

Orbital

Back 11

atom. In reality, we can never know the exact location of
an electron. What we can do instead is describe the space
in which an electron spends most of its time. The threedimensional
space where an electron is found 90% of the time is called an orbital.

Front 12

Covalent Bond:

Back 12

A covalent bond is the sharing of a pair of valence electrons
by two atoms.

Hint 12

Two or more atoms held together
by covalent bonds constitute a molecule, in this case a hydrogen molecule.

Front 13

Molecule:

Back 13

Two or more atoms held together
by covalent bonds constitute a molecule, in this case a hydrogen molecule.

Hint 13

Front 14

Pure elements Vs. Compounds:

Back 14

The molecules H2 and O2 are pure elements rather than
compounds because a compound is a combination of two or
more DIFFERENT elements. Water, with the molecular formula
H2O, is a compound.

Front 15

Nonpolar Covalent Bond:

Back 15

In a covalent bond between two atoms of the same element, the electrons are shared equally because the two atoms have the same electronegativity—the tug-of-war is at a standoff. Such a bond is called a nonpolar covalent bond. For example, the single bond of H2 is nonpolar,
as is the double bond of O2.

Front 16

Polar Covalent Bond

Back 16

When one atom is bonded to a more electronegative atom, the electrons of the bond are not shared equally. This type of bond is called a polar covalent bond. Such bonds vary in their polarity, depending on the relative electronegativity
of the two atoms. For
example, the bonds between the oxygen and hydrogen atoms of a water molecule are quite polar (Figure 2.13).

Hint 16

Front 17

There are No Salt Molecules!

Back 17

A molecule is a compound that is covalently bonded. It's technically incorrect to refer to sodium chloride, which has ionic bonds, as a molecule, but lots of chemists do it anyway. It's kind of like using the wrong fork at a formal dinner. Some people may notice, but most don't notice or don't care. But just so you know, the correct term for ionic compounds is Formula Unit.

Front 18

Electronegativity

Back 18

Atoms in a molecule attract shared electrons to varying degrees,
depending on the element. The attraction of a particular
atom for the electrons of a covalent bond is called its electronegativity. The more electronegative an atom is, the more strongly it pulls shared electrons toward itself. In a covalent bond between two atoms of the same element, the electrons are shared equally because the two atoms have the same electronegativity—the tug-of-war is at a standoff.

Front 19

Hydrogen Bond

Back 19

Among the various kinds of weak chemical bonds, hydrogen
bonds are so important in the chemistry of life that they deserve
special attention. The partial positive charge on a hydrogen
atom that is covalently bonded to an electronegative
atom allows the hydrogen to be attracted to a different electronegative
atom nearby. This noncovalent attraction between
a hydrogen and an electronegative atom is called a
hydrogen bond. In living cells, the electronegative partners
are usually oxygen or nitrogen atoms. Refer to Figure 2.16 to
examine the simple case of hydrogen bonding between water
(H2O) and ammonia (NH3).

Hint 19

Front 20

Chemical Reactions

Back 20

The making and breaking of chemical bonds, leading to
changes in the composition of matter, are called chemical
reactions

Hint 20

An example is the reaction between hydrogen
and oxygen molecules that forms water.

The coefficients indicate the number of molecules involved;
for example, the coefficient 2 in front of the H2 means that the reaction starts with two 'molecules' of hydrogen. Notice
that all atoms of the reactants must be accounted for in the
products. Matter is conserved in a chemical reaction: Reactions
cannot create or destroy matter but can only rearrange
it.

Front 21

Chemical Equilibrium

Back 21

One of the factors affecting the rate of a reaction is the
concentration of reactants. The greater the concentration of
reactant molecules, the more frequently they collide with
one another and have an opportunity to react and form
products. The same holds true for products. As products accumulate,
collisions resulting in the reverse reaction become
more frequent. Eventually, the forward and reverse reactions
occur at the same rate, and the relative concentrations of
products and reactants stop changing. The point at which
the reactions offset one another exactly is called chemical
equilibrium.

Hint 21

This is a dynamic equilibrium; reactions are
still going on, but with no net effect on the concentrations of
reactants and products. Equilibrium does not mean that the
reactants and products are equal in concentration, but only
that their concentrations have stabilized at a particular ratio.
The reaction involving ammonia reaches equilibrium when
ammonia decomposes as rapidly as it forms. In some chemical
reactions, the equilibrium point may lie so far to the right
that these reactions go essentially to completion; that is, virtually
all the reactants are converted to products.

Front 22

What are Functional Groups?

Back 22

Different chemical groups
contribute to function by affecting the molecule’s shape. In
other cases, the chemical groups affect molecular function by
being directly involved in chemical reactions; these important
chemical groups are known as functional groups.
Each functional group participates in chemical reactions in a
characteristic way from one organic molecule to another.

(chapter review gives: Chemical groups attached to the carbon skeletons of organic
molecules participate in chemical reactions (functional
groups) or contribute to function by affecting molecular shape).

Hint 22

Yahoo best answers has a similar definition:

Such a huge number of organic compounds requires organization. They are sorted into organic families defined by functional groups. Functional groups are small structural units within molecules at which most of the compound's chemical reactions occur.

Front 23

Referencing Functional Groups. What are the seven chemical groups most important in biological processes?

Back 23

The seven chemical groups most important in biological
processes are the hydroxyl, carbonyl, carboxyl, amino,
sulfhydryl, phosphate, and methyl groups. The first six groups
can act as functional groups; they are also hydrophilic and
thus increase the solubility of organic compounds in water.
The methyl group is not reactive, but instead often serves as a
recognizable tag on biological molecules.

Front 24

About Hydroxyl:

Back 24

In a hydroxyl group (¬OH), a
hydrogen atom is bonded to an
oxygen atom, which in turn is
bonded to the carbon skeleton of
the organic molecule. (Do not
confuse this functional group with
the hydroxide ion, OH-)

Hint 24

Front 25

About Carbonyl:

Back 25

The carbonyl group ( > CO) consists
of a carbon atom joined to an oxygen
atom by a double bond.

Hint 25

Front 26

About Carboxyl:

Back 26

When an oxygen atom is doublebonded to a carbon atom that is also bonded to an ¬OH group, the entire assembly of atoms is called a carboxyl
group (¬¬¬COOH).

Hint 26

Front 27

About Amino:

Back 27

The amino group (¬NH2)
consists of a nitrogen atom
bonded to two hydrogen
atoms and to the carbon
skeleton.

Hint 27

Front 28

About Sulfhydryl:

Back 28

The sulfhydryl group (¬¬¬SH)
consists of a sulfur atom
bonded to an atom of
hydrogen; it resembles a
hydroxyl group in shape.

Hint 28

Front 29

About Phosphate:

Back 29

In the phosphate group shown here, a phosphorus atom is bonded to four oxygen atoms; one oxygen is bonded to the carbon skeleton; two oxygens carry negative charges (¬OPO3 ^2-). In this text, P represents an attached phosphate group.

Hint 29

Front 30

About Methyl:

Back 30

A methyl group (¬CH3)
consists of a carbon bonded
to three hydrogen atoms. The
carbon of a methyl group may
be attached to a carbon or to
a different atom.

Hint 30

Front 31

What is an Isomer?

Back 31

Variation in the architecture of organic molecules can be seen
in isomers, compounds that have the same numbers of
atoms of the same elements but different structures and
hence different properties.

Front 32

What are the three types of Isomers?

Back 32

Three types of isomers are
structural isomers, cis-trans isomers, and enantiomers.

Front 33

What are Structural Isomers?

Back 33

Structural isomers differ in the covalent arrangements
of their atoms. Compare, for example, the two five-carbon
compounds in Figure 4.7a. Both have the molecular formula
C5H12, but they differ in the covalent arrangement of their
carbon skeletons. The skeleton is straight in one compound
but branched in the other. The number of possible isomers
increases tremendously as carbon skeletons increase in size.
There are only three forms of C5H12 (two of which are shown
in Figure 4.7a), but there are 18 variations of C8H18 and
366,319 possible structural isomers of C20H42. Structural isomers
may also differ in the location of double bonds.

Hint 33

Front 34

What are Cis-Trans Isomers?

Back 34

In cis-trans isomers (formerly called geometric isomers),
carbons have covalent bonds to the same atoms, but these
atoms differ in their spatial arrangements due to the inflexibility of double bonds. Single bonds allow the atoms they
join to rotate freely about the bond axis without changing the compound. In contrast, double bonds do not permit such
rotation. If a double bond joins two carbon atoms, and each
C also has two different atoms (or groups of atoms) attached
to it, then two distinct cis-trans isomers are possible. Consider
a simple molecule with two double-bonded carbons, each of
which has an H and an X attached to it (Figure 4.7b). The
arrangement with both Xs on the same side of the double
bond is called a cis isomer, and that with the Xs on opposite
sides is called a trans isomer. The subtle difference in shape
between such isomers can dramatically affect the biological
activities of organic molecules. For example, the biochemistry
of vision involves a light-induced change of retinal, a
chemical compound in the eye, from the cis isomer to the
trans isomer (see Figure 50.17). Another example involves
trans fats, which are discussed in chapter 5.

Hint 34

Front 35

What are Enantiomers?

Back 35

Enantiomers are isomers that are mirror images of each
other and that differ in shape due to the presence of an
asymmetric carbon, one that is attached to four different atoms
or groups of atoms. (See the middle carbon in the ball-and-stick
models shown in Figure 4.7c.) The four groups can be arranged
in space around the asymmetric carbon in two different ways
that are mirror images. Enantiomers are, in a way, left-handed
and right-handed versions of the molecule. Just as your right
hand won’t fit into a left-handed glove, a “right-handed” molecule
won’t fit into the same space as the “left-handed” version. Usually, only one isomer is biologically active because only that form can bind to specific molecules in an organism.
The concept of enantiomers is important in the pharmaceutical
industry because the two enantiomers of a drug may
not be equally effective, as is the case for both ibuprofen and
the asthma medication albuterol (Figure 4.8). Methamphetamine
also occurs in two enantiomers that have very different
effects. One enantiomer is the highly addictive stimulant
drug known as “crank,” sold illegally in the street drug trade.
The other has a much weaker effect and is even found as an
ingredient in an over-the-counter vapor inhaler for treatment
of nasal congestion! The differing effects of enantiomers in
the body demonstrate that organisms are sensitive to even
the most subtle variations in molecular architecture. Once
again, we see that molecules have emergent properties that
depend on the specific arrangement of their atoms.

Hint 35

Front 36

What are Macromolecules?

Back 36

Given the rich complexity of life on Earth, we might expect
organisms to have an enormous diversity of molecules. Remarkably,
however, the critically important large molecules
of all living things—from bacteria to elephants—fall into just
four main classes: carbohydrates, lipids, proteins, and nucleic
acids.

On the molecular scale, members of three of these
classes—carbohydrates, proteins, and nucleic acids—are huge
and are therefore called Macromolecules. For example, a
protein may consist of thousands of atoms that form a molecular
colossus with a mass well over 100,000 daltons. Considering
the size and complexity of macromolecules, it is
noteworthy that biochemists have determined the detailed
structure of so many of them.

Hint 36

Dalton: a unit used in expressing the molecular weight of proteins, equivalent to atomic mass unit.

Front 37

Polymers & Monomers

Back 37

The macromolecules in three of the four classes of life’s organic
compounds—carbohydrates, proteins, and nucleic
acids—are chain-like molecules called polymers (from the
Greek polys, many, and meros, part). A polymer is a long
molecule consisting of many similar or identical building
blocks linked by covalent bonds, much as a train consists of a
chain of cars. The repeating units that serve as the building
blocks of a polymer are smaller molecules called monomers
(from the Greek monos, single). Some of the molecules that
serve as monomers also have other functions of their own.

Front 38

What are enzymes?

Back 38

Specialized macromolecules that speed up chemical reactions.

Front 39

What is a Dehydration Reaction?

Back 39

Although each class of polymer is made up of a different type
of monomer, the chemical mechanisms by which cells make
and break down polymers are basically the same in all cases.
In cells, these processes are facilitated by enzymes, specialized
macromolecules that speed up chemical reactions.
Monomers are connected by a reaction in which two molecules
are covalently bonded to each other, with the loss of a
water molecule; this is known as a dehydration reaction
(Figure 5.2a). When a bond forms between two monomers,
each monomer contributes part of the water molecule that is
released during the reaction: One monomer provides a
hydroxyl group (—OH), while the other provides a hydrogen
(—H). This reaction is repeated as monomers are added to the
chain one by one, making a polymer.

Hint 39

Wikipedia Defines it as:
In chemistry and the biological sciences, a dehydration reaction is usually defined as a chemical reaction that involves the loss of a water molecule from the reacting molecule.

Front 40

What is Hydrolysis?

Back 40

Polymers are disassembled to monomers by hydrolysis, a
process that is essentially the reverse of the dehydration reaction (Figure 5.2b). Hydrolysis means to break using water
(from the Greek hydro, water, and lysis, break). The bond between
the monomers is broken by the addition of a water molecule,
with the hydrogen from the water attaching to one
monomer and the hydroxyl group attaching to the adjacent
monomer. An example of hydrolysis working within our bodies
is the process of digestion. The bulk of the organic material
in our food is in the form of polymers that are much too large
to enter our cells. Within the digestive tract, various enzymes
attack the polymers, speeding up hydrolysis. The released
monomers are then absorbed into the bloodstream for distribution
to all body cells. Those cells can then use dehydration
reactions to assemble the monomers into new, different polymers
that can perform specific functions required by the cell.

Hint 40

Front 41

What is an Asymmetric Carbon?

Back 41

An asymmetric carbon is a carbon attached to four
different atoms or groups of atoms.

Glucose and galactose, for example, differ only in the placement of parts around one asymmetric carbon (see the purple boxes in Figure 5.3). What seems like a small difference is significant enough to give the two sugars distinctive shapes and behaviors.

Hint 41

Front 42

Most names of sugars end in?

Back 42

ose

Ie. Fructose, sucrose, galactose, etc.

Front 43

What does the Greek word Monos stand for?

Back 43

Single

Front 44

What does the Greek word Sacchar stand for?

Back 44

Sugar

Front 45

What is a Monosaccharide?

Back 45

Monosaccharides (from the Greek monos, single, and
sacchar, sugar) generally have molecular formulas that are some
multiple of the unit CH2O. Glucose (C6H12O6), the most common
monosaccharide, is of central importance in the chemistry of life.

Front 46

What does the prefix di- stand for?

Back 46

two

Front 47

Disaccharide & Glycosidic Linkage

Back 47

A disaccharide consists of two monosaccharides joined
by a glycosidic linkage, a covalent bond formed between
two monosaccharides by a dehydration reaction. For example,
maltose is a disaccharide formed by the linking of two
molecules of glucose (Figure 5.5a). Also known as malt sugar,
maltose is an ingredient used in brewing beer. The most
prevalent disaccharide is sucrose, which is table sugar. Its two
monomers are glucose and fructose (Figure 5.5b). Plants generally
transport carbohydrates from leaves to roots and other
nonphotosynthetic organs in the form of sucrose. Lactose,
the sugar present in milk, is another disaccharide, in this case
a glucose molecule joined to a galactose molecule.

Hint 47

Front 48

What does Poly stand for?

Back 48

Many

Front 49

Polysaccharide

Back 49

Polysaccharides are macromolecules, polymers with a few
hundred to a few thousand monosaccharides joined by glycosidic
linkages. Some polysaccharides serve as storage material,
hydrolyzed as needed to provide sugar for cells. Other polysaccharides
serve as building material for structures that protect the cell or the whole organism. The architecture and
function of a polysaccharide are determined by its sugar
monomers and by the positions of its glycosidic linkages.

Front 50

Starch

Back 50

Plants store starch, a polymer of
glucose monomers, as granules within cellular structures
known as plastids, which include chloroplasts. Synthesizing
starch enables the plant to stockpile surplus glucose. Because
glucose is a major cellular fuel, starch represents stored energy.
The sugar can later be withdrawn from this carbohydrate
“bank” by hydrolysis, which breaks the bonds between
the glucose monomers. Most animals, including humans,
also have enzymes that can hydrolyze plant starch, making
glucose available as a nutrient for cells. Potato tubers and
grains—the fruits of wheat, maize (corn), rice, and other
grasses—are the major sources of starch in the human diet.

Hint 50

Front 51

Glycogen

Back 51

Animals store a polysaccharide called glycogen, a polymer
of glucose that is like amylopectin but more extensively
branched (Figure 5.6b). Humans and other vertebrates store
glycogen mainly in liver and muscle cells. Hydrolysis of glycogen
in these cells releases glucose when the demand for sugar
increases. This stored fuel cannot sustain an animal for long,
however. In humans, for example, glycogen stores are depleted
in about a day unless they are replenished by consumption of
food. This is an issue of concern in low-carbohydrate diets.

Hint 51

Front 52

Cellulose

Back 52

the polysaccharide called cellulose is a major component of the tough walls that enclose plant cells.
On a global scale, plants produce almost 1014 kg (100 billion
tons) of cellulose per year; it is the most abundant organic
compound on Earth. Like starch, cellulose is a polymer of glucose,
but the glycosidic linkages in these two polymers differ.

Hint 52

An example of Cellulose is Insoluble Fiber.

Enzymes that digest starch by hydrolyzing its α linkages
are unable to hydrolyze the β linkages of cellulose because of
the distinctly different shapes of these two molecules. In fact,
few organisms possess enzymes that can digest cellulose. Animals,
including humans, do not; the cellulose in our food
passes through the digestive tract and is eliminated with the
feces. Along the way, the cellulose abrades the wall of the digestive
tract and stimulates the lining to secrete mucus,
which aids in the smooth passage of food through the tract.
Thus, although cellulose is not a nutrient for humans, it is an
important part of a healthful diet. Most fresh fruits, vegetables,
and whole grains are rich in cellulose. On food packages,
“insoluble fiber” refers mainly to cellulose.

Some microorganisms can digest cellulose, breaking it
down into glucose monomers. A cow harbors cellulosedigesting
prokaryotes and protists in its stomach. These microbes
hydrolyze the cellulose of hay and grass and convert
the glucose to other compounds that nourish the cow. Similarly,
a termite, which is unable to digest cellulose by itself,
has prokaryotes or protists living in its gut that can make a
meal of wood. Some fungi can also digest cellulose, thereby
helping recycle chemical elements within Earth’s ecosystems.

Front 53

Chitin

Back 53

Another important structural polysaccharide is chitin,
the carbohydrate used by arthropods (insects, spiders, crustaceans,
and related animals) to build their exoskeletons
(Figure 5.9). An exoskeleton is a hard case that surrounds the
soft parts of an animal. Pure chitin is leathery and flexible,
but it becomes hardened when encrusted with calcium carbonate,
a salt. Chitin is also found in many fungi, which use
this polysaccharide rather than cellulose as the building material
for their cell walls. Chitin is similar to cellulose, with
β linkages, except that the glucose monomer of chitin has a
nitrogen-containing appendage (see Figure 5.9, top right).

Hint 53

Front 54

Lipids

Back 54

Lipids are the one class of large biological molecules that
does not include true polymers, and they are generally not
big enough to be considered macromolecules. The compounds
called lipids are grouped together because they
share one important trait: They mix poorly, if at all, with
water. The hydrophobic behavior of lipids is based on their
molecular structure. Although they may have some polar
bonds associated with oxygen, lipids consist mostly of hydrocarbon
regions. Lipids are varied in form and function. They
include waxes and certain pigments, but we will focus on the
most biologically important types of lipids: fats, phospholipids,
and steroids.

Front 55

Fat & Fatty Acid

Back 55

Although fats are not polymers, they are large molecules assembled
from smaller molecules by dehydration reactions. A fat is
constructed from two kinds of smaller molecules: glycerol and
fatty acids (Figure 5.10a). Glycerol is an alcohol; each of its
three carbons bears a hydroxyl group. A fatty acid has a long
carbon skeleton, usually 16 or 18 carbon atoms in length. The
carbon at one end of the skeleton is part of a carboxyl group,
the functional group that gives these molecules the name fatty
acid. The rest of the skeleton consists of a hydrocarbon chain.
The relatively nonpolar C¬H bonds in the hydrocarbon
chains of fatty acids are the reason fats are hydrophobic. Fats
separate from water because the water molecules hydrogenbond
to one another and exclude the fats. This is the reason
that vegetable oil (a liquid fat) separates from the aqueous vinegar
solution in a bottle of salad dressing.

Hint 55