The Molecules of Life
The elements involved in life processes can, and do, form
millions of different compounds. Thankfully, these millions of compounds
fall into four major groups: carbohydrates, proteins, lipids, and
nucleic acids. Though all of these groups are organized around carbon, each
group has its own special structure and function.
Carbohydrates are compounds that have carbon, hydrogen,
and oxygen atoms in a ratio of about 1:2:1. If you’re stuck on an
SAT II Biology question about whether a compound is a carbohydrate,
just count up the atoms and see if they fit this ratio. Carbohydrates
are often sugars, which provide energy for cellular processes.
Like all of the biologically important classes of compounds,
carbohydrates can be monomers, dimers, or polymers. The names of
most carbohydrates end in “-ose”: glucose, fructose,
sucrose, and maltose are some common examples.
Carbohydrate monomers are known as monosaccharides. This
group includes glucose, C6H12O6,
which is a key substance in biochemistry. Sugars that an animal
eats are converted into glucose, which is then converted into energy
to fuel the animal’s activities by respiration (see Cell Processes).
Glucose has a cousin called fructose with the same chemical
formula. But these two compounds have different structures:
Glucose and fructose differ in one important way: glucose
has a double-bonded oxygen on the top carbon, while fructose has
its double-bonded carbon on the second carbon. This difference is
most apparent when the two monosaccharides are in their ring forms.
Glucose generally forms a hexagonal ring (six sided), while fructose
forms a pentagonal ring (five sided). Whereas fructose is the sugar
most often found in fruits, glucose is most often used as the major
source of energy for cellular activities.
Disaccharides are carbohydrate dimers. These
dimers are formed from two monomers by dehydration synthesis. Any
two monosaccharides can form a disaccharide. For example, maltose
is formed by the dehydration synthesis of two glucose molecules.
Sucrose, common table sugar, comes from the linkage of one molecule
of glucose and one of fructose.
Polysaccharides can consist of as few as three and as
many as several thousand monosaccharides. Depending on their structure
and the monosaccharides they contain, polysaccharides can function
as a means of storing excess energy or provide structural support.
When cells ingest more carbohydrates than they need for
fuel, they link the sugars together to form polysaccharides. The
structure of these polysaccharides is different in plants and animals:
in plants, polysaccharides take the form of starch,
whereas in animals, they are linked in a structure called glycogen.
Polysaccharides can also have structural roles in plants
and animals. Cellulose, which forms the cell walls of plant cells,
is a structural polysaccharide. In animals, the polysaccharide chitin
forms the hard outer armor of insects, crabs, spiders, and other
arthropods. Many fungi also use chitin as a structural carbohydrate.
More than half of the organic compounds in cells are proteins,
which play an important function in almost every cellular process.
Proteins, for example, provide structural support to the cell in
the cytoskeleton and make up many of the hormones that send messages around
the body. Enzymes, which regulate chemical reactions
in the cell, are also proteins.
Proteins are made up of monomers called amino acids. The
names of many, but not all, amino acids end in -ine: methionine,
lysine, serine, etc. Each amino acid consists of a central carbon
atom attached to a set of three designated groups: an atom of hydrogen
(–H), an amino group (–NH2),
and a carboxyl group (–COOH). The final group,
designated (–R) in the diagram below, varies
between different amino acids.
It is possible to make an infinite number of amino acids
by attaching different compounds to the R position
of the central carbon. However, only 20 types of R groups
exist in nature, so there are only 20 naturally occurring amino
All proteins are made of chains of some or all of these
20 amino acids. The bond formed between two amino acids by dehydration
synthesis is known as a peptide bond.
A particular protein has a specific sequence
of amino acids, which is known as its primary
structure. Every protein also winds, coils, and folds in
three-dimensional space in specific and predetermined ways, taking
on a unique secondary (initial winding and coiling) and tertiary
structure (overall folding). In harsh conditions, such as high temperature or
extreme pH, proteins can lose their normal tertiary shape and cease
to function properly. When a protein unfolds in harsh conditions,
it has been “denatured.”
Lipids are carbon compounds that do not dissolve in water.
They are distinguished from other macromolecules by characteristic hydrocarbon chains—long
strings of carbon molecules with hydrogens attached. Such chains
do not dissolve well in water because they are nonpolar.
Triglycerides consist of three long hydrocarbon chains
known as fatty acids attached to each other by a molecule
Because they include three fatty acids, fats and oils
are also known as triglycerides. As you might expect by this point,
glycerol and each fatty acid chain are joined to each other by dehydration
Some fats are saturated, while others are unsaturated.
These terms refer to the presence or absence of double bonds in
the fatty acids of fats. Saturated fats have no double bonds, whereas
unsaturated fats contain one or more such bonds. In general, plant
fats are unsaturated and animal fats are saturated. Saturated fats
are generally solid at room temperature, while unsaturated fats
are typically liquid.
Phospholipids, which are important components of cell
membranes, consist of a glycerol molecule attached to two fatty
acid chains and one phosphate group (PO4–2):
Like all fats, the hydrocarbon tails of phospholipids
do not dissolve in water. However, phosphate groups do dissolve
in water because they are polar. The different solubilities of the
two ends of phospholipid molecules allow them to form the bilayers
that make up the cell membrane.
Steroids are the primary structure in hormones, substances
that play important signaling roles in the body. Structurally, steroids
are made up of four fused carbon rings attached to a hydrocarbon
The linked rings indicate that each carbon atom is attached
to other carbon atoms that form multiple loops. Cholesterol, the
steroid in the image above, is the central steroid from which other
steroids, such as the sex hormones, are synthesized. Cholesterol
is only found in animal cells.
Cells use a class of compounds called nucleic
acids to store and use hereditary information. Individual nucleic
acid monomers, known as nucleotides, consist of three
main units: a nitrogenous base (a compound made with
nitrogen), a phosphate group, and a sugar:
There are two main types of nucleotides, differentiated
by their sugars: deoxyribonucleic acid (DNA) and ribonucleic
acid (RNA). DNA nucleotides have one less oxygen than RNA nucleotides.
The “deoxy” in deoxyribonucleic acid refers to the missing oxygen
molecule. In terms of function, DNA molecules store genetic information
for the cell, while RNA molecules carry genetic messages from the
DNA in the nucleus to the cytoplasm for use in protein synthesis
and other processes.
Within both DNA and RNA, there are further subdivisions
of nucleotides by nitrogenous bases. For DNA, there are four kinds
of nitrogenous bases:
The nitrogenous base of a nucleotide provides it with
its chemical identity, so the nucleotides are called by the name
of their nitrogenous base. RNA also has four nitrogenous bases.
Three—adenine, guanine, and cytosine—are identical to those found
in DNA. The fourth, uracil, replaces thymine.
DNA and RNA
In 1953, James Watson and Francis Crick published the
discovery of the three-dimensional structure of DNA. Watson and
Crick hypothesized that DNA nucleotides are organized into a polymer
that looks like a ladder twisted into a coil. They called this structure
the double helix.
Two separate DNA polymers make up each side of the ladder.
The sugar and phosphate molecules of the DNA form the vertical supports,
while the nitrogenous bases stick out to form the rungs. The rungs
attach to each other by hydrogen bonding.
The nitrogen bases attach to each other according to two
simple rules: adenine (A) pairs with thymine (T), and guanine (G)
pairs with cytosine (C). The exclusivity of the attachments between
nitrogen bases is known as base pairing.
The rules of base pairing are frequently tested on the
SAT II Biology. A test question might ask, “What is the complementary DNA
strand to ‘CAT’?” Following the rules of DNA base pairing, you can
deduce that the answer is “CAT.” (“DOG” is the wrong answer, smart
Unlike the double-stranded DNA, RNA is single stranded.
It looks like a ladder cut down the middle. As you will see when
we discuss protein synthesis in the chapter on Cell Processes, this
structure of RNA is very important to its functions as a messenger
from the DNA in the nucleus to the cytoplasm.
||Adenine, guanine, cytosine, thymine
||Adenine, guanine, cytosine, uracil
||Stores genetic material and passes it from
generation to generation
||Carries messages from the nucleus to the cytoplasm
Summary of the Molecules of Life
||Structure, signaling, catalysis
||Energy storage, signaling, membrane constituents
||Store genetic material
||Energy source, energy storage, structural
||Insulin, transcriptase (an enzyme)