What Are the Four Macromolecules of Life?
By Kevin Beck
Biology – or informally, life itself – is characterized by elegant macromolecules that have evolved over hundreds of millions of years to serve a range of critical functions.
These are often categorized
into four basic types: carbohydrates (or polysaccharides), lipids, proteins and
nucleic acids.
If you have any background in
nutrition, you will recognize the first three of these as the three standard
macronutrients (or "macros," in dieting parlance) listed on
nutritional information labels.
The fourth pertains to two
closely related molecules that serve as the basis for the storage and
translation of genetic information in all living things.
Each of these four
macromolecules of life, or biomolecules, performs a variety of duties; as you
might expect, their different roles are exquisitely related to their various
physical components and arrangements.
Macromolecules
A macromolecule is
a very large molecule, usually consisting of repeated subunits called monomers,
which cannot be reduced to simpler constituents without sacrificing the
"building block" element.
While there is no standard
definition of how large a molecule must be to earn the "macro"
prefix, they generally have, at a minimum, thousands of atoms.
You have almost certainly seen
this kind of construction in the non-natural world; for example, many kinds of
wallpaper, while elaborate in design and physically expansive on the whole,
consist of adjoining subunits that are often less than a square foot or so in
size.
Even more obviously, a chain
can be regarded as a macromolecule in which the individual links are the
"monomers."
An important point about biological
macromolecules is that, with the exception of lipids, their monomer units are
polar, meaning that they have an electric charge that is not distributed
symmetrically.
Schematically, they have
"heads" and "tails" with different physical and chemical properties.
Because the monomers join
head-to-tail to each other, macromolecules themselves are also polar.
Also, all biomolecules have
high amounts of the element carbon.
You may have heard the kind of
life on Earth (in other words, the only kind we know for certain exists
anywhere) referred to as "carbon-based life," and with good reason.
But and nitrogen, oxygen, hydrogen, and phosphorus are indispensable to living
things as well, and a host of other elements are in the mix to lesser degrees.
Carbohydrates
It is a near-certainty that
when you see or hear the word "carbohydrate," the first thing you
think of is "food," and perhaps more specifically, "something in
food a lot of people are intent on getting rid of."
"Lo-carb" and
"no-carb" both became weight-loss buzzwords in the early part of the
21st century, and the term "carbo-loading" has been around the
endurance-sports community since the 1970s.
But in fact, carbohydrates are
far more than just a source of energy for living things.
Carbohydrate molecules all have
the formula (CH2O)n, where n is the number of carbon
atoms present. This means that the C:H:O ratio is 1:2:1.
For example, the simple sugars
glucose, fructose and galactose all have the formula C6H12O6 (the
atoms of these three molecules are, of course, arranged differently).
Carbohydrates are classified as
monosaccharides, disaccharides and polysaccharides.
A monosaccharide is the monomer
unit of carbohydrates, but some carbohydrates consist of only one monomer, such
as glucose, fructose and galactose.
Usually, these monosaccharides
are most stable in a ring form, which is depicted diagrammatically as a
hexagon.
Disaccharides are sugars with
two monomeric units, or a pair of monosaccharides.
These subunits can be the same
(as in maltose, which consists of two joined glucose molecules) or different
(as in sucrose, or table sugar, which consists of one glucose molecule and one
fructose molecule.
Bonds between monosaccharides
are called glycosidic bonds.
Polysaccharides contain three
or more monosaccharides.
The longer these chains are,
the more likely they are to have branches, that is, to not simply be a line of
monosaccharides from end to end.
Examples of polysaccharides
include starch, glycogen, cellulose and chitin.
Starch tends to form in a helix,
or spiral shape; this is common in high-molecular-weight biomolecules in
general.
Cellulose, in contrast, is
linear, consisting of a long chain of glucose monomers with hydrogen bonds
interspersed between carbon atoms at regular intervals.
Cellulose is a component of
plant cells and gives them their rigidity. Humans cannot digest cellulose, and
in the diet it is usually referred to as "fiber."
Chitin is another structural
carbohydrate, found in the outer bodies of arthropods like insects, spiders and
crabs.
Chitin is a modified
carbohydrate, as it is "adulterated" with ample nitrogen atoms.
Glycogen is the body's storage
form of carbohydrate; deposits of glycogen are found in both liver and muscle
tissue.
Thanks to enzyme adaptations in
these tissues, trained athletes are able to store more glycogen than sedentary
people as a result of their high energy needs and nutritional practices.
Proteins
Like carbohydrates, proteins
are a part of most people's everyday vocabulary because of their serving as a
so-called macronutrient.
But proteins are incredibly
versatile, far more so than carbohydrates. In fact, without proteins, there
would be no carbohydrates or lipids because the enzymes needed to synthesize
(as well as digest) these molecules are themselves proteins.
The monomers of proteins are
amino acids. These include a carboxylic acid (-COOH) group and an amino (-NH2)
group.
When amino acids join to each
other, it is via a hydrogen bond between the carboxylic acid group on one of
the amino acids and the amino group of the other, with a molecule of water (H2O)
released in the process.
A growing chain of amino acids
is a polypeptide, and when it is sufficiently long and assumes its
three-dimensional shape, it is a full-fledged protein.
Unlike carbohydrates, proteins
never show branches; they are just a chain of carboxyl groups joined to amino
groups.
Because this chain must have a
beginning and an end, one end has a free amino group and is called the
N-terminal, while the other has a free amino group and is called the
C-terminal.
Because there are 20 amino
acids, and these can be arranged in any order, the composition of proteins is
extremely varied even though no branching occurs.
Proteins have what is called
primary, secondary, tertiary and quarternary structure.
Primary structure refers to the
sequence of amino acids in the protein, and it is genetically determined.
Secondary structure refers to bending or kinking in the chain, usually in a
repetitive fashion.
Some conformations include an
alpha-helix and a beta-pleated sheet, and result from weak hydrogen bonds
between side chains of different amino acids.
Tertiary structure is the
twisting and curling of the protein in three-dimensional space and can involve
disulfide bonds (sulfur to sulfur) and hydrogen bonds, among others.
Finally, quaternary structure
refers to more than one polypeptide chain in the same macromolecule. This
occurs in collagen, which consists of three chains twisted and coiled together
like a rope.
Proteins can serve as enzymes,
which catalyze biochemical reactions in the body; as hormones, such as insulin
and growth hormone; as structural elements; and as cell-membrane components.
Lipids
Lipids are a diverse set of
macromolecules, but they all share the trait of being hydrophobic; that is,
they do not dissolve in water.
This is because lipids are
electrically neutral and therefore nonpolar, whereas water is a polar molecule.
Lipids include triglycerides (fats and oils), phospholipids, carotenoids,
steroids and waxes.
They are involved chiefly in
cell membrane formation and stability, form portions of hormones, and are used
as stored fuel.
Fats, a type of lipid, are the
third type of macronutrient, with carbohydrates and proteins discussed
previously.
Via oxidation of their
so-called fatty acids, they supply 9 calories per gram as opposed to the 4
calories per gram supplied by both carbohydrates and fats.
Lipids are not polymers, so they
come in a variety of forms. Like carbohydrates, they consist of carbon,
hydrogen and oxygen.
Triglycerides consist of three
fatty acids joined to a molecule of glycerol, a three-carbon alcohol. These
fatty-acid side chains are long, simple hydrocarbons.
These chains can have double
bonds, and if they do, that makes the fatty acid unsaturated.
If there is only one such
double bond, the fatty acid is monounsaturated.
If there are two or more, it
is polyunsaturated. These different types of fatty acids have
different health implications for different people owing to their effects on
the walls of blood vessels.
Saturated fats, which have no
double bonds, are solid at room temperature and are usually animal fats; these
tend to cause arterial plaques and may contribute to heart disease.
Fatty acids can be chemically
manipulated, and unsaturated fats such as vegetable oils can be made saturated
so that they are solid and convenient to use at room temperature, like
margarine.
Phospholipids, which have a
hydrophobic lipid at one end and a hydrophilic phosphate at the other, are an
important component of cell membranes.
These membranes consist of a
phospholipid bilayer. The two lipid portions, being hydrophobic, face to the
outside and interior of the cell, while the hydrophilic tails of phosphate meet
in the center of the bilayer.
Other lipids include steroids,
which serve as hormones and hormone precursors (e.g., cholesterol) and contain
a series of distinctive ring structures; and waxes, which include beeswax and lanolin.
Nucleic Acids
Nucleic acids include
deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
These are very similar
structurally as both are polymers in which the monomeric units are nucleotides.
Nucleotides consist of a
pentose sugar group, a phosphate group and a nitrogenous base group.
In both DNA and RNA, these
bases can be one of four types; otherwise, all of the nucleotides of DNA are
identical, as are those of RNA.
DNA and RNA differ in three
main ways. One is that in DNA, the pentose sugar is deoxyribose, and in RNA it
is ribose.
These sugars differ by exactly
one oxygen atom.
The second difference is that
DNA is usually double-stranded, forming the double helix discovered in the
1950s by Watson and Crick's team, but RNA is single-stranded.
The third is that DNA contains
the nitrogenous bases adenine (A), cytosine (C), guanine (G) and thymine (T),
but RNA has uracil (U) substituted for thymine.
DNA stores hereditary
information.
Lengths of nucleotides make
up genes, which contain the information, via the nitrogenous base
sequences, to manufacture specific proteins.
Lots of genes make up chromosomes, and
the sum total of an organism's chromosomes (humans have 23 pairs) is its genome.
DNA is used in the process of
transcription to make a form of RNA called messenger RNA (mRNA).
This stores the coded
information in a slightly different way and moves it out of the cell nucleus
where the DNA is and into the cell cytoplasm, or matrix.
Here, other types of RNA initiate the process of translation, in which proteins are made and dispatched all over the cell.
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.
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