Video Transcript
In this video, we’ll learn what a
biological macromolecule is, we’ll introduce the four types of biological
macromolecules, and we’ll discuss how their essential functions in living things are
supported by their structures.
Let’s start by answering the
question “What is a biological macromolecule?” Well, you’ll recall that complex
multicellular organisms like humans contain several different organ systems. An organ system is a group of
organs. An organ is made of many different
tissues. And tissues are made of many
different cells. Each cell contains many
organelles. An organelle is made of
molecules. Molecules are built from atoms. And even atoms are made of smaller
bits organized together. This concept of organization is
actually centered around the fact that the traits of biological structures are
determined by their components.
Nervous tissue is able to carry out
its function of coordinating our body’s responses because it contains many nerve
cells which are specialized for communication. In the same way, the
characteristics of an atom are determined by the number of protons, neutrons, and
electrons it contains and their arrangement. When it comes to biology, structure
is always directly related to function.
Biological macromolecules are large
organic molecules made of repeating subunits. There are four types of biological
macromolecules that all living things possess. These are carbohydrates, lipids,
proteins, and nucleic acids. Most biological macromolecules are
also considered polymers. Poly- is a word part meaning many,
and -mer means part or unit. Polymers are specifically
macromolecules that are built out of repeating subunits called monomers. Mono- is a word part that means
one. A monomer is one unit within a
polymer. When monomers join together to form
a polymer, we call the process polymerization. The traits of a macromolecule
depend on the traits and arrangement of the molecular subunits that it’s made
of.
Next, let’s get more familiar with
the traits of the different types of macromolecules. We’ll start with carbohydrates and
lipids.
Carbohydrates are molecules that
contain carbon, hydrogen, and oxygen in a particular ratio. Polymer carbohydrates are also
referred to as polysaccharides. Saccharide means sugar. We’ve learned that polymers are
made of repeating units called monomers. The monomers that make up
polysaccharides are called monosaccharides or simple sugars. An example of a monosaccharide that
you’re probably familiar with is glucose, one of the substrates our cells commonly
use for cellular respiration.
Some of the functions of
carbohydrates are storing easily transferrable energy and providing structure to
certain types of cells. Some complex carbohydrates include
starch, cellulose, and glycogen. Let’s take a moment here to note
that the term carbohydrate applies both to simple sugars and to polysaccharides. But when we’re talking about
macromolecules, we’re only referring to the complex polysaccharide carbohydrates and
excluding simple sugars.
Lipids are also made of
combinations of carbon, hydrogen, and oxygen like carbohydrates, but they’re
different in many ways. First, lipids have a different
ratio of these elements. They tend to have more carbon and
hydrogen and less oxygen than carbohydrates do. Lipids are actually characterized
by their long carbon–hydrogen chains and also the fact that lipids are not soluble
in water. A good example of a common lipid is
your typical cooking oil, which famously does not mix with water.
Some of the functions of lipids are
storing energy for longer periods of time and making up the membranes of cells and
organelles. Common biological lipids are
triglycerides, phospholipids, and cholesterol.
When we say that a biological
molecule is organic, we mean that it possesses carbon–hydrogen bonds. So a molecule like glucose is what
we consider organic, but a molecule like water is not. The bond between carbon and
hydrogen is an especially high-energy bond, meaning that the reactions that break
them are usually exothermic and that excess energy can be transferred to other
cellular processes. Between these two types of energy
storage molecules, lipids have a much higher energy-to-mass ratio since they possess
many more of these high-energy carbon–hydrogen bonds.
This is part of the reason why
plants favor easy-to-access starch as their method of long-term energy storage. Plants are stationary organisms, so
they don’t have to carry all of this heavy starch around with them. On the other hand, animals which
need to move around generally use fat for their long-term energy storage. It’s a little harder to make but
much lighter. Imagine if we had potatoes attached
to our bodies. And this difference is an example
of how form and function in macromolecules is related.
Next, let’s take a closer look at
proteins and nucleic acids. Proteins are extremely diverse and
complex biological macromolecules. They are enzymes, antibodies,
hormones, structural molecules, and much more. Proteins are polymers made of
several amino acid monomers. Chains of amino acids are referred
to as polypeptides because of the peptide bonds that form between them. The chain of amino acids must be
folded and shaped into a functional protein. There are 20 different varieties of
amino acids. And their final sequence and
arrangement is what gives each individual protein its traits.
Nucleic acids are DNA and RNA. DNA stands for deoxyribonucleic
acid, and RNA stands for ribonucleic acid. Since the type of macromolecule is
right there in the name, it makes nucleic acids particularly easy to identify. Nucleic acids are polymers made up
of monomers called nucleotides. One nucleic acid can hold thousands
or millions of nucleotides. Nucleic acids are able to contain a
lot of information in the arrangement of the nucleotides, also called a nucleotide
sequence. They’re also very stable
molecules. And, compared to proteins, they
have a structure that’s easy to replicate. This is why nucleic acids are
perfect for storing genetic information. They carry huge sets of
instructions for cells, and they can be copied and passed on to offspring.
Proteins and nucleic acids are very
closely related. You’re likely already aware that
DNA is transcribed into RNA and that RNA is translated into a polypeptide, which is
eventually folded into a functional protein, which would make it seem like billions
of years ago when life began, DNA must have existed first. But scientists interested in the
origins of life know that proteins arose before nucleic acids. However, proteins are varied and
complicated, so scientists theorize that DNA and RNA later developed as ways to
store, copy, and transfer the information needed to make these proteins and not the
other way around. Other scientists have proposed an
RNA world hypothesis in which RNA existed first.
Now that we’ve familiarized
ourselves with the four types of macromolecules, their structures, and their
functions, let’s try a practice question.
Which of the following most
accurately describes the basic structure of a biological macromolecule? (A) Biological macromolecules
are molecules or monomers that are made up of repeating units or polymers. (B) Biological macromolecules
are long or large molecules or polymers that are made up of smaller subunits or
monomers joined together. Biological macromolecules are
small molecules or polymers that are made up of smaller atoms or monomers joined
together.
The correct answer to this
question will be the choice that describes the structure of a biological
macromolecule. And since we’re looking for the
choice that does this the most accurately, we’re going to need to compare all
three answer choices to each other. When we say that a molecule is
biological, we usually mean that it’s organic or that it possesses carbon to
hydrogen bonds. A molecule is two or more atoms
bonded together, and macro- is a word part that means large. So a biological macromolecule
is going to be a large group of atoms bonded together that’s organic.
You’re likely aware that all
living organisms are made of cells, that cells contain many organelles,
organelles are built out of molecules, and we’ve already mentioned that
molecules are made out of two or more atoms bonded together. On our scale of biological
organization, macromolecules would fit right here. So a macromolecule is a large
molecule, which is made up of lots of smaller units, which are molecules
themselves. Many macromolecules are also
considered to be polymers. Poly- is a word part that means
many, and -mer means unit. And those molecular subunits
that make up the polymer are referred to as monomers. Mono- is a prefix that means
one.
A good example of this is a
macromolecule called glycogen. Glycogen is a carbohydrate
polymer made of many repeating subunits bonded together. The monomers that are bonded
together to form glycogen are glucose molecules. So each of these pairs of terms
has the same relationship. Glycogen is made of
glucose. Polymers are made of
monomers. And macromolecules are made of
molecules. Now let’s go back to our answer
choices.
Our first choice says that
monomers are made up of polymers. But we know that monomer is one
unit and polymer is many. So this answer choice is
incorrect. Our second choice says that
macromolecules are large molecules, which we know is true, and also that
polymers are made up of monomers, which is also correct. So this answer choice seems
okay. But since we’re looking for the
most accurate, we need to compare all three. Our last choice states that
macromolecules are small molecules, but we know that the word part macro-
actually means large. So this answer choice is also
not correct.
The choice that best describes
the basic structure of a biological macromolecule is biological macromolecules
are long or large molecules or polymers that are made up of smaller subunits or
monomers joined together.
Next, let’s wrap up our lesson by
taking a moment to review what we’ve learned. In this video, we learned about
biological macromolecules. We reviewed the levels of
organization and determined where in that scale the macromolecule fits in. We learned how these important
molecules are organic and how they’re made up of repeating smaller subunits. And we briefly reviewed the four
types of biological macromolecules, carbohydrates, lipids, proteins, and nucleic
acids and how their structure is directly related to their function.
