You already know that the world of biology is very different if you look
at something from far away, like the body of a dog, than if you look up
close and see organs, tissues, and cells. Each different level of
magnification reveals new features, from the dog's fur color and size, to
the way the cells are organized in its body. If we keep zooming in, we'll
start to see that, in fact, everything is built of the same few basic building
blocks, which join together in complicated ways to form larger
structures that eventually build the cuddly dog you can recognize with the
naked eye. Let's take a look into the very smallest levels, the small and
large molecules of the cell, to understand what these building blocks
are and how they come together. Let's start small with what are referred to
as small molecules or monomers. These have important function on their own,
but can also link up like Lego pieces to form large or macromolecules. There
are four main categories of small molecules: nucleic acids, simple sugars
(such as glucose), fatty acids, and amino acids. Nucleic acids are made up
of a sugar backbone and a base. The base is the monomer. There are four of
these bases: adenine, guanine, cytosine and thymine. You have probably heard of
these as the building blocks of DNA and the related nucleic acid, RNA. RNA
actually uses uracil instead of thymine. The smaller nucleotides also
have critical roles in our cells. One of them, adenosine, which is formed
when adenine binds to ribose sugar, can be modified to form a molecule called
ATP, or adenosine triphosphate. ATP is the most important energy source for
our cells. Now let's move on to the simple sugars, for example, glucose,
fructose, and galactose. When two of these sugar monomers join up, they form
what is called a disaccharide. One example of this is lactose, found in
milk, which contains one glucose molecule and one galactose molecule.
Mono and disaccharides can be broken down all the way to carbon dioxide,
generating a huge amount of energy along the way. If you've ever had a
sugar rush before, you won't be surprised that these are the main
sources of energy for the cell. Breaking down one glucose molecule
produces about 30 of those energy rich ATP molecules. Once cells use up their
glucose stores, they turn to breaking down fatty acids, another type of small
molecule. Fatty acids actually store two times as much energy as the same
massive sugars, which means that breaking them down leads to lots of ATP.
Like sugars and fatty acids, amino acids are another type of small
molecule that can be further broken down for use as fuel. But they have
several other important functions as well. You can think of amino acids as
the Swiss Army knife of the small molecules. Each has the same basic
structure, but then has one of 20 possible attachments coming off of the
main structure. Each attachment has different properties, so different
amino acids can be used for diverse roles. For example, glycine can sense
signals in the brain. Glutamine can also do this and can be converted to
glutamate to make food taste delicious. But why do cells need so much energy? A
major reason is that they need to build these small molecules into larger
macromolecules. First, let's look at how macromolecules are usually
assembled from the monomers. The individual small molecule units can
only attach to another monomer in two places. Stringing several monomers
together end to end gives a polymer, or large macromolecule. We'll next talk
about the four main types of macromolecules: nucleic acids, complex
carbohydrates, proteins, and lipids. As we mentioned, nucleotides can string
together to form nucleic acids, which come in two types— DNA
(deoxyribonucleic acid), which stores are genes, and RNA (ribonucleic acid),
which has many functions including acting as a messenger between the DNA
code and the production of proteins. Let's move on to sugars. Our cells take
the simple sugars, like glucose, from our food and build them into complex
carbohydrates that can be stored. Human cells store carbohydrates as glycogen,
plant cells as starch. We can also directly take in complex carbohydrates
from our diet. These are abundant in foods like bread and potatoes. Those
get broken down and then are either directly used for energy or are built
back up into glycogen for long term storage. The process is dynamic. Amino
acid monomers join directly together to form proteins. As we mentioned, there
are 20 different amino acids. These can link up in many different orders and in
chains of varying length, which means that we can make many thousands of
different types of proteins. These proteins are used for cell structure,
to carry out chemical reactions inside cells, and many other functions. We
saved lipids for last because they are a little unique. To make lipids, fatty
acids do not join up in a line like the other types of monomers. There are
several types of lipids. One type is called a triglyceride, where three
fatty acid monomers join up along a glycerol molecule. These are the main
way our bodies store energy in the form of fat. Two fatty acids joined to a
phosphate group forms a phospholipid, which is the main component of our cell
membranes. You can see that our cells use molecules of a wide range of sizes.
We can get a lot of these from our diet. A big glass of whole milk contains
calcium (an atom), lactose (a disaccharide or simple sugar), proteins
and fats. Our cells can take the simpler molecules and use them for
energy right away or can break them down into smaller pieces. Just like
when you disassemble a Lego house you've made, the individual Lego pieces
can then be used in a whole new construction. For our cells, this means
assembling new macromolecules. Overall, this dynamic cycle of breaking down
and building up macromolecules powers our cells, tissues, organs— our bodies
as a whole.