 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 Then 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 functions on their own but can also link up like Lego pieces to form large or macro molecules 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 mass of 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 send 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 Dioxyribonucleic acid which stores our 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 With 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 and 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