So cells are made up of large chemicals called macromolecules. What does that mean? What are chemicals, anyway? And what is the difference between small and large chemicals? You were not expected to know the answers to all of these questions before starting this class. There was no chemistry prerequisite, so don't worry about that. Yet, you will need to understand the answers to the questions above in order to understand the chemical make-up of cells.
What are chemicals, anyway?
Everything in our world, both living and non-living, is made up of chemicals. In scientific terms, it is said that anything that takes up any space at all and has any mass (like weight) at all is matter. So, a table is matter, you are matter, and a dust particle is matter. All of these things take up some space and have some weight, even if it is a very tiny amount of space and weight like for the dust particle.
Consider a brick of gold as matter. First of all, if this brick of gold is 24 carat, pure gold, it is made up of only one component-- the element, gold. There are lots of elements, like silver, aluminum, iron, lead, mercury, helium, etc. I listed some of the ones you are already familiar with, but there are also some biologically important elements like carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur. Most things in our world are not just made up of one element, but of a combination of elements.
For simplicity for the moment, let's return to the brick of gold. How small could you cut it up and still have matter? You could cut it in half, and then in half again, and in half again, etc. Until you eventually have a tiny shard of gold. The smallest shard of gold that you can still see is still, believe it or not, very large as far as chemicals go. If you could continue to break it down into the smallest bit that still retains the properties of gold, you would have something called an atom. Likewise, the smallest bit of iron that still retains the properties of iron is an iron atom. And the smallest bit of oxygen that still acts like oxygen is an oxygen atom. Most of the time, atoms (which are too small to see even with the best microscopes) are not alone. They combine with other atoms.
What are the small chemicals?
You have now learned that atoms are the tiny building blocks for all matter that makes up our world. When atoms combine with one another, they form chemical bonds to stay together. Once bonded, the atoms travel around together as molecules. Molecules can still be very small-- like oxygen gas, which is just two atoms of oxygen stuck to each other (drawn like O=O to show a special type of bond they make, and written like O2 to show that there are 2 atoms of oxygen). They can also be small and be combinations of different atoms-- like water, which is two atoms of hydrogen bonded to an oxygen atom (it can be depicted as shown at the right, where the oxygen atom is red and the hydrogen atoms are grey and it can be written as H2O). Ammonia is also a small molecule. It is made up of a nitrogen atom stuck to 3 hydrogen atoms. This little animation shows you an ammonia molecule spinning around, where the dark blue is the nitrogen atom and the light blue are the hydrogen atoms.
The chemicals I have just described are small, but they are considered inorganic. Organic molecules, by definition, contain carbon and hydrogen atoms within them. And all living things contain and change organic molecules. For example, sugar is an organic molecule that some living organisms make (like plants), all living organisms acquire in some way, and all living organisms break it down to make cellular energy. Sugar is made up of carbon, hydrogen, and oxygen atoms, so it is considered organic.
What are macromolecules?
Cells are built primarily from the largest organic molecules-- the organic macromolecules. Most of these organic macromolecules are themselves built by assembling smaller organic molecules (like sugars for example) into long chains. There are four types of organic macromolecules:
You should learn this list so well that you don't even hesitate to say what the four types of organic macromolecules are. This list is really important to understanding cells, so really memorize it well. It will pop up again and again throughout the semester.
Carbohydrates are the "sugars." Both the simple sugars (like glucose and table sugar) and complex sugars (like starch). The complex sugars are the macromolecules.
Lipids include the fats and oils. This is the only group that contains molecules that cannot interact/mix with water. You know already that oil and water don't mix, right? Well, the fact that lipids cannot interact well with water gives them many special properties. This group is also the only group that is not built simply by combining smaller chemical subunits into long chains. Lipids also include steroids and another type of molecule that you probably haven't heard about before-- phospholipids. This last type of lipid is important in building cell membranes.
Proteins are an extremely diverse group of macromolecules. These molecules provide us with pigment. They also enable cells to move, to have a specific shape, and to recognize material around them. There are not many popular proteins that you would have heard about... the only ones I am pretty sure you know something about are insulin, hemoglobin, and melanin (skin pigment). Another protein is dystrophin, a protein in muscle that is affected in muscular dystrophy. If you look over these examples, you might notice that they are items that can be affected by genetic disorders (like sickle cell anemia, albinism, and MD). That is because all that genes do is tell our cells what proteins to make. When we display the right set of proteins, we are our unique selves. No two people display the same exact sets of proteins.
Nucleic acids are the macromolecules that make up our genetic code. You have heard of DNA, well it stands for deoxyribonucleic acid. There is also another type of nucleic acid called RNA... we'll get to that next week. But your genes are all made up of DNA, and the DNA molecules are wound up within your chromosomes.
Cells are built as some of each macromolecule lays down in specific places. Membranes get phospholipids and proteins. DNA is stuck in the nucleus of the cell. And so on. You will get this information next week.
What do I mean by a "big" molecule versus a small molecule? Well, you saw an image of a water molecule above. That's a small molecule. Another small (and very important in my day-to-day life!) molecule is caffeine. Here's what caffeine looks like:
Every ball in the diagram, whether it is a gray, blue, red, or white ball, represents one atom. This molecule is a lot larger than a water molecule. Water only had three atoms, while this one has 24 atoms. But this is still considered a small molecule.
So what do I mean when I talk about a large molecule (a macromolecule)? Well, here's a protein. This is a protein that you have in your saliva and that we will study later this semester. It is called amylase. This is a macromolecule, but certainly not the largest macromolecule that we could study. Many proteins are even larger than this one!
There are certainly too many atoms to count in this molecule. In order to fit it on the page, I had to have every atom be a lot smaller than in the original caffeine diagram above. Therefore, I have included a miniature image of caffeine here, so that you can compare its size to the protein.
Don't worry that I might ask you to memorize this! I would never do that. I just thought that you should have some idea of the difference between an atom, a small molecule, and a macromolecule. There are also some easier ways to look at huge molecules like this amylase protein; it is too hard to try to really see anything in this massive diagram to the right.
Finally, just for your information, the macromolecule is so much bigger than the little caffeine molecule that the macromolecule can be seen as a spec when viewed with an electron microscope, while the caffeine molecule isn't visible at all.
In the next lesson we'll work on the macromolecules; you will learn ways to understand them that I hope will make sense to you. This lesson just served to introduce them. You can prepare for the macromolecules by looking over the introduction to the chemistry chapters, and, if you want, viewing your CD that comes with the book. The CD goes over some things I haven't gotten into, like how the atoms and small molecules join together. I will not hold you responsible for these things, but some people need to see how it works to understand it at all-- if you are like that, go ahead and watch this on your CD. By the end of the next lesson, you will have a much better sense of the importance of these molecules as well.
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007