There are four types of organic macromolecules:
You learned that in the last lesson. Now it is time to learn a lot more about them.
One thing to keep in mind is that lipids are different from all the other macromolecules in many ways. But two ways in particular that they are different are: 1) lipids are hydrophobic (meaning that they can't mix with water); and 2) lipids are not formed in the same way that the others form (by sticking building blocks together). If you keep these two differences in mind, it will help you as you progress through this week's material.
On this page you will learn things about organic molecules that apply to all the macromolecules. Then, I go on with the other pages of this lesson to explain each particular type of macromolecule. Because the lipids are so different from the others, I describe them first. That way, you can learn about them separately and be done with them before you go on to learn about the other macromolecules. All of the other macromolecules are formed by sticking building blocks together (kind of like making a necklace out of beads). So, after you learn about lipids, you will learn about the notion of building blocks. Finally, each of the other macromolecules are described on their own pages.
Carbon is pretty amazing
I told you last week that carbon atoms were essential for organic molecules. Well, carbon is really interesting because it can bond to four other atoms. That is not a characteristic of atoms of many other elements-- it is rare. Because it can make so many bonds, it is really good at stringing itself along:
And that drawing above only shows each carbon atom making two bonds. Since they can make 4 bonds, the above drawing could be re-drawn as shown here to the right. Each of these carbons is shown to have four possible bonds sticking off it (black lines).
If we keep adding to this drawing, we can stick in other atoms attaching to the string of carbons. Some of these bonds could be to other carbons, thus giving the chain a branched look, as shown in this diagram to the left. Some of those other bonds could also be to atoms of other elements, like hydrogen atoms (H), so I put those in.
In figure 4.4, your book shows you how carbon chains can run and branch. It even shows you how a carbon chain can bend upon itself to fold into a ring (4.4e).
Your book describes functional groups for you. I want to mention those a little bit here, because in some way they will come up whenever we discuss an organic macromolecule. You learned last week that molecules arise whenever atoms bond together. There are certain atoms that tend to bond together in a particular arrangement in some locations on molecules. For example, oxygen atoms and hydrogen atoms are found all over most organic macromolecules (along with carbon atoms). But we tend to see one oxygen atom bonded to one hydrogen atom in a lot of places along a macromolecule, like:
So in that macromolecule depicted above, there are three places where O is bonded to H (O - H) and sticking off the macromolecule. Because this arrangement of O - H occurs a lot within molecules (it is actually part of the molecule), it is given its own name: the hydroxyl group. This is one example of a functional group.
Before I continue to describe the other functional groups that are important, I want to clarify why you are expected to learn these. You see, as you look at the structure of macromolecules, they will just look like a lot of letters to you. A lot of letters. By learning a few sets of letters that appear together, the functional groups, you will start to see something within all the letters of the macromolecules. You will recognize regions. You will see that some macromolecules are just really combinations of functional groups. Then, suddenly, the macromolecules will make sense to you. Of course, you do not have to memorize the detailed structure of any macromolecule-- you will just have to be able to recognize them. But, how can you recognize something that just looks like lots of atoms? Instead, you will be able to recognize things that look like certain groupings of functional groups.
Another example of a functional group is the carboxyl group. Here is a diagram of a carboxyl group (to the right). In this diagram, X represents the rest of the macromolecule, and the
represents the carboxyl group. So the carboxyl group contains one carbon atom, two oxygen atoms, and one hydrogen atom. It even has a hydroxyl group within it. But, people see this entire combination so much (it can also be written - COOH in a sort of shorthand), that they have named it a carboxyl group.
You will see two other types of groups a lot. Your book lists other functional groups, too, but I don't care about any that aren't on this page. The two other functional groups are the amino group and the phosphate group. The elements that appear most in biological macromolecules are carbon, oxygen, hydrogen (all of which you have already seen), nitrogen, phosphorus, and sulfur. We won't spend much time on sulfur. But nitrogen and phosphorus will pop up again and again this semester. Nitrogen is in the amino group, and phosphorus is in the phosphate group. Here are what those groups look like (the "X" represents the rest of the macromolecule)
Different books sometimes have slightly different ways of writing the functional groups. Your book writes the carboxyl group as X - COO- rather than X - COOH. It also writes the amino group as X - NH3+ rather than X - NH2. That's OK. Don't worry about an extra hydrogen atom or a missing hydrogen atom.
Are you ready to continue? If so, get started now by going on to lipids!
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007