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Table of Contents
Table of Contents
What are Macromolecules?
What are Macromolecules?
You know what I'm about to say - time to break down this word...
Macro means large, and you surely know what molecules are by now. So these are large molecules. That sounds an awful lot like a polymer, doesn't it? Well that's no coincidence. We talked about polymers right before this section for a reason. These macromolecules are polymers, each with its own monomers that make it up. The four macromolecules are carbohydrates, lipids, nucleic acids, and proteins.
Carbohydrates
Carbohydrates
You've definitely heard of these - you might even be trying to avoid them. While it's not healthy to avoid them entirely long term, it is understandable why you might want to cut down on these to try to lose weight.
What is the monomer?
What is the monomer?
Carbohydrates are made up of monosaccharides. That word means 'one (mono) sugar (saccharide)'. So a carbohydrate is made up of single sugars. We typically refer to carbohydrates as sugars, but not all of them are as sweet as you might be imagining.
What are carbohydrates good for?
What are carbohydrates good for?
Not only do carbohydrates consist of glucose and sucrose, but also many other structures that you might not recognize as sugars because they are not at all sweet. One example of this is cellulose, a carbohydrate that plants synthesize for structural support. Celery, for example, is made primarily up of cellulose and water. But celery is not sweet, despite cellulose being a sugar. In reality, our taste buds cannot taste cellulose because we do not receive much of a benefit from the carbohydrate. We cannot digest the carbohydrate, so it just passes through undigested.
Carbohydrates are also good for short-term energy storage. Plants synthesize glucose via photosynthesis not only to build their bodies up with cellulose (and other polymers), but also so that they can break down the glucose via cellular respiration just like we do.
These are only two uses of carbohydrates for organisms that consume or synthesize them. There are many more, and we will see some as we continue throughout the course. For example, carbohydrates are on the outside of some cells, attached to the membrane, and are used for cell signaling and communication.
Lipids
Lipids
You may be more familiar with lipids as fats. Lipids, however, are more than that - they can be used for insulation, energy storage, cell membranes, and more.
What is the monomer?
What is the monomer?
For our course, there are two monomers that make up the lipid polymers we will encounter. The monomers are glycerol and fatty acids. They are shown interacting in this image.
Tryglycerides
Tryglycerides
The image here is a common lipid known as a triglyceride. 'Tri' means three, and you can see that there are 3 fatty acid chains attached to a single glycerol (highlighted in red). These are the two monomers of lipids, but lipids don't always look like this on the right. These are the fats that are in your diet.
Phospholipids
Phospholipids
Phospolipids, such as the one shown here, are going to be a very important lipid that we will encounter. They are responsible for making up cell membranes. The polar head is hydrophilic (hence, polar) whereas the nonpolar tail is hydrophobic. This is incredibly important, and we will see why in cell membranes.
They are able to keep the insides of the cell inside, and the stuff outside of cells on the outside. But more on that later.
Do not bother memorizing the chemical structure of this molecule - just be able to recognize its general shape.
Sterols
Sterols
Sterols make up the final type of lipid we will talk about, and it is actually a subgroup of steroids. Sterols are an important part of cell membranes, as they affect the fluidity of the membrane. We will discuss this more when we get to cell membranes, however.
Nucleic Acids
Nucleic Acids
This is the macromolecule that sounds the most intimidating, and is one of the most important for the course. However, it really is generally considered to be the easiest of the four to understand in AP Biology.
What is the monomer?
What is the monomer?
Nucleic acids (the polymer) are made up of nucleotides (the monomer). Any number of nucleotides chained together form a nucleic acid, but the nucleic acids you may be familiar with are generally made up of millions of nucleotides. These two kinds of nucleic acids are DNA and RNA.
Nucleotides have three parts - the phosphate group, the sugar, and the nitrogenous base (in this diagram, the base is adenine(A), but there can also be thymine(T), guanine(G), or cytosine(C)).
DNA
DNA
DNA, or deoxyribonucleic acid, is probably the most commonly discussed molecule in your school career so far. It is the recipe for life itself, and that is not stated lightly. There will be a lot of time dedicated to learning about the structure of DNA - the double helix. But, for now, I want you to be familiar with the name and be able to recognize a DNA molecule.
Looking at the image, you can see that DNA is double-stranded. It kind of looks like if you took a ladder and twisted it up quite a bit. That twisted-ladder shape is the double helix.
Each strand is made up of a sugar-phosphate backbone (the blue spine of the strands in the image) and the nitrogenous (the colored part in the middle of the strand). Those are really the same parts as the nucleotides. The sugars and phosphates from the nucleotides are linked together from one nucleotide to those of its neighbor. The nitrogenous base (A, T, C, G) sticks out in the middle, and is really the important part of the molecule.
Those bases are going to be more and more important as we continue on throughout the course, but for now, just know those 4 nucleotides that make up DNA - adenine, thymine, guanine, and cytosine. When they chain together, we can represent their sequence with a chain of their letter. So, if a DNA molecule has a chain of adenine, cytosine, cytosine, adenine, thymine, and guanine (in that order), then we would represent that chain as ACCATG.
RNA
RNA
RNA, or ribonucleic acid, sounds very similar to DNA, but there are some key differences. Firstly, the sugar found in the nucleotides are different. Instead of the deoxyribose sugar from DNA (that's where the D comes from), RNA nucleotides contain a ribose sugar (there's the R). You should be familiar with this, but don't need to memorize these sugars.
For us, however, there is a more crucial different here: the nitrogenous bases that can exist within RNA are different from those of DNA. Remember, DNA can be made up of thymine (T), cytosine (C), guanine (G), and adenine (A). RNA, however, cannot contain thymine. Instead, you will see that RNA can contain uracil (U) in addition to the cytosine, guanine, and adenine that the two molecules share.
RNA is also only made up of a single strand, rather than the two strands that make up DNA. It almost looks as if you cut DNA in half down the middle to make RNA.
Proteins
Proteins
Good, we're back to stuff we think about eating all the time! That's less difficult to spell than nucleic acids at least.
What is the monomer?
What is the monomer?
Proteins are polymers, just like all of the other macromolecules we discussed. Just like all polymers, they are made up of building blocks, or monomers. In proteins, the monomers that make them up are called amino acids.
Amino acids are made up of three basic parts: the amino group, the carboxyl group (both named for their functional groups), and the R-group (a.k.a. the side chain). Luckily, all amino acids are identical except for the R-group. That means that all amino acids consist of the same amino group and carboxyl group.
The R-group is what makes amino acids unique from one another - it's just like how the nitrogenous base of a nucleotide is what makes it unique compared to another nucleotide.
There are 20 different amino acids, each with a different R-group. You do not need to memorize any of these amino acids or their structures, but we will encounter some quite frequently in coming units.
So what do proteins do, exactly?
So what do proteins do, exactly?
Well, proteins do everything. If there is something that needs to be done in a cell, it is a protein that is going to do it. Proteins that do things such as catalyzing (or facilitating) chemical reactions are known as enzymes.
Enzymes are so crucial, that we are going to give them their own chapter. However, for now, you just need to know that enzymes are very dependent on their structure. Enzymes can only grab onto molecules to manipulate them if they are shaped precisely correctly. If you change the shape of an enzyme, it may become useless, or perhaps even harmful!
Enzymes appear very intricate and complicated, but I don't need you to memorize the structure of any enzymes - just know the structure of a general amino acid and a chain of them.
As you can see in the amino acid chain shown here, the chain is made up of regular, repeating parts in the middle.
The side chains, or R-groups, are the things that make amino acids unique from one another, remember? Those are highlighted in purple. So this chain consists of 4 amino acids, each different from all others in the chain.
"In simplest terms, a protein is a chain of amino acids. About a million different proteins have been identified so far, and nobody knows how many more are to be found. They are all made from just twenty amino acids, even though hundreds of aminoacids exist in nature that could do the job just as well. Why evolution has wedded us to such a small number of amino acids is one of the great mysteries of biology. For their importance, proteins are surprisingly ill-defined. Although all proteins are made from amino acids, there is no accepted definition as to how many amino acids you need in a chain to qualify as a protein. All that can be said is that a small but unspecified number of amino acids strung together is a peptide. Ten or twelve strung together is a polypeptide. When a polypeptide begins to get bigger than that, it becomes, at some ineffable point, a protein."
Bill Bryson's The Body: A Guide for Occupants
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