Major Elements in Biological Molecules: Proteins, Nucleic Acids, Carbohydrates & Lipids

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  • 0:05 The Major Macromolecules
  • 1:30 Proteins
  • 2:49 Nucleic Acids
  • 4:02 Carbohydrates
  • 5:09 Lipids
  • 6:00 Lesson Summary
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Lesson Transcript
Angela Hartsock

Angela has taught college Microbiology and has a doctoral degree in Microbiology.

Expert Contributor
Brenda Grewe

Brenda has 25 years of experience teaching college level introductory biology and genetics. She earned her PhD in Genetics from Indiana University.

Bacterial cells have complex macromolecules built from simple molecules. In this lesson, we will look at the major elements in the building blocks of proteins, nucleic acids, carbohydrates, and lipids.

The Major Macromolecules

Today we are going to do a dissection, but you don't need your scalpel because we won't be doing the classic frog dissection. Instead, we will be dissecting a bacterial cell. Let's take a look down our microscope to find a bacterial cell. Here you can see a rod-shaped cell. Now imagine that we could pick up this cell and slice it open to see the inside. If you look closely, you will start to make out some major cell structures - maybe a flagella, a cell wall, a cell membrane, some DNA, and even some ribosomes. Zoom in a little bit more and you start to see individual proteins floating around inside the cell catalyzing chemical reactions.

Many of these components of the cell are made up of macromolecules, which simply means large molecules formed by linking together small molecules. The major macromolecules within the cell include proteins, nucleic acids like DNA and RNA, carbohydrates, and lipids. In this lesson, we will discuss the structure of these macromolecules and break them down into their individual units, all the way down to individual atoms. To keep track of the elemental building blocks for our macromolecules, we will use this handy table and fill it in as we go.


Proteins play important roles within a cell. Some make up the structure of cell components, like a flagella or a pilus, and many play the role of enzymes to catalyze the reactions necessary for life.

Let's take a look at a protein. As you can see, they are complicated structures that can be made up of hundreds to thousands of individual units, called amino acids, all linked up in a chain and then folded up like origami into complicated shapes.

The structure may seem complex, but all proteins are actually made up of around 21 different amino acids, just in many different combinations. Every amino acid has the basic structure shown here consisting of carbon, hydrogen, oxygen, and nitrogen. This could be called the backbone of the amino acid. Let's add this information to our table. For the protein row, we will add carbon, hydrogen, oxygen, and nitrogen.

But each of these amino acids has a different molecular group that hangs off one side. Most of the special side groups contain the already mentioned carbon, hydrogen, oxygen, and nitrogen. To help you out, I will just point out the oddballs: these three amino acids have sulfur and selenium. So let's add those to our table.

Nucleic Acids

I am sure you are already familiar with nucleic acids, those incredibly important compounds that include both DNA and RNA. But let's take a closer look and break down these macromolecules to their basic elemental composition.

Let's start with the classic image of a double helix of DNA. If we analyze that macromolecule, we see that there are four basic building blocks that make up the structure: adenine, guanine, cytosine, and thymine. These are part of the DNA nucleotides.

Like DNA, RNA is also made up of nucleotides. It has the same A, G, and C, but instead of thymine, RNA contains uracil. So let's add the structure of uracil to our discussion. Don't forget, these DNA and RNA nucleotides also include a ribose sugar, either deoxyribose for DNA or ribose for RNA, as well as a phosphate molecule.

Let's analyze these structures and add the elements we find to our table. As we can see, the nucleic acid building blocks of DNA and RNA are made up of carbon, hydrogen, oxygen, nitrogen, and phosphorous.

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Additional Activities

Identification of Biological Molecules from Growing Bacterial Cells

In this activity, you will use your knowledge of the four different classes of macromolecules (proteins, nucleic acids, carbohydrates and lipids) to make predictions for the outcomes of an experiment in which bacterial cells are grown in nutrient broth in which the common isotope of carbon, hydrogen, phosphate or sulfur is replaced with the radioactive isotope (radioisotope) of that element.


A student set up four cultures of the bacterium E. coli that were switched from their normal growth medium to nutrient broth containing the radioactive isotope of one the major elements of biological molecules:

  1. Radioactive Carbon: C-14 in place of normal C-12
  2. Radioactive Hydrogen: H-3 (tritium) instead of normal H-1
  3. Radioactive Phosphorus: P-32 instead of normal P-31
  4. Radioactive Sulfur: S-35 instead of normal S-32.

After 40 generations of cell growth and division, the student collected the bacterial cells, lysed them open to release the cell contents, and separated the cell contents into water-soluble and water-insoluble fractions. Any new biological molecules that incorporated the radioisotope are detectable in the fractions by the radioactivity they emit, just as electronic devices such as cell phones can be tracked to their locations by the signal they emit.

Predict the Experimental Results

For each of the bacterial cultures (1-4) grown in the presence of a different radioisotope of a major element (C-14, H-3, P-32, and S-35) of biological molecules, predict a) the macromolecules that will have incorporated the radioisotope and b) whether the(se) macromolecule(s) will be in the water soluble or insoluble fraction.

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