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
Instructor: Angela Hartsock

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

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.


Polysaccharides, 'poly-' meaning multiple and '-saccharide' meaning sugar, are chains of sugar molecules. These macromolecules play a variety of roles within the cell, from making up components of the bacterial cell wall to storing carbon for energy to acting directly as food sources to making up part of the backbone of DNA and RNA. One of the most well-known sugars used within the cell is glucose, but there are many different possible structures and combinations.

Formally, sugars are called carbohydrates, but let's break down the term 'carbohydrate' because, while you might not realize it, the name tells you everything. 'Carbo-' means 'carbon' and '-hydrate' means 'water,' which we know contains hydrogen and oxygen. So carbohydrates are carbon atoms that are hydrated, or in other words bonded with oxygen and hydrogen atoms. So the elements found in carbohydrates are carbon, oxygen, and hydrogen.

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