Lesson Video: Biological Macromolecules Biology

In this video, we will learn how to describe key components found in the cells of organisms, including inorganic and organic compounds and large biological molecules.


Video Transcript

In this video, we’ll learn what a biological macromolecule is, we’ll introduce the four types of biological macromolecules, and we’ll discuss how their essential functions in living things are supported by their structures.

Let’s start by answering the question “What is a biological macromolecule?” Well, you’ll recall that complex multicellular organisms like humans contain several different organ systems. An organ system is a group of organs. An organ is made of many different tissues. And tissues are made of many different cells. Each cell contains many organelles. An organelle is made of molecules. Molecules are built from atoms. And even atoms are made of smaller bits organized together. This concept of organization is actually centered around the fact that the traits of biological structures are determined by their components.

Nervous tissue is able to carry out its function of coordinating our body’s responses because it contains many nerve cells which are specialized for communication. In the same way, the characteristics of an atom are determined by the number of protons, neutrons, and electrons it contains and their arrangement. When it comes to biology, structure is always directly related to function.

Biological macromolecules are large organic molecules made of repeating subunits. There are four types of biological macromolecules that all living things possess. These are carbohydrates, lipids, proteins, and nucleic acids. Most biological macromolecules are also considered polymers. Poly- is a word part meaning many, and -mer means part or unit. Polymers are specifically macromolecules that are built out of repeating subunits called monomers. Mono- is a word part that means one. A monomer is one unit within a polymer. When monomers join together to form a polymer, we call the process polymerization. The traits of a macromolecule depend on the traits and arrangement of the molecular subunits that it’s made of.

Next, let’s get more familiar with the traits of the different types of macromolecules. We’ll start with carbohydrates and lipids.

Carbohydrates are molecules that contain carbon, hydrogen, and oxygen in a particular ratio. Polymer carbohydrates are also referred to as polysaccharides. Saccharide means sugar. We’ve learned that polymers are made of repeating units called monomers. The monomers that make up polysaccharides are called monosaccharides or simple sugars. An example of a monosaccharide that you’re probably familiar with is glucose, one of the substrates our cells commonly use for cellular respiration.

Some of the functions of carbohydrates are storing easily transferrable energy and providing structure to certain types of cells. Some complex carbohydrates include starch, cellulose, and glycogen. Let’s take a moment here to note that the term carbohydrate applies both to simple sugars and to polysaccharides. But when we’re talking about macromolecules, we’re only referring to the complex polysaccharide carbohydrates and excluding simple sugars.

Lipids are also made of combinations of carbon, hydrogen, and oxygen like carbohydrates, but they’re different in many ways. First, lipids have a different ratio of these elements. They tend to have more carbon and hydrogen and less oxygen than carbohydrates do. Lipids are actually characterized by their long carbon–hydrogen chains and also the fact that lipids are not soluble in water. A good example of a common lipid is your typical cooking oil, which famously does not mix with water.

Some of the functions of lipids are storing energy for longer periods of time and making up the membranes of cells and organelles. Common biological lipids are triglycerides, phospholipids, and cholesterol.

When we say that a biological molecule is organic, we mean that it possesses carbon–hydrogen bonds. So a molecule like glucose is what we consider organic, but a molecule like water is not. The bond between carbon and hydrogen is an especially high-energy bond, meaning that the reactions that break them are usually exothermic and that excess energy can be transferred to other cellular processes. Between these two types of energy storage molecules, lipids have a much higher energy-to-mass ratio since they possess many more of these high-energy carbon–hydrogen bonds.

This is part of the reason why plants favor easy-to-access starch as their method of long-term energy storage. Plants are stationary organisms, so they don’t have to carry all of this heavy starch around with them. On the other hand, animals which need to move around generally use fat for their long-term energy storage. It’s a little harder to make but much lighter. Imagine if we had potatoes attached to our bodies. And this difference is an example of how form and function in macromolecules is related.

Next, let’s take a closer look at proteins and nucleic acids. Proteins are extremely diverse and complex biological macromolecules. They are enzymes, antibodies, hormones, structural molecules, and much more. Proteins are polymers made of several amino acid monomers. Chains of amino acids are referred to as polypeptides because of the peptide bonds that form between them. The chain of amino acids must be folded and shaped into a functional protein. There are 20 different varieties of amino acids. And their final sequence and arrangement is what gives each individual protein its traits.

Nucleic acids are DNA and RNA. DNA stands for deoxyribonucleic acid, and RNA stands for ribonucleic acid. Since the type of macromolecule is right there in the name, it makes nucleic acids particularly easy to identify. Nucleic acids are polymers made up of monomers called nucleotides. One nucleic acid can hold thousands or millions of nucleotides. Nucleic acids are able to contain a lot of information in the arrangement of the nucleotides, also called a nucleotide sequence. They’re also very stable molecules. And, compared to proteins, they have a structure that’s easy to replicate. This is why nucleic acids are perfect for storing genetic information. They carry huge sets of instructions for cells, and they can be copied and passed on to offspring.

Proteins and nucleic acids are very closely related. You’re likely already aware that DNA is transcribed into RNA and that RNA is translated into a polypeptide, which is eventually folded into a functional protein, which would make it seem like billions of years ago when life began, DNA must have existed first. But scientists interested in the origins of life know that proteins arose before nucleic acids. However, proteins are varied and complicated, so scientists theorize that DNA and RNA later developed as ways to store, copy, and transfer the information needed to make these proteins and not the other way around. Other scientists have proposed an RNA world hypothesis in which RNA existed first.

Now that we’ve familiarized ourselves with the four types of macromolecules, their structures, and their functions, let’s try a practice question.

Which of the following most accurately describes the basic structure of a biological macromolecule? (A) Biological macromolecules are molecules or monomers that are made up of repeating units or polymers. (B) Biological macromolecules are long or large molecules or polymers that are made up of smaller subunits or monomers joined together. Biological macromolecules are small molecules or polymers that are made up of smaller atoms or monomers joined together.

The correct answer to this question will be the choice that describes the structure of a biological macromolecule. And since we’re looking for the choice that does this the most accurately, we’re going to need to compare all three answer choices to each other. When we say that a molecule is biological, we usually mean that it’s organic or that it possesses carbon to hydrogen bonds. A molecule is two or more atoms bonded together, and macro- is a word part that means large. So a biological macromolecule is going to be a large group of atoms bonded together that’s organic.

You’re likely aware that all living organisms are made of cells, that cells contain many organelles, organelles are built out of molecules, and we’ve already mentioned that molecules are made out of two or more atoms bonded together. On our scale of biological organization, macromolecules would fit right here. So a macromolecule is a large molecule, which is made up of lots of smaller units, which are molecules themselves. Many macromolecules are also considered to be polymers. Poly- is a word part that means many, and -mer means unit. And those molecular subunits that make up the polymer are referred to as monomers. Mono- is a prefix that means one.

A good example of this is a macromolecule called glycogen. Glycogen is a carbohydrate polymer made of many repeating subunits bonded together. The monomers that are bonded together to form glycogen are glucose molecules. So each of these pairs of terms has the same relationship. Glycogen is made of glucose. Polymers are made of monomers. And macromolecules are made of molecules. Now let’s go back to our answer choices.

Our first choice says that monomers are made up of polymers. But we know that monomer is one unit and polymer is many. So this answer choice is incorrect. Our second choice says that macromolecules are large molecules, which we know is true, and also that polymers are made up of monomers, which is also correct. So this answer choice seems okay. But since we’re looking for the most accurate, we need to compare all three. Our last choice states that macromolecules are small molecules, but we know that the word part macro- actually means large. So this answer choice is also not correct.

The choice that best describes the basic structure of a biological macromolecule is biological macromolecules are long or large molecules or polymers that are made up of smaller subunits or monomers joined together.

Next, let’s wrap up our lesson by taking a moment to review what we’ve learned. In this video, we learned about biological macromolecules. We reviewed the levels of organization and determined where in that scale the macromolecule fits in. We learned how these important molecules are organic and how they’re made up of repeating smaller subunits. And we briefly reviewed the four types of biological macromolecules, carbohydrates, lipids, proteins, and nucleic acids and how their structure is directly related to their function.

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