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MLA Full: "What is Life Made of? (Carbon & Biological Molecules): Crash Course Biology #20." YouTube, uploaded by CrashCourse, 14 November 2023,
MLA Inline: (CrashCourse, 2023)
APA Full: CrashCourse. (2023, November 14). What is Life Made of? (Carbon & Biological Molecules): Crash Course Biology #20 [Video]. YouTube.
APA Inline: (CrashCourse, 2023)
Chicago Full: CrashCourse, "What is Life Made of? (Carbon & Biological Molecules): Crash Course Biology #20.", November 14, 2023, YouTube, 13:53,
Despite the diverse appearance and characteristics of organisms on Earth, the chemicals that make up living things are remarkably similar, often identical. In this episode of Crash Course Biology, we’ll look at the building blocks of the four major classes of biomolecules, how those join up to form macromolecules, and how a team of six atoms forms the vast majority of living matter.

Introduction to Life’s Molecules 00:00
Chemical Bonds 2:38
The Major Biological Molecules 6:12
Polymerization 9:27
Hydrolysis 10:54
Review & Credits 12:32

This series was produced in collaboration with HHMI BioInteractive, committed to empowering educators and inspiring students with engaging, accessible, and quality classroom resources. Visit for more information.

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CC Kids:

 (00:00) to (02:00)

Dr. Sammy: The French biochemist and Nobel laureate Dr. Jacques Monod famously said, "All that is true for E. coli, is true for the elephant." And you might be thinking, okay, had Monod every actually seen an elephant? Which is a totally fair question.

On the surface, we see lots of differences between the single-cell bacterium and the elephant, which contains upwards of a quadrillion cells. But what Monod what actually getting at is that on a biochemical level all living things are actually shockingly similar. We can see that unity, which comes from our shared evolutionary history in the identical chemical building blocks that form the DNA and proteins of E. coli, elephants, and, well, us.

Despite our differences, we can use many of the same chemical reactions to break down food, harnessing energy along the way. Of course, we need more energy than E. coli needs and the elephant needs more than us but the rules of the game are the same.

And across all forms of life the games is play with six key elemental players: (audio effect to simulate sports announcer) Now taking the field for Team Life: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur! Give it up for Team C.H.N.O.P.S.! (end audio effect)

Hi, I'm Dr. Sammy, your friendly neighborhood entomologist and this is Crash Course biology and this is some seriously banging theme music.

(theme music)

Dr. Sammy: You're probably familiar with the periodic table. As of 2023, it lists 118 known elements and it isn't unusual for it to be updated. Heck, as recently as 2016 scientists added four new elements to it. Elements are just atoms, by the way, and they are pretty much the simplest form of stuff that exists. An elements is a name for a certain type of atom.

 (02:00) to (04:00)

When is comes to the team of life: (audio effect to simulate sports announcer) Our Atomic Captain is Carbon (end audio effect). Here on Earth, Carbon is the most common element in living things, making it our MVP.

Producers, like plants and algae, pull carbon dioxide from the air or water to make carbon-based molecules or connected groups of atoms. When consumers like mice, or lizards, or you eat your vegetables the molecules can be broken down. Then they get reused to build our own biological molecules. Just like you might break down one Lego creation to build another. These types of repeated uses are called biogeochemical cycles. But carbon isn't just plentiful, it takes a lot more to be MVP in the game of life. One of carbon's most amazing talents is its ability to be bound up in larger molecules. See, to really pull together as a team to make life work our six elements need to make and break chemical bonds.

Carbon wears number six on the periodic table and it's quite the team player. It can form four lengths, called covalent bonds, with other atoms. Think of covalent bonds like the molecular version of a rodeo. In this case, Carbon is the cowpoke lassoing bucking atoms.

(audio effect to simulate sports announcer) Hydrogen wears atomic number one on the periodic table. (end audio effect) It's the smallest element and it can only form one covalent bond. So if we arrange four hydrogens around the carbon and connect them with lines to represent the bonds, we've successfully built methane, the simplest organic molecule. So our new chemical, methane, has both carbon and hydrogen, which makes it an organic compound. Also know as an organic molecule.

Forming an organic compound is one of the top moves in the team's playbook because these compounds are the chemical basis for life. Most components of food, for example, are made up of organic compounds, including proteins, carbohydrates, and fats. And we're not talking about Organic, like the kind you'd find on a food label at a grocery store. That just refers to the lack of synthetic additives. Same name, totally different thing. 

 (04:00) to (06:00)

But those labels can be wildly misleading so we're not going to get into that here.

No matter how they're built organic compounds will always include carbon in at least some capacity. And the compounds change based on the number of players involved. If we put four carbons in a row and connect them with covalent bonds and then add in enough hydrogens to give each carbon four bonds BAM we get butane. Which is helpful for all kinds of things including using a lighter to kick start your campfire.

Organic compounds don't have to be straight chains fo carbon atoms though. If we rearrange butane we can make a branched organic compound called isobutane. This compound is often used in gasoline. Butane and isobutane have the exact same chemical formula with four carbons and ten hydrogens a piece. This makes them isomers, compounds with the same chemical formula but a different arrangement of atoms. Molecules can take all kinds of shapes and it can get pretty elaborate. 

We all know that there is no "I" in "team" and our MVP Carbon often needs a little help from another member of the starting lineup. (audio effect to simulate sports announcer) Wearing atomic number eight here comes Oxygen! (end audio effect). Oxygen is sort of like the team ball hog and doesn't collaborate as much with carbon. It's only going to create two covalent bonds, which can make team-ups a little trickier.

Take carbon dioxide for example, it has the chemical formula CO2. So if carbon likes to make four bonds and oxygen two, what do we do? We run some special plays. We put double covalent bonds between carbon and each oxygen so the four bonds are shared between two other atoms. Score! (short triumphant music)

(audio effect to simulate sports announcer) And now, the moment you've all been waiting for, give it up for the last member of our starting lineup. With atomic number seven on its jersey, Nitrogen! (end audio effect). It makes three bonds. For example, it teams up with three hydrogens to make ammonia.

We'll mention our second stringers, phosphorus and sulfur, a bit later. They're also essential to our team.

 (06:00) to (08:00)

Our starting lineup can't win the game all on its own. And winning the game –in case you lost track– means sustaining all life on earth. So these guys are kind of a big deal.

The upshot is that when our team coordinates their plays we get the four major classes of biological molecules that make up nearly everything in our bodies and not just our bodies: elephant bodies, plant bodies, and every other living thing that you can think of.

So what are these majestic molecules and how do they work like complicated Lego kits to build you and me?

The four types of biological molecules are lipids, carbohydrates, proteins, and nucleic acids. And they are made from our team C.H.N.O.P.S. and then they in turn make us.

Lipids are organic molecules that can't dissolve in water, like, cooking oil or beeswax. They're usually made of smaller blocks called fatty acids which consist of long chains of carbon atoms.

Some lipids are chemical messengers, like the hormones estrogen and testosterone which help prepare our bodies for things like puberty and reproductive processes.

Oh, so that's what I had to blame for all those pimples thanks a lot lipids.

Carbohydrates are our second class of biological molecules. They're used as sweeteners and they are never-ending at the olive garden meaning: they're the main components of pasta and all of those endless bread sticks. You might know them as carbs. In our bodies, carbs can be used to store energy for release as needed.

Their mini Lego piece is a monosaccharide: simple sugars like glucose or fructose. Stick a bunch of monosaccharides together and you get complex carbs like those in pasta and bread.

Our brains are particularly hungry for glucose and they should be considering that the brain consumes twenty percent of the body's metabolic fuel.

Our third type of biological molecule–called nucleic acids–store and transmit genetic information which is basically instructions that we inherit from our parents that tell our bodies how to work. DNA stores this information.

 (08:00) to (10:00)

And the nucleic acid RNA tells the body what to do with it.

All of this rests on the mini Lego piece called the nucleotide which is made of a simple carbohydrate, a phosphate group, and some nitrogen rich rings. Did you see that?

Phosphorus, coming in for the assist! We love a team player.

Our fourth and final class are proteins like the hormone insulin that helps regulate our blood sugar or the antibodies that help our immune system fight off infections.

Some also have structural roles like the collagen that makes up much of our skin. And while those proteins are tiny some huddle together so much that you can actually see them, like the protein keratin that makes your hair and your fingernails.

Proteins are built from amino-acids, and all amino-acids are pretty similar but each one has its own special group that makes it unique. Like, tryptophan the amino-acid in that turkey leg that used to take the blame for making you tired. Even though your sleep hormones are made out of this stuff, eating it doesn't mean you're going to get drowsy. That turned out to be a myth. The sleepiness was just because you were really full.

And finally, taking the field, we have have Sulfur!

It's in some of these amino acid building blocks, giving us the "S" in "CHNOPS." As you can see, these atomic Lego pieces don't act alone. They continue to build even bigger teams of molecules. Like, say we need our bodies to interpret our DNA, which we do need, all the time.

The process is kinda like a text chain full of coded emojis. DNA sends the messages that get interpreted by RNA, a code-breaker extraordinaire.

These messages are not just your average gossip, they're instructions for amino acids. So the amino acids join together, end-to-end, making a polymer, which is a large molecule containing many repeating molecules. And the name for a molecule containing many other molecules, by the way, is a macromolecule.

 (10:00) to (12:00)

DNA is the champion of polymerization, or linking up repeating molecules to make macromolecules. In humans, Chromosome 1, our largest chromosome, contains a single polymer made of 498 million nucleotide blocks. That's a lot of Legos. It works out to over 16 billion atoms, working together to store the genetic information in just one of 46 chromosomes. There are more atoms in that single chromosome of yours than there are humans on earth! Now that's a macro molecule.

We also have biological molecules to thank for the sweeter things in life. Some carbohydrates can be made of just two Lego blocks, like sucrose, the carb that makes the fruit in your smoothie taste sweet. Or carbs can macromolecules, like polysaccharides, which help plants and animals get their shape and store their energy.

Let's check out how our bodies break down molecules. It works the same way in polymers, but for simplicity, we'll look at sucrose. When you sip hot chocolate, proteins in your digestive system break sucrose into the monosaccharides, or simpler sugars, glucose and fructose. They do this by adding water to the covalent bond holding them together. These are called hydrolysis reactions, with "hydro" meaning water and "lysis" meaning to pull apart or loosen.

Hydrolysis is a process that cleaves one big molecule into two smaller molecules using water. Hydrolysis lets us reuse the building blocks from the producers and consumers that we eat to build up our own macromolecules. Each amino acid from a plant protein can be recycled to make the specific proteins that we need. And the different proteins that organisms make contribute to the uniqueness among all living things.

So hydrolysis is how our bodies break bonds between building blocks. But we also need to make new bonds, putting the Lego pieces back together in new combinations. For that, there's the opposite of hydrolysis: dehydration reactions.

 (12:00) to (13:53)

Just like the dehydration you might feel on a hot summer day, this process involves water loss. It removes water from the building blocks to make them stick together. In chemical terms, a water molecule is squeezed out from two building blocks, joining them into a bigger molecule.

For example: dehydration reactions occur when three fatty acids team up by attaching to a molecule of glycerol, a type of carbohydrate, squeezing out water to make what's called a triglyceride. This process helps bears get ready for hibernation, when their bodies live on fat stored during summer and fall.

Life can take on a lot of different shapes, but despite differences in the DNA code, it's the same nucleotides carrying the message that makes every organism on this planet. And the similarity doesn't stop there!

From a tiny amoeba to an enormous blue whale, we'll see similar lipids holding stuff inside cells, DNA and RNA working together to make proteins, and structural proteins helping cells keep their shape.

And behind it all, the six atoms team, CHNOPS! Our key players in the game of life. Chemically speaking, that is.

In the next episode, we're gonna take a deep dive into, well, water, and how our friend H2O makes life better for everyone. I'll see you then!

This series was produced in collaboration with HHMI BioInteractive. If you're an educator, visit for classroom resources and professional development related to the topics covered in this course.

Thanks for watching this episode of Crash Course Biology, which was filmed at our studio here in Indianapolis, Indiana, and was made with the help of all of these nice people! If you want to help keep Crash Course free for everyone, forever, you can join our team on Patreon.

[Outro Music]