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Here on Earth, life is dizzyingly diverse—but it’s also surprisingly organized. A sense of order structures life and its processes, from the tiniest cell to the total sum of every living thing. In this episode of Crash Course Biology, we’ll uncover the levels of biological organization, discover soil’s superpowers, and find out why the biosphere is kind of like a really, really long train.

Life's Layers 00:00
Themes of Life 1:54
Form Fits Function 4:08
Regulation 5:29
Levels of Organization 6:39
Life's Flows 9:55
Review & Credits 12:15

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:
Here on Earth, we’ve got it all: plants that eat spiders, spiders that eat birds, and birds that drink blood.

There are trees that live for thousands of years, and some mayflies that live for only 12 hours, and that seemingly immortal mold in the corner of your basement that you’d rather not think about. Variety is the spice of, well, life.

But in all its spiciness, life also has order. It’s organized in layers, each one rich, complex, and connected, like the most delicious multi-layer dip you’ve ever tasted. We call those layers the levels of biological organization.

And we are all swimming in them, from the cells in our bodies to the totality of all living things on our planet. And understanding this organization is important to understanding life as we know it. Hi, I’m Dr.

Sammy, your friendly neighborhood entomologist, and this is Crash Course Biology. Now where’s my theme music? [THEME MUSIC] You may not see the family resemblance, but you and I both share our family tree with the Venus Flytrap, the Blobfish, and yes, even our old friend the Platypus. That’s because all life on Earth shares a common ancestor.

And that makes for some interesting family reunions, with Grandpa Bacterium dozing off in the corner, Auntie Fungi recalling the good old days before mammals showed up, and all the young’ns crawling and flying around, getting skinned knees and crying and stuff. Good times, good times… As we page through life’s family album, some shared properties pop up again and again, which we’re calling the themes of life. Callie—cue dramatic music. [Static] Callie?

C’mon Callie... [Dramatic music plays] —there we go! There we go! First, life’s physical forms tend to fit their function, like how a shark’s tail helps it swim through the water.

Second, life is governed by regulation— mechanisms that keep life’s processes in check. And third, life depends on a flow of information, energy, and chemicals. As we explore biology, we’ll encounter these themes over and over.

To see how this plays out across the levels of biological organization, let’s start here, with our distant cousin, the humpback whale. She, like you and I, is an organism: an individual life form. Yep, she’s alive all right.

But zoom in and out on our whale, pan up and down, and you’ll find layers of life. I’m not just talking about the barnacles growing on her chin, either. Although, wow, those are some strange designs.

Is that…the Mona Lisa? Starting from the bottom, the levels of biological organization begin on the microscopic level with the molecules inside your cells, and they end with the sum of every living thing on our planet. Qualities called emergent properties arise at each of these levels.

They’re kind of like special powers, appearing only when all the parts are together. Like how a boy band’s harmonies only work if all the members are there. What are you doing here, Jimin?

Are you here to watch me film again? You’re so sweet! Life itself is an emergent property.

It starts with cells—the basic building blocks of all living things— way before we have an entire whale in front of us. But all living things have cells, or are cells, in the case of single-celled organisms like bacteria. And groups of similar cells make up tissues.

Tissues make up organs, such as the whale’s heart or brain. As groups of organs work together, they form a connected system, in this case an organ system. Additional functions emerge from that cooperation, which one organ couldn't complete on its own.

Life is full of these kinds of interlocking parts, like the whale’s nervous system, for example, which sends messages to the rest of her body to swim that-a-way, towards that tasty swarm of krill. Or to belt out a tune! [Whale wails.] She should collab with BTS! It’d be pretty awesome.

Which reminds me of that first theme: Form fits function. Throughout the levels of biological organization, the structure of a thing tends to enable the thing to do what it needs to do to survive. This match between form and function is explained by evolution— a process of gradual change in a population as they adapt to their environment over generations.

Evolution explains life’s unity and diversity, and it’s a theme of life so important that it gets its own episode later in our series. For now, let’s look at an example that you might be familiar with: teeth, and how different forms of teeth evolved to fit unique functions. Our krill-eating whale doesn’t need teeth; she uses bristles called baleen to filter out krill from big gulps of ocean water.

A manatee, meanwhile, prefers the seagrass buffet, so she has a mouthful of molars for the job, continuously replaced throughout her lifetime. And hyenas’ thick teeth are a perfect match for crushing zebra bones. Evolution just doesn’t give out features all willy-nilly.

Those features have to do something useful. Form fits function on many scales, even down to the microscopic level. Like how DNA molecules look like spiral staircases —which is not just cool, but also perfect for protecting and copying genetic information, and how a plant’s leaves get structure from its cell's thick outer walls.

We see another theme even at these microscopic levels of life, too: regulation. Regulation is the process that maintains internal functions in an organism and makes sure that nothing gets too far out of alignment. When you’re caught outside on a hot day, your cells keep a constant flow of information about the conditions inside and outside of your body.

Your nerve cells sense your rising temperature and send messages to your brain. Your brain then activates your sweat glands, which cool your skin. Sweat is one of your body’s ways of regulating your temperature so that all of your internal systems remain stable.

We call that a state of homeostasis. Regulation happens with gene expression, too. A cell’s master instructions, which are encoded in DNA, might include genes for, say, growing a thick mustache.

Meaning: there’s a time and a place for a thick mustache. And it’s not on a baby. Maybe one day; but not today, baby… not today.

So, the expression of those genes is regulated. The instructions only get read and put into action when the signals say so. So you might be a late-stage teenager before your participation in No-Shave November is particularly impressive.

And regulation happens at higher levels of organization, too. The humpback whale, for example, is connected to all the other individuals of her species, called a population. All the populations in an area—whether they’re seaweed, snails, or sharks— make up communities.

The interactions between those communities and their physical landscape makes up ecosystems. And the stuff that happens in a population’s environment regulates that population’s numbers. Like, a population of rabbits might have a baby boom one year, but their population can’t grow forever.

The ecosystem just doesn’t have an infinite food supply, and that regulates how many rabbits can live there. The levels of life’s organization don’t end there. Keep zooming out, and we find biomes: large regions of ecosystems sharing a similar climate.

Earth is home to a diverse array of biomes: including rainforests, oceans, deserts, savannas, and more. And finally, we arrive at the top level, the mega-system linking every other level of life, the biosphere: the collective sum of all ecosystems, where all living things do all that living. It includes most of the land, water, and the lower parts of the Earth’s atmosphere, and all the nested levels of life within them.

To get a sense for how the biosphere works, let’s head to the Thought Bubble. Imagine the biosphere as a never-ending train, each individual train car might hold a different form of life, but all the cars are connected and moving on the same rails. Our infinite train has three really important rules. 1.

You’ve got to have a ticket to ride, and only living things get tickets. 2. If you’re energy, feel free to hop on, but you can’t stick around; you’re gonna change forms. And 3.

If you’re chemicals, well, congratulations, the train is your home now. And forever! When the train first started rolling a long, long, long time ago, it was conducted by a single-celled organism.

As life’s processes continued, and more and more complex organisms began to appear, new cars were added to the train. Energy flows in and out of our biosphere train. The sun’s energy constantly streams in through the windows, getting captured and used by living things — like plants, and then animals — and eventually exiting as heat.

Once it’s used to do some work in one car, it has to pass to the next – but it’s a one-way journey. After it’s used by one passenger, it can’t be re-used by them. That’s why life is dependent on a constant flow of energy from the sun into the system.

The energy keeps the train moving along the tracks, so to speak. But for chemicals, the biosphere train is a closed system. Chemicals, like water, carbon, and nitrogen, pass through living and non-living things over and over, in what are called biogeochemical cycles.

They’ll drift back and forth from train car to train car, forever. Thanks, Thought Bubble. For as long as life has existed, elements such as carbon, nitrogen, and phosphorus have been recycled.

So, all the atoms that you’re made up of and all that you’ve encountered are hand-me-downs from billions of years ago. There’s no chance of getting new stuff without a visit from a meteorite. And that brings us to our final theme of life: flow.

Life depends on the flow of information, energy, and chemicals. We see the flow of information through life’s processes. Genes, for example, carry instructions for building proteins.

Cells use proteins to receive and send information to each other, like walkie-talkies in your body. And when an elk smells a wolf and responds with a posture that reads “Yikes,” her herd mates receive the message, like a big game of telephone spoken in body language. And as we saw in the Thought Bubble, energy flows in one direction through the biosphere, while chemicals are recycled again and again.

That flow knits all living things and environments together in a massive, interconnected web. And if this feels up in the clouds: well, it literally is, but it’s also down in the soil beneath your feet. It’s a lot to take in, I know, but it’s your lucky day I’ve got us a couple tickets to the Theater of Life… So, there’s this amazing biogeochemist named Dr.

Asmeret Asefaw Berhe. She’s been studying soil since she was just 18 years old. Which was no small feat for someone whose home country of Eritrea was on the verge of a war for independence at the time.

But it may explain why she became interested in how landmines affect soil, even after wars end. Many people think that soil is just a layer of dirt in the garden, or something you accidentally track onto the carpet. But Dr.

Berhe asks: what are the many important roles that soil plays in supporting life on this planet? Her work illustrates how we endanger ourselves when we don’t take care of the soil. Healthy soils act as a carbon sink — a storage container for carbon dioxide.

It hangs onto large amounts that would otherwise be released into the atmosphere, where it would worsen global climate change. But Dr. Berhe’s research shows that soil degraded from human activity — like deforestation, intensive agriculture, and war — does a worse and worse job of storing carbon, leaving more heat-trapping CO2 in the air.

So, she advocates for land management practices that maximize the amount of carbon that the soil can hold onto. By finding ways to “stop treating soil like dirt” — Dr. Berhe’s words, not mine — we look out for ourselves as well other life on this planet.

What a performance. So we see in the soil, life’s dizzying diversity… yet also its sense of order and unity. You’ve got the regulation of nutrients that plants need to survive, tiny communities of organisms whose structures help them live in their soil ecosystem, and chemicals and energy flowing through a massive system of roots.

So the next time you’re gardening, remember that life’s layers are all around you — like a delicious worm-filled bean dip… You know, I’m gonna work on my analogies. I promise you I'll get better. It’s like what, episode 4?

We got this. And what’s happening in the soil is happening throughout all of life’s layers. We see that physical forms of living things tend to fit their function.

We discover, again and again, that life’s processes are regulated. And we find a flow of information, energy, and chemicals. This won’t be the last time that we encounter these themes.

Next time, we’ll learn how ecology reveals relationships between organisms and their environment — and unravel an elephant mystery along the way. I’ll see ya 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 made with the help of all these nice people. And of course, Jimin.

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