Previous: The History of Museums: Crash Course Art History #3
Next: Why We Can Exist | Crash Course Pods: The Universe #2



View count:28,926
Last sync:2024-06-14 09:45


Citation formatting is not guaranteed to be accurate.
MLA Full: "Plants Are Hardcore: Plant Anatomy & Physiology: Crash Course Biology #42." YouTube, uploaded by CrashCourse, 7 May 2024,
MLA Inline: (CrashCourse, 2024)
APA Full: CrashCourse. (2024, May 7). Plants Are Hardcore: Plant Anatomy & Physiology: Crash Course Biology #42 [Video]. YouTube.
APA Inline: (CrashCourse, 2024)
Chicago Full: CrashCourse, "Plants Are Hardcore: Plant Anatomy & Physiology: Crash Course Biology #42.", May 7, 2024, YouTube, 13:05,
Plants may not seem like they’re doing much, but if you look closer, you’ll find a whole world just lurking beyond the surface. We’re talking chemical defenses, highways, and even ways to change the weather. In this episode, we’ll learn how plants get resources, get rid of waste, stay defended, govern themselves, and much more.

Introduction: How Plants Work 00:00
Meet the angiosperms 00:58
How plants transport nutrients 02:04
How plants eliminate waste 05:28
Plant defenses 08:05
Plant Communication and Reproduction 09:55
Review & Credits 11:47

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.

Check out our Biology playlist here:

Watch this series in Spanish on our Crash Course en Español channel here:


Crash Course is on Patreon! You can support us directly by signing up at

Thanks to the following patrons for their generous monthly contributions that help keep Crash Course free for everyone forever:
Leah H., David Fanska, Andrew Woods, DL Singfield, Ken Davidian, Stephen Akuffo, Toni Miles, Steve Segreto, Kyle & Katherine Callahan, Laurel Stevens, Burt Humburg, Perry Joyce, Scott Harrison, Mark & Susan Billian, Alan Bridgeman, Breanna Bosso, Matt Curls, Jennifer Killen, Jon Allen, Sarah & Nathan Catchings, team dorsey, Bernardo Garza, Trevin Beattie, Eric Koslow, Indija-ka Siriwardena, Jason Rostoker, Siobhán, Ken Penttinen, Nathan Taylor, Barrett & Laura Nuzum, Les Aker, William McGraw, Vaso, ClareG, Rizwan Kassim, Constance Urist, Alex Hackman, Pineapples of Solidarity, Katie Dean, Stephen McCandless, Wai Jack Sin, Ian Dundore, Caleb Weeks

Want to find Crash Course elsewhere on the internet?
Instagram -
Facebook -
Twitter -

CC Kids:
Meet Barbacenia.

Plants that grow between a rock  and a hard place… literally. They’ve got hairy roots that  ooze rock-dissolving acids,   letting them chisel their way  into Brazilian mountaintops.

So, instead of getting nutrients  from the soil like most plants,   they’re busy carving into boulders. Now, the petunias in your grandma’s yard aren’t  out there dissolving stones to make a living. But the rest of the plant  world is no less hardcore.

Plants transport water against gravity, defend  themselves against predators, and all-in-all   function through super-specialized,  interlocking systems in their bodies. Basically, the closer you look at plants,  the more mind-blowing they become. Hi, I’m Dr.

Sammy, your friendly  neighborhood entomologist,   and this is Crash Course Biology.  Speaking of mindblowers, Callieeeeeeeee,   drop that theme music, please! We’re  dropping theme music, not our manners. [THEME MUSIC] Plants are wildly diverse—there  are nearly 400 thousand species,   and that’s just the ones we know of. So, today, we’re zooming in on one category:  angiosperms, which grow flowers and fruits.

But don’t worry, you can go check out Crash Course  Botany to learn about our other plant friends.   I may be biased, but I highly recommend  it. And sorry, ferns, we still love you. You see, angiosperms are the  dominant plant life on Earth.

Almost every plant you would  recognize is an angiosperm,   whether it’s a tomato, oak tree, or wildflower. And they grow basically everywhere on  Earth, including the Arctic tundra. Now, just like our bodies need  systems to help them eat, breathe,   poop, fight germs, and so on, plants rely  on integrated systems in their bodies.

Like, you might have heard of photosynthesis,   the process plants use to turn sunshine, carbon  dioxide from the air, and water from the soil,   into sugars for the plant’s food  and basic building material. But it’s not a one-and-done:  photosynthesize and chill. Plants then need to move those sugars throughout  their bodies to keep things running smoothly. [Chapter 3 - How plants transport nutrients] For this, angiosperms use veins called phloem.

You can actually see phloem in those  stringy things on peeled bananas. Though in most cases, they’re inside  the plant where you can’t see them,   like here in this redwood tree. The phloem moves sugars from the  leaves to the rest of the plant.

This can happen either through diffusion,   where molecules passively spread into an  area where there aren’t a lot of them. Or, through active transport, where the  plant uses energy and special proteins to   drag molecules to where they need to go,  even if there’s already a bunch there. But let’s back up a second.

For photosynthesis to even begin,   plants need to move stuff in a totally  different, way more impressive direction. You see, they have to suck water  from the soil, into the roots,   and upwards to get to the leaves,  where photosynthesis happens. That means plants are working against gravity.

Now, this isn’t a huge issue for  a low-lying plant like a moss. But for an angiosperm like  a 100-meter-tall redwood? It has to get that water all the  way to the leaves at the very top.

I don’t know about you, but I’ve  never seen a waterfall go in reverse. Keeping those sky-high leaves hydrated is a  bit of a journey, and it begins at the roots. Roots, by the way, are one of the main types  of plant organs, along with stems and leaves.

An organ is just a structure made of  tissues working together to do a job. And tissues are just groups of cells working  together, so organs are like a system of teams. Plants —and animals for that matter — are all  pretty much organized into these kinds of systems.

But back to defying gravity. Roots need to absorb lots of water,  so most angiosperm roots have a bunch   of little hairs to suck up as much as  possible through many different entryways. Again, this transportation  happens through diffusion:   If there’s more water in the soil than the roots,  the H2O will passively travel into the plant.

Meanwhile, nutrients like salts  and nitrogen may get into the   roots through either diffusion or through  active transport, which requires energy. But that just gets us as far as the roots! From there, the solution of water and  minerals heads to another type of plant   vein, called xylem, which will carry  everything to the rest of the plant.

But again, that usually means going up. So, how does that work? The first thing to know is that water may  not be immune to gravity, but it is sticky.

On the molecular level, water molecules  are slightly attracted to each other,   so they tend to group up like  the cliques in a teen movie. That’s called cohesion. And when water molecules are gently  attracted to other things — like,   say, the walls of xylem — that’s adhesion.

The second thing to know? Water may come into a plant through  the roots, but if it’s not used for   photosynthesis, it leaves through… well,  the leaves in the form of water vapor. That’s called transpiration, and it’s  yet another process powered by diffusion.

When you put cohesion, adhesion,  and transpiration together,   it’s a more magical trio than  peanut butter, jelly, and bread. [Sammy sings “Peanut Butter Jelly”] Sorry. Ahem. As water - in the form of vapor  - transpires out of the leaves,   those sticky water molecules pull  on the molecules behind them.

And those molecules pull on the ones behind them. And suddenly, there’s this giant conga  line that drags water all the way from   the soil to the roots to the leaves  at the tops of the tallest trees. Take that, gravity!

So, that was a quick and dirty tour of some of plants’ resource acquisition  and nutrient transport systems. But, just like us, plants don’t only need to  eat stuff and move those nutrients around. They also need to get rid of stuff.

And no, I’m not saying plants poop. I’m just saying, plants… expel waste products. Which, yes, sounds like a fancy  phrase for poop, but it’s not!

Like, take transpiration again. A huge amount of water that’s no longer needed  can actually leave a plant through transpiration. We’re talking enough water for a  forest to create its own weather.

See, in the Amazon rainforest, it rains. A lot. Typically, rainy seasons are caused by  seasonal winds carrying moist ocean air inland.

But the rainy season in the  Amazon actually starts two to   three months before that — thanks to local trees. Scientists estimate there are nearly 400  billion trees in the Amazon — about 50 for every   person on Earth! — and all of them are releasing  water from their leaves on a daily basis. Enough that it collects in the atmosphere  and condenses to form rain clouds!

As the clouds dump rain onto the  forest, this raises the humidity,   which in turn warms up the  atmosphere just a little. This causes air to rise and start circulating,  like the bubbles at the bottom of a teapot. This circulation then causes the local wind  patterns to change and starts dragging in wet   air from the ocean and kicking off  the rainy season months in advance.

It goes to show how big of a deal  transpiration is — and how adding   or removing trees from an area can  do more than just shape the obvious   parts of an ecosystem: On a big enough  scale, it can even change the weather. So, leftover water transpires out of plants  through pores in their leaves called stomata. But the stomata are also where carbon  dioxide and oxygen enter and leave the plant.

You guessed it, through diffusion. And for plants, as for people,   maintaining the right balance of stuff  we need vs. stuff we don’t can be tricky. For every molecule of carbon dioxide  a plant takes in through its stomata,   it can lose around 400 water molecules  that it may want to hang onto.

It needs that water not only for photosynthesis,   but because water helps plants  keep their structure and shape. So, to maintain the right balance,  plants have evolved strategies   for how to take in air through their  stomata without losing too much water. For example, cacti only open their  stomata at night when the air is cool;   if they opened them in the daytime, they  would lose a lot more water in the heat.

OK, we’ve seen lots of  interlocking systems at this point. Plant systems are taking in resources,  moving ‘em around, and getting rid of waste. Check, check, and check.

But how do plants get rid of  other things… like predators? Yes, you adorable bunny you, you are  a threat to my strawberry plants. Rooted in just one spot, plants  can’t just up and book it when   lil thumper comes wiggling his  little nose in their direction.

So to fight against local predators, plants  might have stabby thorns on their stems,   or fuzzy hairs that make it harder for  very hungry caterpillars to reach a leaf. Some plants even use chemical weapons —  special compounds that make them taste awful,   or that make them poisonous to would-be diners. And they ramp up production of those nasty  compounds when they sense plant-eaters are around.

Plants can warn each other of danger, too. For instance, when a giraffe checks into  the Acacia tree buffet, the tree releases   chemicals on the wind that warns its  buddies that leaf-eaters are in the area. Any Acacia that gets the message  starts producing gross compounds.

So, ultimately, giraffes have to go upwind to  find tasty trees that haven’t gotten the memo. And they can even call for help. Some plants, like tomatoes, detect compounds  from the saliva of a caterpillar and   release a chemical signal which summons the  caterpillar’s worst enemy: a parasitoid wasp.

This wasp stings the caterpillar paralyzing it   and allows its babies to chow  down on the nutritious meal. And when it comes to microscopic  threats like bacteria, viruses,   or fungal infections, plants have immune  systems to keep themselves safe, too. Their cells have proteins and special molecules  that can recognize and neutralize invaders.

So bacteria and bunnies don’t stand a chance,   usually…there’s no fool-proof defense  in the evolutionary arms race. As the armor grows thicker,  the swords grow sharper still. Now, all of that said, for any of  the systems we’ve mentioned to work,   plant cells have to be  communicating with each other: If each plant part were just vibing on its own,   they wouldn’t know how to coordinate  to take down a germ or make plant food.

Plant cells get information about the  outside world from receptor proteins,   which change in response  to the plant’s environment. Like, a protein that’s sensitive to light   will change shape when the sun  comes up and when it goes down. And that can cause a chain reaction that  leads to bigger changes in the plant — like,   cueing a cactus that it’s  safe to open its stomata.

Plants can also coordinate their bodies  with hormones, just like humans do. These signaling molecules travel around and  trigger responses in any cell they can bind to. And there are more varieties of  hormones than there are flavors   of jelly beans in one of those  giant jugs you win in a raffle.

Some hormones, for example, help plants reproduce. Like, check out these strawberry plants. The flowers are their reproductive organs.

In fact, that’s true for all angiosperms! Which does make giving your crush a bouquet of  roses a little weird…but at least they’re pretty. Anyway, when the plant is ready to reproduce,   the hormone florigen lets flowers  know they should start blooming.

That yellow, powdery stuff called  pollen contains plant sperm cells,   and the hormone auxin helps it develop. When that pollen hits an egg  in another part of the flower,   it’s only a matter of time until seeds  and baby strawberry plants are on the way. But plants reproduce in other ways, too.

Some essentially make clones of themselves,  in a process known as asexual reproduction. And some plants do both! Like strawberry plants, for instance.

And there are pros and cons  to both types of reproduction. Asexual reproduction is generally simpler: it  doesn’t require the organism to find a mate. But sexual reproduction can also introduce  genetic diversity into a population,   which helps a strawberry patch survive long-term.

So, being able to reproduce both ways  gives plants the best of both worlds. So, while it can seem like  plants are just chilling,   minding their own business… there’s  a lot more going on than we realize. Our leafy green neighbors are constantly  moving nutrients throughout their bodies,   defending themselves against danger,   and communicating within and among themselves  to keep these overlapping systems functioning.

And in the end, plant life doesn’t  just make our world more beautiful: It can shape the weather and climate,   act as important sources of food  and medicine, and reshape our world. Next time, we’ll be jumping into the  amazingly complex world of animals,   starting with how they get the stuff they  need and get rid of the stuff they don’t. I’ll see you then.

Peace! 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   in Indianapolis, Indiana, and was made  with the help of all these nice people. If you want to help keep Crash  Course free for everyone,   forever, you can join our community on Patreon.