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Fungi: Death Becomes Them | Crash Course biology| Khan Academy

August 22, 2019


– Hello and welcome to the
wonderful world of fungi or fungi, both are
acceptable pronunciations, but I say fungi because
it’s fungus not fungus though fungus is also fun to say. Fungi are a little bit like plants and more like animals
than you might think. They diverged from protists
about a billion years ago, and today scientists
estimate that there are about 1.5 million species
of fungi on the Earth though in a formal
taxonomic way, we only know about 100,000 or so of them,
and those that we have met are wonderful, weird and
in some cases deadly. The fact is death is pretty
much what fungi are all about. Sure, there are the fun fungi like the single-celled saccharomyces,
also known as yeast. Without them, we wouldn’t
have beer or wine or bread. It’s also true that
fungi are responsible for all kinds of diseases from athlete’s foot to potentially deadly
histoplasmosis, aka spelunker’s lung, caused by fungus found in
bird and bat droppings. Fungi can even make people crazy. When the fungus claviceps
purpurea grows on grains used to make bread or beer, it
causes gangrene, nervous spasms, burning sensations, hallucinations
and temporary insanity. One compound in this
fungus, lysergic acid, is the raw material used to make LSD. Finally, there’s the destruction that some fungi bring onto other animals. More than six million
bats in North America have died since just 2007
due to a fungal disease called white-nose syndrome, and
a fungus has been implicated in several extinctions of amphibians and threatens many more,
perhaps as many as 1/3 of all amphibians on Earth. But, none of this is what I mean when I talk about fungi and death. While some members of the
fungus family are total bummers, all of them together perform perhaps the most vital function
in the global food web. They feast on the deceased
remains of almost all organisms on the planet, and by
doing that, they convert the organic matter that we’re all made of back into soil from which
new life will spring. So fungi, they thrive on death, and in the process,
make all life possible. (lively music) Aha, you didn’t expect to
see me in the chair so soon, but before we go any deeper
into the kingdom fungi, I wanted to make a toast to Louis Pasteur in the form of a biolo-graphy. (lively piano music) By Pasteur’s time, beer had been brewed for thousands of years in
cultures all over the world. Some experts think it may
have been the very reason that our hunter/gatherer
ancestors started farming and cobbled together
civilization in the first place. But for all those
millennia, no one understood how its most important ingredient worked. Until brewers could actually
see what yeast were doing, the magic of fermentation
was essentially magic. Pasteur himself was
never a big beer drinker, but part of his academic
duties in France required him to help find solutions to problems for the local alcohol industry,
and as part of this work, in 1857, he began studying
yeast under a microscope and discovered that they were
in fact living organisms. In a series of experiments
on the new found creatures, he found that in the
absence of free oxygen, yeast were able to obtain energy by decomposing substances
that contained oxygen. We now know that Pasteur was
observing yeast undergoing the process of anaerobic
respiration, aka fermentation, breaking down the sugars in
grains like malted barley and converting them into
alcohol, carbon dioxide and the range of flavors
that we associate with beer. Along the way, Pasteur also
discovered that beer was often contaminated by
other bacteria and fungi. The growth of these
beer-spoiling microbes he found could be thwarted for up to 90 days by keeping the beer between
55 and 60 degrees Celsius for a short period of time. Today, we call that heating
process pasteurization, and it’s used in everything from milk to canned foods to syrups to wines. For our purposes, the
thing to hold onto here is Pasteur discovered that
yeasts decompose sugars to get energy, and it
turns out, most fungi spend most of their time decomposing
all kinds of organic matter. Often the matter is dead
when fungi get to it but not always. When a tree or a person
or a deer keels over, fungi move in and start
the work of decomposition. Same goes for that orange you forgot at the bottom of the fruit bowl. If it weren’t for this fungal function, plants and the animals that
eat them couldn’t exist because the elements that they take from the soil would never return. Thankfully, the decomposition
performed by fungi recycles the nutrients for the enjoyment of plants and animals as
well as for other fungi. All of this points to
one of the main traits that all fungi have in common. From single-celled yeast to
giant multicellular mushrooms, fungi, like us, are heterotrophs,
but instead of eating, they absorb nutrition
from their surroundings. They do this mostly by
secreting powerful enzymes that break down complex molecules into smaller organic compounds which they use to feed,
grow and reproduce. Most multicellular fungi contain networks of tiny tubular filaments called hyphae then grow through and within
whatever they’re feasting on. Unlike plant cell walls
which are made of cellulose, the cell walls of fungi are strengthened by the nitrogenous carbohydrate chitin. The same material found
in the exoskeletons of insects, spiders and other arthropods. The inner woven mass of hyphae that grows into the food source
is called the mycelium, and it’s structured to
maximize its surface area which, as we’ve learned,
in both plants and animals is the name of the game when
it comes to absorbing stuff. Mycelia are so densely packed
that one cubic centimeter of rich soil can contain enough hyphae to stretch out one kilometer
if you laid them end-to-end. As hyphae secrete the digestive enzymes, fungi use the food to
synthesize more proteins, and the hyphae continue
to grow allowing the fungi to conquer new territory
and grow even more. As a result, fungi can get
crazy big, record holding big. A single honey mushroom in
the Blue Mountains of Oregon is thought to occupy some 2,386 acres. By area, the largest
organism on the planet. Now, there are all kinds of crazy ways that fungi are classified,
but probably the easiest and most useful is organizing them by how they interact with other organisms. The straight up decomposers
that break down dead stuff. The mutualists which form
beneficial relationships with other organisms especially plants. Then, there are the
predators and the parasites. Decomposer fungi secrete
enzymes that break down and absorb nutrients from
nonliving organic material such as that tree that nobody
heard fall in the forest. In fact, the ability of
fungi to break down lignin, which is like what makes wood woody, and break it into glucose
and other simple sugars is crucial for the cycle of life. They’re pretty much the only
organism that can do that. They can even decompose proteins
into component amino acids. Basically, all the black bits
in the soil in your backyard are tiny fragments of former
plants digested by fungi. Mutualist fungi are a smaller group. Many have specialized
hyphae called haustoria that tangle themselves with plant roots for the benefit of both organisms. These guys help plants absorb nutrients, especially phosphates,
by breaking them down more efficiently than
the roots can themselves. In turn, the fungi send out their hyphae into the plant’s root
tissue and withdraws like a finder’s fee basically in
the form of energy rich sugars. These mutualistic relationships
are known as mycorrhizae from the Greek words (speaks
in foreign language) or fungus and (speaks in foreign language) or root. Mycorrhizae are enormously important in natural ecosystems as
well as in agriculture. Almost all vascular plants,
in fact, have fungi attached to their roots and rely on
them for essential nutrients. Growers of barley, the
main ingredient in beer, will even inoculate barley seed beds with specific mycorrhizal
fungi to help promote growth. Other fungi aren’t nearly
so kind to their hosts. Predatory fungi actively
capture prey with their hyphae. The soil fungus arthrobotrys
uses modified hoops on its filaments to snare nematodes and absorb their inner tissue. Then, there are the parasites. Those fungi that feed on living organisms without killing them at least for a while. Take one of my personal favorites, the zombie ant fungus or ophiocordyceps. It shoots spores into an ant where their hyphae grow into its body and absorb nutrients from
nonessential ant organs. When the fungus is ready to reproduce, it invades the ant’s brain
and directs it to march to a cool, moist location in the forest where its so-called fruiting spores erupt through the ant’s head to
spread even more spores, mmm. And just to prove that even
fungi have superheroes, in 2012, scientists discovered
that these zombie spores have themselves been targeted
by another parasitic fungus. Not a lot is known about
this ant saving fungus other than it sterilizes
many of the zombie spores through a process likened
to chemical castration. That is so messed up, weird. All right now, since I brought that up, we should talk briefly about fungus sex. Fungi reproduce any way they can either sexually or asexually. Some species even do it both ways. But whichever way they choose,
most propagate themselves by producing enormous numbers of spores much like we saw in nonvascular plants and the simplest vascular
plants the ferns. But, and this is a big but,
sexual reproduction in fungi isn’t like sex in any other
organism we’ve studied so far. The concepts of male and
female don’t apply here at all. Some fungi reproduce on their own. Others can reproduce
with any other individual that happens to be around,
and still others can only mate with a member of a different
so-called mating type. They’re not different sexes. They just have different
molecular mechanisms that either make them compatible or not. Sometimes these types
are called plus and minus and other times one and two. In any case, it’s still
considered sexual reproduction because each parent
contributes genetic information when they make with the spore making. It all starts with this
beautiful chemical mating dance as the mycelium from one
fungus sends out pheromones that are picked up and bound to receptors by another willing and able partner. This binding compels each mycelium to send its hyphae toward the other. When they meet, they fuse
the cytoplasm of their cells. A stage of reproduction called plasmogamy. Some time between hours
and centuries later, yes, it can literally
take hundreds of years for fungi to have sex, this
union leads to the production of spores that each fungus
is then able to disperse. Certain types of fungi,
including the tasty morel, produce spores in a saclike asci contained in fruiting
bodies known as ascocarps. That is the part that you pick when you’re wandering through the forest. Some fungi shoot their
spores off into the breeze. Other spores float away on the water. More enterprising spores will hitch a ride on passing critters
hopefully to be dropped off somewhere where there’s
plenty of nutrients to absorb, so they too can grow,
send out sexual pheromones when their time comes and let
their hyphae do the tango. Finally, for some fungi,
sexual reproduction just isn’t all it’s cracked up to be. They’d rather just get
on with it themselves. Some of these grow filamented structures that produce spores by mitosis. These structures are visible,
and they’re called molds, the stuff on the orange in
the bottom of the fruit bowl or on the heel of the piece of bread that you left for a roommate
who decided to leave it for the other roommate who
though that you’d rather have it. In the unicellular yeast, the
asexual reproduction occurs by old fashioned cell division
or the formation of buds that get pinched off
into separate organisms. Since some species of yeast,
like our beer making friend saccharomyces cerevisiae,
convert sugars into alcohol, brewers create conditions
that encourage high rates of yeast production like
giving them lots of sugar and oxygen since more
yeast means more alcohol. So yeah fungi, they feast on
death, and they can make us go insane and turn ants into
unholy zombies of the night, but because of their hard
work and strange ways, they make possible stuff like agriculture and beer and everything
else worth living for.

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