Chapter 9: Fungal Physiology and Metabolism

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Have you ever looked at a mushroom popping up or maybe, you know, some fuzz growing on old bread in the fridge and just wondered, what is actually going on inside there?

Right.

They seem so simple, maybe even a bit alien.

But underneath, there's this whole hidden world,

incredibly complex life, really.

Yeah, a whole kingdom humming with tiny machines and pretty surprising survival tricks.

Absolutely.

Fungi are, you could say, master chemists doing things few other organisms can.

It's a fascinating internal world.

Exactly.

And that's what we're doing today.

Welcome to the deep dive.

Yeah.

We're pulling back the curtain on fungal physiology and metabolism.

We're using a great chapter from the fifth kingdom by Bryce Kendrick as our guide.

Think of it as a clear, engaging tour inside the fungal cell.

Our mission today to give you a solid handle on their building blocks, how they get energy, how they grow, reproduce, all that stuff.

And how knowing this helps us fight fungal infections, for example, will describe everything so you can picture it, even the microscopic bits.

Yeah, no diagrams needed.

And this isn't just for scientists.

If you're curious about biology, medicine, even just the world around you.

You'll definitely have some aha moments.

It might change how you see that fuzzy bread, right?

OK, let's start with the basics, the building blocks.

What are fungi actually made of?

Well, like all complex life, they use familiar stuff, but with their own twists.

First up, proteins, the real workhorses.

Proteins, right, big molecules.

Huge, complex molecules.

Think of them like long chains made from 20 different types of amino acids all linked together.

OK.

And the key thing is how these chains fold up into very specific 3D shapes.

That shape is everything.

Like origami, kind of.

The fold determines the function.

Exactly.

That precise fold creates little pockets, docking stations, you could call them, or active sites, crucial for enzymes, which speed up reactions.

Guys, so enzymes are proteins?

Many are, yes.

Proteins also provide structure, hold things together, and they can team up with other molecules, too, like nucleic acids or carbohydrates.

All right, let's talk nucleic acids, then.

The genetic code stuff.

DNA and RNA.

Right.

DNA is the master blueprint, that famous double helix, like a twisted ladder.

The wungs are made of chemical bases linked to a sugar phosphate backbone.

And the sugar in DNA is deoxyribone.

Correct.

And the sequence of bases.

That's the code.

Every three bases, a codon tells the cell which amino acid to add when making a protein.

And RNA.

It's a bit different.

Yeah, usually single -stranded.

Uses a different sugar, rebose, and swaps one base, cyanine, for uracil.

RNA has several jobs.

Messenger RNA, mRNA, carries the instructions from DNA to the protein -making spots.

Transfer RNA, tRNA, brings the amino acids.

And ribosomal RNA, RNA, actually forms the ribosomes, the protein factories themselves.

Where do fungi keep all this?

Mostly the nucleus.

Mostly the nucleus, yes.

But also some DNA in the mitochondria, those power plants in the cell, which is a cool hint about their ancient origins, actually.

Interesting.

And fungi have what?

Less DNA, but more RNA.

Yeah.

Generally, yes.

Lower DNA content compared to many organisms, but lots of RNA, especially ribosomal RNA, shows they're busy making proteins.

Okay, next building block.

Carbohydrates.

Sugars, basically.

Sugars and polymers of sugars, yeah.

Long chains.

They're used for structure and storing energy.

Think, CH2O -hydrated carbon.

And the big one for fungi is chitin, right?

In the cell wall.

Chitin, exactly.

A really strong polymer.

Forms the main structural part of the cell wall in most true fungi, like the Dikaria.

It's tough, protective.

Like an insect's shell.

Kinda, yeah.

But it's worth noting, some fungus -like organisms, the umicids, they use something more like cellulose instead.

And for energy storage, not just structure.

Right.

Their main storage carb is glycogen, just like in animals, actually.

They also use a sugar called trehalose and sugar alcohols like mannitol for quick energy access.

Cool.

Last one.

Lipids.

Fats and oils.

Yep.

Characterized by these long hydrocarbon chains.

Used for energy storage, obviously, but critically important for building cell membranes.

Like phospholipids.

Phospholipids and single lipids, yes.

They form the membranes that enclose the cell and all its internal compartments.

Keep everything organized.

And you mentioned something specific for fungi here.

Isoprenoids.

Ah, yes.

Isoprenoid lipids.

These are built from repeating five carbon units, isoprene, streamed them together in different ways.

I need you.

Things like terpenes, carotenoids, which give some fungi their color, are in this group, and importantly, sterols.

Sterols.

Like cholesterol in this.

Similar idea, but the main one in fungi is ergosterol.

It's absolutely vital for their membrane structure and function.

And, spoiler alert, it's a key target for anti -fungal drugs.

Ah, okay.

I couldn't remember ergosterol.

So those are the building blocks.

Now, how do fungi actually use them?

How do they power themselves?

Metabolism.

Exactly.

Metabolism is just the sum total of all the chemical reactions keeping the fungus alive.

Two sides to it.

Enabolism.

Dueling things up.

Like making more fungus.

Right.

Converting food into fungal biomass.

And the other side is catabolism breaking things down to get energy, producing ATP, the energy currency, and other useful molecules like NADH.

And driving all this are enzymes again.

Enzymes are key.

They speed up reactions incredibly, like millions of times faster, making life possible, really.

They often work in teams in sequences called metabolic pathways.

And fungi have some special enzymes.

You mentioned breaking down tough stuff.

They do.

They're amazing at producing enzymes like ligninases and celluloses.

Lignin is that super tough woody stuff in plants and cellulose is the main fiber.

Very few organisms can break those down.

But fungi can.

Make some great decomposers, right?

Absolutely essential decomposers.

Okay.

So how do they get energy from simple sugars like glucose?

Mainly two pathways.

The Emden -Meierhoff pathway gives them ATP directly.

Useful energy.

The other, the Hexosu monophosphate pathway, is more about producing building supplies That's NADPH for making fats and ribose for making DNA and RNA.

Got it.

Two routes for glucose breakdown.

And most fungi use oxygen.

Like us.

Most do, yes.

Aerobic respiration happens largely in the mitochondria.

There's the citric acid cycle breaking down carbon compounds and then the electron transport chain which generates a lot of ATP using oxygen.

Okay, standard respiration.

But you mentioned antifungals targeting this.

Precisely.

Some drugs mess with that electron transport chain or uncouple energy production.

It's a vulnerability.

But what if there's no oxygen?

Like deep in the soil maybe?

Good question.

Then they switch gears.

Fermentation.

It's a way to keep some energy flowing without oxygen.

Producing alcohol.

Like yeast does.

Alcohol and CO2 is a classic fungal fermentation, yeah.

Famous in yeast like saccharomyces, but other fungi do it too.

Or some produce lactic acid, especially umicetes and zygomycetes.

It's their backup plan.

Adaptable.

Okay, so that's energy.

What about building their own complex molecules?

Biosynthesis.

You mentioned something really interesting about lysine.

Ah yes, the lysine story.

This is fascinating from an evolutionary perspective.

There are two completely different ways fungi make the essential amino acid lysine.

Two separate pathways.

Through the same molecule.

Totally different biochemical routes.

Umicotan fungi, the true fungi, use one pathway.

The AAA pathway.

Umicetes.

Plants.

Algae.

They use a different one.

The DAP pathway.

Wow.

That sounds like a major evolutionary split.

It really is.

It's strong evidence that these groups have very distinct ancient origins.

And animals, like us, we can't make lysine at all.

We have to eat it.

We're lysineoxytrophs.

It shows how different fungi are.

That is really cool.

Okay, what about building their walls?

The hyphal wall.

You said chitin is key.

Right.

The wall is crucial.

It's their skeleton, providing strength and protection.

For most true fungi, chitin is the main ingredient.

They link sugar monomers together enzymatically to build those strong chains.

And they need nutrients to do all this building.

Right, where do they get them?

They're heterotrophs.

They can't make their own food from sunlight or CO2 or fixed nitrogen from the air like some bacteria can.

They need ready -made organic carbon sugars, cellulose, even lignin and nitrogen sources from their environment.

And they're good at getting it, especially breaking down those tough materials like lignin, cellulose, even keratin.

Exceptionally good.

That ability to degrade recalcitrant polymers is a fungal superpower.

Makes them vital recyclers.

Now, you also mentioned secondary metabolism.

Sounds like optional extras.

In a way, yes.

Secondary metabolites aren't strictly needed for basic growth.

They're often specific to certain fungi or life stages derived from primary metabolism building blocks, and they tend to accumulate.

And critically, they're often biologically active.

Like penicillin.

Yeah.

Or toxins.

Exactly.

Famous examples include penicillin, the immunosuppressant cyclosporine, toxins like aflatoxin, hallucinogens like psilocybin, all products of fungal secondary metabolism.

Why make them then if they're not essential for just living?

Good question.

The idea is when growth slows down, maybe food is running low, some metabolic pathways don't fully switch off, precursors build up.

The fungus then sense these into making these extra compounds.

So not just waste products.

They might have a purpose.

Very likely.

Penicillin helps penicillium fight off bacteria.

Aflatoxin might give aspergillus an edge against insects or animals.

They can be weapons, signaling molecules, part of the fungus's toolkit for interacting with its world.

Fascinating.

Okay, so that's the chemistry.

Let's move on to the life cycle.

Growth and reproduction.

How do we define fungal growth?

It's basically an irreversible increase in size, volume, but it also involves changes inside metabolism shape, function.

For typical fungi, it's about hyphal growth.

The branching threads.

Right.

Those threads hyphae elongate at their tips, branch out, lay down more wall material, and fill up with protoplasm and nuclei.

And you measure this how?

Colony size?

Weight?

You can, yes.

Dry mass, colony diameter on agar plates or cell counts if you're looking at yeasts.

Growth usually happens in stages, germination from a spore, then active feeding and growth assimilative growth, and finally making new spores.

Sporulation.

Let's talk spores.

You said they're a fungal trademark.

Absolutely.

They're key to survival and dispersal.

Fungi make tons of them, incredible variety, and spores often have a dormant period.

Dormancy, just waiting.

Waiting for the right conditions, yes.

That's exogenous dormancy imposed by the environment, but sometimes it's endogenous built in, like needing a cold spell or a specific chemical trigger, maybe even passing through an animal's gut for dung fungi.

And germination.

When they wake up.

It starts with taking up water, then stored food reserves get used, enzymes activate to grow off the wall, often at a weak spot called a germ pore.

A little germ tube emerges the first hypha, and it has to find food fast.

Then that hypha starts growing at the tip.

Exactly.

Elongation happens right at the hyphal apex, cytoplasm, nuclei, everything streams forward.

They pump out exoenzymes to digest whatever they're growing on, absorb the nutrients, and keep extending and branching.

Branching is key for exploring, right, leads to those circular colonies.

Crucial for exploring and exploiting the substrate, yes.

And the growth is highly localized.

New wall material is deposited almost exclusively right at or very near the tip.

How do we know that?

Clever experiments.

Using fluorescent antibodies that stick to new wall material or feeding them radioactive building blocks and seeing where they end up, it all points to the tip.

That's why you get fairy rings.

The fungus grows outwards from a central point.

Do hyphae have a lead tip, like a main branch?

Sometimes yes, apical dominance, just like in plants.

The main tip grows fastest.

Branching versus elongation can be influenced by nutrients like nitrogen or sulfur availability.

And what about those cross walls?

The septa?

The septa, found in most higher fungi, the Dicaria, they form like an iris diaphragm closing inwards, dividing the hyphae into compartments, but they usually have pores.

Pores, so things can still move through.

Yes, cytoplasm, organelles, even nuclei can often squeeze through the pores, so the whole fungal network, the mycelium, stays interconnected.

Okay, what about growth rates?

Fungi seem to grow fast sometimes.

Some grow incredibly fast, like Neurospora.

In ideal conditions, they enter an exponential growth phase, but in a closed system, like a lab culture, growth eventually slows and declines.

They run out of food or poison themselves with waste products that's staling.

And environment matters hugely, right?

Temperature.

Hugely.

Every fungus has its preferred temperature range.

Minimum, optimum, maximum, we group them.

Cycrophiles, cold -loving, mesophiles, middle range, most common, thermophiles, heat -loving.

Finding the optimum is tricky, though.

It can be.

The temperature that gives the fastest start might not yield the biggest final mass.

It's a trade -off.

Temperature also affects fruiting, dormancy, germination,

everything.

What about light?

Light's effects are really varied.

And it can inhibit growth, stimulate it, or trigger specific things like sporulation or fruiting body development.

Some fungi even bend towards light -positive phototropism, like Plobilis, the dung -canon fungus.

Timing its spores.

Exactly.

Aiming for open areas to disperse its spores.

And some fungi have internal clocks, circadian rhythms.

They show daily cycles of growth or sporulation, even if you keep them in constant darkness.

Wow.

OK, so after all this growth, they reproduce,

make spores.

Right.

When conditions change, food runs low, waste builds up, or they get an environmental cue, they switch from feeding mode to reproductive mode.

Sporulation.

Is that a complex process?

It can be quite specific.

Take Aspergillus niger.

It needs particular nutrient shifts to trigger the formation of its specialized spore -bearing structures step -by -step.

The conditions for making spores are often narrower than for just growing.

And the spores themselves are different chemically from the hyphae.

Often yes.

Spores might accumulate more storage compounds like trehalose or protective pigments like carotenoids compared to the vegetative mycelium.

They're packed for survival and dispersal.

Right.

Makes sense.

Now, what about fungal sex?

Is it all just spores?

Ugh.

The fungal love story.

It's often chemically mediated.

Fungi use diffusable substances broadly called hormones, sometimes pheromones when acting between individuals to trigger and coordinate sexual reproduction?

Chemical signals.

Like perfume.

Kind of.

In Allomyces, the female gametes release sirenin, which attracts the male gametes through the water.

The males even break it down so they can keep sensing the gradient.

Lover.

Any other examples?

Oh yes.

A clea, an oomocytes, has this amazing back and forth hormonal conversation.

Hormone A from the female triggers male structures.

Then Hormone B from the male triggers female structures.

A chemical dialogue.

A bing bong match.

Pretty much.

Zygomyces like Mekor use trisporic acid released by compatible partners to induce sexual structures.

Even yeasts like Saccharomyces use peptide hormones to stop each other from budding and make them sticky for conjugation.

Seems like hormones are pretty common, then.

Very likely the norm.

Evidence exists across Ascomycetes and Basidiomycetes, too, using various types of hormones.

Molecular tools are now letting us find these signals even in fungi, where we only knew the asexual stage before.

It's unlocking a lot of mysteries about fungal life cycles.

This deep understanding must be useful, right?

Especially when fungi cause problems.

Antifungals.

Absolutely critical.

Understanding fungal physiology allows us to design targeted antifungal compounds.

We've moved from just randomly screening chemicals to designing drugs that hit specific fungal weak points.

Like what kind of weak points?

Several key targets.

Some older compounds are general enzyme poisons, like heavy metals, but they're not very specific.

More targeted are the polyene antibiotics, like nystatin.

How do they work?

They bind specifically to ergosterol, that unique of fungal sterol in their membranes, and basically punch holes in the membrane.

Since we use cholesterol, they're relatively safe for us.

And bacteria or umesutes lacking sterols aren't affected.

So ergosterol is a major target.

A huge target.

Other drugs, sterol inhibitors, block the synthesis of ergosterol in the first place, disrupting membrane building.

What else?

The cell wall.

Keetin.

Yes.

Keetin synthesis inhibitors like polyoxins exist.

They interfere with the enzyme that builds ketin.

Though their practical use in medicine or agriculture can be limited sometimes.

Can we target protein making?

Or cell division?

Yes.

Cyclohexamide blocks protein synthesis by binding to ribosomes.

And drugs like Benamol disrupt nuclear division by binding to tubulin, a key protein in the mitotic spindle that pulls chromosomes apart.

Interestingly, Benamol doesn't work well on umesutes, showing subtle differences even there.

And finally, the mitochondria, the powerhouses.

Also a target.

Carboxans, for instance, interfere with mitochondrial respiration in many fungi, disrupting their energy supply.

So knowing the detailed biochemistry really lets us design specific weapons.

Exactly.

And the more we learn about fungal physiology, the better we can design highly specific antifungals that only harm the fungus, leaving other organisms untouched.

Plus, we might uncover new uses for all those unique fungal metabolites.

It's a huge field.

Okay, let's try to wrap this up.

What an amazing dive into the fungal engine room.

It really is.

We've seen fungi as master chemists with unique building blocks, diverse ways to get energy and build themselves up, really intricate growth focused at those high full tips.

And in complex reproduction, using spores and those fascinating chemical signals, the hormones for sex.

And we connected it to the practical side, how knowing all this helps us develop targeted antifungals and potentially harness fungal power in new ways.

So for you listening, this deep dive isn't just academic trivia, it's fundamental biology that impacts medicine, farming,

biotech.

Understanding this kingdom helps us solve real world problems and discover new solutions.

And here's something to think about.

We know so little about most fungi out there, right?

Especially the ones we've only seen in their asexual stage.

That's true.

The vast majority are still mysteries.

So imagine what other incredible biochemistry, what unique molecules, what hidden life cycles, and maybe even new medicines or industrial tools are waiting to be discovered if we just keep digging deeper into how fungi live and, well, talk to each other.

It's a frontier that's still wide open.

So much potential left in the fifth kingdom.

Keep exploring, keep questioning,

and keep learning.

Thank you for joining us on this deep dive into the hidden complexities of fungi.