Chapter 3: Cell Structure and Function

Welcome to the Deep Dive.

Today, we're really diving into something fundamental,

microbiology basics for healthcare.

We're looking at chapter three, the cellular building blocks, basically the tiny life forms that make such a big difference in patient outcomes.

Yeah, this is absolutely essential stuff.

Forget just memorizing terms.

We're looking for the practical differences.

You know, how is a bacterial cell actually different from one of ours?

Because understanding that structure, how it eats, how it works, that's the foundation for diagnosing infections and maybe more importantly, figuring out how antibiotics can work.

Exactly.

We need to know their weak spots.

That's the mission.

Find the targets.

So we'll walk you through the key parts, the cell structures, how they get energy, how they move, and crucially, how they share information, especially resistance information.

We'll be talking peptidoglycan, ribosomes, all that good stuff.

Let's get started.

Okay.

Basics first, the cell theory, right?

The cell is the smallest unit of life.

All living things are made of cells and cells only come from other cells.

And the first big division we always talk about, prokaryotes versus eukaryotes.

Right.

Prokaryotes think bacteria, no nucleus wrapped in a membrane.

Eukaryotes like our cells, fungi, protists, they do have that membrane bound nucleus.

But you know, despite that huge difference, they share some core things.

Both use DNA and RNA for their genetic blueprint.

Both need a plasma membrane to control what gets in and out.

Both have ways to make energy metabolism.

But it's those differences, especially right at the surface that we often target.

Okay.

Let's zoom in on that surface.

The plasma membrane first.

You can picture it, right?

That phospholipid bilayer, those water living heads pointing out, the water heating tails tucked inside.

It's super thin, maybe eight nanometers.

And even there, you see subtle but important differences.

Our cells, eukaryotic cells, embed cholesterol in that membrane.

It helps control how fluid or stiff it is.

You know, it's permeability.

But prokaryotes don't usually have cholesterol.

Right.

Instead, many use similar molecules called hoponoids.

They basically do the same job stabilizing those fatty acid tails, making the membrane a bit less permeable.

It's a neat example of a structural substitution.

Okay.

So membrane is the inner boundary.

Then outside of that, at least for Exactly.

And the key component, the thing that really defines bacterial cell walls,

is peptoglycan.

It's this mesh -like polymer sugars and peptides linked together.

And how much peptidoglycan they have is the whole basis for gram staining.

Ah, right.

Let's visualize that.

Gram positive bacteria.

Think thick and relatively simple.

They have this really substantial peptidoglycan layer, could be 20, even 80 nanometers thick, sitting right outside the membrane.

And woven into it are things like tycoic acids, which are unique to them.

Okay.

Thick wall.

Now gram negative.

More complex, definitely.

Their peptidoglycan layer is much thinner.

Maybe only five or 10 nanometers.

Oh, wow.

Much thinner.

Yeah.

And it's sandwiched.

You have the inner plasma membrane, then this thin peptidoglycan layer, and then another membrane outside of the outer end.

And that outer one, it's a big deal, isn't it?

Huge deal.

It's loaded with lipopolysaccharides, LPS.

Which are endotoxins.

Precisely.

Bacterial endotoxins.

And here's the clinical tie -in.

If you hit a bunch of gram negative bacteria hard with antibiotics, they break open and release a flood of this LPS.

That LPS acts like a powerful alarm signal to our immune system, causes fever, inflammation, and if it's a massive release, it can trigger endotoxin shock.

A really dangerous systemic reaction.

So killing the bacteria can, paradoxically, make the patient acutely worse in the short term.

It's a critical consideration.

Plus, that outer membrane has defenses, little protein channels called porins.

They let small nutrients through but can block bigger molecules.

Like some antibiotics.

Exactly.

Some larger antibiotic molecules just can't get through those porins easily.

It's a built -in resistance mechanism.

And you mentioned that space between the two membranes in gram negatives.

The periplasmic space.

Yeah, that's a really interesting little compartment.

It's packed with enzymes.

Some are for nutrient processing, but others are for defense.

Like detoxification.

Exactly.

And critically, this is often where bacteria stash enzymes like beta -lactamase.

The enzyme that chews up penicillin and related antibiotics.

That's the one.

So the resistance mechanism is literally located right there in that protected space, ready to neutralize the drug before it even gets inside the cell properly.

Okay, so we've got the barriers down.

How do these guys move and stick to things?

Appendages, right?

Right.

You've got flagella, these long whip -like tails for swimming.

Helical filaments.

And then pili or fimbriae.

Yeah, those are shorter, more rigid, hair -like things.

They're mainly for sticking adhesion.

Sticking to surfaces, sticking to our cells.

And then there's a special type, the sexpelis, used for, well, bacterial conjugation, sharing DNA.

We'll definitely come back to that.

So the flagella let them move?

Is it just random swimming?

Not usually, no.

It's often directed.

The flagella rotate, kind of like a propeller.

This allows the bacterium to do a run move purposefully or tumble to change direction, and they move in response to stimuli.

That's taxis.

Exactly.

Movement towards or away from something.

Chemotaxis, if it's a chemical signal, like moving towards food or away from a toxin.

Phototaxis, if it's light.

And that ability to stick using pili and other things leads us to biofilms.

Ah, biofilms, yes.

Clinically, these are a huge challenge.

Think of them as organized communities of microbes.

They stick to a surface.

Like a catheter or a heart valve or teeth.

Any surface, really.

First, it's kind of a loose attachment, maybe reversible.

Then they anchor down more firmly, often using those pili's.

Then they start multiplying.

And crucially, they secrete this protective slime layer around themselves, an extracellular matrix, usually made of polysaccharides.

A shield.

A very effective shield.

The NIH estimates something like 80 % of chronic microbial infections involve biofilms.

And the scary part is the resistance.

How resist - The bacteria living inside a biofilm can be up to a thousand times more resistant to antibiotics than the same bacteria floating freely.

A thousand times why?

Multiple reasons.

The matrix itself can act as a physical barrier, preventing the drug from even reaching the cells deep inside.

Plus, the cells within the biofilm often slow down their metabolism, they're less active, and many antibiotics target processes in actively growing cells.

Like penicillin targets cell wall synthesis, if they're not actively building walls.

The penicillin is much less effective.

And the matrix can also protect them from immune cells like phagocytes.

They're really tough nuts to crack.

Understanding that structure is definitely key, then.

Okay, let's move inside the cell.

The cytoplasm.

It's mostly water, right?

Kind of a gel where all the chemistry happens.

Yeah, 70 -80 % water.

It's crowded with ribosomes, enzymes, nutrients, waste products.

In eukaryotes, you also have a complex internal scaffolding.

The cytoskeleton actin filaments, microtubules give shape, helps move things around involved in cell division.

Prokaryotes have simpler cytoskeletal elements, but the cytoplasm is still the main arena for biochemistry.

And eukaryotes have those specialized compartments, the organelles, the nucleus obviously holding the DNA.

Control center, yeah.

With its own double membrane, the nuclear envelope, and the nucleolus inside where ribosomes are made.

And the power plants.

Yeah.

Mitochondria.

Right.

Double membrane again, the air one folded into cristae to maximize surface area for energy production.

They're the site of aerobic respiration in eukaryotes.

Interesting tidbit,

mitochondria actually have their own DNA and ribosomes.

Oh, right.

And those ribosomes are different, aren't they?

This is important.

Very important.

This is a key point for selective toxicity.

Eukaryotic cells, our cells, have ribosomes that are described as ADS in size.

They're built from 60S and 40S subunits.

Okay.

ADS and S.

What about bacteria?

Bacteria, prokaryotes have smaller ribosomes.

They're 70S made from 50S and 30S subunits.

70S versus 80S.

That difference sounds small, but it's huge therapeutically.

It's fundamental.

This is exactly what many antibiotics exploit.

Drugs like macrolides think erythromycin or azithromycin or aminoglycosides like tryptomycin or gentamicin.

They are designed to bind specifically to parts of that bacterial 70S ribosome.

But not our ADS ribosomes.

Ideally, yes.

They interfere with bacterial protein synthesis, stopping the bacteria from making essential proteins while leaving our own protein synthesis machinery relatively untouched.

It's a beautiful example of targeting a specific molecular difference.

That makes sense.

Okay.

So these internal factories where the ribosomes or mitochondria all exist within these fluid environments, inside and outside the cell, and the membrane manages that, right?

Transport.

Absolutely.

The membrane is the gatekeeper.

We talk about fluid compartments.

The intracellular fluid, ICF, inside the cells, that's about two thirds of our body water, rich in potassium and phosphate.

And the extracellular fluid, ECF, outside plasma, lymph, the fluid between cells, which is high in sodium and chloride.

The membrane maintains these differences.

How does stuff get across?

Passive transport.

Right.

Passive means no energy required from the cell.

It's driven by concentration gradients.

Things move from where they're more concentrated to where they're less concentrated.

Simplest form is diffusion, just molecules spreading out.

But some things need help.

Yeah.

Facilitated diffusion.

Still passive, still down the gradient, but requires a helper protein.

Either a channel protein that forms a pore or a karyo protein that binds the molecule and changes shape to move it across.

These carriers can get saturated, like turnstiles, if the concentration gets too high.

And osmosis is passive, too, right?

Water movement.

Exactly.

Osmosis is specifically the diffusion of water across a selectively permeable membrane.

Water moves to try and dilute the side with more solutes, more dissolved stuff.

So if you put a cell in a very salty solution, hypertonic, water rushes out.

Put it in pure water, hypertonic, water rushes in, maybe even bursting it.

Okay, that's passive.

What about active transport?

Active means the cell has to spend energy, usually ATP.

Why?

Because it's moving something against its concentration gradient, from low concentration to high, like pumping water uphill.

The classic example is the sodium potassium pump.

Perfect example.

It uses ATP to actively pump three sodium ions out of the cell for every two potassium ions it pumps in.

This maintains those crucial concentration differences and the electrical charge across the membrane.

And then there's moving really big stuff, bulk transport.

Right.

For things too large for pumps or channels, endocytosis is bringing things into the cell.

Phagocytosis is cell eating, engulfing large particles like a bacterium.

Penocytosis is cell drinking, taking in droplets of extracellular fluid.

And receptor -mediated endocytosis is very specific, using receptors on the surface to grab particular molecules before engulfing them.

And the reverse, getting stuff out.

That's exocytosis.

Vesicles inside the cell fuse with the plasma membrane and release their contents, maybe waste products, maybe hormones or neurotransmitters to the outside.

Okay, structure, transport, now energy, metabolism, how they power all this.

Metabolism is just the sum of all the chemical reactions.

We split it into catabolism, breaking down complex molecules, which usually releases energy, and anabolism, building up molecules, which usually requires energy.

And where they get that initial energy divides them too.

Yeah.

Chemotrophs versus phototrophs.

Exactly.

Chemotrophs get energy from breaking chemical bonds in nutrients.

Phototrophs get energy from sunlight, like plants.

And all these reactions rely on enzymes.

Pretty much all of them, yeah.

Enzymes are catalysts, mostly proteins.

They speed up reactions dramatically by lowering the energy needed to get the reaction started, the activation energy.

Each enzyme is typically very specific.

It have an active site with a unique shape that fits only its specific target molecule, the substrate, like a lock and key.

Some need helpers, though.

Coenzymes are cofactors.

Right.

Some enzymes called apoenzymes are inactive on their own.

They need a non -protein helper.

If it's an organic molecule, often derived from a vitamin, we call it a coenzyme.

If it's an inorganic ion, like magnesium or zinc, it's a cofactor.

When the coenzyme binds its helper, it becomes the active hollow enzyme.

And we can interfere with enzymes.

That's key for drugs.

Absolutely key.

Enzyme inhibition is a major strategy.

Things like temperature and pH affect enzyme activity, of course.

But we can also use inhibitor molecules.

Two main types.

Competitive and non -competitive.

Yep.

Competitive inhibition is when the inhibitor molecule resembles the actual substrate and literally competes with it for binding to the site.

If the inhibitor gets there first, the substrate can't bind and the reaction stops.

Okay.

Direct competition.

Non -competitive.

Non -competitive is sneakier.

The inhibitor binds to the enzyme at a different location, not the active site.

This other location is called the allosteric site.

Binding there causes the enzyme to change its overall shape.

Which changes the shape of the active site.

Exactly.

So even though the inhibitor isn't blocking the active site indirectly, it changes it so the substrate no longer fits properly.

The reaction is inhibited indirectly.

Do we have a good clinical example of this?

Oh, a classic one.

Sulfa drugs, sulfonamides.

These are a prime example of competitive inhibition.

How do they work?

Okay.

Many bacteria need to synthesize their own folic acid, which is essential for making DNA and RNA.

One crucial step uses a molecule called pebea.

Sulfa drugs look very, very similar to pebea.

So they're pebe mimics.

Exactly.

They act as anti -metabolites.

They compete with the real pebea for the active site of the enzyme involved in that step.

If the sulfa drug binds, the enzyme is blocked, the bacteria can't make folic acid, and they can't grow or reproduce.

And since we get folic acid from our diet, it doesn't harm ourselves in the same way.

Selective toxicity again.

Precisely.

It selectively targets a pathway essential for the bacteria, but not for us.

Okay.

So that's how they run reactions.

Yeah.

How do they get the energy, the ATP, to power everything, especially through aerobic respiration?

Right.

Aerobic respiration is the main ATP generator if oxygen is available.

Starting with glucose, it's typically broken down in three main stages.

Goal is to get theoretically up to maybe 38 ATP per glucose molecule.

The other one.

Glycolysis happens out in the cytoplasm.

Glucose, a six -carbon sugar, gets split into two molecules of pyruvic acid, which is a three -carbon molecule.

This yields a little bit of ATP directly a net of two ATP and some energy carriers called NADH.

Okay.

Glucose to pyruvic acid.

Then?

Then, if oxygen is present, that pyruvic acid moves into the next stage, often called the Krebs cycle, or a citric acid cycle.

It gets converted into acetyl -CoA first, then enters the cycle.

The cycle basically finishes breaking down the carbons, releasing them as CO2.

It doesn't make much ATP directly, but it generates a lot more energy carriers, more NADH, and another one called FADH2.

So glycolysis and Krebs break down the fuel and load up these energy carriers.

Stage three is the payoff.

That's the electron transport chain, or ETC.

This is where the big ATP harvest happens.

In our cells, it's in the mitochondria.

In bacteria, it's embedded in their plasma membrane.

How does it work?

Those carriers, NADH and FADH2, drop off high -energy electrons at the start of the chain.

The electrons get passed down a series of protein carriers, like a bucket brigade.

Each step releases a bit of energy.

And that energy is used to make ATP?

Yes.

That energy is used to pump protons, H +, across the membrane, creating a strong gradient.

Then, those protons flow back across the membrane through a special enzyme called ATP synthesis, and the energy of that flow is captured to make lots of ATP.

And oxygen's role?

Oxygen is the final electron acceptor.

At the very end of the chain, it takes the used low -energy electrons and combines with protons to form water.

Without oxygen to take those electrons, the whole chain backs up.

So no oxygen, no big ATP payoff from the ETC.

What do they do then?

They have alternatives.

Anaerobic respiration is similar to aerobic, but uses a different inorganic molecule instead of oxygen as the final electron acceptor, maybe nitrate or sulfate.

Still uses an ETC, yields less ATP than aerobic.

And fermentation?

Fermentation is quite different.

It doesn't use an ETC.

After glycolysis makes pyruvic acid, fermentation pathways just use an organic molecule, often derived from the pyruvic acid itself, as the final electron acceptor.

The main goal is just to regenerate the NAD plus needed to keep glycolysis running.

So much less ATP?

Way less.

Only the couple of ATP made during glycolysis.

But fermentation produces characteristic waste products, things like lactic acid, ethanol, acetic acid.

Different microbes do different types of fermentation, which we can actually use to help identify them in the lab.

Okay, makes sense.

Last big area, genetics.

How they store and use information and how they change.

The central dogma.

Right.

The flow of genetic information.

DNA makes, RNA makes protein.

Transcription is the first step.

DNA to RNA.

Yep.

A specific gene segment on the DNA is copied into a messenger RNA, mRNA molecule.

The enzyme RNA polymerase does this.

It finds the start signal, the promoter sequence on the DNA, unwinds the DNA locally, and builds the mRNA copy, stopping when it hits a terminator sequence.

Then translation.

RNA to protein.

That's where the mRNA message is read by ribosomes.

The mRNA sequence is read in three letter words called codons.

Transfer RNA, tRNA molecules, each carrying a specific amino acid and having an anticodon that matches an mRNA codon, bring the correct amino acids in order.

The ribosome links them together into a polypeptide chain, which then folds into a functional protein.

And how do they pass this DNA on replication first?

DNA replication has to happen before the cell divides.

It's semi -conservative.

The DNA double helix unwinds and each strand serves as a template to build a new complementary strand.

So each new DNA molecule ends up with one old strand and one brand new strand.

Then cell division.

Very different in prokaryotes and eukaryotes.

Very different.

Prokaryotes mostly do binary fission.

Simple, rapid division.

The cell just replicates its single circular chromosome, grows bigger, and splits into two genetically identical daughter cells.

Very efficient.

Eukaryotes are more complex.

The cell cycle.

Right.

A regulated cycle with phases, G1 growth, sDNA synthesis replication, G2, more growth, prep for division, and M, mitosis or meiosis.

Mitosis is for normal body cell division.

It carefully separates the replicated chromosomes, so each daughter cell gets a complete identical set, maintains the chromosome number.

Meiosis is special.

It's for producing gametes, sperm and eggs and animals.

It involves two rounds of division and reduces the chromosome number by half, creating haploid cells.

Crucially, meiosis introduces genetic variation through crossing over, swapping bits between homologous chromosomes and random shuffling of chromosome.

Variation is key.

That brings us to how bacteria change and adapt so quickly.

Genotype versus phenotype.

Good distinction.

Genotype is the actual genetic makeup the DNA sequence an organism has.

Phenotype is the observable characteristics, what it looks like, how it behaves, its biochemistry.

And remember, the environment can influence the phenotype even without changing the genotype.

That's phenotypic plasticity.

But the genotype can change.

Mutations.

Mutations are the ultimate source of new genetic variation.

Their changes in the DNA sequence can be spontaneous, just errors during replication, or induced by mutagens like radiation or certain chemicals.

What kinds matter most?

Even small changes can have big effects.

Point mutations are changes in a single DNA base.

Sometimes they're silent and don't change the amino acid.

Sometimes they change one amino acid.

Missense.

Sometimes they create a premature stop signal nonsense, which is usually bad.

And frameshift mutations.

Those are often catastrophic.

They happen if you insert or delete one or two bases.

Because the DNA is read in groups of three codons, adding or removing a base shifts that whole reading frame downstream.

Usually leads to a completely garbled protein sequence from that point on.

Then there are bigger jumps.

Transposons.

Yeah, transposons or jumping genes.

These are segments of DNA that can actually cut themselves out of one location in the genome and insert themselves somewhere else.

And the scary part is, they can sometimes carry other genes with them.

Like antibiotic resistance genes.

Exactly.

A transposon carrying a resistance gene can jump from the chromosome onto a plasmid, or vice versa, spreading resistance within the cell's DNA.

And bacteria are notoriously good at sharing genes, right?

Morosomal gene transfer.

This is critical for resistance spread.

Absolutely critical.

It's how resistance can spread so rapidly, even between different species of bacteria.

Three main ways they do this.

Okay, number one.

Transformation.

This is when a bacterium takes up naked DNA fragments directly from its environment.

Maybe released from dead, salized bacteria nearby.

If that DNA fragment contains a useful gene, like for resistance, the recipient cell can incorporate it into its own genome.

Number two.

Transduction.

This involves viruses that infect bacteriophages or phages.

Sometimes when a phage is assembling new virus particles inside a bacterium, it accidentally packages a piece of the bacterial DNA instead of its own viral DNA.

When this faulty phage then infects a new bacterium, it injects that piece of bacterial DNA from the previous host.

So the virus acts like a delivery service for bacterial genes?

Pretty much, yeah.

And number three.

Conjugation.

The bacterial sex one.

Sort of.

It requires direct cell -to -cell contact.

One bacterium, the donor, extends a structure called a sexpelis to connect to a recipient cell.

Then it can transfer genetic material, usually a plasmid, a small circular piece of extra DNA,

directly into the recipient.

Plasmids very often carry resistance genes.

Wow.

So transformation, transduction, conjugation.

Three ways they swap useful DNA around.

And that ability, combined with their rapid reproduction and the selective pressure of antibiotics, is why resistance can emerge and spread so frighteningly fast.

Okay.

Let's try to wrap this up for someone studying this for healthcare.

What are the absolute must -knows from this chapter?

I'd say focus on the differences, especially at the surface.

The gram -positive versus gram -negative cell wall structure, that thick peptidoglycan, versus the thin layer plus the outer membrane with its LPS and porins.

Understand the LPsendotoxin risk.

Know about the paraplasmic space and beta -lactamase and gram -negatives.

And inside.

The ribosome difference.

70S and bacteria, 80S and eukaryotes.

That's the target for so many crucial antibiotics.

And then grasp the power of genetic change mutations, especially transposons carrying resistance, and the mechanisms of horizontal gene transfer.

That's how they adapt and fight back.

Right.

It all links back to how we can selectively target them.

Yeah.

But here's a final thought to chew on.

We talked about that 70S ribosome difference being key for antibiotics like erythromycin.

Crucial target.

But we also mentioned, kind of in passing, that our own mitochondria, the power plants inside our eukaryotic cells, also have 70S ribosomes.

Likely because of their evolutionary origins.

That's true.

They do.

Remnants of their bacterial ancestry.

So if you're using an antibiotic designed to hammer bacterial 70S ribosomes, what might the unintended, maybe long -term, consequences be for the patient's own mitochondrial function and energy production, even if the drug is successfully killing the bacteria?

That's, yeah, that's a really important consideration in pharmacology and toxicology.

It highlights that even selective targets might not be perfectly selective.

Something definitely worth thinking about as you learn more about these drugs and patient care.

Absolutely.

Well, that brings us to the end of this deep dive into the cellular building blocks.

We hope this helps solidify those core concepts.

Thanks for tuning in.

We'll catch you in the next one.