Describe the bacterial cell wall and flagella

1. The Bacterial Cell Wall

WHY does a bacterium need a wall?

WHAT is peptidoglycan made of?

Glycan backbone: alternating sugars NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid), joined in long chains.
Peptide cross-links: short chains of amino acids hanging off each NAM, which bond to peptides on neighbouring strands.
Result: ONE giant bag-shaped molecule (the sacculus) wrapping the whole cell — strong in every direction.

HOW the wall divides bacteria into two groups

The Gram stain (crystal violet → iodine → alcohol wash → safranin) sorts bacteria by wall structure:

Feature	Gram-positive	Gram-negative
Peptidoglycan layer	Thick (many layers)	Thin (1–2 layers)
Outer membrane	Absent	Present (with lipopolysaccharide, LPS)
Gram stain colour	Purple (retains crystal violet)	Pink/red (takes up safranin)
Periplasmic space	Small	Large

Figure — Describe the bacterial cell wall and flagella

2. The Bacterial Flagellum

WHY is it built as a spinning corkscrew, not a whip?

WHAT are the three main parts?

Filament — the long external helix of flagellin; the "propeller blade".
Hook — a flexible joint connecting filament to motor.
Basal body — rings embedded in the membrane and wall forming the motor.

HOW is it powered? — derive the energy source

Formula Proton-motive force drives rotation

The motor is NOT powered by ATP directly. It is driven by the proton-motive force (pmf): protons ( $H^+$ ) flow back into the cell through the motor and turn it, like water turning a water wheel.

The pmf has two parts, derived from the free energy of moving one proton across the membrane: $\Delta G = RT\ln\frac{[H^+]_{in}}{[H^+]_{out}} + zF\Delta\psi$ Using $\text{pH} = -\log_{10}[H^+]$ and $\ln x = 2.303\log_{10}x$ , the energy per unit charge (the pmf, in volts) is: $\text{pmf} = \Delta\psi - \frac{2.303\,RT}{F}\,\Delta\text{pH}$ At 25 °C, $\dfrac{2.303RT}{F}\approx 59\text{ mV}$ , so: $\boxed{\text{pmf} = \Delta\psi - 59\,\Delta\text{pH}\ \ (\text{mV})}$ Reading it: a charge difference ( $\Delta\psi$ , inside negative) plus a proton concentration difference ( $\Delta$ pH, more protons outside) together store the energy that spins the motor.

Common mistake Steel-manning the common errors

"Bacterial flagella beat like a sperm tail / eukaryotic flagella." Why it feels right: both are called "flagella" and both cause movement, so surely they work the same. The fix: the eukaryotic flagellum has a 9+2 microtubule core that bends using ATP (dynein). The bacterial flagellum is a rigid rotating flagellin helix powered by proton flow. Same name, totally different machine.

"The cell wall controls what enters the cell." Why it feels right: it's the outermost barrier, so it seems like the gatekeeper. The fix: the wall is porous and mainly gives shape/anti-burst strength. The plasma membrane controls selective entry/exit.

"Peptidoglycan is made of pure protein because of 'peptido'." Why it feels right: "peptide" → protein. The fix: it is a glycopeptide — sugar chains (NAG/NAM) cross-linked by short peptides. Both sugar AND peptide.

Recall Feynman: explain to a 12-year-old

A bacterium is like a juice box that's too full — water keeps pushing in and it wants to pop. So it wears a tough net jacket (the cell wall) that holds it together and gives it shape. Some bacteria also have a tiny twisty tail (the flagellum) that spins like a boat propeller to push them toward food. The propeller doesn't run on a battery (ATP) — it runs on protons sliding through a little gate, like a water wheel turned by a stream.

Flashcards

What molecule gives the bacterial cell wall its strength?

Peptidoglycan (murein), sugar chains cross-linked by short peptides.

Which two sugars alternate in the peptidoglycan backbone?

NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid).

Why would a bacterium burst without its wall?

It lives in hypotonic surroundings; water enters by osmosis creating turgor pressure that the membrane alone can't resist.

Gram-positive cells appear what colour and why?

Purple — thick peptidoglycan traps the crystal violet–iodine complex during the alcohol wash.

Gram-negative cells appear what colour and why?

Pink/red — thin peptidoglycan + outer membrane lets alcohol wash out the violet, so safranin counterstains them.

How does penicillin kill bacteria selectively?

It blocks transpeptidase, preventing peptidoglycan cross-linking; human cells lack peptidoglycan so are unharmed.

What protein makes up the bacterial flagellar filament?

Flagellin.

What are the three main parts of a bacterial flagellum?

Filament, hook, and basal body (the motor).

What powers the bacterial flagellar motor?

The proton-motive force (proton flow inward), NOT ATP directly.

Write the pmf formula at 25 °C.

pmf = Δψ − 59·ΔpH (in mV).

How is the bacterial flagellum mechanically different from a eukaryotic one?

Bacterial = rigid flagellin helix that ROTATES on proton flow; eukaryotic = 9+2 microtubules that BEND using ATP/dynein.

What does CCW vs CW flagellar rotation cause?

CCW = bundled flagella → smooth run; CW = unbundled → tumble/reorient (basis of chemotaxis).

What does lysozyme do to the wall?

Cuts the NAG–NAM bond, breaking the glycan backbone so the cell lyses.

Connections

Prokaryotic vs Eukaryotic Cells — walls and flagella are classic prokaryotic structures.
Osmosis and Turgor Pressure — explains WHY a wall is needed.
Proton-Motive Force / Chemiosmosis — the same energy currency powering ATP synthase.
Antibiotics and Selective Toxicity — penicillin's mechanism.
Plant Cell Wall (cellulose) — contrast: same job, different material.
Eukaryotic Flagella and Cilia (9+2) — contrast the movement machinery.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, bacteria ek chhota sa pressurised bag hota hai. Aksar wo hypotonic (paani-zyada) environment me rehta hai, to osmosis se paani andar ghusta hai aur turgor pressure banta hai. Agar sirf membrane ho to cell phat jaaye — isiliye bahar ek strong cell wall hoti hai, jiska main material hai peptidoglycan (murein). Ye sugar chains (NAG aur NAM) ko chhote peptides se jod kar ek poora net jaisa jacket banata hai jo shape deta hai aur bursting rokta hai.

Gram stain se bacteria do groups me bat jaate hain: Gram-positive me thick peptidoglycan hota hai, isliye wo crystal violet ko trap karke purple dikhte hain. Gram-negative me thin peptidoglycan + ek outer membrane hota hai, alcohol wash dye nikaal deta hai, isliye wo safranin se pink dikhte hain. Ek important baat: penicillin peptidoglycan ke cross-links banane wali enzyme ko block karta hai, isliye bacteria phat jaata hai — aur kyunki humare cells me wall hi nahi hoti, hume nuksaan nahi hota (selective toxicity).

Ab flagellum — ye ek lambi twisty tail hai jo flagellin protein se bani hoti hai. Iske 3 parts: filament (propeller blade), hook (joint), aur basal body (motor wall/membrane me lagga). Yaad rakho: ye ATP se direct nahi chalti — ye proton-motive force se chalti hai, yaani $H^+$ ions andar bahte hain aur motor ko ghumate hain, bilkul water-wheel ki tarah. Formula simple hai: $\text{pmf} = \Delta\psi - 59\,\Delta\text{pH}$ mV. CCW ghoomne par flagella bundle ban kar seedha "run" karte hain, CW par "tumble" karke direction badalte hain — yahi chemotaxis ka basis hai (khaane ki taraf jaana). Bacterial flagellum ghoomta hai, eukaryotic flagellum jhukhta (bend) hai — ye difference exam me bahut puchha jaata hai.

Test yourself — Prokaryotic vs Eukaryotic Cells

Connections