Compare prokaryotic and eukaryotic cells — Biology notes

WHAT are we comparing?

WHY this single criterion? Because a membrane around the DNA changes everything downstream: it separates transcription (inside nucleus) from translation (outside), allows large genomes, internal compartments, and bigger cell size. So "nucleus or not" predicts almost all other differences.

HOW they differ — feature by feature (with WHY)

Figure — Compare prokaryotic and eukaryotic cells

Feature	Prokaryotic	Eukaryotic	WHY it matters
Nucleus	Absent (nucleoid)	Present (membrane-bound)	Separates DNA processing from rest of cell
Size (typical)	~0.2–5 µm (rough)	~10–100 µm (rough; yeast can be <10 µm)	Bigger cell needs internal compartments — but these are averages, not strict limits
DNA shape	Usually a single circular chromosome (exceptions: Borrelia, Streptomyces have linear/multiple)	Multiple linear chromosomes	Linear ends need telomeres
Membrane organelles	Absent	Present (mito, ER, Golgi, lysosome)	Specialised "rooms" do specialised jobs
Ribosomes	70S	80S (cytoplasm)	Antibiotics target 70S → selective killing
Cell wall	Usually present (peptidoglycan in bacteria)	Plants/fungi yes; animals no	Different building material
Reproduction	Binary fission	Mitosis / meiosis	No spindle vs spindle apparatus
Histones	Usually absent (Archaea have histone-like proteins)	Present (DNA wound on histones)	Packaging large genome

DERIVE the size/efficiency argument from scratch

WHY are prokaryotes small but eukaryotes can be large?

A cell exchanges nutrients/waste across its surface, but its needs scale with volume.

For a sphere of radius $r$ : $\text{Surface area } A = 4\pi r^2, \qquad \text{Volume } V = \tfrac{4}{3}\pi r^3$

The crucial quantity is the surface-area-to-volume ratio: $\frac{A}{V} = \frac{4\pi r^2}{\tfrac{4}{3}\pi r^3} = \frac{3}{r}$

Why this step? As $r$ grows, $A/V = 3/r$ falls. A big simple cell can't pass enough material across its membrane to feed its volume → it would starve.

HOW eukaryotes escape this limit: they add internal membranes (ER, organelles) that provide extra surface inside the cell, so they support large volume without relying only on the outer membrane. (Note: small cell size is a tendency, not a law — some prokaryotes are larger and some eukaryotes like yeast are very small.)

Worked examples

Common mistakes (Steel-manned)

Flashcards

What single feature defines a eukaryote vs prokaryote?

A true membrane-bound nucleus (and membrane-bound organelles).

Where is prokaryotic DNA located?

In the nucleoid region (not membrane-bound), usually a single circular chromosome.

Name two prokaryotes that break the "single circular chromosome" rule.

Borrelia and Streptomyces (linear chromosomes); some species also have multiple chromosomes/plasmids.

Ribosome size in prokaryotes vs eukaryotic cytoplasm?

70S vs 80S.

What is the size of a mammalian mitochondrial ribosome?

~55S (28S + 39S) — NOT 70S. Chloroplast ribosomes are 70S.

Why are Svedberg units non-additive (30S+50S=70S)?

S measures sedimentation rate which depends on shape and size, not just mass.

Formula for surface-area-to-volume ratio of a sphere?

A/V = 3/r.

Why must large eukaryotic cells use internal membranes?

Because A/V = 3/r falls as r grows; internal membranes add surface to feed the large volume.

Why does penicillin harm bacteria but not human cells?

It targets peptidoglycan cell walls, which human cells lack.

DNA shape: prokaryote vs eukaryote?

Usually single circular vs multiple linear chromosomes (with prokaryotic exceptions).

Are the size ranges strict boundaries?

No — ~0.2–5 µm (prokaryote) and ~10–100 µm (eukaryote) are rough averages; e.g. yeast can be <10 µm.

How do prokaryotes reproduce?

Binary fission (no spindle).

Do prokaryotes have ribosomes?

Yes (70S) — they lack a nucleus, not ribosomes.

Recall Feynman: explain to a 12-year-old

Imagine two houses. The bacteria house is one open room — your bed, your homework, and your snacks are all on the same floor, no walls. It's tiny, so it's quick to clean and you can run errands fast. The animal/plant house is a mansion with separate locked rooms: a bedroom that keeps your diary (DNA) safe behind a door (nucleus), a kitchen for energy, a recycling room. Big house = need rooms to stay organised. The ONLY first thing to remember: does the diary have its own locked room (nucleus) or not? Everything else follows from that.

Connections

Cell Theory — all cells share a membrane, cytoplasm, DNA, ribosomes.
Endosymbiotic Theory — mitochondria/chloroplasts have their own ribosomes (chloroplast 70S, mammalian mito ~55S) → were once prokaryotes.
Bacterial Cell Wall and Peptidoglycan — basis of antibiotic selectivity.
Surface Area to Volume Ratio — limits cell size, drives organelle evolution.
Transcription and Translation — coupled in prokaryotes, separated by nucleus in eukaryotes.
Binary Fission vs Mitosis — reproduction mechanisms.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, pure comparison ka asli funda sirf EK cheez par tika hai: cell ke DNA ke aas-paas membrane wala "kamra" (nucleus) hai ya nahi. Agar nahi hai → woh prokaryote (bacteria); agar hai → eukaryote (animal, plant, fungi). Prokaryote ek chhote studio room jaisa hai — DNA, ribosomes, sab khule mein padा hai, aur cell chhota hota hai. Eukaryote ek badi kothi jaisi hai jisme alag-alag labeled rooms (organelles: nucleus, mitochondria, ER, Golgi) hote hain. Yaad rakho size ranges (prokaryote ~0.2–5 µm, eukaryote ~10–100 µm) rough averages hain — yeast jaisa eukaryote 10 µm se chhota bhi ho sakta hai, koi strict boundary nahi.

Size ka logic samajh lo, kyunki exam mein poochhte hain. Sphere ke liye surface-area-to-volume ratio $A/V = 3/r$ hota hai. Jaise-jaise $r$ badhta hai, ratio girta hai, matlab badi simple cell apne andar tak nutrients pahuncha nahi paati — woh "bhookhi" mar jaayegi. Isliye prokaryotes choti rehti hain (fast exchange, fast reproduction), aur eukaryotes andar internal membranes (organelles) laga ke extra surface banate hain taaki bade ho sakein.

Do classic points jo medical/board exam mein kaam aate hain: (1) Ribosome — prokaryote mein 70S, eukaryote cytoplasm mein 80S. Yaad rakho S (Svedberg) add nahi hote, isliye 30S+50S = 70S, 40S+60S = 80S. Ek trap: mammal ke mitochondrial ribosome ~55S hote hain (28S+39S), 70S NAHI — sirf chloroplast ribosome 70S hota hai. (2) Antibiotics kaam kyun karte hain? Penicillin bacteria ki peptidoglycan wall todta hai jo humare cells mein hoti hi nahi; streptomycin 70S ribosome ko block karta hai jo humare 80S se alag hai. Isi liye dawai bacteria ko maarti hai par humein nahi.

Galti se bachna: "prokaryote ke paas DNA ya ribosome nahi hota" — GALAT. Unke paas DNA (usually circular) aur 70S ribosomes hote hain, bas membrane-bound nucleus aur organelles nahi hote. Aur "har prokaryote ke paas ek hi circular chromosome hota hai" bhi pura sach nahi — Borrelia aur Streptomyces ke paas linear chromosomes hote hain. Keyword hamesha membrane-bound yaad rakhna.

Test yourself — Prokaryotic vs Eukaryotic Cells

Connections