Spec Mapping — OCR H420 Module 2.1.1 — Cell structure, content statements covering prokaryotic cell ultrastructure and comparison of prokaryotic and eukaryotic cells (refer to the official OCR H420 specification document for exact wording). This lesson is the final piece of the 2.1.1 jigsaw and is examined synoptically with Module 4.1 (communicable diseases, antibiotic action) and Module 6.1.3 (manipulating genomes via plasmids).
So far we have focused on eukaryotic cell structure. OCR 2.1.1 also requires a detailed understanding of prokaryotic cells and the ability to compare them with eukaryotes. Prokaryotes (bacteria and archaea) are the smallest, simplest, and most abundant cells on Earth, and their architecture differs from eukaryotes in important ways.
The historical sweep here is dense. Antonie van Leeuwenhoek (1670s) was the first to observe bacteria, which he called "animalcules". Christian Gram (1884) developed the staining technique that still divides bacteria into the two major classes by cell-wall structure. Carl Woese (1977) used 16S rRNA sequencing to split prokaryotes into the two domains Bacteria and Archaea, overturning a century of taxonomy. Lynn Margulis (1967) re-interpreted mitochondria and chloroplasts as descendants of engulfed prokaryotes — and the prokaryotic features you will learn here (70S ribosomes, circular DNA, double membrane, binary fission) are exactly the features those organelles still display. The intellectual link between prokaryotic cell biology and the endosymbiotic theory is one of the most beautiful synoptic threads in the spec.
Key Definition: A prokaryotic cell is a cell that lacks a membrane-bound nucleus and membrane-bound organelles. Prokaryotes include the domains Bacteria and Archaea. Their name (from Greek pro = before, karyon = kernel) reflects the evolutionary view that they predate eukaryotic cells.
SVG: bacterial cell cross-section
The diagram includes every feature OCR examines on a "label this prokaryotic cell" question: capsule, peptidoglycan wall, plasma membrane, nucleoid, plasmid, 70S ribosomes, pili/fimbriae, and flagellum. Note the absence of membrane-bound organelles — no nucleus, no mitochondria, no ER, no Golgi — which is the defining structural feature of all prokaryotes.
General Features of Prokaryotic Cells
Typical prokaryotic features include:
Small size — most are 0.2 to 10 µm across (often around 1–5 µm). Roughly ten to a hundred times smaller in linear dimension than a typical eukaryotic cell.
No true nucleus — DNA is free in the cytoplasm, concentrated in an area called the nucleoid.
Genetic material is a single, circular DNA molecule, generally described as "naked" because it is not bound to histones (though archaea have histone-like proteins).
Additional small circular DNA molecules called plasmids are common.
No membrane-bound organelles — no mitochondria, ER, Golgi, lysosomes, or chloroplasts.
70S ribosomes are scattered throughout the cytoplasm (smaller than the 80S ribosomes of eukaryotes).
A cell wall of peptidoglycan (murein) lies outside the plasma membrane.
May have additional surface structures: capsule, pili/fimbriae, and flagella.
Structure in Detail
The Cell Wall
Made of peptidoglycan (also known as murein) — a polymer of sugars (N-acetylglucosamine and N-acetylmuramic acid) cross-linked by short peptide chains.
Provides structural support and prevents the cell from bursting under osmotic pressure.
Chemically distinct from the cellulose wall of plants or the chitin wall of fungi.
The presence and structure of peptidoglycan is the basis of the Gram stain:
Gram-positive bacteria have a thick peptidoglycan layer and stain purple.
Gram-negative bacteria have a thin peptidoglycan layer plus an outer lipopolysaccharide membrane; they stain pink.
Many antibiotics (e.g., penicillin) work by inhibiting enzymes that cross-link peptidoglycan, weakening the wall so the cell lyses under osmotic pressure. Because human cells do not have cell walls, penicillin is highly selective.
Capsule (Slime Layer)
Some (not all) bacteria have a polysaccharide capsule outside the cell wall.
Functions:
Protection against phagocytosis by host immune cells.
Protection against desiccation.
Aids attachment to surfaces and to host tissues (important in biofilm formation and pathogenesis).
Capsules are often major virulence factors; for example, Streptococcus pneumoniae without its capsule is harmless, but with its capsule it causes severe pneumonia.
Pili (Fimbriae)
Short, hair-like projections of protein (pilin) on the cell surface.
Attachment pili allow bacteria to stick to host cells or surfaces (e.g., uropathogenic E. coli attaching to the bladder wall).
Sex pili (conjugation pili) are used during conjugation, where one bacterium transfers plasmid DNA to another. This is a major route of horizontal gene transfer and of antibiotic resistance spread.
Flagella
Long, whip-like structures for locomotion, made of the protein flagellin.
Structurally very different from eukaryotic flagella: they are not surrounded by a membrane, and they rotate like a propeller, driven by a proton gradient across the plasma membrane, rather than bending like eukaryotic cilia.
A bacterium can reverse the direction of rotation to change between smooth swimming and tumbling, enabling chemotaxis (movement towards food or away from harmful substances).
Plasma Membrane
A phospholipid bilayer, similar in basic structure to that of eukaryotes, but usually lacks cholesterol.
Site of many processes that in eukaryotes occur on organelle membranes: respiration (electron transport chain and ATP synthesis occur on the prokaryotic plasma membrane rather than on mitochondrial inner membranes) and in photosynthetic prokaryotes, photosynthesis (on infoldings).
Mesosomes
Mesosomes are infoldings of the plasma membrane seen in some bacterial electron micrographs.
Historically, they were thought to increase the surface area for respiration and perhaps anchor DNA.
Modern evidence suggests that many mesosomes seen in TEMs are actually fixation artefacts rather than real structures. However, OCR requires that you know the term and the suggested role.
Nucleoid and Circular DNA
The nucleoid is the area of the cytoplasm containing the circular chromosome. It is not membrane-bound and so is not a true nucleus.
The chromosome is typically a single circular molecule of DNA, supercoiled and anchored at several points to the plasma membrane.
The DNA is considered "naked" because it is not wound around histones (although archaea have histone-like proteins, a reason why some biologists argue they should be studied separately).
Plasmids
Small, circular, double-stranded DNA molecules independent of the main chromosome.
Replicate separately from the chromosome.
Carry genes that are often non-essential for basic metabolism but give a selective advantage — for example, antibiotic resistance, heavy metal tolerance, production of toxins, or the ability to metabolise unusual compounds.
Can be transferred between bacteria via conjugation using a sex pilus, spreading adaptive traits rapidly.
Are widely exploited in genetic engineering as vectors to introduce genes into bacteria.
70S Ribosomes
Smaller than the 80S ribosomes of eukaryotic cytoplasm.
Consist of a 50S large subunit and a 30S small subunit.
Translate mRNA into protein — functionally equivalent to 80S ribosomes but different in composition.
The differences between 70S and 80S ribosomes are exploited by antibiotics such as tetracycline, streptomycin, erythromycin, and chloramphenicol, which bind selectively to 70S ribosomes and inhibit bacterial protein synthesis without affecting human ribosomes.
Food Storage Granules
Some prokaryotes store carbon and energy as glycogen granules, poly-β-hydroxybutyrate, or polyphosphate granules within the cytoplasm.
Comparison: Prokaryotic vs Eukaryotic Cells
Feature
Prokaryotic cell
Eukaryotic cell
Typical size
0.2–10 µm (usually 1–5 µm)
10–100 µm
Nucleus
No true nucleus (nucleoid region only)
True membrane-bound nucleus
DNA organisation
Single circular chromosome, no histones
Multiple linear chromosomes, wound on histones
Plasmids
Common
Rare (some yeast and other exceptions)
Ribosomes
70S (50S + 30S)
80S (60S + 40S) in cytoplasm; 70S inside mitochondria and chloroplasts
Membrane-bound organelles
Absent
Present (nucleus, ER, Golgi, mitochondria, lysosomes, chloroplasts in plants)
Cell wall
Peptidoglycan (murein)
Cellulose in plants, chitin in fungi, absent in animals
Capsule
Often present
Absent
Pili / fimbriae
Often present
Absent
Flagella
Simple; flagellin; rotate like propellers
Complex; microtubule-based 9 + 2 axoneme; beat by bending
On infolded plasma membranes (e.g., thylakoids in cyanobacteria)
In chloroplast thylakoids
Exam Tip: Always quote numerical values (e.g., 1–5 µm for prokaryotes vs 10–100 µm for eukaryotes; 70S vs 80S) where possible. Precise quantitative details are credited at A-Level.
Binary Fission
Prokaryotes reproduce asexually by binary fission — not the same as mitosis.
Stages of Binary Fission
The circular chromosome is replicated; both copies attach to the plasma membrane.
The cell elongates, physically separating the two chromosome copies.
A septum of new plasma membrane and cell wall grows inward at the midpoint, driven by the protein FtsZ (a prokaryotic relative of tubulin).
The cell divides into two genetically identical daughter cells.
Under ideal conditions, many bacteria can divide every 20 minutes, giving rapid population growth (exponential growth curve).
Differences from Mitosis
No nuclear envelope breakdown (because there is no nucleus).
No spindle formation.
No condensation of chromosomes into visible, rod-like structures.
Generally much faster than eukaryotic mitosis.
Mermaid Comparison Diagram
flowchart LR
P[Prokaryotic cell] --> P1[No nucleus, DNA in nucleoid]
P --> P2[Circular DNA, naked]
P --> P3[Plasmids common]
P --> P4[70S ribosomes]
P --> P5[Peptidoglycan cell wall]
P --> P6[No membrane-bound organelles]
P --> P7[Simple flagella flagellin, rotate]
P --> P8[Binary fission]
E[Eukaryotic cell] --> E1[True nucleus with envelope]
E --> E2[Linear DNA on histones]
E --> E3[No plasmids usually]
E --> E4[80S ribosomes cytoplasm; 70S in mitochondria/chloroplasts]
E --> E5[Cellulose, chitin, or no wall]
E --> E6[Many membrane-bound organelles]
E --> E7[Complex flagella 9+2 microtubules, bend]
E --> E8[Mitosis or meiosis]
The Endosymbiotic Theory
A striking point of A-Level depth is that mitochondria and chloroplasts themselves have several prokaryotic features — they contain their own circular DNA, 70S ribosomes, double membranes, and they reproduce by binary fission within the cell. The endosymbiotic theory, proposed in detail by Lynn Margulis, suggests that:
Around 1.5–2 billion years ago, an ancestral eukaryotic cell engulfed a free-living aerobic bacterium, which was not digested but became the ancestor of modern mitochondria.
Later, one such cell engulfed a photosynthetic cyanobacterium, which became the ancestor of modern chloroplasts.
Over time, most of the engulfed organisms' genes were transferred to the host nucleus, though some remained behind as mitochondrial and chloroplast DNA.
This theory is supported by:
70S ribosomes inside mitochondria and chloroplasts.
Circular DNA in both organelles.
Double membranes (the inner membrane is of prokaryotic origin; the outer is host).
Division by binary fission, independently of the cell cycle.
Responsiveness to antibiotics that target 70S ribosomes (e.g., erythromycin can inhibit mitochondrial protein synthesis).