Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.4.5Inquiry Process

Prokaryotic Cell Structure — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 6 of 160

📚 IB Biology HL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
0%read

Part 1 of 5Core Components

Prokaryotes are small, simple cells with no nucleus and no membrane-bound organelles.

Cell wall
  • Cell wall A rigid layer of peptidoglycan outside the membrane that prevents bursting in dilute surroundings and keeps the cell shape.
Plasma membrane
  • Plasma membrane A phospholipid bilayer controlling which substances enter and leave, and the site of many reactions such as respiration.
Cytoplasm
  • Cytoplasm A watery solution of enzymes, salts and ribosomes in which the metabolic reactions of the cell take place.
70S ribosomes
  • 70S ribosomes Small ribosomes lying free in the cytoplasm that translate messenger RNA into polypeptides for the cell.
Nucleoid
  • Nucleoid A region holding one circular, naked DNA molecule that is not surrounded by a nuclear envelope.
Plasmid
  • Plasmid A small circular DNA ring carrying extra genes, such as antibiotic resistance, that can pass between bacteria.

Part 2 of 5Surface Structures

Surface Structures
Flagellum
  • Flagellum A long rotating protein filament that propels the bacterium through liquid towards favourable conditions.
Pili
  • Pili Short hair-like protein projections used to attach to surfaces, and in conjugation to link two bacterial cells.
Capsule
  • Capsule A sticky outer layer of polysaccharide in some species that protects against drying and engulfment by white blood cells.
Mesosome idea
  • Mesosome idea Folds once drawn in the membrane are now regarded as artefacts of specimen preparation, not real structures.

Part 3 of 5Reading a Prokaryote in a Micrograph

1
Look first at the overall size; most bacteria measure about 1 to 5 micrometres, far smaller than a typical eukaryotic cell.
2
Search for a nucleus; its absence together with a pale nucleoid region of DNA marks the cell as prokaryotic.
3
Check for a clear cell wall outside the membrane, which is nearly always present in bacteria.
4
Note any flagella, pili or capsule, since these help decide the species and its method of movement or attachment.
5
Finish by labelling the structures and writing the function of each beside its label.
FeatureProkaryote
DNANaked, circular, in nucleoid
Ribosomes70S
WallPeptidoglycan
OrganellesNone membrane-bound

Part 4 of 5Worked Example

Question

  • An electron micrograph shows a rod-shaped cell with no nucleus, a thick outer layer, a ring of DNA in a pale area and many small dark granules.
  • Identify the cell type and justify the identification.

  1. The absence of a nucleus and of any membrane-bound organelles points to a prokaryote.
  2. The pale area holding a ring of DNA is the nucleoid, which has no nuclear envelope.
  3. The thick outer layer outside the plasma membrane is the cell wall, made of peptidoglycan.
  4. The small dark granules are 70S ribosomes, which are smaller than eukaryotic ribosomes.

Answer The cell is a bacterium (prokaryote): no nucleus, naked circular DNA in a nucleoid, peptidoglycan wall and 70S ribosomes.

Part 5 of 5Exam Checklist & Tips

1.4.5 – Key Points for the Exam

  • Prokaryote A cell without a nucleus or membrane-bound organelles.
  • Nucleoid The region of a prokaryotic cell containing its naked DNA.
  • Peptidoglycan The sugar-amino acid polymer forming bacterial cell walls.
  • Plasmid A small circular extra-chromosomal DNA molecule in bacteria.
  • 70S ribosome The smaller ribosome type found in prokaryotes.

Common Mistakes

  • Stating that prokaryotes have no DNA; they do, but it is not enclosed by a nuclear envelope.
  • Saying the bacterial wall is made of cellulose; it is peptidoglycan.
  • Calling the nucleoid a nucleus.
  • Forgetting that prokaryotes still carry ribosomes, which are 70S not 80S.

Last updated:

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A  ⇔  A = I / M  ⇔  I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.

\n