Cell Biology learning route · Core lesson 2 of 13


The most useful distinction is the organisation of genetic material. A typical eukaryotic cell encloses its main chromosomes inside a membrane-bound nucleus. A prokaryotic cell has no such nucleus: its chromosome occupies a nucleoid region. Both are living cellular systems, with a plasma membrane, cytoplasm and protein-making machinery.

First review cell theory and cellular continuity. Here you will compare structures, interpret unfamiliar specimens and avoid using size or a cell wall as a shortcut for classification.

Compare the organisation, not just the names

Prokaryotic bacterial cell compared with a eukaryotic animal cell, showing nucleoid DNA, nucleus, plasma membranes, ribosomes and mitochondria
Original schematics, not to scale. The nucleus has its own envelope; the bacterial nucleoid does not. Both examples have a plasma membrane, cytoplasm, DNA and ribosomes. Not every bacterium has the wall shown.
FeatureProkaryotic cellsEukaryotic cells
ExamplesBacteria and ArchaeaAnimals, plants, fungi and protists
Main DNA compartmentNucleoid; no nuclear envelopeNucleus in a typical cell
Membrane-bound organellesNo typical eukaryotic nucleus, ER or Golgi; internal membranes occur in some groupsCompartments such as ER, Golgi and mitochondria, depending on cell type
Cytosolic ribosomesTypically 70STypically 80S; organellar ribosomes differ
ChromosomesOften a main circular chromosome; exceptions existTypically several linear nuclear chromosomes
Cell wallsCommon; most bacterial walls contain peptidoglycan, archaeal walls do notPlant and fungal walls differ; animal cells lack walls
Typical dimensionsOften around 0.1–5 µmOften around 10–100 µm
Cell divisionBinary fission is common; no mitosisNuclear division may involve mitosis or meiosis

These are general comparisons, not rules that every specimen must satisfy. Cell size overlaps, chromosome arrangements vary and differentiated eukaryotic cells may lose particular structures. Classification should combine organisation, ancestry and molecular evidence.

What both kinds of cells need

A plasma membrane provides a controlled boundary. Cytoplasm supports reactions. Ribosomes translate RNA messages into proteins. DNA provides inherited information. This common plan explains why prokaryotes can grow, respond and reproduce without a nucleus. “No nucleus” does not mean “no DNA”, and “no mitochondria” does not mean “no ATP production”.

Some energy-converting reactions in bacteria use their plasma membrane or specialised internal membranes. In many eukaryotes, mitochondria provide compartments for aerobic respiration. These locations differ while the wider need for usable chemical energy remains.

Why compartmentalisation matters

Internal membranes separate reactions that require different conditions and organise the movement of molecules. For example, a secreted protein can enter the rough endoplasmic reticulum and then move through the Golgi system. A bacterium can still produce and export proteins, but does not possess that typical eukaryotic ER–Golgi pathway. Do not describe bacteria as inactive or chemically unsophisticated.

How to identify an unfamiliar cell

  1. Locate genetic material. Is there evidence of a nuclear envelope rather than just a dark stain?
  2. Look for supporting compartments. Mitochondria, ER or a chloroplast support eukaryotic organisation.
  3. Use the specimen context. A specialised mammalian red blood cell is different from a free-living bacterium.
  4. State uncertainty. Failure to see a nucleus in a low-resolution image is not proof that none exists.

Worked comparison

Specimen A is 2 µm long and contains DNA in a region without an envelope. Specimen B is 35 µm across and contains an enclosed nucleus plus mitochondria. A is consistent with a prokaryote; B is eukaryotic. The decisive evidence is organisation, while the dimensions support the comparison.

For another constructed example, if two spherical cells have diameters 2 µm and 20 µm, the diameter ratio is 10, but the volume ratio is 10³ = 1,000. This assumes similar spherical geometry. A tenfold size statement must specify whether it concerns length, area or volume.

Important exceptions and misleading shortcuts

  • A wall is not enough: bacteria, plants and fungi can all have walls of different composition.
  • Unicellular does not mean prokaryotic: yeast and many protists are unicellular eukaryotes.
  • Photosynthesis does not mean chloroplasts: cyanobacteria photosynthesise without chloroplasts.
  • Archaea are not bacteria: both have prokaryotic organisation, but are distinct evolutionary groups.
  • 70S and 80S are sedimentation coefficients: the numbers are not additive measures of physical size. Bacterial 30S and 50S subunits form a 70S ribosome.
  • Specialisation matters: mature mammalian red blood cells lack nuclei; their developmental origin remains eukaryotic.

Connect the comparison to endosymbiosis

Mitochondria and chloroplasts have bacterial ancestry and retain some genetic information. This helps explain why a eukaryotic cell can contain ribosomes that differ from those in its cytosol. The organelles now depend on many proteins encoded by nuclear genes, so they should not be treated as independent bacteria. Study the three-domain system for the wider classification context.

20 MCQs with answers and explanations

Choose and justify an answer before opening the explanation. These original practice questions are separate from the scored quiz below.

1. Which feature best identifies typical eukaryotic organisation?

  • A. A plasma membrane alone
  • B. A membrane-bound nucleus
  • C. Small size alone
  • D. A cell wall alone
Show answer and explanation

B. A membrane-bound nucleus. The nuclear envelope distinguishes the typical organisation.

2. Where is the main chromosome located in a bacterium?

  • A. In a nucleoid region
  • B. Inside a chloroplast
  • C. Inside a nuclear envelope
  • D. In the Golgi apparatus
Show answer and explanation

A. In a nucleoid region. A nucleoid lacks a surrounding nuclear membrane.

3. Which groups have prokaryotic organisation?

  • A. Animals and fungi
  • B. Plants and animals
  • C. Only viruses
  • D. Bacteria and Archaea
Show answer and explanation

D. Bacteria and Archaea. These two groups lack a typical membrane-bound nucleus.

4. Which organism is a unicellular eukaryote?

  • A. A typical bacterium
  • B. A bacteriophage
  • C. Yeast
  • D. An archaeon
Show answer and explanation

C. Yeast. Unicellular organisation does not imply prokaryotic organisation.

5. What do typical bacterial and animal cells both possess?

  • A. Chloroplasts
  • B. Ribosomes
  • C. A cellulose wall
  • D. A Golgi apparatus
Show answer and explanation

B. Ribosomes. Both require protein synthesis.

6. What is the usual cytosolic ribosome type in eukaryotes?

  • A. 80S
  • B. 70S
  • C. 30S only
  • D. 50S only
Show answer and explanation

A. 80S. Cytosolic eukaryotic ribosomes are typically 80S.

7. What do 30S and 50S bacterial subunits form?

  • A. An 80S ribosome by simple addition
  • B. A nucleus
  • C. A chromosome
  • D. A 70S ribosome
Show answer and explanation

D. A 70S ribosome. Sedimentation coefficients are not additive lengths.

8. Most bacterial walls contain which distinctive material?

  • A. Cellulose
  • B. Chitin
  • C. Peptidoglycan
  • D. Starch
Show answer and explanation

C. Peptidoglycan. Peptidoglycan is characteristic of most bacterial walls.

9. Why does a cell wall not prove that a cell is prokaryotic?

  • A. All walls are nuclear envelopes
  • B. Plants and fungi can also have walls
  • C. Only animals have walls
  • D. Walls contain no molecules
Show answer and explanation

B. Plants and fungi can also have walls. Several unrelated groups have walls of different composition.

10. Where can bacteria carry out energy-converting membrane reactions?

  • A. At the plasma membrane or specialised internal membranes
  • B. Only inside mitochondria
  • C. Only inside chloroplasts
  • D. Only in a nucleus
Show answer and explanation

A. At the plasma membrane or specialised internal membranes. A lack of mitochondria does not prevent energy metabolism.

11. Which is a typical eukaryotic compartment?

  • A. Nucleoid
  • B. Bacterial capsule
  • C. Peptidoglycan layer
  • D. Golgi apparatus
Show answer and explanation

D. Golgi apparatus. Golgi organisation is part of the eukaryotic endomembrane system.

12. Which statement about chromosomes is most accurate?

  • A. All organisms have one circular chromosome
  • B. Bacterial DNA never forms chromosomes
  • C. Many bacteria have a main circular chromosome, with exceptions
  • D. Every eukaryotic chromosome is circular
Show answer and explanation

C. Many bacteria have a main circular chromosome, with exceptions. General comparisons should not erase biological variation.

13. How do many bacterial cells reproduce?

  • A. Mitosis within a nucleus
  • B. Binary fission
  • C. Meiosis in a chloroplast
  • D. Fusion of Golgi bodies
Show answer and explanation

B. Binary fission. Binary fission is a common prokaryotic division mechanism.

14. Why can cyanobacteria photosynthesise without being plants?

  • A. Photosynthesis can occur without chloroplasts
  • B. They secretly have animal nuclei
  • C. All photosynthetic organisms are plants
  • D. Their walls are made of chlorophyll
Show answer and explanation

A. Photosynthesis can occur without chloroplasts. Their photosynthetic membranes are not chloroplast organelles.

15. Which evidence is stronger than small size for classifying a specimen?

  • A. A green stain
  • B. A round outline
  • C. An image printed on small paper
  • D. DNA in a region lacking a nuclear envelope
Show answer and explanation

D. DNA in a region lacking a nuclear envelope. Organisation is more diagnostic than size alone.

16. A 20 µm sphere and a 2 µm sphere have what volume ratio?

  • A. 10:1
  • B. 100:1
  • C. 1000:1
  • D. 2:1
Show answer and explanation

C. 1000:1. For similar spheres, volume scales with diameter cubed.

17. Which eukaryotic example complicates the no-nucleus shortcut?

  • A. A yeast nucleus
  • B. A mature mammalian red blood cell
  • C. A leaf chloroplast
  • D. An animal Golgi apparatus
Show answer and explanation

B. A mature mammalian red blood cell. The specialised mature cell loses its nucleus during development.

18. Why are Archaea not simply a type of bacterium?

  • A. Their evolutionary and molecular characteristics differ
  • B. They are all multicellular animals
  • C. They all contain nuclei
  • D. They lack all genetic material
Show answer and explanation

A. Their evolutionary and molecular characteristics differ. Shared prokaryotic organisation does not erase distinct ancestry.

19. Which feature supports eukaryotic organisation in a micrograph?

  • A. A wall without other evidence
  • B. A small spherical outline
  • C. A plasma membrane alone
  • D. A nucleus plus mitochondria
Show answer and explanation

D. A nucleus plus mitochondria. Multiple characteristic compartments strengthen the conclusion.

20. What benefit can compartmentalisation provide?

  • A. Elimination of all membranes
  • B. Removal of all proteins
  • C. Different reaction conditions in different spaces
  • D. Prevention of every chemical reaction
Show answer and explanation

C. Different reaction conditions in different spaces. Membrane-bound compartments organise specialised processes.

Independent 20-question interactive quiz

Select one answer per question and choose Check my answers. Your score, unanswered count and explanations appear below. Reset clears the attempt. Your answers stay in this page; no account or upload is needed.

1. Specimen X has a nucleoid and cytosolic 70S ribosomes. Best interpretation?
2. A specimen has ER and a nuclear envelope. It is most likely what?
3. Which statement about bacterial DNA is correct?
4. Which observation cannot distinguish plant cells from bacteria by itself?
5. A single-celled pond organism has a nucleus. How is it classified structurally?
6. Why is a 3 µm diameter insufficient to establish bacterial identity?
7. Which structure is not membrane-bound?
8. Which pairing is correct?
9. Which claim should be rejected?
10. How do typical archaeal walls differ from most bacterial walls?
11. Where would a typical animal secretory protein enter a membrane pathway?
12. What does failure to see a nucleus at low resolution show?
13. Which statement about mitochondria in eukaryotes is best?
14. Two similar spherical cells have diameters 1 µm and 5 µm. Volume ratio?
15. Which comparison uses the right quantity?
16. Which label is appropriate for bacterial DNA territory?
17. Which feature is shared by typical bacteria and plant cells?
18. Which bacterial process demonstrates that no mitochondria does not mean no energy metabolism?
19. Which statement about cytosolic 80S ribosomes is appropriately qualified?
20. What is the strongest general identification strategy?

Frequently asked questions

What is the main difference between prokaryotic and eukaryotic cells?

Typical eukaryotic cells contain a membrane-bound nucleus. Prokaryotic cells have a nucleoid without a nuclear envelope.

Do bacteria have DNA and ribosomes?

Yes. A lack of a nucleus does not mean a lack of DNA; bacterial ribosomes support protein synthesis.

Are all unicellular organisms prokaryotic?

No. Yeast and many protists are unicellular eukaryotes.

Are plant cells prokaryotic because they have walls?

No. Plant cells are eukaryotic. A wall alone does not distinguish prokaryotes from eukaryotes.

Are 70S and 80S ribosome values additive sizes?

No. S values describe sedimentation behaviour. Subunit values cannot be added as ordinary length measurements.

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Sources and scope

Reviewed 23 October 2026. Original explanations, schematics and assessment questions by PreachBio; sources below support the underlying biology. The core concepts support international biology study, but this page does not claim complete alignment with every examination specification. All numerical examples are constructed teaching examples.