Cell Theory, Prokaryotes & Eukaryotes

Overview

This is the section’s foundational page — read it first. Cell theory states that all living organisms are composed of cells, the cell is the basic structural and functional unit of life, and all cells arise from pre-existing cells. From this starting point, the single organising idea that explains most of what follows in this section is compartmentalisation: eukaryotic cells solve the problem of running many incompatible chemical processes simultaneously by physically separating them into membrane-bound compartments, while prokaryotic cells largely forgo this strategy and instead rely on a much smaller genome, faster reproduction, and biochemical simplicity.

Key Concepts

The three tenets of cell theory

1. All living organisms are composed of one or more cells.
2. The cell is the basic structural, functional, and organisational unit of all organisms.
3. All cells arise only from pre-existing cells (omnis cellula e cellula — Virchow), ruling out spontaneous generation as a mechanism of ongoing cell formation.

Historically, cell theory itself is a favourite context for questions on the transition from spontaneous generation to a mechanistic account of life, associated with Hooke’s first observation of (dead) cork cells, Leeuwenhoek’s observation of living “animalcules,” and Schleiden/Schwann’s formalisation of the theory — but the conceptual content (the three tenets above) is what’s actually tested, not a detailed history of discovery.

Prokaryotic cell organisation

Prokaryotic cells (Bacteria and Archaea) lack a membrane-bound nucleus and membrane-bound organelles. Their genome is typically a single circular chromosome located in a nucleoid region — not membrane-enclosed, simply a zone of DNA concentration. They are generally smaller (~1–5 μm) than eukaryotic cells (~10–100 μm), which matters functionally: smaller cells have a higher surface-area-to-volume ratio, supporting faster nutrient exchange and (partly) explaining prokaryotes’ generally faster growth/division rates.

Structural features distinguishing prokaryotes:

• Cell wall: peptidoglycan-based (bacteria) or pseudopeptidoglycan/other polymers (archaea) — chemically distinct from the cellulose (plant) or chitin (fungal) walls found in eukaryotes.
• Ribosomes: 70S (30S + 50S subunits) — smaller than the eukaryotic 80S ribosome, a distinction directly exploited by many antibiotics (e.g. streptomycin, erythromycin target the 70S ribosome specifically, sparing the host’s 80S cytoplasmic ribosomes).
• Plasmids: small, circular, extrachromosomal DNA molecules, often carrying non-essential but conditionally advantageous genes (e.g. antibiotic resistance) and capable of independent replication and horizontal transfer between cells.
• No cytoskeleton in the eukaryotic sense (though bacterial homologues of actin/tubulin, e.g. FtsZ, exist and perform analogous structural roles).

Source: ecampusontario.pressbooks.pub

Eukaryotic cell organisation

Eukaryotic cells possess a membrane-bound nucleus housing linear chromosomes complexed with histone proteins (chromatin), plus an extensive system of membrane-bound organelles (endoplasmic reticulum, Golgi apparatus, mitochondria, and — in plants/algae — chloroplasts; see Endomembrane System & Protein Trafficking and Mitochondria & Chloroplasts for full treatment of each). This compartmentalisation lets eukaryotic cells run mutually incompatible reactions simultaneously — for instance, concentrating and protecting the genome from cytoplasmic mechanical/chemical stress, or maintaining the distinct redox and pH environments each organelle’s chemistry requires.

Eukaryotic cells also possess a true cytoskeleton (actin filaments, microtubules, intermediate filaments — see Cytoskeleton & Motor Proteins) providing structural support, intracellular transport tracks, and the machinery for cell division and, in animal cells, shape change and motility.

Source: opentextbc.ca (OpenStax-derived Biology OER)

Why the prokaryote/eukaryote distinction is not simply “simple vs. complex”

A common misconception worth correcting directly: prokaryotes are not “primitive” eukaryotes-in-waiting. Both lineages are highly evolved for their respective strategies. Prokaryotic biochemical diversity (metabolic pathway diversity in particular — nitrogen fixation, chemolithotrophy, extreme-environment tolerance) vastly exceeds that of eukaryotes; what eukaryotes gained in compartmentalised complexity, they generally lost in metabolic versatility and division speed. Mitochondria & Chloroplasts: Structure & Endosymbiotic Origin covers the specific evolutionary event (endosymbiosis) that gave eukaryotic cells two of their signature organelles, each still carrying vestiges of a free-living prokaryotic ancestor.

Comparative Structures

Common Exam Questions

• “Why do antibiotics targeting the 70S ribosome not harm human cells?” — the correct answer names the ribosome-size distinction (70S vs. 80S) specifically, and often additionally notes that human mitochondria retain 70S ribosomes, which is why some such antibiotics do carry mitochondrial-toxicity side effects at high doses — a frequently tested nuance.
• “Which feature most fundamentally distinguishes eukaryotes from prokaryotes?” — compartmentalisation (membrane-bound organelles, especially the nucleus), not simply “bigger” or “more complex,” which are consequences rather than the defining feature itself.
• Questions describing a newly discovered microorganism and asking you to classify it as prokaryotic or eukaryotic test recognition of the diagnostic structural features in the comparison table above, not memorised taxonomic names.

Visual Reference

Interactive

• A side-by-side prokaryotic/eukaryotic cell cutaway diagram with clickable structures that reveal a short description on click — the natural entry point widget for the whole section.

Static

(Static images are placed inline in Key Concepts above, next to the concept each one illustrates, rather than collected here. Only the animal-cell version of the eukaryotic cell diagram is placed; a plant-cell version is not yet sourced.)

Practice Problems

1. A newly isolated microorganism is found to have a single circular chromosome, no membrane-bound organelles, and 70S ribosomes, but is unusually large (15 μm) for a prokaryote. Would you still classify it as prokaryotic? Justify your answer using the diagnostic features that actually define the category.

2. Explain, in terms of surface-area-to-volume ratio, why prokaryotic cells are generally under stronger evolutionary pressure to stay small than eukaryotic cells are.

3. A student claims “eukaryotic cells are more evolutionarily advanced than prokaryotic cells.” Identify what is misleading about this framing, using at least one specific counter-example of prokaryotic capability that eukaryotes generally lack.