Core: Cells
IB Biology HLΒ· Theme A: Unity and Diversity, Unit 1, Topic 2Β· 45 min read
1. Cell Theory: Core Principlesβ β ββββ± 15 min
Cell Theory
The unifying biological principle that establishes cells as the fundamental structural and functional unit of all living organisms
Example:
All living things, from bacteria to blue whales, are built from one or more cells
Cell theory has three core, universally accepted tenets that form the backbone of modern biology:
All living organisms are composed of one or more cells
The cell is the basic unit of structure and organization in organisms
All cells arise from pre-existing, living cells
A newly discovered microscopic structure has a cell membrane, cytoplasm, DNA, ribosomes, and can reproduce independently. Does this fit the core tenets of cell theory?
- 1
Check the structure against each tenet of cell theory:
- 2
- The structure is a single functional unit, so it satisfies the rule that the cell is the basic unit of life.
- 3
- It can reproduce independently from pre-existing units, so it satisfies the rule that all cells come from pre-existing cells.
- 4
Conclusion: This is a living cell, so it fits all core tenets of cell theory.
Exam tip:
Always list all three tenets when asked to state cell theory β examiners award one mark per correct tenet.
2. Prokaryotic vs Eukaryotic Cellsβ β β βββ± 20 min
The most fundamental division of cell types distinguishes prokaryotes (domains Bacteria and Archaea) from eukaryotes (all complex organisms, including plants, animals, fungi, and protists). Key structural differences are frequently tested in IB exams.
Prokaryotic Cell
A cell that lacks a membrane-bound nucleus and specialized membrane-bound organelles
Example:
Escherichia coli (E. coli) bacteria, found in the human gut, is a common prokaryote
Feature | Prokaryotic Cell | Eukaryotic Cell |
|---|---|---|
Nucleus | No (nucleoid region, no membrane) | Yes, membrane-bound |
Membrane-bound organelles | None | Yes (mitochondria, ER, chloroplasts etc) |
Ribosome size | 70S | 80S |
Cell wall | Almost always present (peptidoglycan) | Present in plants/fungi, absent in animals |
DNA structure | Circular, not bound to proteins | Linear, bound to histone proteins |
An unknown cell is observed to have 80S ribosomes, linear DNA, and a cell wall. Could this be a prokaryotic cell? Justify your answer.
- 1
Recall the defining distinguishing features of prokaryotic cells: prokaryotes only have 70S ribosomes and circular naked DNA.
- 2
Compare to the observed features: 80S ribosomes and linear DNA are unique to eukaryotic cells.
- 3
Cell wall presence does not rule out a eukaryote, because plants and fungi have eukaryotic cells with cell walls.
- 4
Conclusion: This cannot be a prokaryotic cell, it is a eukaryotic cell.
Exam tip:
Ribosome size (70S vs 80S) is a very common exam question β always double check you have not reversed the values.
3. Atypical Cells: Exceptions to Cell Theoryβ β β β ββ± 20 min
While cell theory is universally accepted, there are several specialized atypical cell types that do not fit all explicit or implicit rules of classic cell theory. These are common long-answer exam questions.
Striated muscle: Large fibres with multiple nuclei, challenges 'one nucleus per cell' assumption
Giant algae (Acetabularia): Single cell up to 10 cm, challenges 'all cells are small' assumption
Aseptate fungal hyphae: Continuous cytoplasm with many nuclei, challenges 'organisms are made of discrete cells' tenet
Mature mammalian red blood cells: No nucleus, challenges 'all cells contain genetic material' rule
Explain why Acetabularia (giant algae) is considered an exception to cell theory.
- 1
Classic cell theory implicitly assumes all cells are small and microscopic, because a single nucleus can only control a limited cytoplasmic volume.
- 2
Acetabularia grows up to 10 cm in length, but is a single-celled organism with only one nucleus.
- 3
This contradicts the implicit assumption that all cells are small, so it is classified as an exception.
4. Common Pitfalls
Wrong move:
Stating that all living cells have a nucleus
Why:
Mature mammalian red blood cells and phloem sieve tube elements are a-nucleate, and examiners expect recognition of this exception
Correct move:
State that almost all cells contain a nucleus with genetic material, and note common exceptions when prompted
Wrong move:
Confusing 70S and 80S ribosome sizes for prokaryotes and eukaryotes
Why:
S values are sedimentation measures, not direct size, leading to frequent reversal of the values
Correct move:
Remember: Prokaryotes are smaller organisms with smaller ribosomes: 70 < 80, so 70S = prokaryote, 80S = eukaryote
Wrong move:
Claiming cell wall presence means a cell is prokaryotic
Why:
Presence of a cell wall is not unique to prokaryotes; plant and fungal eukaryotes also have cell walls
Correct move:
Use ribosome size or presence of membrane-bound organelles to distinguish prokaryotes from eukaryotes, not cell wall presence
Wrong move:
Claiming viruses are exceptions to cell theory
Why:
Viruses are not cells at all, so they do not violate any tenets of cell theory, which only describes living cells
Correct move:
Only recognize atypical cell types (giant algae, striated muscle etc) as exceptions to cell theory
Wrong move:
Forgetting prokaryotes have ribosomes
Why:
Students often associate ribosomes only with eukaryotes, but all cells need ribosomes to synthesize proteins
Correct move:
Remember that all living cells (prokaryotic and eukaryotic) contain ribosomes
5. Quick Reference Cheatsheet
Concept | Key Exam Details |
|---|---|
Cell Theory Tenets |
|
Prokaryote Features | 70S ribosomes, no nucleus, circular naked DNA, no membrane organelles |
Eukaryote Features | 80S ribosomes, membrane-bound nucleus, linear histone-bound DNA, membrane organelles |
Common Exceptions | Striated muscle (multi-nucleate), Acetabularia (giant single cell), Aseptate hyphae (multi-nucleate), Mature RBC (anucleate) |
6. Frequently Asked
Why are viruses not considered exceptions to cell theory?
Viruses are not made of cells at all, so they do not violate any tenets of cell theory (which only describes living cells). They are non-living infectious particles, not atypical cells.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2025 Β· 1
Compare prokaryote and eukaryote structure
- 2024 Β· 2
Discuss exceptions to cell theory
Going deeper
What's Next
Understanding core cell structure and cell theory is the foundation for all further topics in IB Biology HL. Next, you will build on this knowledge to explore cell membrane structure and transport, which relies on understanding the differences between prokaryotic and eukaryotic cell components. This core knowledge also underpins future topics including cell division, cellular respiration, photosynthesis, and cell specialization. Recognizing atypical cells will also prepare you to answer extended response questions about cell differentiation and specialized tissue structure in later units.