Core: Cells
IB Biology HL· Theme B: Form and Function, Unit 2 Core Cells· 25 min read
1. Cell Theory and Its Exceptions★☆☆☆☆⏱ 8 min
Cell Theory
Three core unifying tenets for biology: (1) all living organisms are composed of one or more cells, (2) the cell is the basic unit of organization in living organisms, (3) all cells arise from pre-existing, living cells.
Example:
Both a unicellular E. coli bacterium and a multicellular redwood tree follow the core tenets of cell theory.
While cell theory is a foundational principle of biology, there are well-documented accepted exceptions that are frequently tested in IB exams.
Striated muscle fibers: multinucleated, large size challenges the 'basic unit' tenet
Giant algae (Acetabularia): single-celled, up to 10 cm long, challenges the idea that large organisms are multicellular
Aseptate fungal hyphae: no cross cell walls, continuous cytoplasm with multiple nuclei
Explain why Acetabularia is considered an exception to cell theory.
- 1
- Recall that one generalisation of cell theory is that all large living organisms are made of many small, discrete cells.
- 2
- Acetabularia is a single-celled alga that can grow to 10 cm in total length, which is much larger than most cells.
- 3
- Its large size as a single cell contradicts the generalization that large organisms are always multicellular, making it an accepted exception.
Exam tip:
Always name at least two exceptions for 3-mark extended response questions to gain full marks.
2. Prokaryotic vs Eukaryotic Cell Structure★★☆☆☆⏱ 10 min
Prokaryotic Cell
Unicellular cells that lack a membrane-bound nucleus and membrane-bound specialized organelles.
Example:
Escherichia coli, a common gut bacterium, is a typical prokaryotic cell.
Eukaryotic cells share several differences from prokaryotic cells that are tested repeatedly in IB exams, both in multiple choice and extended response.
Structural Feature | Prokaryote | Eukaryote |
|---|---|---|
Nucleus | Absent (nucleoid region only) | Present (membrane-bound) |
Typical diameter | 0.1–5 μm | 10–100 μm |
Membrane-bound organelles | Absent | Present (mitochondria, ER, etc) |
Ribosome size | 70S | 80S |
Cell wall | Almost always present (peptidoglycan) | Present in plants/fungi, absent in animals |
A student observes a 100 μm cell with a membrane-bound nucleus and 80S ribosomes. Identify if this cell is prokaryotic or eukaryotic, and justify your answer.
- 1
- Check cell size: typical prokaryotes are 0.1–5 μm, while eukaryotes are 10–100 μm. 100 μm falls in the eukaryotic size range.
- 2
- Check for nucleus: prokaryotes have no membrane-bound nucleus, while eukaryotes do. This cell has a nucleus.
- 3
- Check ribosome size: prokaryotes have 70S ribosomes, while eukaryotes have 80S ribosomes. This cell matches the eukaryotic ribosome size.
- 4
- Conclusion: The cell is eukaryotic, as it matches all key defining features of eukaryotic cells.
Exam tip:
Ribosome size is a common Paper 1 trick question: remember 70S = prokaryote, 80S = eukaryote.
3. Surface Area to Volume Ratio★★☆☆☆⏱ 7 min
As a cell increases in size, its volume grows much faster than its surface area. All nutrient and waste exchange occurs across the cell membrane, so the ratio of surface area to volume (SA:V) limits maximum cell size.
A lower SA:V means less membrane is available per unit of volume to exchange substances, making cell metabolism inefficient. This is why cells remain small, and multicellular organisms develop specialized exchange surfaces for large-scale transport.
Calculate the SA:V ratio for a cube-shaped cell with side length 2 μm.
- 1
- Calculate total surface area of the cube. A cube has 6 identical faces, each with area equal to side × side:
- 2
- 3
- Calculate volume of the cube:
- 4
- 5
- Simplify to get the final SA:V ratio:
- 6
Exam tip:
Always simplify your ratio to the smallest whole numbers for full marks in calculation questions.
4. Common Pitfalls
Wrong move:
Claiming that viruses are an exception to cell theory because they lack cells.
Why:
Cell theory only applies to living organisms. Viruses are not classified as living, so they do not violate the theory.
Correct move:
Only list accepted exceptions (striated muscle, giant algae, aseptate fungi) when asked.
Wrong move:
Mixing up prokaryotic and eukaryotic ribosome sizes, writing 80S for prokaryotes.
Why:
This is one of the most common mistakes tested in IB Paper 1 multiple choice.
Correct move:
Memorize: 70S = Prokaryote, 80S = Eukaryote.
Wrong move:
Claiming that SA:V increases as cell size increases.
Why:
Volume increases faster than surface area when cell size grows, so the ratio decreases, not increases.
Correct move:
Remember: larger cell = lower SA:V, smaller cell = higher SA:V.
Wrong move:
Stating that all cells follow all tenets of cell theory with no exceptions.
Why:
IB syllabi explicitly require knowledge of accepted exceptions, so omitting them loses marks in extended response.
Correct move:
Acknowledge accepted exceptions when discussing cell theory in exam answers.
5. Quick Reference Cheatsheet
Concept | Key Exam Points |
|---|---|
Cell Theory Tenets |
|
Accepted Exceptions | Striated muscle, giant algae, aseptate fungal hyphae |
Prokaryote Features | No nucleus, 70S ribosomes, 0.1-5 μm, no membrane organelles |
Eukaryote Features | Nucleus, 80S ribosomes, 10-100 μm, membrane-bound organelles |
SA:V Rule | Larger cell = lower SA:V, limits maximum cell size |
6. Frequently Asked
Why are viruses not considered exceptions to cell theory?
Cell theory only applies to living organisms. Viruses are not classified as living, so they do not violate the tenets of the theory.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 1
Compare prokaryote and eukaryote structure
- 2023 · 2
Discuss exceptions to cell theory
- 2024 · 1
Calculate SA:V for a cell
Going deeper
What's Next
Understanding core cell properties is the foundation for all further topics in IB Biology. The core theme of form matching function that you explored here runs through every topic, from physiology to genetics. Mastering cell theory, prokaryote/eukaryote comparisons, and SA:V ratio gives you a strong base for more complex topics like membrane transport, cell signaling, and cell division. These foundational concepts are also frequently revisited in Paper 3 extended response and data analysis questions.