Cell Structure
IB Biology SLΒ· 15 min read
1. Cell Theory and Core Cell Propertiesβ β ββββ± 4 min
Cell Theory
Three core tenets describing the fundamental properties of all living cells: 1. All living organisms are composed of one or more cells, 2. The cell is the basic unit of life, 3. All cells arise from pre-existing cells.
Example:
Striated muscle fibers are a widely cited exception, as they are large, functional cells with multiple nuclei.
All cells, regardless of type, share three core features: a phospholipid cell membrane that separates the inside of the cell from the external environment, cytoplasm (a gel-like matrix of dissolved solutes), and genetic material to store hereditary information. Accepted exceptions to cell theory do not disprove the core tenets, they are just specialized adaptations.
A student observes that striated muscle fibers are large cells with multiple nuclei. They claim this contradicts cell theory. Evaluate their claim.
- 1
Recall the three core tenets of cell theory to test the claim against.
- 2
This observation does not contradict any core tenet: the muscle fiber is still a functional unit of life, it arises from pre-existing cells, and it is still a form of living cell.
- 3
The claim is incorrect. Multiple nuclei are a specialized adaptation, not a counterexample to cell theory.
Exam tip:
Always be prepared to name specific common exceptions (striated muscle, aseptate fungal hyphae, giant algae) when asked about cell theory in exams.
2. Prokaryotic Cell Structureβ β ββββ± 4 min
Prokaryotic Cell
A small, unicellular cell that lacks a membrane-bound nucleus and membrane-bound specialized organelles. Bacteria and archaea are the two domains of prokaryotes.
Example:
Escherichia coli, a common gut bacterium, is a model prokaryote.
Prokaryotic cells are typically 0.1-5 ΞΌm in diameter, much smaller than most eukaryotic cells. Their genetic material is a single circular chromosome located in the nucleoid region, which is not enclosed by a membrane. Most prokaryotes also have a rigid cell wall made of peptidoglycan outside the cell membrane.
State three structural features found in prokaryotic cells that are not present in animal eukaryotic cells.
- 1
- Prokaryotes have a peptidoglycan cell wall, which animal cells do not have at all.
- 2
- Prokaryotes have unbound circular DNA in the nucleoid region, while animal cells have linear DNA enclosed in a nuclear membrane.
- 3
- Prokaryotes have 70S ribosomes, while animal eukaryotic cytoplasmic ribosomes are 80S. Other valid answers include pili or prokaryotic flagella.
3. Eukaryotic Organelle Structure and Functionβ β β βββ± 5 min
Eukaryotic Cell
A cell that contains a membrane-bound nucleus and membrane-bound specialized organelles that compartmentalize cellular functions, increasing efficiency.
Organelle | Key Structure | Core Function |
|---|---|---|
Nucleus | Double nuclear envelope with pores | Stores genetic material, controls gene expression |
Mitochondrion | Double membrane, inner folded into cristae | Site of aerobic respiration, produces ATP |
Rough ER | Membrane network with bound 80S ribosomes | Synthesizes proteins for secretion |
Golgi Apparatus | Stack of flattened membrane cisternae | Modifies and packages proteins for export |
Chloroplast | Double membrane, internal thylakoid stacks | Site of photosynthesis in plant cells |
Explain which two organelles are most abundant in a cell that actively secretes protein enzymes.
- 1
Enzymes are proteins, so they are synthesized by ribosomes attached to the rough endoplasmic reticulum (RER). High secretion requires many RER ribosomes.
- 2
After synthesis, proteins are transported to the Golgi apparatus, where they are modified and packaged into secretory vesicles for release from the cell.
- 3
Thus, rough endoplasmic reticulum and Golgi apparatus are the two organelles most strongly associated with active protein secretion.
Exam tip:
Always link a specific structural feature to function when asked. For example: "Cristae in mitochondria increase surface area for respiration enzymes" earns full marks.
4. Plant vs Animal Cell Structureβ β ββββ± 2 min
Plant and animal cells are both eukaryotic, so they share most organelles, but have key differences related to their specialized functions:
Plant cells have a rigid cellulose cell wall outside the cell membrane; animal cells have no cell wall
Mature plant cells have a large central vacuole for water storage; animal cells only have small temporary vacuoles
Photosynthetic plant cells have chloroplasts; animal cells never have chloroplasts
Animal cells have centrioles for spindle formation; most plant cells lack centrioles
An unknown cell observed under an electron microscope has a cell wall, nucleus, and 80S ribosomes. Can this be an animal cell? Explain.
- 1
No, this cannot be an animal cell.
- 2
Animal cells never produce a permanent cell wall as part of their structure.
- 3
The presence of a cell wall, 80S ribosomes and a nucleus confirms this is a eukaryotic cell from a plant, fungus, or protist, not an animal.
5. Common Pitfalls
Wrong move:
Claiming prokaryotes do not have ribosomes
Why:
Prokaryotes require ribosomes to synthesize proteins; they only have smaller ribosomes than eukaryotes
Correct move:
State that prokaryotes have 70S ribosomes, while eukaryotes have 80S cytoplasmic ribosomes
Wrong move:
Confusing cell wall composition across groups
Why:
IB exams regularly test differences in cell wall composition between kingdoms
Correct move:
Remember: prokaryotes = peptidoglycan, plants = cellulose, fungi = chitin
Wrong move:
Stating the nucleus is the site of protein synthesis
Why:
Students often mix up the location of DNA with the site of translation
Correct move:
The nucleus stores DNA and performs transcription; protein synthesis occurs at ribosomes
Wrong move:
Claiming all plant cells have chloroplasts
Why:
Chloroplasts are only required for photosynthesis, so are not present in all plant tissues
Correct move:
Only photosynthetic plant cells (e.g. leaf mesophyll) have chloroplasts; root cells do not
6. Quick Reference Cheatsheet
Feature | Prokaryote | Animal Eukaryote | Plant Eukaryote |
|---|---|---|---|
Membrane-bound nucleus | No (nucleoid) | Yes | Yes |
Membrane organelles | No | Yes | Yes |
Ribosome size | 70S | 80S | 80S |
Cell wall | Yes (peptidoglycan) | No | Yes (cellulose) |
Chloroplasts | No | No | Yes (photosynthetic cells) |
Central vacuole | No | No (small only) | Yes |
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
Organelle identification from micrograph
- 2024 Β· 2
Compare prokaryote and eukaryote structure
- 2023 Β· 1
Exceptions to cell theory
Going deeper
What's Next
Understanding cell structure is the foundation for all subsequent topics in IB Biology, from membrane transport to cell division, molecular biology, and organism physiology. The compartmentalization of organelles you learned here underpins how cells carry out specialized functions in multicellular organisms, which we explore in topics on cell differentiation and tissue organization. Mastering organelle identification and function will also help you interpret common electron micrograph questions that appear regularly in both Paper 1 and Paper 2 exams. Next, you will build on this knowledge to explore how molecules move across cell membranes and how eukaryotic cells evolved from prokaryotic ancestors.