Cell Biology & Ultrastructure

How to Use This Guide

• Cell Theory & Types — the three tenets, prokaryote vs eukaryote
• Eukaryotic Ultrastructure — every organelle, its function, and what IB asks you to draw
• Cell Division HL — the cell cycle, mitosis stages, mitosis vs meiosis
• MCQ Practice — styled like real IB Paper 1 questions
• Exam Alerts — the exact traps that cost marks in A1/D1 questions

Aligned to IB Biology 2025 syllabus — A1.1 Unity and Diversity — A1.2 Cell Ultrastructure — D1.1 Cell Division

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Jump to section: Cell Theory & Types · Eukaryotic Ultrastructure · Cell Division · Exam Strategy

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Section 1: Cell Theory & Types

The Three Tenets of Cell Theory

Cell theory is the foundational framework of modern biology. Every IB Biology student must be able to state all three tenets precisely.

Prokaryotes vs Eukaryotes

The most fundamental division in cell biology is between prokaryotic and eukaryotic cells. Expect at least one MCQ on this distinction in every IB Paper 1.

Feature	Prokaryote	Eukaryote
Membrane-bound nucleus	Absent — nucleoid region only	Present
DNA location	Circular DNA in nucleoid region; plasmids possible	Linear DNA in membrane-bound nucleus
Ribosomes	70S (smaller)	80S cytoplasmic (70S in mitochondria/chloroplasts)
Membrane-bound organelles	Absent	Present (mitochondria, ER, Golgi, etc.)
Cell wall	Usually present (peptidoglycan in bacteria)	Present in plants/fungi (different composition); absent in animals
Size	Typically 1–10 µm	Typically 10–100 µm
Examples	Bacteria, archaea	Animals, plants, fungi, protists

Prokaryotic Cell Structure (Bacteria)

The following components are testable for IB. You may be asked to label a diagram.

Structure	Description
Nucleoid region	Area containing the circular, naked (no histone) DNA
Plasmid	Small, circular, extra-chromosomal DNA; not always present
Ribosomes (70S)	Site of protein synthesis; smaller than eukaryotic ribosomes
Cell wall	Rigid layer of peptidoglycan providing structural support
Plasma membrane	Phospholipid bilayer controlling substance entry/exit
Capsule	Outer polysaccharide layer; aids attachment and protection (not always present)
Flagella	Long protein filaments used for locomotion
Pili	Short protein projections used for attachment and conjugation

Scale and Measurement

Cells are measured in micrometres (µm) and nanometres (nm).

$1 \text{ mm} = 1000\ \mu\text{m} = 1{,}000{,}000 \text{ nm}$

• Typical bacterial cell: 1–10 µm
• Typical eukaryotic cell: 10–100 µm
• Mitochondrion: ~1–10 µm (similar to a bacterium — relevant to endosymbiotic theory)

MCQ Practice — Section 1

Question 1. Which of the following is a feature found in prokaryotic cells but NOT in eukaryotic cells?

A. Ribosomes

B. Plasma membrane

C. Circular DNA with no associated histones

D. Cell wall

Reveal answer

C — Circular, non-histone-associated DNA is a prokaryotic feature. Eukaryotic DNA is linear and wound around histone proteins. Options A, B, and D are found in both cell types (though cell walls differ in composition).

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Question 2. A student views a cell under the electron microscope and observes ribosomes, a nucleoid region, and peptidoglycan in the cell wall. What type of cell is this?

A. Plant cell

B. Fungal cell

C. Animal cell

D. Bacterial cell

Reveal answer

D — Peptidoglycan is a uniquely prokaryotic cell wall component (specifically bacterial). The nucleoid region (not a membrane-bound nucleus) confirms prokaryote. All three features together identify this as a bacterium.

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Quick Recall — Section 1

Try to answer without scrolling up:

1. State the three tenets of cell theory.
2. Do prokaryotes have a nucleus?
3. What type of ribosomes do prokaryotes have?

Reveal answers

1. (1) All organisms are composed of one or more cells. (2) The cell is the basic structural and functional unit of life. (3) All cells arise from pre-existing cells.
2. No — they have a nucleoid region, which is an area of cytoplasm containing circular DNA. There is no membrane-bound nucleus.
3. 70S ribosomes (smaller than the 80S ribosomes found in eukaryotic cytoplasm).

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Section 2: Eukaryotic Ultrastructure

Animal vs Plant Cell — Key Differences

Feature	Animal Cell	Plant Cell
Cell wall	Absent	Present (cellulose)
Chloroplasts	Absent	Present
Large central vacuole	Absent (small vacuoles only)	Present — provides turgor pressure
Centrioles	Present	Absent in most plants
Shape	Irregular	More regular, often rectangular

Organelle Functions — Testable Table

This table is frequently tested. Know the function of every organelle listed.

Organelle	Key Function	IB Detail
Nucleus	Stores DNA; directs protein synthesis	Double membrane (nuclear envelope) with nuclear pores; contains nucleolus (rRNA synthesis)
Mitochondria	ATP production via aerobic respiration	Double membrane; inner membrane folded into cristae; matrix contains enzymes for Krebs cycle
Rough ER (RER)	Protein synthesis and transport	Covered with 80S ribosomes; continuous with outer nuclear membrane
Smooth ER (SER)	Lipid synthesis; detoxification	No ribosomes; prominent in liver cells and steroid-producing cells
Golgi apparatus	Modifies, packages, and routes proteins	Receives vesicles from RER; sorts proteins to lysosomes, plasma membrane, or secretion
Ribosomes (80S)	Protein synthesis	Free in cytoplasm or bound to RER; 70S in mitochondria and chloroplasts
Lysosomes	Intracellular digestion	Contain hydrolytic enzymes; bud from Golgi; digest old organelles (autophagy) and pathogens
Large central vacuole	Turgor pressure; storage	Plant cells only; filled with cell sap; pushes cytoplasm against cell wall
Chloroplasts	Photosynthesis	Plant cells only; double membrane; thylakoid membrane + stroma; 70S ribosomes
Cell wall	Structural support; prevents over-expansion	Plants: cellulose; Fungi: chitin; Bacteria: peptidoglycan
Centrioles	Organise spindle fibres during cell division	Animal cells only; absent in most plant cells; found in pairs (centrosome)

Generalised Eukaryotic Cell Diagram

Magnification and Scale

IB Paper 1 and Paper 2 both test magnification calculations.

$\text{Magnification} = \frac{\text{image size}}{\text{actual size}}$

Rearranged:

$\text{Actual size} = \frac{\text{image size}}{\text{magnification}}$

MCQ Practice — Section 2

Question 1. Which of the following correctly identifies a structural difference between rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER)?

A. RER has a double membrane; SER has a single membrane.

B. RER is covered with ribosomes; SER has no ribosomes.

C. RER produces lipids; SER synthesises proteins.

D. RER is found only in animal cells; SER is found in all eukaryotes.

Reveal answer

B — The defining structural feature is the presence (RER) or absence (SER) of ribosomes on the cytoplasmic face. C reverses the functions (RER is associated with protein synthesis/transport; SER with lipid synthesis). A and D are incorrect.

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Question 2. A cell was treated with a drug that destroys the Golgi apparatus. Which process would be MOST directly impaired?

A. DNA replication

B. ATP production

C. Secretion of proteins to the cell exterior

D. Transcription of mRNA

Reveal answer

C — The Golgi apparatus modifies, packages, and routes proteins for secretion (exocytosis), to the plasma membrane, or to lysosomes. Without it, the secretory pathway is blocked. DNA replication (A) and transcription (D) are nuclear processes; ATP production (B) occurs in mitochondria.

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Quick Recall — Section 2

Try to answer without scrolling up:

1. Which organelle modifies and packages proteins for export?
2. What structural feature distinguishes mitochondria from most other organelles?
3. Which organelle is absent in plant cells but present in animal cells?

Reveal answers

1. The Golgi apparatus.
2. Mitochondria have a double membrane — an outer membrane and an inner membrane folded into cristae.
3. Centrioles — present in animal cells, absent in most plant cells. (Chloroplasts and large central vacuoles are the reverse — plant only.)

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Section 3: Cell Division HL

Syllabus reference: D1.1 Cell Division

The Cell Cycle

The cell cycle describes the ordered series of events a cell undergoes from formation to division. It has two major phases:

1. Interphase — the cell grows and replicates its DNA
2. Mitotic phase (M phase) — the cell divides

Interphase Sub-stages

Sub-stage	What Happens
G1 (Gap 1)	Cell grows; organelles replicate; proteins are synthesised
S (Synthesis)	DNA replication — each chromosome is duplicated to form two sister chromatids
G2 (Gap 2)	Further growth; cell prepares for division; DNA checked for errors

Stages of Mitosis

Mitosis produces two genetically identical daughter cells from one parent cell ($2n \rightarrow 2n$).

Mitosis vs Meiosis — Key Differences

Feature	Mitosis	Meiosis
Purpose	Growth; repair; asexual reproduction	Production of gametes (sexual reproduction)
Number of divisions	1	2 (Meiosis I + Meiosis II)
Starting cell	Diploid ($2n$)	Diploid ($2n$)
Daughter cells produced	2	4
Ploidy of daughter cells	Diploid ($2n$)	Haploid ($n$)
Genetic outcome	Genetically identical to parent	Genetically varied (crossing over + independent assortment)
Crossing over	Does not occur	Occurs in prophase I (between homologous chromosomes)
Homologous pairs align at equator?	No	Yes — in metaphase I

MCQ Practice — Section 3

Question 1. During which phase of the cell cycle does DNA replication occur?

A. G1 phase

B. G2 phase

C. S phase

D. Prophase

Reveal answer

C — DNA replication takes place during S (Synthesis) phase of interphase. By the time prophase begins, each chromosome already consists of two identical sister chromatids.

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Question 2. A cell has a diploid number of 8 ($2n = 8$). After completing meiosis, what is the chromosome number in each daughter cell?

A. 8

B. 4

C. 2

D. 16

Reveal answer

B — Meiosis halves the chromosome number (diploid → haploid). If $2n = 8$, then $n = 4$. The four resulting cells each contain 4 chromosomes.

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Quick Recall — Section 3

Try to answer without scrolling up:

1. Name the three sub-stages of interphase.
2. What happens to sister chromatids during anaphase of mitosis?
3. How does the chromosome number change from parent cell to daughter cells in meiosis?

Reveal answers

1. G1 (first gap/growth), S (DNA synthesis/replication), G2 (second gap/growth and preparation).
2. Sister chromatids separate — spindle fibres shorten and pull each chromatid to opposite poles of the cell, so each pole receives one complete set of chromosomes.
3. Meiosis halves the chromosome number: $2n \rightarrow n$. A diploid parent cell produces four haploid daughter cells.

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Section 4: Exam Strategy

Top 5 Mistakes on A1/D1 Questions

1. Writing “nucleus” instead of “nucleoid region” for prokaryotes. Prokaryotes have no membrane-bound nucleus. The IB mark scheme will not accept “nucleus” for a prokaryotic cell. Use the correct term: nucleoid region.

2. Confusing 70S and 80S ribosomes. Remember: prokaryotes, mitochondria, and chloroplasts all use 70S ribosomes. Eukaryotic cytoplasmic ribosomes are 80S. The endosymbiotic theory uses this fact as evidence.

3. Stating that DNA replication happens during mitosis. It does not — replication is complete before mitosis begins (S phase of interphase). During mitosis, the already-replicated chromosomes are simply sorted and separated.

4. Confusing what separates in anaphase. In mitotic anaphase: sister chromatids. In meiosis I anaphase: homologous chromosomes. Drawing a quick sketch and counting chromatid strands often resolves the confusion.

5. Losing drawing marks for incorrect proportions or missing double membranes. The nucleus and mitochondria both have double membranes — always draw two lines, not one. A nucleus should be drawn as roughly 10–20% of the cell’s diameter.

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Virtual Lab Alignment: Labster Simulations

Labster Simulation	IB HL Topic	What It Covers
Cell Organelles: Be a Cell Biologist	A1: Ultrastructure of eukaryotic cells	Organelle function, endosymbiosis evidence
Cell Division: Observe Mitosis	A1/A2: Cell cycle and mitosis	Cell cycle stages, mitosis, cytokinesis
Cell Membrane: The Gatekeeper	A1: Fluid mosaic model	Phospholipid bilayer, membrane proteins, transport mechanisms
Confocal Microscopy: Imaging the Cell	A1: Techniques for visualising cell structure	Resolution, microscopy methods HL

How to use these simulations for IB exam prep:

• The Cell Organelles simulation tests whether you can match structure to function — exactly what Paper 1 MCQs test
• Use the Cell Membrane simulation to visualise facilitated diffusion vs active transport before answering Paper 2 data-based questions
• The microscopy simulation helps with Paper 3 questions on experimental methods