What is meant by the ultrastructure of cells in the context of IB DP Biology?

In IB DP Biology, the ultrastructure of cells refers to the detailed structure of the cell and its organelles as revealed by electron microscopy. This includes the intricate architecture of cell components like the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and ribosomes, which are not visible under a light microscope.

How does electron microscopy contribute to our understanding of cell ultrastructure?

Electron microscopy provides a much higher resolution than light microscopy, allowing scientists to observe the ultrastructure of cells in great detail. This technology enables the visualization of small organelles and complex structures within cells, such as the double membrane of mitochondria or the layered structure of the Golgi apparatus, which are crucial for understanding cell function and biology.

What are the main differences between prokaryotic and eukaryotic cell ultrastructure?

Prokaryotic cells, such as bacteria, lack membrane-bound organelles and have a simpler ultrastructure compared to eukaryotic cells. Eukaryotic cells, found in plants, animals, and fungi, have a nucleus and various organelles like mitochondria and chloroplasts, each with specific functions. The complexity of eukaryotic cell ultrastructure supports more advanced cellular processes and specialization.

Why is understanding the ultrastructure of cells important in cell biology?

Understanding the ultrastructure of cells is crucial in cell biology because it provides insights into how cells function at a molecular level. Knowledge of cell ultrastructure helps explain how organelles interact, how cellular processes are carried out, and how cells respond to their environment. This understanding is fundamental for fields like medicine, genetics, and biotechnology.

What role do ribosomes play in the ultrastructure of cells?

Ribosomes are essential components of the cell's ultrastructure, responsible for protein synthesis. They can be found floating freely in the cytoplasm or attached to the endoplasmic reticulum, forming rough ER. Ribosomes translate genetic information from mRNA to synthesize proteins, which are vital for various cellular functions and processes, making them crucial for maintaining cell health and activity.

Why is "Claiming prokaryotes have no ribosomes." a common mistake in Ultrastructure of Cells?

Why it happens: Confusing 'no organelles' with 'no ribosomes'. How to avoid it: Prokaryotes HAVE ribosomes (70S) — they are not membrane-bound, so not 'organelles' in the strict sense.

Why is "Drawing mitochondria with a single membrane." a common mistake in Ultrastructure of Cells?

Why it happens: Confusion with vesicles. How to avoid it: Mitochondria have a DOUBLE membrane — outer smooth, inner folded into cristae.

Why is "Claiming plants don't have mitochondria." a common mistake in Ultrastructure of Cells?

Why it happens: Belief that plants only do photosynthesis. How to avoid it: Plants do BOTH — chloroplasts make sugar, mitochondria respire it. Both present.

IB DP Biology (BI)

Ultrastructure of Cells

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Short Notes - Ultrastructure of Cells

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Detailed Study Notes

Detailed notes on Cell Biology for IB DP Biology, covering key concepts, explanations, examples, and exam-focused revision points.

Ultrastructure of Cells — IB Biology SL: prokaryotes vs eukaryotes, organelles, electron micrographs

Maps to Theme B (Form and Function) of the 2025+ syllabus. Compares prokaryotic and eukaryotic cell architecture, the function of every named organelle, and the differences between plant and animal cells.

At a glance

PROKARYOTES (bacteria): no nucleus; 70S ribosomes; circular DNA in nucleoid; cell wall of peptidoglycan.
EUKARYOTES (plant, animal, fungi, protists): membrane-bound nucleus; 80S ribosomes; compartmentalised organelles.
ANIMAL ONLY: centrioles, no chloroplasts, no large vacuole, no cell wall.
PLANT ONLY: cell wall (cellulose), large central vacuole, chloroplasts.
ENDOMEMBRANE: nucleus → rough ER → Golgi → vesicles → membrane (protein secretion).
MITOCHONDRION: ATP synthesis by aerobic respiration; double membrane; own DNA.
ELECTRON MICROSCOPE >> light microscope in resolution (uses electrons, λ much shorter).

What you’ll learn

Mapped to the IB DP Biology SL subject guide (2025 onwards (first assessment May 2025)).

B2.2.1 — Compare prokaryotic and eukaryotic ultrastructure.
B2.2.2 — Name and describe the function of each eukaryotic organelle.
B2.2.3 — Distinguish plant from animal cells.
B2.2.4 — Interpret electron micrographs.

Prokaryotes vs eukaryotes

Two architectures for life.

Prokaryotic cells (Bacteria, Archaea). Small (1–5 μm), no membrane-bound organelles.

Cell wall of peptidoglycan (in bacteria).
Plasma membrane beneath the wall.
Cytoplasm containing 70S ribosomes.
Nucleoid — region with one circular DNA chromosome (not membrane-bound).
Plasmids — small extra circles of DNA (can confer antibiotic resistance).
Pili (attachment) and flagella (movement).

Eukaryotic cells (animals, plants, fungi, protists). Larger (10–100 μm), with a true nucleus and many membrane-bound organelles.

Endomembrane system (protein secretion pathway):

Nucleus — DNA → mRNA via transcription.
Rough ER — ribosomes attached; protein synthesis + folding.
Golgi apparatus — modifies, sorts, and packages proteins into vesicles.
Vesicles — transport to plasma membrane; secretion by exocytosis.

Other organelles:

Mitochondrion — aerobic respiration (ATP). Double membrane; inner folded into cristae.
Smooth ER — lipid synthesis; detoxification.
Lysosomes (animal cells) — digestive enzymes; break down worn organelles.
Cytoskeleton — microfilaments and microtubules; shape, transport, division.

Prokaryote: no nucleus; 70S ribosomes; nucleoid + plasmids.
Eukaryote: nucleus; 80S ribosomes; compartmentalised.
Endomembrane pathway exports proteins.

Plant vs animal cells

Extra plant structures.

Plant cells only:

Cell wall — cellulose; rigid; gives shape; freely permeable.
Large central vacuole — turgor (water pressure); storage of pigments and waste.
Chloroplasts — photosynthesis; double membrane; contain thylakoids stacked into grana.
Plasmodesmata — channels connecting adjacent plant cells.

Animal cells only:

Centrioles — organise spindle in mitosis.
Lysosomes — common in animals; rare in plants.
Small temporary vacuoles rather than one large one.

Comparison summary.

Feature	Plant	Animal
Cell wall	Yes (cellulose)	No
Chloroplast	Yes	No
Large vacuole	Yes (central)	No (small only)
Centrioles	No	Yes
Shape	Fixed (rectangular)	Flexible (variable)

Resolution and magnification. Light microscopes resolve to ~200 nm; electron microscopes to ~0.1 nm. The shorter wavelength of electrons gives higher resolution — letting us see ribosomes (~20 nm) and membranes (~7 nm) only with electron microscopy.

Plant: cell wall + vacuole + chloroplast.
Animal: centrioles + flexible shape.
Electron microscope resolution ≫ light microscope.

Quick recap

Prokaryotes lack nucleus; circular DNA in nucleoid.
Eukaryotes have nucleus + organelles.
Endomembrane: nucleus → rough ER → Golgi → vesicle → membrane.
Plant cells have walls, vacuoles, chloroplasts.
EM resolves down to 0.1 nm vs LM 200 nm.

Memorise this

Verbatim phrases, formulae and definitions IB DP mark schemes credit (key for AO1 knowledge marks on Paper 1).

Prokaryote: 70S ribosomes; nucleoid; peptidoglycan wall
Eukaryote: 80S ribosomes; membrane-bound nucleus
Mitochondrion: double membrane; cristae; own DNA
Plant only: wall, vacuole, chloroplast

How it’s examined

Paper 1: MCQ identifying organelles from electron micrographs; prokaryote vs eukaryote facts. Paper 2: 'compare and contrast' table; 'describe the role of the rough ER in protein secretion'.

Sources: IB Diploma Programme Biology subject guide (IBO, 2023 — first assessment 2025). Last reviewed 2026-05-31.

Take this whole topic with you

Step-by-step worked examples — Ultrastructure of Cells

Step-by-step solutions to past-paper-style questions on ultrastructure of cells, written exactly the way a tutor would explain them at the board.

1Trace a protein from gene to secretion
Stretch• endomembrane
Question
Outline how a protein hormone is synthesised and exported from a eukaryotic cell. (4 marks)
Step-by-step solution
1. Step 1
  Gene in nucleus is transcribed to mRNA which exits via the nuclear pore.
2. Step 2
  mRNA translated by ribosomes attached to rough ER; protein folds in ER lumen.
3. Step 3
  Vesicles bud off from ER → carry protein to Golgi for modification (e.g. glycosylation).
4. Step 4
  Secretory vesicles from Golgi fuse with plasma membrane → exocytosis releases protein outside.
Answer
Nucleus → rough ER → Golgi → vesicle → plasma membrane (exocytosis).
2Compare prokaryotic and eukaryotic cells
Building confidence• comparison
Question
Give THREE differences between prokaryotic and eukaryotic cells. (3 marks)
Step-by-step solution
1. Step 1
  Nucleus: prokaryote has nucleoid (no membrane); eukaryote has membrane-bound nucleus.
2. Step 2
  Ribosomes: 70S in prokaryote; 80S in eukaryote.
3. Step 3
  Organelles: prokaryote lacks membrane-bound organelles; eukaryote has mitochondria, ER, Golgi.
Answer
Nucleus (none vs membrane-bound); ribosomes (70S vs 80S); membrane-bound organelles (absent vs present).
3Identify organelles on electron micrograph
Getting started• EM
Question
On an electron micrograph, you see a double-membraned structure with inner folds. Identify it and state its function. (2 marks)
Step-by-step solution
1. Step 1
  Mitochondrion.
2. Step 2
  Site of aerobic respiration; produces ATP.
Answer
Mitochondrion — aerobic respiration / ATP production.
4Distinguishing plant from animal
Building confidence• comparison
Question
List FOUR features that distinguish a plant cell from an animal cell. (4 marks)
Step-by-step solution
1. Step 1
  Cellulose cell wall (plant only).
2. Step 2
  Chloroplasts (plant only).
3. Step 3
  Large central vacuole (plant only).
4. Step 4
  Centrioles (animal only) / fixed rectangular shape (plant) vs flexible (animal).
Answer
Cell wall, chloroplasts, large vacuole, centrioles (animal-only).

Key Definitions and Keywords — Ultrastructure of Cells

Definitions to memorise and the exact keywords mark schemes credit for ultrastructure of cells answers — sharpened from recent examiner reports for the 2026 IB DP Biology SL sitting.

Prokaryotic cell
Examiner keyword
Cell without a membrane-bound nucleus or organelles; DNA in nucleoid.
Eukaryotic cell
Examiner keyword
Cell with a membrane-bound nucleus and other membrane-bound organelles.
Endomembrane system
Examiner keyword
Connected network of nuclear envelope, ER, Golgi and vesicles that synthesises, modifies and exports proteins.

Common Mistakes and Misconceptions — Ultrastructure of Cells

The traps other students keep falling into on ultrastructure of cells questions — taken from recent IB DP Biology SL examiner reports and mark schemes — and how to avoid them.

✕Claiming prokaryotes have no ribosomes.
Why it happens
Confusing 'no organelles' with 'no ribosomes'.
How to avoid it
Prokaryotes HAVE ribosomes (70S) — they are not membrane-bound, so not 'organelles' in the strict sense.
✕Drawing mitochondria with a single membrane.
Why it happens
Confusion with vesicles.
How to avoid it
Mitochondria have a DOUBLE membrane — outer smooth, inner folded into cristae.
✕Claiming plants don't have mitochondria.
Why it happens
Belief that plants only do photosynthesis.
How to avoid it
Plants do BOTH — chloroplasts make sugar, mitochondria respire it. Both present.

Ultrastructure of Cells — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Ultrastructure of Cells

Short Notes - Ultrastructure of Cells

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Trace a protein from gene to secretion

2Compare prokaryotic and eukaryotic cells

3Identify organelles on electron micrograph

4Distinguishing plant from animal

Prokaryotic cell

Eukaryotic cell

Endomembrane system

✕Claiming prokaryotes have no ribosomes.

✕Drawing mitochondria with a single membrane.

✕Claiming plants don't have mitochondria.

Ultrastructure of Cells — frequently asked questions