IB DP Biology: Organelles, compartmentalization and cell specialization (DP Biology) – Revision Notes

These notes condense sections B2.2 and B2.3 of the IB Diploma Programme Biology guide, first assessment 2025. For full explanations and worked examples, read the study guide. Test yourself with the practice set, then use the IB Biology hub, the printable checklist and the free diagnostics. B2.2.1 to B2.2.3 and B2.3.1 to B2.3.6 are SL and HL. Everything labelled HL only is additional higher level (B2.2.4 to B2.2.9, B2.3.7 to B2.3.10).

Key definitions

Term	Meaning
Organelle	Discrete subunit of a cell with a specific function
Not organelles	Cell wall, cytoskeleton, cytoplasm
Compartmentalization	Separating cell regions by membranes so reactions occur apart
Differentiation	Development of an unspecialized cell into a specialized one by gene expression
Stem cell	Cell able to divide endlessly and to differentiate along different pathways
Niche	Microenvironment that maintains stem cells or promotes their proliferation and differentiation
Totipotent	Can form every cell type, including a whole organism
Pluripotent	Can form most body cell types, not a whole organism
Multipotent	Can form a limited range of related cell types

B2.2 Organelles: recall points

B2.2.1. Nuclei, vesicles, ribosomes, mitochondria and the plasma membrane are organelles. Cell fractionation uses a homogenate and an ultracentrifuge. Denser structures pellet at lower speeds.

B2.2.2. Eukaryote: transcription in the nucleus, then post-transcriptional modification of mRNA, then export, then translation in the cytoplasm. Prokaryote: no nucleus, so ribosomes can meet mRNA immediately and no modification is possible first.

B2.2.3. Advantages of compartments:

• Enzymes and substrates are concentrated.
• Each compartment can have its own conditions, such as pH.
• Incompatible reactions are separated.

Examples: lysosomes hold hydrolytic enzymes that would damage the cytoplasm. Phagocytic vacuoles hold engulfed material, then fuse with lysosomes.

HL only: mitochondrion, chloroplast and nucleus

Structure	Adaptation	Why it helps
Mitochondrion	Small intermembrane space	Protons build up quickly, so a steep gradient forms
Mitochondrion	Cristae	More surface for electron transport chains and ATP synthase
Mitochondrion	Matrix	Krebs cycle enzymes and substrates concentrated
Chloroplast	Thylakoid membranes	Large area carries photosystems
Chloroplast	Small thylakoid volume	Protons build up quickly inside
Chloroplast	Stroma	Calvin cycle enzymes and substrates concentrated
Nuclear envelope	Double membrane with pores	Controls transport of mRNA and proteins, and can break into vesicles in mitosis and meiosis

HL only: the protein route

1. Ribosomes make a protein, free or on the rough ER.
2. Protein from rough ER ribosomes enters the ER lumen and folds.
3. A transport vesicle carries it to the Golgi apparatus.
4. The Golgi apparatus processes and sorts the protein.
5. A vesicle carries the protein to the plasma membrane, where exocytosis releases it.

Free ribosomes make proteins kept in the cell. Ribosomes on rough ER make proteins for secretion, for lysosomes or for the membrane. Clathrin assembles into a cage that helps shape a coated vesicle.

B2.3 Cell specialization: recall points

B2.3.1. All cells hold the same genome. Differentiation depends on which genes are expressed. Gradients in an early embryo mean cells at different positions receive different signals, so they express different genes.

B2.3.2. Two properties only: endless division and differentiation along different pathways.

B2.3.3. The niche either maintains stem cells or promotes their proliferation and differentiation. Two examples:

• Bone marrow: haematopoietic stem cells give blood cells.
• Hair follicle: stem cells supply cells for hair growth and renewal.

B2.3.4. Early embryo cells: totipotent, then soon pluripotent. Adult tissue: multipotent.

B2.3.5. Know the size range in humans, from smallest to largest, and link size to function:

Cell	Size note	Link to function
Erythrocyte	About 7 µm across	Small and flat, so oxygen exchanges quickly
White blood cell	Somewhat larger	Room for a nucleus and engulfed material
Egg	About 0.1 mm across	Stores food for early development
Sperm	Small head, long tail	Motile, delivers a haploid nucleus
Neuron	Axon can exceed a metre	Carries signals over long distances
Striated muscle fibre	Up to several centimetres	Contracts along a long, multinucleate cell

Surface area-to-volume ratio

For a cube with side s:

Quantity	Formula
Surface area	6s²
Volume	s³
Ratio	6/s

Exchange across the surface depends on area. The need depends on volume. As a cell grows, volume rises faster than area, so exchange cannot keep up. This sets a limit on cell size. A model with cubes is a simplified version of real cells.

Method for a ratio question. (1) Write the formula. (2) Substitute the side. (3) Divide. (4) State units. (5) Comment on what the number means for exchange.

Method for magnification. actual size = image size ÷ magnification. Convert units before you divide. 1 mm = 1000 µm.

Reminder. A cube of side 3 µm has area 54 µm², volume 27 µm³ and ratio 2 µm⁻¹. A cube of side 6 µm has area 216 µm², volume 216 µm³ and ratio 1 µm⁻¹. Doubling the side halves the ratio.

HL only: specialized cells

Method for explaining a specialized cell. Name the feature, state what it changes, then link that to the cell’s job. For example: microvilli (feature) increase surface area (change), so more solute is reabsorbed from the filtrate per unit of cell volume (job).

Method for a protein-route data question. Find the time when each structure holds its highest share. The order of the peaks gives the order of the route. State the first and last stage, then say whether the protein is secreted or retained.

Cell	Key adaptations
Erythrocyte	Flattened biconcave disc gives high SA:V
Proximal convoluted tubule cell	Microvilli and membrane invaginations raise surface area for reabsorption
Type I pneumocyte	Extremely thin, so diffusion distance is short
Type II pneumocyte	Lamellar bodies release surfactant into the alveolar lumen
Cardiac muscle cell	Myofibrils, branched, usually one or two nuclei
Striated muscle fibre	Myofibrils, unbranched, very long, many nuclei
Sperm	Flagellum, mitochondria for ATP, acrosome with enzymes
Egg	Large, with stored food in the cytoplasm and a protective outer layer

The alveolar epithelium shows that one tissue can contain more than one cell type, because its overall function needs different adaptations. Your answer on the muscle fibre as a cell depends on the definition you use: it is multinucleate and formed by fusion, but it has a single plasma membrane.

Must-know distinctions

• Organelle versus non-organelle: the cell wall is outside the plasma membrane and is not an organelle.
• Totipotent versus pluripotent: only totipotent cells can form extra-embryonic tissue and a whole organism.
• Pluripotent versus multipotent: many cell types versus a limited range.
• Free versus rough ER ribosomes: where the protein ends up.
• Surface area versus SA:V ratio: area gets bigger as cells grow, but the ratio gets smaller.

Quick self-test

1. Name two structures that are not organelles.
2. State one reason why eukaryotic mRNA is made in a separate compartment from translation.
3. A cell is modelled as a cube of side 4 µm. Find its ratio of surface area to volume.
4. A micrograph shows a nucleus 30 mm across at ×6000. Calculate its actual diameter in µm.
5. Give two advantages of compartmentalization shown by lysosomes.
6. (HL only) State why a small intermembrane space helps mitochondria make ATP.
7. (HL only) Which type of ribosome makes a protein that is secreted?
8. State one location of a stem cell niche in an adult human.
9. Distinguish pluripotent from multipotent.
10. (HL only) State the function of lamellar bodies.

Answers

1. Any two of: cell wall, cytoskeleton, cytoplasm.
2. The mRNA can be modified after transcription, before ribosomes reach it. Prokaryotes lack this separation.
3. Area = 6 × 4² = 96 µm². Volume = 4³ = 64 µm³. Ratio = 96 ÷ 64 = 1.5 µm⁻¹.
4. 30 mm ÷ 6000 = 0.005 mm = 5 µm.
5. Hydrolytic enzymes are kept apart from the rest of the cell, and they can work at their own acidic pH. The enzymes are concentrated, too.
6. Protons pumped into a small space raise their concentration quickly, so a steep gradient forms to drive ATP synthase.
7. Ribosomes attached to the rough ER.
8. Bone marrow or hair follicle.
9. Pluripotent cells can give most cell types of the body. Multipotent cells give only a limited range of related types.
10. They release surfactant into the alveolar lumen, which reduces surface tension.

Where marks are usually lost

• Calling the cytoplasm or cell wall an organelle.
• Writing “bigger cells have a bigger ratio”, when the ratio falls.
• Stating only the area or the volume change, with no link to the exchange need.
• Forgetting to convert mm to µm in magnification questions.
• Describing the mitochondrion as “having a large surface area” without saying that cristae are folds of the inner membrane.
• Saying surfactant “helps gas exchange” without stating that it reduces surface tension.
• Omitting the units on a ratio, such as µm⁻¹.
• Treating differentiation as a change in DNA, when it is a change in gene expression.
• Giving a “yes” or “no” about the muscle fibre as a cell, with no reasons.

Official syllabus

International Baccalaureate Organization, Diploma Programme Biology guide, first assessment 2025, published February 2023, sections B2.2 and B2.3.