Eukaryotic cells aren't just bags of cytoplasm. They are factories with rooms — and a reason for the wall between each.
Every sub-topic below feeds at least one of these questions.
How are organelles in cells adapted to their functions?
What are the advantages of compartmentalisation in cells?
The required syllabus content for B2.2, in order. Each card is one lesson-sized checkpoint.
Nature of Science: Students should recognize that progress in science often follows development of new techniques.
In prokaryotes this is not possible—mRNA may immediately meet ribosomes.
Advantages of compartmentalisation in the cytoplasm of cells
Organelles are compartmentalised cellular structures with specific functions — and the IB has a particular list of what does and doesn't count.
Per the IB list. Each is a discrete structural unit with a defined function. (The plasma membrane is included by the IB, though not by most cell biologists, because it does compartmentalise.)
Cell wall is extracellular. Cytoplasm is the bulk fluid, not compartmentalised. Cytoskeleton is dispersed throughout the cell, not bounded.
The function of individual organelles became studyable only when ultracentrifuges and cell fractionation techniques were developed in the 20th century. Each could now be isolated and its biochemistry examined separately.
Separating transcription from translation allows post-transcriptional modification of mRNA — something prokaryotes can't do.
In eukaryotes, transcription happens inside the nucleus. mRNA is then post-transcriptionally modified (capping, splicing, polyadenylation) before being exported through nuclear pores to the cytoplasm, where ribosomes translate it. This separation:
In prokaryotes there is no nucleus. mRNA is translated by ribosomes as it is being transcribed — no opportunity for post-transcriptional editing.
Compartmentalising the cytoplasm enables concentrations, conditions and reactions that would be impossible in a single mixed pool.
Membrane-bound organelles full of digestive enzymes. The enzymes would shred the cell if released — the membrane keeps them in. When a cell dies, lysosomes burst and the enzymes self-digest the cell (autolysis).
White blood cells engulf bacteria by phagocytosis, forming a phagocytic vacuole. Lysosomes fuse with the vacuole, releasing their enzymes to digest the bacterium. Compartmentalisation makes the destruction safe for the host cell.
An extra 6 sub-topics for HL — same syllabus, deeper mechanism.
Adaptations of the mitochondrion for production of ATP by aerobic cell respiration
Adaptations of the chloroplast for photosynthesis
Functional benefits of the double membrane of the nucleus
Structure and function of free ribosomes and of the rough endoplasmic reticulum
Structure and function of the Golgi apparatus
Structure and function of vesicles in cells
Every part of the mitochondrion's structure serves the chemistry of aerobic respiration.
Protein channels let pyruvate in. Impermeable to H⁺ — allowing protons to accumulate in the intermembrane space.
The narrow space between membranes means even small numbers of protons rapidly create a large gradient.
Holds the electron transport chain and ATP synthase. Cristae fold dramatically increase surface area for these protein complexes.
Contains the enzymes of the link reaction and Krebs cycle, plus mitochondrial DNA and 70S ribosomes that synthesise some respiratory proteins.
Same logic as mitochondria — every structural feature serves the chemistry, this time splitting water and fixing carbon.
Flattened membrane discs containing chlorophyll, electron transport chain, and ATP synthase. Stacking into grana massively increases surface area for light absorption.
Like the mitochondrial intermembrane space — a small volume allows rapid proton gradient buildup.
The fluid surrounding the thylakoids. Contains chloroplast DNA, 70S ribosomes, and all the enzymes of the Calvin cycle (light-independent reactions).
Controls movement of CO₂ in and O₂ out; isolates chloroplast biochemistry from the cytoplasm.
Why two membranes instead of one? It enables selective transport and graceful disassembly during cell division.
The location of a ribosome determines where its protein ends up — keep-it-in-the-cell vs ship-it-out.
Ribosomes have two subunits (small and large), each made of rRNA and proteins. The small subunit binds mRNA; the large subunit catalyses peptide bond formation and has the polypeptide exit tunnel.
Float freely in the cytoplasm. Synthesise proteins that stay in the cell — glycolysis enzymes, cytoskeletal proteins, housekeeping enzymes — or that enter the nucleus.
Attached to the rough ER. Synthesise proteins for secretion, for insertion into membranes, or for lysosomes. Proteins enter the ER lumen, get packaged into vesicles, and travel to the Golgi.
Vesicles from the rough ER arrive at the Golgi, get modified and sorted, and leave as secretory vesicles bound for a specific destination.
The Golgi apparatus is a stack of flattened membrane sacs (cisternae). It sits between the rough ER and the plasma membrane.
Two competing models describe how proteins move through the stack: the vesicle transport model (cisternae stay put; vesicles ferry proteins between them) and the cisternal maturation model (whole cisternae move from cis to trans, transforming as they go).
Vesicles are small phospholipid-bilayer sacs that carry cargo around the cell. Specific vesicles are built with the help of clathrin.
Vesicles transport proteins from the rough ER to the Golgi, between Golgi cisternae, and from the Golgi to the plasma membrane or to lysosomes. They also form during endocytosis to bring material into the cell.
Clathrin is a triskelion-shaped protein (three legs splayed out from a centre). When recruited to a region of membrane, clathrin proteins polymerise into a basket-like cage around that region, forcing the membrane to curve inward and bud off as a vesicle. The clathrin cage then disassembles, leaving a free vesicle ready to deliver its cargo.
If you can't define one of these in a sentence, that's where to revise next.
“What are examples of structure–function correlations at each level of biological organization?”
“What separation techniques are used by biologists?”