A eukaryotic cell is a building with rooms. The walls are membranes, and the point of a room is to keep one job away from another.
A discrete sub-unit of a cell with a specific function. Not everything inside a cell qualifies — and the exceptions are the ones examiners ask about.
Nucleus, mitochondria, chloroplasts, ER, Golgi, lysosomes, vesicles, ribosomes and the plasma membrane. A membrane is not required — ribosomes count.
Cytoskeleton: a network of protein filaments, not a discrete unit. Cell wall: outside the membrane, not metabolic. Cytoplasm: the medium everything sits in.
Break cells open, spin the soup in an ultracentrifuge. Heavier organelles pellet first. Each fraction can then be tested for what it does — how organelle functions were discovered.
Each supernatant goes on to the next spin. The pellet from each step is a near-pure sample of one organelle, so you can ask it what it does: feed the mitochondrial fraction glucose and oxygen and it makes ATP. Discovery of function came from isolation.
A membrane costs lipid and energy to maintain. A cell pays it for four reasons.
The envelope separates transcription from translation, so mRNA can be spliced and capped before a ribosome sees it. Prokaryotes can't: their ribosomes bind mRNA while it's still being made.
A small volume packs enzymes and substrates together; reactions run faster than they would diluted across the whole cytoplasm.
Hydrolytic enzymes at pH 4.5 in a lysosome would digest the cell if loose. Oxidising reactions, low pH, high Ca²⁺ — each gets its own room.
Each compartment can hold a different pH, ion mix or redox state, and the cell can move things between rooms in vesicles on demand.
DIPFDDR. Detection, Ingestion, Phagosome forms, Fusion with a lysosome, Digestion, Discharge, Recruitment. The bacterium is never loose in the cytoplasm; it goes from one compartment into another. That is compartmentalisation doing immunity.
Choose where the protein has to end up, then step it along the line. Tap any organelle on the way for its job.
Nucleus, free ribosomes, rough ER, Golgi, transport and secretory vesicles, lysosome, coated pit or plasma membrane.
One line, four exits. Every protein starts as mRNA leaving the nucleus. The first decision is which ribosome: free, and it stays in the cytoplasm; on the rough ER, and it enters the membrane system for transport, secretion or the membrane itself. After that the Golgi sorts it by address.
The nuclear envelope is the only compartment that has to open on a schedule.
Thousands of nuclear pores let mRNA and ribosome subunits out and enzymes, histones and signals in — and let the cell set the rate of each.
In prophase the envelope fragments into vesicles so the spindle can reach the chromosomes; it re-forms around each daughter nucleus in telophase. A single rigid membrane couldn't.
The outer membrane is continuous with the rough ER, so the envelope is really a flattened ER sac wrapped round the chromatin. Proteins from the ER can reach the nucleus without a vesicle.
Both make ATP by pushing protons across a membrane. So both need a big membrane, a tiny space to pump into, and a sealed compartment for enzymes.
Tap the outer membrane, intermembrane space, inner membrane / cristae, matrix, or the DNA and ribosomes.
| Design need | Mitochondrion | Chloroplast |
|---|---|---|
| Big membrane for the pumps | Inner membrane folded into cristae — electron transport chain and ATP synthase | Thylakoid membranes, stacked into grana — photosystems and ATP synthase |
| Tiny space to pump into | Intermembrane space: small volume, so the proton gradient builds fast | Thylakoid lumen: small volume, same reason |
| Sealed room for enzymes | Matrix: Krebs cycle enzymes and substrates concentrated together | Stroma: Calvin cycle enzymes and substrates concentrated together |
| Own genes | Circular DNA, 70S ribosomes | Circular DNA, 70S ribosomes |
Same three sentences, swap the nouns. Any "explain how the structure of X is adapted" question on either organelle is answered by surface area, small volume, compartment — plus the double membrane that keeps it all apart from the cytoplasm.
Membranes don't spontaneously bud. A protein scaffold bends them.
Clathrin is the scaffold. Three-legged proteins assemble into a cage on the cytoplasmic face, forcing the membrane to curve, then pinching it off. Used at the plasma membrane (endocytosis, receptor uptake) and at the trans-Golgi (making vesicles for lysosomes). The coat is stripped and recycled once the vesicle is free.
A common 3-mark answer describes the bilayer. A 7-mark answer explains every placement by polarity. Tap the points you'd have written.
Tap a node to light up its links. The pink dashed link is the one idea every HL question here comes back to.
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
Single best answer. You get the reasoning as soon as you commit. HL items are marked.