Biology HL Paper 1A (May 2025, TZ2) · Question 15
FocusedMatching part: 15
6.1 Organelles, Compartmentalization and Trafficking
Trace proteins through the endomembrane system and relate organelle membranes to energy conversion.
Estimated time: 70 minutes
IB syllabus: B2.2 · SL and HL
Compartments Concentrate and Isolate
Membrane-bound organelles occur in eukaryotes, not prokaryotes. Compartments concentrate substrates and enzymes, maintain pH or ion conditions unlike the cytosol, and prevent incompatible reactions from interfering. The nuclear envelope separates transcription and RNA processing from translation. Lysosomes keep acid hydrolases away from cytoplasmic proteins. Compartmentalization increases control, although transport between compartments becomes essential.
A lysosome contains proteases, lipases, nucleases and carbohydrate-digesting enzymes whose optimum conditions are acidic. It fuses with vesicles containing worn organelles or engulfed particles, hydrolyses their macromolecules and returns reusable monomers to the cytosol. Describing a lysosome as a 'suicide packet' is incomplete: controlled recycling and defence are normal functions, while uncontrolled leakage is damaging.
HL extensionB2.2 AHL
The Endomembrane System Is a Directed Logistics Network
Ribosomes are structurally identical whether free or ER-bound. Translation begins in the cytosol; a signal sequence on a growing polypeptide can direct the ribosome to rough ER. Proteins destined for secretion, membranes or much of the endomembrane system enter the ER, fold, and undergo quality control. Proteins used in the cytosol are generally completed on free ribosomes.
Transport vesicles bud from ER and fuse with the receiving face of the Golgi apparatus. Golgi cisternae modify carbohydrate groups, sort cargo and package it for lysosomes, the plasma membrane or secretion. Protein coats such as clathrin help bend a donor membrane and select cargo; targeting proteins and complementary fusion machinery make delivery selective rather than random.
The nuclear envelope has two bilayers. Its outer membrane is continuous with rough ER, while nuclear pores regulate transport of RNAs, ribosomal subunits, nucleotides and proteins. Large cargo does not simply diffuse through an open hole; receptor-mediated transport allows the nucleus to maintain a distinct protein and RNA composition.
Energy Organelles Use Membranes to Build Proton Gradients
A mitochondrion has an outer membrane and a highly folded inner membrane. Matrix enzymes carry out the link reaction and Krebs cycle, while electron carriers and ATP synthase occupy the inner membrane. Proton pumping into the small intermembrane space creates an electrochemical gradient; return through ATP synthase couples that stored energy to ATP formation.
A chloroplast envelope has two membranes and encloses stroma containing Calvin-cycle enzymes. A third membrane system forms thylakoids, grana and intergranal lamellae. Photosystems, electron carriers and ATP synthase in thylakoid membranes generate and use a proton gradient across a very small lumen. In both organelles, large membrane area and small gradient-containing volume make chemiosmosis effective.
Mitochondria and chloroplasts also contain circular DNA, 70S ribosomes and divide by fission, consistent with endosymbiotic origins. Their retained genomes do not make them independent: many organelle proteins are encoded in the nucleus, synthesized in the cytosol and imported through membrane complexes.
Compartment audit
Use the membrane view to identify which structures create barriers, selective routes and recognition surfaces.
Exchange · gradients · inheritance
Cell function laboratory
Test Yourself
A mutation prevents a secreted enzyme's ER signal sequence from being recognized but leaves its catalytic region intact. What is the most likely immediate outcome?
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