What the exam asks
In Section I (Multiple Choice) and Section II (especially Free-Response Question 4, Conceptual Analysis, and Question 5, Analyze a Model), you will encounter stimuli showing radioactively labeled amino acids, genetic mutations affecting transport vesicles, or cells specialized for secretion. You must be ready to:
- Explain how structural features (like membrane folding or compartmentalization) increase efficiency.
- Trace the exact path of a secreted or membrane-bound protein from synthesis to exocytosis.
- Predict the downstream consequences on the cell or organism if an organelle or enzyme fails.
Core ideas
Ribosomes and common ancestry
Ribosomes are non-membrane-bound structures composed of ribosomal RNA (rRNA) and proteins. They consist of a large and a small subunit that assemble around an mRNA molecule to synthesize polypeptide chains according to the mRNA sequence.
- Universal presence: Almost all cells—prokaryotic and eukaryotic—possess ribosomes. This shared machinery is primary evidence of the common ancestry of all known life.
- Location matters: Free ribosomes suspended in the cytosol synthesize proteins that function within the cytosol. Bound ribosomes attached to the cytosolic side of the rough endoplasmic reticulum (ER) or nuclear envelope synthesize proteins destined for membranes, the organelles of the endomembrane system (such as lysosomes), or secretion outside the cell.
The endomembrane system
The endomembrane system is an interactive network of membranes and organelles in eukaryotes that works together to synthesize, fold, modify, package, and transport lipids, proteins, and polysaccharides. Its components include the nuclear envelope, endoplasmic reticulum, Golgi complex, lysosomes, vacuoles, transport vesicles, and the plasma membrane.
| Component | Structural features | Core functions |
|---|---|---|
| Rough ER | Network of membranes continuous with the nuclear envelope; studded with ribosomes. | Compartmentalizes the cell; folds and chemically modifies newly synthesized polypeptides (e.g., adding carbohydrates); packages proteins into transport vesicles. |
| Smooth ER | Tubular membrane network lacking ribosomes; rich in embedded enzymes. | Synthesizes lipids (phospholipids, steroids); metabolizes carbohydrates; detoxifies drugs and poisons. |
| Golgi complex | Flattened, stacked membrane sacs (cisternae) with distinct directional polarity (cis face near ER, trans face toward plasma membrane). | Receives vesicles at the cis face; further folds and modifies proteins and lipids (e.g., glycosylation, targeting tags); sorts and packages them into vesicles at the trans face. |
| Lysosomes | Membrane-bound sacs containing hydrolytic enzymes that function best at an acidic pH (). | Intracellular digestion of macromolecules, recycling of damaged organelles (autophagy), and programmed cell death (apoptosis). |
| Vacuoles | Large membrane-bound vesicles (central vacuole in plants; food or contractile vacuoles in animals and protists). | Plants: Water and ion storage, macromolecule retention, and maintaining turgor pressure against the cell wall. Animals/Protists: Storage of nutrients/waste or pumping out excess water. |