AP Biology: Membrane?Bound Organelles (Nucleus, ER, Golgi, Lysosomes, Mitochondria, Chloroplasts)

Membrane?Bound Organelles (Nucleus, ER, Golgi, Lysosomes, Mitochondria, Chloroplasts)

Concept Summary

• Nucleus: Double-membrane-bound organelle housing DNA, directing cellular activities via transcription and ribosome assembly.
• Endoplasmic Reticulum (ER): Network of membranous tubules; rough ER (with ribosomes) synthesizes proteins, smooth ER synthesizes lipids and detoxifies drugs.
• Golgi Apparatus: Stack of flattened cisternae modifying, sorting, and packaging proteins/lipids for secretion or delivery to other organelles.
• Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes for intracellular digestion, autophagy, and apoptosis.
• Mitochondria: Double-membrane organelles generating ATP via oxidative phosphorylation; contain their own DNA and ribosomes.
• Chloroplasts: Double-membrane plastids in plants/algae conducting photosynthesis via thylakoid membranes and stroma; contain their own DNA.

Core Questions

WHAT (definitional)

Q: What is the nuclear envelope? A: Double lipid bilayer perforated by nuclear pores, regulating transport between nucleus and cytoplasm. Trap/Clarification: The envelope is not a single membrane—it’s two distinct bilayers with a perinuclear space.

Q: What distinguishes rough ER from smooth ER? A: Rough ER has bound ribosomes (synthesizing secretory/membrane proteins); smooth ER lacks ribosomes (lipid synthesis, detoxification). Trap/Clarification: Ribosomes on rough ER are not permanent—they attach/detach based on protein synthesis demands.

WHY (causal/explanatory)

Q: Why is the Golgi’s cis-to-trans polarity important? A: Ensures sequential processing (e.g., glycosylation) and accurate sorting of proteins/lipids to their final destinations. Trap/Clarification: The Golgi is not a static structure—cisternae mature and move (cis-trans), not vesicles jumping between fixed stacks.

Q: Why do mitochondria and chloroplasts have their own DNA? A: Endosymbiotic origin: both evolved from engulfed prokaryotes, retaining circular DNA and 70S ribosomes for autonomous replication/protein synthesis. Trap/Clarification: Their DNA encodes only some proteins—most mitochondrial/chloroplast proteins are nuclear-encoded and imported.

HOW (process/application)

Q: How does the ER-Golgi pathway target proteins to lysosomes? A: Rough ER synthesizes lysosomal enzymes-Golgi adds mannose-6-phosphate (M6P) tags-M6P receptors in trans-Golgi direct vesicles to lysosomes. Trap/Clarification: M6P tags are not added in the ER—glycosylation begins in ER but M6P is added in the cis-Golgi.

Q: How do mitochondria generate ATP? A: Chemiosmosis: Electron transport chain (ETC) pumps protons into intermembrane space-ATP synthase uses proton gradient (proton-motive force) to phosphorylate ADP. Trap/Clarification: ATP is not made directly by the ETC—it’s the proton gradient that drives ATP synthase.

CAN (conditions/possibilities)

Q: Can lysosomes digest entire organelles? A: Yes, via autophagy: lysosomes fuse with autophagosomes (double-membrane vesicles) to degrade damaged organelles (e.g., mitochondria). Trap/Clarification: Autophagy is not the same as phagocytosis—autophagy recycles intracellular components, not extracellular material.

Q: Under what conditions do chloroplasts produce ATP without glucose? A: In the light-dependent reactions: thylakoid ETC generates ATP via photophosphorylation (using light energy, not glucose). Trap/Clarification: Chloroplast ATP is not used for general cellular work—it’s primarily for the Calvin cycle (CO? fixation).

Quick Facts & Traps

• Fact: Signal peptides (N-terminal sequences) direct nascent proteins to the ER; cleaved after translocation. Trap: Signal peptides are not the same as nuclear localization signals (NLS)—NLS targets proteins to the nucleus, not ER.

• Fact: Peroxisomes (not lysosomes) detoxify H?O? via catalase; they also break down fatty acids via ?-oxidation. Trap: Peroxisomes are not part of the endomembrane system—they self-replicate and import proteins post-translationally.

• Fact: Cristae (mitochondrial inner membrane folds) increase surface area for ETC/ATP synthase complexes. Trap: Cristae are not static—their shape changes with metabolic demand (e.g., more cristae in active cells).

• Fact: Thylakoids (chloroplast inner membranes) contain chlorophyll and ETC components for light-dependent reactions. Trap: Thylakoid membranes are not derived from the inner chloroplast membrane—they’re a separate, third membrane system.

• Fact: Lysosomal storage diseases (e.g., Tay-Sachs) result from missing hydrolytic enzymes, causing substrate accumulation. Trap: These diseases are not due to overactive lysosomes—they’re caused by defective enzyme function.

Rapid-Fire True/False

• Statement: The smooth ER synthesizes all proteins destined for secretion. Answer: FALSE Why the common mistake happens: Confuses rough ER (protein synthesis) with smooth ER (lipid synthesis/detox).

• Statement: Mitochondria and chloroplasts divide independently of the cell cycle. Answer: TRUE Why the common mistake happens: Assumes organelle division is tied to mitosis/meiosis, but both replicate via binary fission.

• Statement: The Golgi apparatus modifies proteins by adding phosphate groups to mannose residues. Answer: TRUE Why the common mistake happens: Overlooks that M6P tags (mannose-6-phosphate) are added in the Golgi, not the ER.