Test 3 ch15 Learning Objectves Flashcards
Evolution of the Endomembrane System (Nucleus, ER, Golgi and Lysosomes)
The endomembrane system likely evolved through the infolding of the plasma membrane in an ancestral prokaryotic cell. This led to the formation of internal compartments such as the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, and lysosomes.
Evidence for this theory:
The nuclear envelope is continuous with the ER, suggesting that it arose from membrane invagination.
The ER, Golgi, and lysosomes share vesicular transport systems, indicating a common evolutionary origin.
Some proteins required for membrane trafficking and sorting are conserved across eukaryotes.
Evolution of Mitochondria and Chloroplasts
The endosymbiotic theory proposes that mitochondria and chloroplasts originated from free-living prokaryotic cells that were engulfed by an ancestral eukaryote.
Evidence supporting endosymbiosis:
Double membranes: Similar to Gram-negative bacteria.
Own circular DNA: Resembling bacterial genomes.
Independent division by fission: Like bacteria.
Ribosomes and DNA polymerases similar to prokaryotes: Differing from eukaryotic counterparts.
Advantages and disadvantages to Multiple Compartments
Advatages:
Specialization of functions (e.g., lysosomes digest, mitochondria generate energy).
Compartmentalization prevents harmful reactions from interfering with other cell processes.
Enhances efficiency by localizing enzymes and substrates.
Disadvantages:
Requires complex transport and signaling systems.
Energy-intensive due to vesicular transport and protein targeting.
More prone to mutations or defects in trafficking pathways.
Ribosome Structure and Role in Translation
Structure: Ribosomes consist of two subunits (large and small) composed of rRNA and proteins.
Function: They catalyze peptide bond formation, translating mRNA into proteins.
Location: Found free in the cytosol or bound to the ER.
Importance of protein sorting
Protein sorting ensures that proteins reach their correct cellular destination, maintaining cellular function and preventing diseases caused by mislocalization.
Role of signal sequences in protein targeting
Signal sequences are short amino acid stretches that direct proteins to specific compartments.
No signal sequence → protein remains in the cytosol.
Experimental evidence: When signal sequences are removed, proteins fail to localize correctly, and when added artificially, they reach the intended compartment.
Free v. Membrane bound Ribosomes
Free ribosomes: Synthesize cytosolic and nuclear proteins.
Membrane-bound ribosomes (on ER): Synthesize proteins destined for the ER, Golgi, lysosomes, plasma membrane, or secretion.
Protein Transport into the nucleus
- Translated on free ribosomes.
- If a nuclear localization signal (NLS) is present, it binds a nuclear import receptor.
- The receptor interacts with nuclear pore fibrils.
- Active transport through the nuclear pore complex occurs via GTP hydrolysis.
Protein Transport into Mitochondria
- Translated on free ribosomes.
- Unfolded protein binds mitochondrial import receptor.
- Transported through protein translocator complexes.
- Chaperones assist in folding inside the matrix.
Translocation of Water-Soluble and Transmembrane Proteins in ER
Water-soluble proteins: Fully translocated into the ER lumen.
Transmembrane proteins: Contain stop-transfer sequences, which integrate them into the membrane.
Protein Modification in the ER and Golgi
ER modifications: Glycosylation (adding sugars), folding via chaperones.
Golgi modifications: Further glycosylation, sorting of proteins to final destinations.
Structure and Function of the Golgi Apparatus
Structure: Flattened membrane sacs (cis, medial, trans Golgi).
Function: Protein modification, sorting, and transport.
Formation and Targeting of Transport Vesicles
- Clathrin-coated vesicles form by cargo receptors binding cargo proteins.
- Adaptins recruit clathrin, pulling the membrane outward.
- Dynamin pinches off the vesicle.
- Vesicles move via motor proteins along microtubules.
- Rab proteins and SNARE proteins ensure correct membrane targeting.
Secretory Pathway (Protein Translation to Secretion)
- Protein synthesis starts in the cytosol.
- If an ER signal sequence is present, ribosome binds ER.
- Protein moves through ER, gets modified.
- Transported via vesicles to Golgi for further modification.
- Sorted into vesicles for secretion.
Constitutive vs. Regulated Exocytosis
Constitutive exocytosis: Continuous secretion; supplies lipids and proteins to plasma membrane.
Regulated exocytosis: Triggered by signals (e.g., hormones, neurotransmitters).
Pinocytosis, Phagocytosis, and Receptor-Mediated Endocytosis
Pinocytosis (“cell drinking”): Small vesicles take up fluid and molecules.
Phagocytosis (“cell eating”): Large particles engulfed (e.g., macrophages engulf bacteria).
Receptor-mediated endocytosis: Specific molecules bind receptors, forming clathrin-coated vesicles.
Structure and Function of Endosomes and Lysosomes
Endosomes: Sorting stations directing vesicles to lysosomes or recycling.
Lysosomes: Contain acid hydrolases for degradation.
Acid Hydrolases, Lysosomal Targeting, and Role of pH
Acid hydrolases: Enzymes that degrade macromolecules.
Targeting to lysosomes: Mannose-6-phosphate (M6P) is added in the Golgi, directing them to lysosomes.
pH dependence: Lysosomes maintain a low pH (via proton pumps) to activate hydrolases.
