Cell Biology Chapter 12 Flashcards

1
Q

Membrane flow and maintenance

A

Membrane Flow: Endocytosis and exocytosis have opposite effects in terms of membrane flow: Exocytosis adds lipids and proteins to the plasma membrane; Endocytosis removes lipids and proteins to the plasma membrane; The steady-state composition of the plasma membrane results from a balance between the two processes

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2
Q

Trafficking directions what does it mean, between which organelles

A

Anterograde Transport: Anterograde transport - movement of material toward the plasma membrane; As a secretory granule fuses with the plasma membrane and discharges its contents (exocytosis), a bit of membrane from the ER becomes part of the plasma membrane; This flow of lipids toward the plasma membrane must be balanced
Retrograde Transport; Retrograde transport is the flow of vesicles from Golgi cisternae back to the ER; This allows the cell to balance the flow of lipids toward the plasma membrane; It also ensures a supply of materials for forming new vesicles

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3
Q

Roles of the ER and Golgi Complex in Protein Trafficking

A

Proteins synthesized in the rough ER must be directed to a variety of locations; Once a protein reaches its destination, it must be prevented from leaving; Each protein contains a specific “tag” targeting it to a transport vesicle that will take it to the correct location; A tag may be an amino acid sequence, a hydrophobic domain, oligosaccharide side chain, or some other feature; Membrane lipids may also be tagged to help vesicles reach their destinations; Sorting of proteins begins in the ER and early compartments of the Golgi; There are mechanisms to retrieve or retain compartment-specific proteins; The final sorting of material that will leave the Golgi complex occurs in the TGN

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4
Q

Secretory pathways and their differences

A

Two methods (unique to eukaryotes) for transporting materials across the plasma membrane are: Exocytosis - the process by which secretory vesicles release their contents outside the cell; Exocytosis, inside to outside of the cell. Triggered by local increases in Ca2+ concentration; Endocytosis - the process by which cells internalize external materials; Endocytosis, outside to inside of the cell. Cellular uptake of material dissolved or suspended in fluid. Uses clathrin coated pits.

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5
Q

Exocytosis and mechanisms (Calcium?)

A

Secretory Pathways Transport Molecules to the Exterior of the Cell: Secretory pathways move proteins from the ER through the Golgi complex to secretory vesicles and secretory granules; The secretory granules then discharge their contents to the exterior of the cell; Constitutive Secretion: After budding from the TGN, some vesicles move directly to the cell surface and immediately fuse with the plasma membrane; Constitutive secretion - is this unregulated process which is continuous and independent of external signals; Regulated Secretion: Secretory vesicles involved in regulated secretion accumulate in the cell and fuse with the plasma membrane only in response to specific signals; An important example is neurotransmitter release; Regulated secretory vesicles form by budding from the TGN as immature secretory vesicles; The mature secretory vesicles move close to the site of secretion and remain there until receiving a signal; The signal triggers vesicles to release their contents by fusion with the plasma membrane
Polarized Secretion - In many cases, exocytosis of specific proteins is limited to a specific surface of the cell; Exocytosis Releases Intracellular Molecules: Outside the Cell: In exocytosis, proteins in a vesicle are released to the exterior of the cell as the vesicle fuses with the plasma membrane; The Process of Exocytosis
1. Vesicles containing products for secretion move to the cell surface; 2. The membrane of the vesicle fuses with the plasma membrane; 3. Fusion with the plasma membrane discharges the contents of the vesicle
The membrane of the vesicle becomes part of the cell membrane; Orientation of Membrane: When the vesicle fuses with the plasma membrane; The lumenal (inner) membrane of the vesicle becomes part of the outer surface of the plasma membrane; Glycolipids and glycoproteins that were formed in the ER and Golgi lumens will face the extracellular space; The Role of Calcium in Triggering Exocytosis: Fusion of regulated secretory vesicles with the plasma membrane is generally triggered by an extracellular signal; In most cases, a hormone or neurotransmitter binds receptors on the cell surface and triggers a second messenger inside the cell; A transient elevation in Ca2+ appears to be an essential step in the signaling cascade

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6
Q

Endocytosis Imports Extracellular Molecules by formation of vesicles; Receptor-mediated endocytosis internalization, recycling, mechanisms

A

Endocytosis Imports Extracellular Molecules by Forming Vesicles from the Plasma Membrane: Most eukaryotic cells carry out one or more forms of endocytosis for uptake of extracellular material; (1) A small segment of the plasma membrane folds inward; (2–4) Then it pinches off to form an endocytic vesicle containing ingested substances or particles; Endocytic Vesicles; Endocytic vesicles develop into early endosomes, which fuse with vesicles from the TGN; They acquire digestive enzymes and form new lysosomes; In phagocytosis, solid particles are ingested; Phagocytosis: he ingestion of large particles up to and including whole cells or microorganisms is called phagocytosis; Phagocytosis - cellular “eating”
Plasma membrane engulfs particle and forms phagosome. Fuses with the lysosome to degrade contents. (e.g. uptake of bacteria); Autophagy – regulated destruction of cellular contents
Autophagosome (double membrane) forms around organelle, fuses with lysosome, (autolysosome) and contents degraded; Receptor-Mediated Endocytosis
Cells acquire some substances by receptor-mediated endocytosis or clathrin-dependent endocytosis; Cells use receptors on the outer cell surface to internalize many macromolecules; Process of Receptor-Mediated Endocytosis: 1. Specific molecules (ligands) bind to their receptors on the outer surface of the cell; 2. Receptor-ligand complexes diffuse laterally and encounter specialized regions called coated pits, sites for collection and internalization of these complexes; Accumulation of complexes in the pits triggers the accumulation of additional proteins on the cytosolic surface of the membrane; 3. These proteins—adaptor protein, clathrin, dynamin—induce curvature and invagination of the pit; 4. Eventually, the pit pinches off forming a coated vesicle; 5. The clathrin coat is released, leaving an uncoated vesicle; 6. Coat proteins and dynamin are recycled to the plasma membrane, and the uncoated vesicle fuses with an early endosome
The process is very rapid, and coated pits can be very numerous in cells; After Internalization: Uncoated vesicles fuse with vesicles budding from the TGN to form early endosomes; Early endosomes are sites for sorting and recycling of materials brought into the cell; Early endosomes continue to acquire lysosomal proteins from the TGN and mature to form late endosomes, which then develop into lysosomes; Recycling Plasma Membrane Receptors; Receptors from the plasma membrane are recycled as a result of acidification of the early endosome; The pH gradually declines as the endosome matures, facilitated by an ATP-dependent proton pump; The lower pH dissociates ligand and receptors, allowing receptors to be returned to the membrane

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7
Q

Vesicle formation and coating mechanims: Clathrin, COPI, COPII

A

Most vesicles in protein and lipid transport are called coated vesicles because of the layers of proteins covering their cytosolic surfaces; Coated vesicles are involved in vesicular traffic throughout the endomembrane system, as well as in exocytosis and endocytosis; There are several types of coat proteins
Coat Proteins; The most studied coat proteins are clathrin, COPI, and COPII; The type of coat protein on a vesicle helps to determine the destination of the vesicle; They also: Induce curvature needed for the formation of the vesicles; Prevent nonspecific fusion of the vesicle with another membrane; Coated Vesicles
Coat proteins seem to be specific to certain cell locations/organelles: COPII-coated - Bud from the ER; Transport Vesicles to Golgi; COPI-coated - Bud from the Golgi; Retrograde Transport Vesicles (move “backward”) to ER; Clathrin-coated - Bud from the Golgi AND Plasma Membrane; Secretory Vesicles (release material from cell); Endosomes (engulf materials into cell); Lysosomes (break down cellular materials)

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8
Q

Vesicle Docking

A

Coat proteins must be lost following budding to allow for docking and fusion to the target membrane; Rab GTPase on the vesicle binds a specific Tethering Protein Receptor on target membrane: Dictates specificity, Brings into proximity to SNARES, v-SNARE and t-SNARE bind pulling vesicle to membrane surface
Membrane Fusion: Membrane fusion requires very close contact between vesicle and target membrane, mediated by SNARES; SNARES bring vesicle very close to the target membrane; SNARES wrap around each other to displace water (bringing hydrophobic membranes together); Lipids of both membranes are able to interact, forming one continuous membrane; Cargo and membrane delivered to the target

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