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40 Cards in this Set
- Front
- Back
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What is the function of lysosomes?
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1. Autophagy: degradation of organelles through use of hydrolytic enzymes.
2. Phagocytosis: uptake of large particulate matter. 3. Endocytosis: uptake of small particulate matter. 4. Recycling of entire cells. |
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How are proteins targeted to the lysosome?
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A Mannose-6- Phosphate tag is added to the protein in 2 steps:
1. N-acetylglucosamine Phosphate is added to the 6C of mannose residue on the protein. 2. the sugar residue is then removed, leaving the phosphate on the 6C of the mannose. B. N-acetylglucoseamine phosphate transferase (enzyme) then recognizes the signal patch that is on all lysosomal proteins (This patch is a series of amino acids that interact on the folded protein) |
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Explain the process of transport of lysosomal enzymes from TGN to the lysosome.
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The enzymes are transported via clathrin-coated vesicles. They are escorted from the TGN by GGA's which are adaptor proteins. The outer end of the GGA binds to the clathrin. The inner surface of the GGA binds to the sorting signal in the tails of the M6P receptors (MPR). MPRs bind to lysosomal enzymes within the vesicle lumen. This is all contingent upon binding of ARF1: a GTP-binding protein. After the vesicle has budded from the TGN, the clathrin coat is lost and the uncoated vesicle proceeds to its destination. Before it reaches its destination, the MPRs dissociate from the lysosomal enzymes and return to TGN for the next round of transport.
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Describe Autophagy.
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The organelle to be destroyed is surrounded by a double membrane to produce an autophagosome. The outer membrane fuses with the lysosome to produce an autophagolysosome. The enclosed organelle is then degraded by hydrolytic enzymes. It is not called a residual body. The breakdown products are then made available to the cell. They are either eliminated by exocytosis or retained within the cytoplasm forever as lipofuscin granules, which are associated with the aging process.
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Describe phagocytosis in prokaryotes.
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Cell eating: large matter. The food vacuole forms a phagosome and once that fuses with the lysosome it is called a phagolysosome. Primarily a feeding mechanism.
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Describe phagocytosis in eukaryotes.
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Macrophages and neutrophils: defense mechanisms. Also receptor-mediated. The invading organisms are killed via
1. Lysosomal enzymes/low pH 2. O2 free radicals made within the lumen of the phagosome. The organelles are engulfed through contractile activity of actin-containing microfilaments that underlie PM. |
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Describe the plant vacuoles.
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1. Store ions, amino acids, sugars, and toxic compounds.
2. Maintains + turgor pressure. 3. 80-90% of cell. 4. Some can by lytic. 5. Intracellular digestion: the pH is kept low by V-type H+ ATPase within tonoplast that pumps protons into the fluid. |
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Describe Exocytosis.
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Fusion of the vesicle to plasma membrane and the discharge of vesicle contents to the outside of the cell. Can be Constitutive secretion or Regulated secretion. One cell uses both pathways: 1 fusing to PM, other's vesicles are building u pin conventration underneath PM. All regulated proteins end up in same vesicle on 1 side of PM:
the coats are lost really fast, and some sort of targeting is occuring but its not really understood. |
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Describe the Housekeeping-receptors of endocytosis. What happens to the receptor after endocytosis?
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Function in uptake of material that will be used by the cell. Ex. LDL and transferrin. After endocytosis the bound materials are delivered to the cell and the receptor returns to the cell surface of the cell. The receptor dissociates from the ligand because of the high H+ concentration in the early endosome.
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Describe signaling receptors in endocytosis. What happens to them after endocytosis?
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Bind extracellular ligands that carry messages to change cell activity. After endocytosis they are destroyed (receptor-down regulation). The receptors are marked with a ubiquitin tag at the cytosolic tail.
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What is the transformation of early to late endosome caused by?
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1. Decrease in pH of lumen
2. exchange of Rab proteins 3. Morphological changes in internal structures. |
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What is the function of coated pits in RME?
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Stuff that enters the cell become bound to receptors that collect in these pits, which are specialized domains of the PM.
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Describe the coated pit.
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Consists of clathrin building blocks: hexagons and pentagons.
Each clathrin molecule: 3 heavy and 3 light chains. They have a layer of adapters between the clathrin and the surface of the vesicle that faces the cytoplasm. AP2 is important adapter: Its µ subunits engages the tails of the PM receptors and their cargo to the emerging coated vesicle. The beta subunit: binds and recruits clathrin molecules. |
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What are the accessory proteins that occur in coated vesicles?
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Dynamin: GTP-binding protein.
Amphysin: recruits Dynamin. Synaptojanin: bind to both of these and is important in neurons. |
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What are the 2 types of Endocytosis?
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Pinocytosis: cell-drinking
Receptor-mediated endocytosis: RME, uptake of ligands that bind to the external surface of PM. |
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What are the 4 subcompartments where proteins can be targeted to in Mitochondria?
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OMM-outer mitoch. membrane
IMM- inner mitoch. membrane IMS- intermembrane space Matrix |
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How are Matrix proteins targeted to mitochondria
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Have a presequence: removable N-terminal sequence that has (+) residues.
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How are IMM proteins targeted to mitochondria?
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Have an internal sequence that remains part of the molecule
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Describe the signal sequence that targets proteins to mitochondria. (# aa, etc. )
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20-60 amino acids, 3-5 of these are (+) charged. The amino acids form an amphipathic alpha helix with the (+) ones clustered to one side.
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What are the 2 cytosolic factors that are important in targeting stuff to the mitochondria?
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MSF and HSP70
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Describe the role of MSF in targeting stuff to mitochondria.
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It binds to the signal sequence and helps in targeting to the IMM. Keeps the protein in an unfolded state and requires ATP to do this. Interacts with TOM 37/70.
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Describe the role of HSP70 in targeting stuff to mitochondria.
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Binds to the N-terminal sequence- but not always. Targets Mitochondrial membrane. Keeps protein unfolded. Also has role in the matrix:
1. Functions as "molecular ratchet": prevents retrograde movement: 1 way diffusion. 2. "molecular motor" model: exerts a physical pulling force on the protein. ATP hydrolysis creates a power stroke. |
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Explain role of TOM40, TIM22, and TIM 23
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TOM40: in OM: makes up majority of pore in OM.
TIM22: Binds integral proteins of IMM and inserts them into lipid bilayer. TIM23: binds proteins with N-terminus presequence (matrix bound) and translocates them through the IMM and into the matrix. |
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What are the energy requirements of targeting stuff to the mitochondria?
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1. ATP in cytosol: HSP70
2. ATP in matrix: mtHSP70 3. Proton motive force: IMM: *movement into the matrix is powred by an electric potential across the IMM acting on the (+) targeting sequence. A conformational change in TIM23: causes it to be charged and "opens" when it binds ? |
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What are the 6 compartments that stuff is targeted to in the chloroplast?
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OM
IM IMS Stroma Thylakoid membrane Thylakoid lumen |
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Describe the stroma-targeting sequence.
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All cells that are targeted through the envelope have this. It is an N-terminal signal sequence that gets removed later by peptidase once enters the stroma. 30-100 aa. Different types of aa involved.
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Describe the sequences that proteins who are targeted to the thylakoid lumen or membrane have.
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Have the stroma-targeting sequence which gets removed by peptidase once in the stroma. Also have a thylakoid transfer domain that targets them to the lumen or membrane.
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What are the 2 compartments that proteins are targeted to in the peroxisome?
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The boundary membrane and the internal matrix.
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Where are proteins destined for the matrix made?
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On free ribosome in the cytosol and targeted to the peroxisome using a signal sequence. Translocated in the folded state. Can have either a C-terminal sequence (90%) that is not cleaved or an N-terminal sequence that is cleaved by peptidase.
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Describe the peroxisomal targeting sequence.
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Each signal is recognized and bound to adaptor protein in cytosol:
PTS1: PEX5 PTS2: PEX7 The adaptor then interacts with docking proteins in the translocation complex: PEX5: PEX13 PEX7: PEX14 PEX17 is the point of convergence for both pathways and required for all transport into peroxisome. |
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What do PEX proteins do?
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Bind and hydrolyze ATP. Some ATP hydrolysis is required for the recycling of PEX 5 and 7 to cytosol.
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Is HSP70 required for peroxisomal targeting?
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Yes, if there is none, then binding occurs but no translocation.
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What goes in through nuclear pores?
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Histones, TF, enzymes for transcription and replication
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What goes out through nuclear pores?
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mRNA, tRNA, subunits = exportins
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Describe the 2 types of movement within nuclear pores.
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1. Passive: very small stuff moves through "spokes in the pore.
2. Passive and gated movement: requires exportins and importins. |
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Describe import through nuclear pores.
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Requires alpha and beta importins. Alpha binds to NES (leu rich), Beta binds to Alpha. Beta is recognized by the pore. Moves through the pores. Once inside the pore, RAN (G-protein): GTP binds to Beta. Alpha and cargo dissociate, while Beta and RAN stay together. Alpha binds to exportin. RAn and exportin = complex formation for export.
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Describe export through nuclear pores.
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Beta/RAN GTP and Alpha/RAN GTP move through pore. The pore recognizes Beta and exportin. Once in the cytosol: RAN GAP hydrolyzes GTP/RAN. In GDP form RAn releases B importin. So alpha importin and Beta importin are now recycled and free in the cytoplasm to continue importing proteins.
** Movement down concentration gradient and only energy required is hydrolysis of GTP in the cytosol and [GTP] maintenance in nucleus. |
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What are the functions of the nucleus?
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1. Stores DNA
2. Organizes DNA for: transcription, replication, proper packaging, ribosome assembly in NOR. 3. Regulates movement in and out of nucleus 4. Compartmentalizes nuclear function from cytoplasm. |
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Describe the features of the interphase nucleus.
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1. Chromatin: euchromatin vs heterochromatin.
2. nuclear envelope: 2 membranes 3. Nuclear pores 4. Nucleolus: rRNA synthesis 5. Nucleoplasm 6. Nuclear lamins (RBC), IF (animals) |
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Discuss the functions of the lamins in the nuclear envelope
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Lamin A, B, C: IF
During interphase, stabilizes the envelope. Chromatin attachment. NE breakdown and reformation, which depends on phosphorylation state of lamins: P: breakdown de-P: stable |
