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24 Cards in this Set
- Front
- Back
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What is the cell wall like in a growing/expanding cell?
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Less rigid and exhibits long-term irreversible stretching
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Explain what cell wall stress relaxation is.
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Turgor pressure of the cell stretches, pushing out the cell wall and exerting a force on it (0.3-1 megaPa). The cell wall pushes back, like a spring, to create physical stress/tension back on the cell (10-100 megaPa)
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What happens during the biochemical loosening of the cell wall?
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This allows tiny slippage of cellulose microfibrils past each other which leads to a very minute increase in the surface area of the cell wall. This expansion leads to a decrease in turgor pressure which causes an overall decrease in stress on the wall.
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How does the decrease in turgor pressure drive cell wall expansion?
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It causes a decrease in overall water potential of the cell. Prior to the stress/relaxation event, the cell is at equilibrium with external environment. After the event, the turgor pressure decreases, causing Ycell to be negative, which draws water into the cell (which expands the cell)
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What is necessary for cell expansion?
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Turgor pressure- to draw water in and expand cell
Accumulation of solute to maintain its levels |
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What is the acid-growth hypothesis?
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Says that stress/relaxation event is induced by acidification of cell wall through a proton pump. Auxin mediates proton movement across membrane. The short term effects of Auxin are creating the proton gradient. Long term effects are protein synthesis
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How quickly can Auxin work?
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Can stimulate stem growth in 10-12 minutes
Proton efflux occurs at onset of growth: where pH decreases, elongation increases Maximal growth rates require more time though: because of protein synthesis of H pumps. Also there has to be sucrose or another OAS to maintain solute potential. |
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What is the optimal Auxin concentration?
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Stems: 10^-6
Roots: 10^-9 - -11 |
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What 5 predictions does the acid growth hypothesis lead to?
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Acid buffers alone can promote short term growth
Auxin increases rate of H+ extrusion Neutral buffers should inhibit Auxin-induced growth Compounds that stimulate H+ extrusion should stimulate growth (ex. fusicoccin) Cell wall should loosen via expansin in acidic pH |
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How does Auxin work in the short term?
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It indirectly activates the H+ pumps:
1. Auxin binds to pump at ABP1 and increases its activity: more H+ pumped out 2. Increases trafficking of H+ pumps to the membrane that are in vesicles, ER, etc. and prevents them from being endocytosed. |
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How does Auxin work long term?
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It stimulates the synthesis of new H+ pumps
1. Auxin targets repressor protein that is bound to the transcription factor that is bound to the promoter on gene. It causes transcription to occur to make the new pumps by tagging the repressor with Ub. for degradation. |
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How does Ubiquitin-mediated degradation work?
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Ub= protein that is covalently attached to other proteins, marking them for degradation by 26S proteasome
The process of adding Ub requires 3 enzymes: E1 (activating), E2 (conjugating), and E3 (Ub ligase). E3 is very diverse so it's called the specificity factor |
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Explain the pathway of Ub degradation with SCF.
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1.SKP1 binds to F-box and CUL1- acts as a scaffolding protein
2. CUL1 binds to E2 3. F-box binds to SKP1 (specificity factor) 4. Roc binds to E2 When Auxin binds to F-box and scaffolding protein (TRL1), it activates the E3 complex so that it can begin breaking down repressor protein |
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What does a lower pH do in the cell wall?
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1. Disrupts cellulose microfibrils and hemicellulose interactions
2. Activates other hydrolytic enzymes (ex. pectinases) which creates an enhancement of expansion by further weakening cell wall irreversibly 3. Disrupts salt bridge formation |
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What stops growth from occurring?
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1. Auxin is gone
2. Negative feedback loops 3. De-sensitization |
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How is the direction of expansion controlled?
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By microfibril orientation. The newest ones (closest to PM) determine this. In animal cells these are randomly organized which results in dispersive growth and a spherical shape. In plants they are horizontal so growth is perpendicular resulting in long, vertical cell
*Older layers of cell wall are weakened and pulled in the direction of growth over time to become more aligned |
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What causes the orientation of microfibrils?
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Cortical microtubules near the PM.
1. Directly associate with cellulose synthase of the particle rosettes via motor proteins 2. Indirect: MT form a channel/track that cellulose synthase follows. |
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How does polarized growth occur in root hairs and other cells?
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These cells must have:
a high Ca gradient Localized cell wall loosening Localized exocytosis of cell wall material *Cytoskeleton is important for delivering cell wall expansion and patterning of growth: uses Ras protein pathway |
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Describe Ras proteins.
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In a super family of small monomeric G proteins.
GTP bound = active form They utilize accessory proteins: GAPs, GEFs, and GDIs. GAPs: hydrolyze GTP to GDP GEFs: conformational change that causes GDP to be removed so GTP can be taken up from cytosol GDIs: prevents release of GTP |
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What are the 5 families of Ras proteins?
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RAB and ARF: regulate vesicle signaling
RAN: nuclear import RAS and RHO: cell signaling cascades |
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Discuss the protein families found in plants
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No RTKs, use RPK instead
No Ras homologs: have a unique family of Rho proteins called Rops No GPCR has been identified: but there are a few heteromeric G proteins. ABA receptor may be GPCR-like. |
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Discuss RPKs in plants
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1. A lot of genes in AT encode for RPKs (genes are duplicated). Most with unknown function
2. Ser/Thr kinase: autophosphorylates on cytoplasmic domain of adjacent monomer: interacts with Rops 3. WAK: wall associated protein: Ser/Thr kinase receptors. 5 highly conserved genes in AT. Expressed in expanding cells (required for expansion). Regulated by environment and developmental causes: ex. wounding and pathogens can cause it. *Plants still have MAP kinase cascade pathways, but use them different because don't have Ras to regulate them. |
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Discuss Rops
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ROPs = G proteins associated with signaling
Many are bound to membrane. Regulated by GEFs, GDIs, GAPs. Can activate kinases like ROK1. Associate with many effectors: ex. CCR lignin synthesis. |
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Give a summary of cell expansion and elongation.
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Depends on:
Position in plant ( at apices) cell type and genetics environment light vs. dark hormones * All require cell signal cascades: many are not understood yet. |
