BBMB 405 : EXAM 1
97 Cards in this Set
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Overall, what is produced by the light reactions of photosynthesis?
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oxygen, chemical energy (ATP, NADPH)
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Overall, what is produced by the dark reactions of photosynthesis?
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hexose, ADP, NADP+
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In the Calvin Cycle/dark reactions, what are NADPH and ATP used for?
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to drive CO2 reduction to form hexose
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How is reducing power generated in the light reactions?
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electron transfer through photosystems I and II
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Where do the primary events of photosynthesis take place?
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thylakoid membranes
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What is the stroma?
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-space between the inner membrane and the thylakoid membranes
-location of the dark reactioins
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What are stroma lamellae?
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regions of the thylakoid membrane linking different grana (stacks of thylakoids)
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What are the major components of thylakoid membranes?
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galactolipids and sulfolipids
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Describe the major structural features of chlorophyll.
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-magnesium center
-pyrrole ring structure, with one reduced pyrrole ring
-an additional 5 carbon ring fused to a pyrrole ring
-a phytol group
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Why are chlorophylls such effective photoreceptors?
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the conjugated double bonds in their structure
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What happens when an electron is excited, and how does being near the photosystems change this?
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-normally when an electron is excited, it releases heat and returns to ground state
-in photosystems, there is an acceptor molecule nearby
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What is photoinduced charge separation?
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-the excited electron moves from excited (donor) molecule to the acceptor, thus resulting in a positive charge on the donor and a negative charge on the acceptor
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What is the name given to the site of charge separation?
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reaction center
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What initiates charge separation?
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-a special pair of chlorphylls
-absorb a very specific wavelength of light, which often gives the special pair its name
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What is the makeup of the reaction center in bacteria?
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-4 bacteriochlorophyll
-2 bacteriophytin
-2 quinones
-1 ferrous non-heme iron
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What is the major difference between bacteriochlorophyll and chlorophyll?
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-magnesium center
-pyrrole ring structure, with one reduced pyrrole ring
-an additional 5 carbon ring fused to a pyrrole ring
-a phytol group
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Describe the structure of bacteriophytin.
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1. P960 absorbs light and transfers an electron to bacteriophytin (BPh) via bacteriochlorophyll (BCh), giving P960+ and BPh-
2. Quinone A quickly grabs the electron from BPh-, forming BPh and Qa-. One of the hemes on the reduced cytochromes gives an electron to P960+, thus restoring P960…
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What are the steps of the electron chain in the photosynthetic bacterial reaction center?
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-normally when an electron is excited, it releases heat and returns to ground state
-in photosystems, there is an acceptor molecule nearby
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What are the steps of the electron chain in the photosynthetic bacterial reaction center?
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1. P960 absorbs light and transfers an electron to bacteriophytin (BPh) via bacteriochlorophyll (BCh), giving P960+ and BPh-
2. Quinone A quickly grabs the electron from BPh-, forming BPh and Qa-. One of the hemes on the reduced cytochromes gives an electron to P960+, thus restoring P960…
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What is a major difference between the electron transport chain in bacteria and in plants?
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-in plants, the electron flow is not cyclic
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Summarize the electron transport chain in plants.
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-PSI forms NADPH using electrons PSII took from 2 H2O
-electrons move from PSII to PSI through cytochrome bf
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What does Photosystem II do?
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-transfers electrons from water to plastoquinone
-generates a proton gradient
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Why does PSII need to used light energy?
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-the reaction driven by PSII is thermodynamically uphill, the light energy allows it to go forward
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How many chlorophyll molecules does PSII contain?
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>30
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Describe the structure of PSII.
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-the special pair and the water-oxidizing complex (WOC/manganese center) are pointing towards the thylakoid space
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What is the structure of the water-oxidizing complex?
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-a calcium ion
-4 manganese ions
-4 water molecules
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Describe the steps of electron flow through PSII.
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1. P680 absorbs light and transfers an excited electron to a nearby pheophytin.
2. The electron is transferred to plastoquinone Qa and then to a mobile plastoquinone Qb.
3. Another photon triggers another electron moving through the system to Qb, where, with the uptake of 2H+, it forms …
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How is P680+ restored to P680?
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-P680+ extracts an electron from tyrosine (in WOC), forming a radical
-tyrosine radical removes electron from manganese ion
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How many electrons from H2O are needed to reduced 2 molecules of Q to QH2?
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4
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What is the purpose of cytochrome bf?
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-transfers electrons from QH2 to plastocyanin (Pc)
-releases the protons from QH2 into the thylakoid lumen
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What does Photosystem I do?
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-uses light energy to generate reduced ferredoxin
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What is ferredoxin?
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-a soluble protein containing a 2Fe-2S cluster
-a powerful reductant
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How is PSI linked to cytochrome bf?
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-the reduced plastocyanin gives its electrons to P700+ to restore P700
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After being reduced, what does ferredoxin do?
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-carries the electrons to Ferredoxin-NADP+ reductase, a flavoprotein
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What does Ferredoxin-NADP+ reductase do
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-converts NADP+ into NADPH
-has an FAD prosthetic group
-two molecules of reduced ferredoxin form FADH2, which then converts NADP+ into NADPH
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Why is NADPH used over FADH2 in Ferredoxin-NADP+ reductase?
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-it carries two electrons and is more widely used
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What is the major difference between the ATP synthase of the chloroplast and that of the mitochondria?
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-the membrane orientation is flipped in photosynthesis so as to release ATP into the stroma for the dark reactions
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What happens when NADP+ is in short supply or when the concentration of NADPH is high?
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-cyclic electron flow
-leads to the production of ATP instead of NADPH
-ferredoxin, instead of moving to the reductase, moves back to cytochrome bf and reduces plastocyanin, bringing in a proton and contributing to the gradient
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What results from the absorption of eight protons?
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-one O2
-two NADPH
-three ATP (2.7 photons per ATP)
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What is the purpose of accessory pigments? How do they work?
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-funnel energy into the reaction centers
-resonance energy transfer allows energy to move from the site of initial absorbance to the reaction center
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How do inhibitors of PSII work?
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-block electron flow
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What are the herbicides diruon and atrazine?
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-inhibitors of PSII
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How do inhibitors of PSI work?
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-accept electrons from PSI, thus inhibiting NADP+ reduction
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What is the herbicide paraquat?
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-inhibitor of PSI
-becomes a radical and damages the membrane
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What are 4 light-driven changes in the stroma of chloroplasts?
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1. increased pH (from protons pumped into the thylakoid lumen) --> promotes carbamate formation on lysine of Rubisco
2. increased Mg2+ (transfer from lumen to stroma) --> promotes formation of magnesium carbamate
3. increased NADPH (product of light reactions) --> activates phosphoribul…
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What is the net effect of the light reactions?
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-activation of the rate-controlling reaction of the Calvin cycle
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Overall, what happens in the Calvin cycle?
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-carbon dioxide and water is used to synthesize hexoses
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What are the three stages of the Calvin cycle?
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1. Fixation
2. Reduction
3. Regeneration
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What is the first step of the Calvin cycle?
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-carbon dioxide reacts with ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate
-catalyzed by rubisco
-occurs on the stromal surface of the thylakoid membrane
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What is rubisco?
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-Ribulose 1,5-bisphosphate carboxylase/oxygenase
-a slow enzyme that catalyzes the rate-limiting step of hexose synthesis
-made of 8 large subunits and 8 small subunits
-most abundant protein
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Why does rubisco depend on magnesium and carbamate?
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-helps with positioning of rubisco
-activates rubisco so that it reacts with CO2
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What happens to rubisco when CO2 isn't present?
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-rubisco binds ribulose 1,5-bisphosphate too tightly for enzyme activity
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How is rubisco forced to have the correct structure for enzyme activity?
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-rubisco activase uses ATP to force correct structure
-ATP dependence of rubisco links light and dark reactions
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What makes rubisco "catalytically imperfect"?
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-it catalyzes a wasteful oxygenase reaction
-instead of two 3-phosphoglycerate we get phosphoglycolate and one 3-phosphoglycerate
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How is the phosphoglycolate salvaged?
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-it's dephosphorylated and transported to a peroxisome
-oxygen reacts with glycolate to form glyoxylate and hydrogen peroxide, which regenerates oxygen
-glyoxylate is transaminated to glycine
-2 clycines lose CO2 (wasteful) and NH4+ to generate serine
-serine loses another NH4+ to mak…
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3-phosphoglycerate --> 1,3-bisphosphoglycerate
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phosphoglycerate kinase (requires ATP)
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1,3-bisphosphoglycerate --> glyceraldehyde 3-phosphate
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NADPH-specific glyceraldehyde 3-phosphate dehydrogenase (requires NADPH)
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glyceraldehyde 3-phosphate --> dihydroxyacetone phosphate
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triose phosphate isomerase
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glyceraldehyde 3-phosphate --> fructose 1,6-bisphosphate
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aldolase
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fructose 1,6-bisphosphate --> fructose 6-phosphate
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fructose 1,6-bisphosphatase
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fructose 6-phosphate --> glucose 6-phosphate
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phosphoglucose isomerase
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glucose 6-phosphate --> glucose 1-phosphate
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phosphoglucomutase
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fructose 6-phosphate + glyceraldehyde 3-phosphate --> erythrose 4-phosphate + xylulose 5-phosphate
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transketolase
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erythrose 4-phosphate + dihydroxyacetone phosphate --> sedoheptulose 1,7-bisophosphate
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aldolase
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sedoheptulose 1,7-bisphosphate + H2O --> sedohetpulose 7-phosphate
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sedoheptulose 1,7-bisphosphatase
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sedoheptulose 7-phosphate + glyceraldehyde 3-phosphate --> ribose 5-phosphate + xyulose 5-phosphate
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transketolase
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ribose 5-phosphate --> ribulose 5-phosphate
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phosphopentose isomerase
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xylulose 5-phosphate --> ribulose 5-phosphate
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phosphopentose epimerase
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ribulose 5-phosphate --> ribulose 1,5-bisphosphate
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phosphoribulose kinase (requires ATP)
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Summarize the regeneration part of the Calvin cycle.
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F6P+2GAP+DHAP+3ATP --> 3Ru15BP+3ADP
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How many/what molecules are needed to bring carbon dioxide to the level of a hexose? How many rounds of the Calvin cycle?
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-three ATP, 2 NADPH
-6 rounds of the Calvin cycle are needed
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Give the net reaction of the Calvin Cycle.
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6CO2 + 18 ATP + 12 NADPH +12 H2O --> C6H12O6 + 18 ADP + 18Pi + 12 NADP+ + 6H+
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What role does thioredoxin play in the Calvin cycle? How is it made?
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-presence of reduced form of thioredoxin regulates rubisco
-produced by reduced ferredoxin
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What is the purpose of the C4 pathway?
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-accelerates photosynthesis by concentrating carbon dioxide
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How does the C4 pathway work? Why is it needed?
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-found mostly in tropical plants
-high temps lead to higher oxygenase activity of rubisco
-uses the Hatch-Slack pathway (using conversions between OAA, malate, and pyruvate as transporters) to facilitate a 20 fold increase in CO2 in the bundle sheath cells to favor carboxylase activity
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How does the CAM pathway work? Why is it needed?
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-found mostly in desert plants
-close stomata during day to conserve water and open at night to absorb H2O
-store CO2 as malate during the night
-during the day, malate is converted back to pyruvate and a CO2, which then goes to the Calvin Cycle
-results in photosynthesis without wate…
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What does the pentose phosphate (HMP) pathway do?
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-generates NADPH and synthesizes five carbon sugars
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What is the physiological significance of the HMP pathway?
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-produces NADPH for reductive reactions
-generates pentoses for RNA, DNA, coenzyme, and nucleotide synthesis
-uses diet-derived pentoses
-reduces peroxides
-keeps cysteine of proteins as -SH
-keeps hemoglobin in the Fe2+ state
-helps with the respiratory burst in neutrophils
-help…
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Where does the HMP pathway occur?
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-cytoplasm
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What are the two phases of the HMP pathway?
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-oxidative generation of NADPH
-nonoxidative inter-conversion of sugars
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When glucose 6-phosphate is converted into ribulose 5-phosphate, what is generated?
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-2 molecules of NADPH
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How are the HMP pathway and glycolysis linked?
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-transketolase
-transaldolase
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What happens to excess ribulose 5-phosphate created by the HMP pathway?
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-can be converted completely into glycolytic intermediates
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glucose 6-phosphate + NADP+ --> 6-phosphoglucono-d-lactone + NADPH
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glucose 6-phosphate dehydrogenase
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6-phosphoglucono-d-lactone + H2O --> 6-phosphogluconate
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lactonase
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6-phosphogluconate + NADP+ --> ribulose 5-phosphate + CO2 + NADPH
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6-phosphogluconate dehydrogenase
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ribulose 5-phosphate --> ribose 5-phosphate
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phosphopentose isomerase
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ribulose 5-phosphate --> xylulose 5-phosphate
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phosphopentose epimerase
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xylulose 5-phosphate + ribose 5-phosphate --> sedoheptulose 7-phosphate + GAP
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transketolase
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sedoheptulose 7-phosphate + GAP --> fructose 6-phosphate + erythrose 4-phosphate
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transaldolase
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xylulose 5-phosphate + erythrose 4-phosphate --> fructose 6-phosphate + GAP
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transketolase
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The metabolism of _________________ by the pentose phosphate pathway is coordinated with glycolysis.
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glucose 6-phosphate
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How is the rate of the pentose pathway controlled? Why is this control exerted?
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-controlled by the level of NADP+
-greater concentrations of NADP+ leads to more HMP pathway activity (controls dehydrogenation of glucose 6-phosphate)
-ensures that NADPH is not generated unless the supply needed for reductive biosynthesis or protection against oxidative stress is depl…
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When does Mode 1 of the HMP pathway occur? What makes it unique? What is the situation that might be occurring in the cell?
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-occurs when much more ribose 5-phosphate than NADPH is needed
-most of G6P is converted to F6P and GAP by glycolytic pathway; doesn't go through oxidation reactions
-cell needs to make nucleic acids (ex. a rapidly dividing cell)
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When does Mode 2 of the HMP pathway occur? What makes it unique? What is the situation that might be occurring in the cell?
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-occurs when the needs for NADPH and ribose 5-phosphate are balanced
-oxidation reactions only (G6P-->Ru5P-->R5P); gives off CO2 and 2 NADPH
-cell is very metabolically active (ex. a liver cell)
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When does Mode 3 of the HMP pathway occur? What makes it unique? What is the situation that might be occurring in the cell?
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-occurs when much more NADPH than ribose 5-phosphate is required
-the pathway becomes cyclic; G6P is completely oxidized to CO2; three sets of reactions
-red blood cell (no nucleic acids); cells with active fatty acid synthesis (adipose tissue)
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When does Mode 4 of the HMP pathway occur? What makes it unique? What is the situation that might be occurring in the cell?
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-occurs when both NADPH and ATP are required
-very similar to cyclic pathway, but instead of regenerating G6P, the molecules enter the glycolytic pathway to form pyruvate, generating ATP
-ex. brain cells
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BBMB 405: FINAL EXAM