ch 11 phototrophic energy metabolism: photosynthesis Flashcards
(110 cards)
1
Q
photoheterotrophs
A
organisms that acquire energy from sunlight but depend on organic sources, rather than CO2, of reduced carbon
2
Q
photoautotrophs
A
organisms that use solar energy to synthesize energy-rich organic molecules using starting materials such and CO2 and H2O
3
Q
two major biochemical processes
A
energy transduction reactions
carbon assimilation reactions
4
Q
energy transduction reaction
A
light energy is captured and converted into chemical energy in the form of ATP and coenzyme NADPH
5
Q
carbon assimilation reactions
A
carbohydrates are formed from CO2 and H2O
aka carbon fixation
6
Q
sucrose
A
major transport carb in plants
7
Q
starch
A
major storage carb in plants
8
Q
amyloplasts
A
specialized for storing starch
9
Q
chromoplasts
A
give flowers and fruits their distinctive colors
10
Q
proplastids
A
can develop into proteinoplasts (storing proteins) and elaioplasts (storing lipids)
11
Q
three membrane systems of the chloroplasts
A
outer membrane
inner membrane
intermembrane space
12
Q
outer membrane
A
contains porins that permit the passage of solutes with molecular weights up to about 5000 and is freely soluble to most small organic molecules and ions
13
Q
inner membrane
A
encloses the stroma (a gel-like teeming with enzymes for carbon, nitrogen, and sulfur reduction and assimilation)
forms a significant permeability barrier
14
Q
intermembrane space
A
region btw outer and inner membrane
freely permeable
15
Q
carboxysome
A
polyhedral proteinaceous structure that contains enzymes important for carbon fixation in cyanobacteria
16
Q
resonance energy transfer
A
mechanism whereby the excitation energy of a photoexcited molecule is transferred to an electron in an adjacent molecules, exciting that electron to a high-energy orbital
17
Q
photochemical reduction
A
transfer of photoexcited electrons from one molecule to another
18
Q
chlorophyll a
A
has a broad absorbency spectrum
has a methyl group
19
Q
chlorophyll b
A
has a CHO group
shifts the absorption to the center of the spectrum
20
Q
bacteriochlorophyll
A
type of chlorophyll found in bacteria
absorption toward the near-uv and the far-red regions
21
Q
accessory pigments
A
absorb light of wavelengths not absorbed by chlorophyll
give distinctive colors to plant tissue
22
Q
carotenoids
A
beta-carotene
lutein
absorb blue, therefore yellow or orange in color
23
Q
phycobilins
A
absorb green-to-orange light
phycoerythrin
phycocyanin
24
Q
contents of a photosystem
A
- chlorophyll
- accessory pigments
- chlorophyll-binding proteins that stabilize the chlorophyll in a photosystem
- other proteins that bind components of the ets
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antenna pigments
absorbs photons and passes the energy to a neighboring chlorophyll molecule of accessory pigment by resonance energy transfer
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reaction center
portion of a photosystem containing the two chlorophyll molecules that initiate electron transfer, utilizing the energy gather by other chlorophyll molecules and accessory pigments
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light-harvesting complex (LHC)
collection of light-absorbing pigments, usually chlorophylls and carotenoids, linked together by proteins
does not contain a reaction center
absorbs light and funnels the energy to a nearby photosystem
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photosystem complex
photosystem plus LHC
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emerson enhancement effect
achievement of greater photosynthetic activity with red light of two slightly different wavelengths than is possible by summing the activities obtained with the individual wavelengths separately
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photosystem I (PSI)
700nm
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photosystem II (PSII)
680nm
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special pair
two chlorophyll a molecules, located in the reaction center of a photosystem, that catalyze the conversion of solar energy into chemical energy
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PSII role in ETC
- uses electrons from water to reduce a plastoquinone (Qb) to platsoquinol (QbH2)
- PSII is a dimer
- associated with LCHII
- captured energy in the reaction center lowers the reduction potential of a P680 molecules, making it a better electron donor
- solar energy gets harvested and converted to electrochemical potential energy in the form of the charge separation
- the electron is passed to Qa
- Qb received two electrons from Qa and picks up two protons from the stroma to form QbH2
- 2 photons + Qb +2H+ + 2e- -> QbH2
- P680+ is reduced by an electron from water (two water molecules donate four electrons)
- the protons accumulating in the lumen contribute to an electrochemical proton gradient across the thylakoid membrane, and the O2 diffuses out of the chloroplast
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PSII summary reaction
4 photons + 2H2O + 2Qb + 4H+stroma -> O2 + 2QbH2 + 4H+lumen
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cytochrome b6/f complex
multiprotein complex within the thylakoid membrane that transfers electrons from a plastoquinol (QbH2) to plastocyanin as part of the energy transduction reactions of photosynthesis
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chloroplast Q cycle
QbH2 passes one electron to cytochrome b6 and the other to an iron-sulfur protein
these electrons are passed to cytochrome f and then to the plastocyanin
eight protons get pumped into the thylakoid lumen
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ferredoxin
immediate electron donor to NADP+
gets photoexcited electrons from reduced plastocyanin
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net reaction of PSI
4 photons + 4PC(Cu+) + 4Fd(Fe3+) -> 4PC(Cu2+) + 4Fd(Fe2+)
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final step in photoreduction
the transfer of electrons from ferredoxin to NADP+, producing the NADPH needed for carbon reduction and assimilation
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ferredoxin-NADP+ reductase (FNR)
enzyme located on the stroma side of the thylakoid membrane that catalyzes the transfer of electrons from ferredoxin to NADP+
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noncyclic electron flow
continuous, unidirectional flow of electrons from water to NADP+ during the energy transduction reactions of photosynthesis, with light providing the energy that drives the transfer
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net equation of noncyclic electron flow
8 photons + 2H2O + 6H+stroma + 2NADP+ -> 8H+lumen + O2 + 2NADPH
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photophosphorylation
light-dependent generation of ATP driven by an electrochemical proton gradient established and maintained as excited electrons of chlorophyll return to their ground state via an electron transport chain
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ATP synthase
catalyzes the reverse process in which the exergonic flow of protons down their electrochemical gradient is used to drive ATP synthesis
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CFoCF1 complex
ATP synthase complex found in chloroplast thylakoid membranes
catalyzes the process by which exergonic flow of protons down their electrochemical gradient is used to drive ATP synthesis
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CF1
hydrophilic group of polypeptides protruding from the stromal side of the thylakoid membrane and containing three catalytic sites for ATP synthesis
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CFo
hydrophobic assembly of polypeptides anchored to the thylakoid membrane
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proton translocator
channel through which protons flow across a membrane driven by an electrochemical gradient
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subunits of CFo
subunit I and II - form a stalk that connects CFo and CF1
subunit III - a ring of polypeptides next to subunit IV, the rotation of which is coupled to ATP synthesis, similar to mitochondria
subunit IV - proton translocator, protons flow back to the stroma
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H+/ATP reaction equation
4H+lumen + ADP + Pi -> 4H+stroma + ATP + H2O
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cyclic electron flow
light-driven transfer of electrons from photosystem I through a sequence of electron carriers that returns them to a chlorophyll molecule of the same photosystem, with the released energy used to drive ATP synthesis
- used when NADPH consumption is low, or when more ATP is needed
52
summary of photosystem II complex
- an assembly of chlorophyll molecules, accessor pigments, and proteins that contains the P680 reaction center chlorophyll
- water is oxidized and split. electrons flow from water to P680
- following photon absorption, P680 donates an electron to plastoquinone, reducing it to plastoquinol
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summary of cytochrome b6/f complex
- an ETC that transfers electrons from plastoquinol to plastocyanin
- links PSII to PSI
- electron flow is linked to proton pumping across the thylakoid membrane to establish a proton gradient
- optional cyclical electron flow allows additional ATP synthesis
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summary of photosystem I complex
- an assembly of chlorophyll molecules, accessory pigment, and proteins that contain the P700 reaction center chlorophyll
- accepts electrons from plastocyanin
- following photoexcitation, P700 donates electrons to ferredoxin
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summary of ferredoxin-NADP+ reductase
- enzyme on the stromal side of the thylakoid membrane
- catalyzes the transfer of electrons from two ferredoxin proteins and one proton to one NADP+ molecule
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summary of ATP synthase complex (CFoCF1)
- a proton channel and ATP synthase
- couples proton flow from the thylakoid lumen to the stroma with the synthesis of ATP
- ATP accumulates in the stroma
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calvin cycle
cyclic series of reactions used by photosynthetic organisms for the fixation of carbon dioxide and its reduction to form carbohydrates
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3 stages of calvin cycle
1. the carboxylation (fixation) of the initial acceptor molecule ribulose-1,5-biphosphate and immediate hydrolysis to generate two molecules of 3-phosphoglycerate
2. the reduction of 3-phosphoglycerate to form glyceraldehyde-3-phosphate
3. the regeneration of initial acceptor ribulose-1,5-biphosphate to allow continued carbon assimilation
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first stage of calvin cycle
- begins with the covalent attachment of CO2 to ribulose-1,5-biphosphate
- leads to production of two 3-carbon molecules, 3-phosphoglycerate
- ribulose-1,5-biphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction
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second stage of calvin cycle
- the 3-phosphoglycerate molecules formed during carbon dioxide fixation are reduced to glyceraldehyde-3-phosphate
- this reaction is basically the reverse of glycolysis
- coenzyme NADPH is used
- step 2a: phosphoglycerokinase catalyzes the transfer of a phosphate group from ATP to 3-phosphoglycerate. this generates an activated intermediate, 1,3-biphosphoglycerate
- step 2b: glyceraldehyde-3-phosphate dehydrogenase catalyzes the transfer of two electrons and one proton from NADPH to 1,3-biphosphoglycerate, reducing it to glyceraldehyde-3-phosphate (G3P)
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stage 3 of the calvin cycle
- 3 molecules of the acceptor pentose ribulose-1,5-biphosphate are regenerated from 5 glyceraldehyde-3-phosphates
- reactions are catalyzed by aldolases, transketolases, phosphatases, and isomerases
- phosphoribulokinase (PRK) phosphorylates each ribulose-5-phosphate to form the ribulose-1,5-biphosphate
- consumes 3 ATP
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net reaction of calvin cycle
3CO2 + 9 ATP + 6 NADPH + 5H2O -> G3P + 9ADP + 6NADP+ + 8Pi
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net reaction for energy transduction
26 photons + 9ADP + 9 Pi + 6 NADP + 6 H2O -> 3 O2 + 9 ATP + 6 NADPH + 9 H2O
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controls of calvin cycle
regulation of key enzymes:
- enzymes are not synthesized in tissues that are not exposed to light
- rubisco, sedoheptulose biphosphate, and PRK - stimulated by high pH and high conc of Mg2+
regulation based on ferredoxin:
- in the light, electrons donated by water are used to reduce ferredoxin, then transferred to thioredoxin
- which causes a conformational change in glyceraldehyde-3-phosphate dehydrogenase, sedoheptulose biphosphate, and PRK
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when light is driving the movement of electrons from water to ferredoxin
- protons are pumped from the stroma to the lumen and the pH rises from 7.2 to 8.0
- magnesium diffuses from the lumen to the stroma, raising the conc fivefold
- eventually high levels of reduced ferredoxin, NADPH, and ATP accumulate
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rubisco activase
removes inhibitory sugar-phosphate compounds from te rubisco active site
- has ATPase activity, which is sensitive to the ADP/ATP ratio
- in the dark, accumulated ADP inhibits rubisco activase, leaving rubisco inactive
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most abundant protein in the chloroplast inner membrane
triose phosphate/phosphate translocator
- catalyzes the exchange of triose phosphates in the stroma for Pi in the cytosol
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antiport system
exports triose phosphates only if Pi for making new triose phosphates returns to the stroma
- triose phosphates remaining in the stroma are used for starch synthesis
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glucose-1-phosphate
a hexose required for both starch and sucrose synthesis
- formed from G3P and DHAP in a three-step process
- these reactions can occur in the cytosol or stroma
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why must hexoses and hexose phosphates be made in the same location
because they cannot cross the chloroplast inner membrane
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S-1
- trioses undergo a condensation reaction to form fructose-1,6-biphosphate using the enzyme aldolase
- fructose-1,6-biphosphate is hydrolyzed to fructose-6-phosphate using the enzyme fructose-1,6-biphosphatase
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two forms of fructose-1,6-biphosphatases
cytoplasmic
stromal
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isoenzymes/isozymes
proteins that carry out the same enzymatic function
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S-2
fructose-6-phosphate can be converted to glucose-6-phosphate
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S-3
glucose-6-phosphate is converted to glucose-1-phosphate
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glucose is regularly converted into
sucrose - transport carb
starch, or glycogen in photosynthetic bacteria - storage carb
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sucrose is synthesized in
the cytosol of a photosynthetic cell
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S-4c
glucose is produced by activation with UTP to produce UDP-glucose
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S-5c
the glucose of UDP-glucose is transferred to fructose-6-phosphate to form sucrose-6-phosphate
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S-6c
the hydrolytic removal of the phosphate group generates free sucrose
- sucrose is then transported to nonphotosynthetic parts of the plant to provide energy and reduced carbon
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starch synthesis occurs in
plastids
triose phosphates are converted to glucose-1-phosphates which is used for starch synthesis
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S-4s
glucose-1-phosphate reacts with ATP to generate ADP-glucose
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S-5s
the activated glucose is added to a growing starch chain by starch synthases
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why is sucrose synthesis controlled
to prevent conflict with degradation pathways
- sucrose phosphate synthase is stimulated by glucose-6-phosphate and inhibited by sucrose-6-phosphate, UDP, and Pi
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how is starch biosynthesis controlled
ADP-glucose phosphorylase is stimulated by glyceraldehyde-3-phospahte and inhibited by Pi
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other products of photosynthesis
ATP and NADPH generated by photosynthetic energy transduction can be consumed for: the synthesis of fatty acids, chlorophyll, and carotenoids
- reduction of nitrite to ammonia serves for amino acid synthesis
- reduction of sulfate to sulfide serves for amino acid synthesis
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primary reaction catalyzed by rubisco
the addition of CO2 and H2O to ribulose-1,5-biphosphate, forming two 3-phosphoglycerate
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rubisco can act as a
carboxylase or an oxygenase
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3 strategies phototrophs use to deal with the enzyme's oxygenase activity
1. photorespiratory glycolate pathway
2. C4 photosynthesis
3. crassulacean acid metabolism
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phosphoglycolate
two-carbon compound produced by the oxygenase activity of rubisco. bc it cannot be metabolized during the next step of the calvin cycle, the production of phosphoglycolate decreases photosynthetic efficiency
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glycolate pathway role
returns about 75% of phosphoglycolate to the calvin cycle as 3-phosphoglycerate
- prevents toxic buildup of phosphoglycolate
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GP-1
phosphoglycolate is rapidly dephosphorylated by a phosphatase in the stroma
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GP-2
the resulting glycolate diffuses to a leaf peroxisome, where an oxidase converts it to glyoxylate
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GP-C
GP-2 is accompanied by O2 uptake and H2O2 generation, which is immediately degraded by catalase
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GP-3
aminotransferase transfers an amino group to glyoxylate, forming glycine
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GP-4
glycine diffuses from the peroxisome to a mitochondrion, where a decarboxylase and a hydroxymethyl transferase convert two glycine to one serine, along with formation of NADH and release of CO2 and NH3
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GP-5
serine diffuses back to the peroxisome, where another aminotransferase removes an amino group to form hydroxypryuvate
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GP-6
a reductase reduces it to glycerate
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GP-7
glycerate diffuses to the chloroplast and is phosphorylated by glycerate kinase to form 3-phosphoglycerate
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plants in what environment are most affected by rubiscos oxygenase activity
hot, arid environments under intense illumination
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hatch-slack cycle
a short carboxylation/decarboxylation pathway, with oxaloacetate as the intermediate of carbon fixation
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C4 plants
plants that use the hatch-slack cycle to produce oxaloacetate (4-carbon compound)
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C3 plants
have 3-phosphoglycerate (3 carbon compound) as the first detectable product of carbon fixation
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two types of photosynthetic cells in C4 plant leaves
- mesophyll cell: CO2 fixation here uses an enzyme other than rubisco; these cells are exposed to CO2 and O2
- bundle sheath cells: these are relatively isolated from the atmosphere, and the entire calvin cycle is confined to these cells
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HS-1
begins with carboxylation of phosphoenolpyruvate to form oxaloacetate
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HS-2
oxaloacetate is rapidly converted to malate by NADPH-dependent malate dehydrogenase
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malate
stable four-carbon acid that carries carbon from mesophyll cells to chloroplasts of bundle sheath cells, where decarboxylation by NADP+ malic enzyme releases CO2
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HS-3
malate moves to bundle sheath cells, where decarboxylation of NADP+ malic enzyme releases CO2, which is refixed and reduced by the calvin cycle
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HS-4
the pyruvate produced in HS-3 diffuses into mesophyll cells, where it is phosphorylated, to regenerate PEP at the expense of one ATP
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crassulacean acid metabolism (CAM) plants
open stromata only at night to minimize water loss
- CO2 enters mesophyll cells and goes through the first two steps of the hatch-slack cycle to produce malate
- the accumulated malate is stored in vacuoles
- during the day the stomata are closed, and the malate diffuses into the cytosol, where the HS cycle continues
- CO2 released diffuses to chloroplast stroma, where it is refixed and reduced in the calvin cycle
- CAM plants may assimilate over 25 times as much carbon as a C3 plant does
