Chapter 19 Flashcards
(113 cards)
1
Q
Photosynthesis is the process whereby light energy drives the reduction of CO2 to yield …. In plants and cyanobacteria, photosynthesis oxidizes water to …
A
carbohydrates; O2;
2
Q
in plants, the photosynthetic machinery consists of protein complexes embedded int he … and enzymes dissolved in the … of chloroplasts
A
thylakoid membrane; stroma
3
Q
… and other light-absorbing pigments are organized in light-harvesting complexes that funnel light energy to … (RCs)
A
chlorophyll; photosynthetic rxn centers
4
Q
The purple bacterial photosynthetic reaction center (PbRC) undergoes photooxidation when it absorbs a photon. The excited electron passes through a series of electron carriers before reducing …. The reduced ubiquinone is reoxidized by cytochrome bc1, which in the process translocates … protons from the cytosol to the periplasmic space via a Q cycle. The electron is then returned to the PbRC via an electron transport chain resulting in no net oxidation–reduction.
A
ubiquinone; four
5
Q
In plants and cyanobacteria, photosystems I and II (PSI and PSII) operate in electrical series in an arrangement known as the… The oxidation of water by the Mn-containing oxygen-evolving center (OEC) is driven by the photooxidation of PSII.
A
Z-scheme.
6
Q
The electrons released by the photooxidation of PSII are transferred, via plastoquinone, to the … complex, which mediates a proton-translocating Q cycle while passing the electrons to ….
A
cytochrome b6 f; plastocyanin
7
Q
The photooxidation of PSI drives the electrons obtained from plastocyanin to .. and then to NADP+ to produce …. In cyclic electron fl ow, however, electrons return to cytochrome b6 f, thereby bypassing the need for …
A
ferredoxin; NADPH; PSII photooxidation.
8
Q
The reaction centers of the PbRC, PSII, and PSI have similar structures and mechanisms and therefore appear to have arisen from …
A
a common ancestor
9
Q
In …, the protons released by the oxidation of H2O and proton translocation into the thylakoid lumen generate a transmembrane …that is tapped by chloroplast ATP synthase to drive the phosphorylation of ADP. A similar process occurs in purple photosynthetic bacteria.
A
photophosphorylation; proton gradient
10
Q
The dark reactions use the ATP and NADPH produced in the light reactions to power the synthesis of … from CO2. In the first phase of the Calvin cycle, CO2 reacts with ribulose-1,5bisphosphate (RuBP) to ultimately yield … The remaining reactions of the cycle regenerate the … acceptor of CO2.
A
carbohydrates ; glyceraldehyde-3-phosphate (GAP); RuBP
11
Q
…, the key enzyme of the dark reactions, is regulated by pH, [Mg2+], and the inhibitory compound 2-carboxyarabinitol1-phosphate (CA1P). The two bisphosphatases of the Calvin cycle are controlled by the ..of the chloroplast via disulfide interchange reactions mediated in part by thioredoxin.
A
RuBP carboxylase; redox state
12
Q
…, in which plants consume O2 and evolve CO2, uses the ATP and NADPH produced by the light reactions. …plants minimize the oxygenase activity of RuBP carboxylase (RuBisCO) by concentrating CO2 in their photosynthetic cells. …plants use a related mechanism to conserve water
A
Photorespiration; C4; CAM
13
Q
…, in which light energy drives the reduction of carbon, is essentially the reverse of oxidative carbohydrate metabolism
A
Photosynthesis
14
Q
Moreover, photosynthesis, over the eons, generated all of the oxygen in the earth’s atmosphere (recall that the early earth’s atmosphere was devoid of O2; Section 1-1A). It is estimated that photosynthesis annually fi xes ∼1011 tons of carbon, which represents the storage of over 1018 kJ of energy. About half of this activity is carried out by …, mainly …
A
phytoplankton; cyanobacteria
15
Q
The two stages of photosynthesis are traditionally referred to as the … and …:
A
light reactions; dark reactions
16
Q
In the light reactions, specialized pigment molecules capture light energy and are thereby …. A series of electron-transfer reactions, which culminate with the reduction of… to …, generate a transmembrane proton gradient whose energy is tapped to synthesize ATP from ADP + Pi. The oxidized pigment molecules are reduced by …, thereby generating O2.
A
oxidized; NADP+; NADPH; H2O
17
Q
The dark reactions use …and …to reduce CO2 and incorporate it into the three-carbon precursors of carbohydrates
A
NADPH; ATP
18
Q
both processes occur in the light and are therefore better described as … and … reactions
A
light-dependent; light-independent
19
Q
The site of photosynthesis in eukaryotes (algae and higher plants) is the
…
A
chloroplast
20
Q
The inner membrane of the chloroplast encloses the …, a concentrated solution of enzymes, including those required for carbohydrate synthesis. The stroma also contains the …, …, and … involved in the synthesis of several chloroplast proteins.
A
stroma; DNA; RNA; ribosomes
21
Q
The stroma, in turn, surrounds a third membranous compartment, the ….
A
thylakoid
22
Q
The thylakoid is probably a single highly folded vesicle, although in most organisms it appears to consist of stacks of disklike sacs named … (singular, granum), which are interconnected by unstacked …
A
grana; stromal lamellae
23
Q
In photosynthetic bacteria, the machinery for the light reactions is located in the …
A
plasma membrane
24
Q
The principal photoreceptor in photosynthesis is ….
A
chlorophyll
25
The major chlorophyll forms in plants and cyanobacteria, ...(Chl a) and ….(Chl b), and the major forms in photosynthetic bacteria, ... (BChl a) and … (BChl b), also diff er from heme and from each other in the degree of saturation of Rings II and IV and in the substituents of Rings I, II, and IV.
chlorophyll a; chlorophyll b; bacteriochlorophyll a; bacteriochlorophyll b
26
The primary r eactions of photosynthesis, as is explained in Section 19-2B, take place at …..
photosynthetic reaction centers;
27
Yet photosynthetic assemblies contain far more ... molecules than are contained in reaction centers. This is because most chlorophyll molecules do not participate directly in photochemical reactions but function to gather light; that is, they act as ...
chlorophyll; light-harvesting antennas.
28
These antenna chlorophylls pass the energy of absorbed ...(units of light) from molecule to molecule until it reaches a photosynthetic reaction center
photons
29
Even in bright sunlight, an RC directly intercepts only ∼1 photon per second, a metabolically insignificant rate. Hence, a complex of antenna pigments, or ..., is essential.
lightharvesting complex (LHC)
30
Most LHCs contain other light-absorbing substances besides chlorophyll. These ...“fi ll in” the absorption spectra of the antenna complexes, covering the spectral regions where chlorophylls do not absorb strongly (Fig. 19-3).
accessory pigments
31
...: accessory pigments which are linear polyenes such as β-carotene
components of all green plants and many photosynthetic bacteria and are therefore the most common accessory pigments.
carotenoids
32
Photosynthesis is a process in which electrons from excited chlorophyll molecules are passed through a series of acceptors that convert ...energy to …. energy.
electronic; chemical
33
Electromagnetic radiation is propagated as discrete ...(photons) whose energy E is given by Planck’s law:
... where h is Planck’s constant (6.626 × 10−34 J ∙ s), c is the speed of light in vacuum (2.998 × 108 m ∙ s−1), ν is the frequency of the radiation, and λ is its wavelength
quanta; E = hv =hc/λ
34
When a molecule absorbs a photon, one of its electrons is promoted from its … state molecular orbital to one of higher energy. However, a given molecule can absorb photons of only certain wavelengths because, as is required by the law of conservation of energy, the energy difference between the two states must exactly match the energy of ...
ground (lowest energy); the absorbed photon.
35
(An electronically excited molecule can dissipate its excitation energy in several ways) ..., a common mode of decay in which electronic energy is converted to the kinetic energy of molecular motion; that is, to heat. Many molecules relax in this manner to their ground states. Chlorophyll molecules, however, usually relax only to their lowest excited states. Consequently, the photosynthetically applicable excitation energy of a chlorophyll molecule that has absorbed a photon in its short-wavelength band, which corresponds to its second excited state, is no different than if it had absorbed a photon in its less energetic long-wavelength band.
Internal conversion
36
(An electronically excited molecule can dissipate its excitation energy in several ways) ..., in which an electronically excited molecule decays to its ground state by emitting a photon. A fluorescently emitted photon generally has a longer wavelength (lower energy) than that initially absorbed. Fluorescence accounts for the dissipation of only 3 to 6% of the light energy absorbed by living plants.
Fluorescence
37
(An electronically excited molecule can dissipate its excitation energy in several ways) ... (also known as ...), in which an excited molecule directly transfers its excitation energy to nearby unexcited molecules with similar electronic properties. This process occurs through interactions between the molecular orbitals of the participating molecules. Light energy is funneled to RCs through exciton transfer among .... The energy (excitation) is trapped at the RC chlorophylls because they have slightly lower excited state energies than the antenna chlorophylls. This energy difference is lost as heat.
Exciton transfer; resonance energy transfer; antenna pigments
38
(An electronically excited molecule can dissipate its excitation energy in several ways) ..., in which a light-excited donor molecule is oxidized by transferring an electron to an acceptor molecule, which is thereby reduced. This process occurs because the transferred electron is less tightly bound to the donor in its excited state than it is in the ground state. In photosynthesis, excited chlorophyll (Chl*) is such a donor. The energy of the absorbed photon is thereby … transferred to the photosynthetic reaction system. Photooxidized chlorophyll, Chl+, a cationic free radical, eventually returns to its reduced state by oxidizing some other molecule.
Photooxidation; chemically
39
During the electron-transfer process, cytoplasmic protons are translocated across the .... Dissipation of the resulting proton gradient drives ATP synthesis
plasma membrane
40
The RCs from several species of purple photosynthetic bacteria each contain three hydrophobic subunits known as H, L, and M. The L and M subunits collectively bind four molecules of bacteriochlorophyll, two molecules of … (BPheo; bacteriochlorophyll in which the Mg2+ ion is replaced by two protons), one Fe(II) ion, and two molecules of the redox coenzyme ubiquinone (Fig. 18-10b) or one molecule of ubiquinone and one of the related ...
bacteriopheophytin ; menaquinone
41
The most striking aspect of the PbRC is that the groups are arranged with nearly perfect.... Two of the BChl molecules, the so-called ..., are closely associated; they are nearly parallel and have an Mg—Mg distance of ∼7 Å. The special pair is named for the wavelength (in nanometers) at which its absorbance ... on photooxidation
twofold symmetry; special pair; maximally decreases
42
(The photochemical events mediated by the PbRC occur as follows:) 1. The primary photochemical event of bacterial photosynthesis is the ..by the special pair (e.g., P960). The excited electron is delocalized over both its BChl molecules.
absorption of a photon
43
(The photochemical events mediated by the PbRC occur as follows:) P960*, the excited state of P960, has but a fleeting existence. Within ∼3 picoseconds (ps; 10−12 s), P960* transfers an electron to the BPheo to yield P960+ BPheo b− (the intervening BChl group probably plays a role in ..., although it is not itself reduced; it is therefore known as the ...
conveying electrons; accessory BChl
44
(The photochemical events mediated by the PbRC occur as follows:) During the next 200 ps, the electron migrates to the menaquinone (or, in many species, the second ubiquinone), designated QA, to form the anionic semiquinone radical Q− A∙. All these electron transfers, are to progressively ....which makes the process all but ....
lower energy states; irreversible
45
electron transfer in the PbRC is so efficient that its overall ... (ratio of molecules reacted to photons absorbed) is virtually ...%.
quantum yield; 100
46
Qa-∙, which occupies a hydrophobic pocket in the PbRC, transfers its excited electron to the more solvent-exposed ..., QB, to form Qb-∙ (the Fe ion positioned between QA and QB does not directly participate in these redox reactions). QA never becomes fully ...; it shuttles between its oxidized and semiquinone forms.
ubiquinone; reduced
47
QB is a molecular ...that converts two light-driven one-electron excitations to a two- electron chemical reduction.
transducer
48
The electrons taken up by QBH2 are eventually returned to... via an electron-transport chain. The available redox carriers include a membrane-bound pool of ubiquinone molecules, a cytochrome bc1 complex, and cytochrome
P960+
49
Since electron transport in purple photosynthetic bacteria is a cyclic process (Fig. 19-10), it results in no ...
net oxidation–reduction
50
photon absorption by the PbRC generates a transmembrane ... Light- dependent synthesis of ATP is driven by the dissipation of this gradient
H+ gradient.
51
In plants and cyanobacteria, photosynthesis is a ...process that uses the reducing power generated by the light-driven oxidation of ...to produce .... This multistep process involves two photosynthetic reaction centers (RCs) that each bear considerable resemblance to PbRCs
noncyclic; H2O; NADPH
52
the photosynthetic RCs are ... (PSII), which oxidizes H2O, and ... (PSI), which reduces NADP+. Each photosystem is independently activated by ..., with electrons fl owing from PSII to PSI. PSII and PSI therefore operate in ... to couple H2O oxidation with NADP+ reduction.
photosystem II; photosystem I; light; electrical series
53
The components involved in electron transport from H2O to NADPH are largely organized into three thylakoid membrane-bound particles (Fig. 19-11): ..., a ...complex, and ...
PSII; cytochrome b6 f; PSI
54
Electrons are transferred between the complexes via mobile ..., much as occurs in the respiratory electron-transport chain. The ubiquinone analog plastoquinone (Q), via its reduction to ...(QH2), links PSII to the cytochrome b6 f complex, which, in turn, interacts with PSI through the mobile peripheral membrane protein ...(PC
electron carriers; plastoquinol; plastocyanin
55
Electrons eventually reach ...FNR), where they are used to reduce NADP+. The oxidation of water and the passage of electrons through a … cycle generate a transmembrane proton gradient, with the greater [H+] in the thylakoid lumen. The free energy of the proton gradient is tapped by chloroplast ATP synthase
ferredoxin–NADP+ reductase; Q;
56
The various prosthetic groups of the photosynthetic apparatus of plants can be arranged in a diagram known as the ... As in other electron-transport systems, electrons flow from low to high reduction potentials. The ...nature of the Z-scheme reflects the two loci for photochemical events (one at PSII, one at PSI) that are required to drive electrons from H2O to NADP+.
Z-scheme; zigzag
57
PSII is a …
symmetric dimer
58
...: Pheo a’s; Chl a with its Mg2+ replaced by two protons
pheophytin a’s
59
Mn4CaO5 complex known as the ...center (OEC).
oxygen-evolving
60
The electron ejected by photooxidation of P680 is replaced by an electron derived from H2O via the OEC. The OEC of PSII is also known as the ... because it breaks down two water molecules to O2, four protons, and four electrons
water-splitting enzyme
61
each OEC must undergo four lightdependent reactions—that is, four ...—before releasing O2.
electron transfers
62
...which produces an X-ray beam of such enormous intensity that it vaporizes anything in its path
X-ray free-electron laser,
63
Water-splitting, a reaction that is essential for sustaining most organisms, is driven by the excitation of the ...
PSII RC.
64
The four electrons abstracted from 2 H2O by the OEC therefore lead to the translocation of ...H+ from the stroma to the thylakoid lumen. Electron transport via the ...generates much of the electrochemical proton gradient that drives the synthesis of ATP in chloroplasts
eight; cytochrome b6 f complex
65
...(PC), a peripheral membrane protein located on the thylakoid lumenal surface
plastocyanin
66
..and ..., together with other transmembrane subunits, also bind the cofactors of the core antenna system
PsaA; PsaB
67
rather than the reduced forms of either QK-A or QK-B dissociating from PSI, both of the quinones directly pass their photoexcited electron to a chain of three spectroscopically identifi ed ... designated FX, FA, and FB
[4Fe–4S] clusters
68
The o bservation that both branches of PSI’s electron-transfer pathways are active, in contrast to only one active branch in PSII and the PbRC, is rationalized by the observation that the two quinones at the ends of each branch are functionally equivalent in PSI but functionally diff erent in … and the ...
PSII and the PbRC.
69
The ..., which are mostly β-carotenes, are deeply buried in the membrane, where they are in van der Waals contact with Chl a rings. This permits effi cient energy transfer from photoexcited carotenoids to Chl a.
carotenoids
70
( Electrons ejected from FB in PSI may follow either of two alternative pathways) 1. . Most electrons follow a noncyclic pathway by reducing an ∼100-residue, [2Fe–2S]-containing, soluble protein called …. that is located in the stroma. Reduced Fd, in turn, reduces NADP+ in a rxn mediated by the monomeric FAD containing … to yield the final product of the chloroplast light rxns, ...
ferredoxin; ferredoxin–NADP+ reductase; NADPH
71
( Electrons ejected from FB in PSI may follow either of two alternative pathways) 1. Two reduced Fd molecules successively deliver one electron each to the FAD of FNR, which thereby sequentially assumes the neutral … and fully reduced states before transferring the two electrons and a proton to the NADP+ via what is formally a ...
semiquinone; hydride ion transfer.
72
( Electrons ejected from FB in PSI may follow either of two alternative pathways) 2. Some electrons are returned from PSI, via cytochrome b6, to the plastoquinone pool, thereby traversing a cyclic pathway that translocates protons across the … Note that the cyclic pathway is independent of the action of PSII and hence does not result in the evolution of O2. This accounts for the observation that chloroplasts absorb more than eight photons per O2 molecule evolved.
thylakoid membrane; O2; 8
73
The cyclic electron fl ow presumably functions to increase the amount of ATP produced relative to that of NADPH and thus permits the cell to .. of the two substances produced according to its needs
adjust the relative amounts
74
Chloroplasts generate ATP in much the same way as mitochondria, that is, by coupling the dissipation of a proton gradient to the enzymatic synthesis of ATP (Section 18-3). This light-dependent process is known as .... Like oxidative phosphorylation, it requires an intact thylakoid membrane and can be uncoupled from light-driven electron transport by compounds such as 2,4-dinitrophenol
photophosphorylation
75
In fact, the chloroplast ATP synthase, which is also known as the ...complex (C for chloroplast), is remarkably similar to the mitochondrial F1F0 complex.
CF1CF0
76
At saturating light intensities, chloroplasts generate proton gradients of ∼3.5 pH units across their thylakoid membranes as a result of two processes:
1. The evolution of a molecule of O2 from two H2O molecules releases ..protons into the thylakoid lumen.
2. The transport of the liberated four electrons through the cytochrome b6 f complex occurs with the translocation of ..protons from the stroma to the thylakoid lumen.
four; eight
77
Altogether, ∼...protons enter the lumen per molecule of O2 produced by noncyclic electron transport.
12
78
The thylakoid membrane, in contrast to the inner mitochondrial membrane, is permeable to ions such as Mg2+ and Cl−. Translocation of protons and electrons across the thylakoid membrane is consequently accompanied by the passage of these ions to maintain ...(Mg2+ out and Cl− in). This all but eliminates the membrane potential. The electrochemical gradient in chloroplasts is therefore almost entirely a result of the ...
electrical neutrality; pH (concentration) gradient.
79
Chloroplast ATP synthase, according to most estimates, produces one ATP for every … protons it transports from the thylakoid lumen to the stroma. Noncyclic electron transport in chloroplasts therefore results in the production of ∼12/3 = … molecules of ATP per molecule of O2 evolved (cyclic electron transport generates more ATP because more protons are translocated to the thylakoid lumen via the Q cycle mediated by cytochrome b6 f).
three; 4
80
Noncyclic electron transport, of course, also yields NADPH (2 NADPH for every 4 electrons liberated from 2 H2O by the OEC). Each NADPH has the free energy to produce 2.5 ATP (Section 18-3C; although NADPH is not used to drive ATP synthesis), for a total of 5 more ATP equivalents per O2 produced. Consequently, a total of..... ATP equivalents are generated per O2 produced. A minimum of ...photons is required for each electron traversing the system from H2O to NADPH, that is, ….photons per O2 produced
9; 2; 8
81
the overall effi ciency of the light reactions is 9 ATP/8–10 photons, or approximately ...ATP per absorbed photon.
one
82
…: The metabolic pathway by which plants incorporate CO2 into carbohydrates
calvin cycle
83
the first stable radioactive compound formed in calvin is ... (3PG), which is initially labeled only in its carboxyl group. This result immediately suggested that the 3PG was formed by the carboxylation of a C2 compound.
3-phosphoglycerate
84
The actual carboxylation reaction involves a pentose derived from... (Ru5P; at right). The resulting C6 product splits into two C3 compounds, both of which turned out to be 3PG. ATP and NADPH, the products of the light reactions, are required to convert 3PG to ...(GAP), which is used to synthesize carbohydrates as well as re-form Ru5P (Fig. 19-25). The entire pathway, which involves the carboxylation of a pentose, the formation of carbohydrate products, and the regeneration of the pentose, is known as the ... or the...
ribulose-5-phosphate; glyceraldehyde-3-phosphate; Calvin cycle; reductive pentose phosphate cycle.
85
Stage 1 of calvin cycle:
The production phase (top line of Fig. 19-26), in which three molecules of Ru5P react with three molecules of CO2 to yield six molecules of glyceraldehyde-3-phosphate (GAP) at the expense of nine ATP and six NADPH molecules. The cyclic nature of the entire pathway makes this process equivalent to the synthesis of ....At this point, GAP can be bled off from the cycle for use in biosynthesis.
one GAP from three CO2 molecules.
86
Stage 2 of calvin cycle:
. The... phase, in which the carbon atoms of the remaining five GAPs are shuffled in a remarkable series of reactions, similar to those of the pentose phosphate pathway, to re-form the three ...with which the cycle began.
The overall stoichiometry for the process is therefore
5 C3 → 3 C 5
recovery; Ru5Ps
87
The fi rst reaction of the Calvin cycle is the phosphorylation of Ru5P by ...to form ribulose-1,5-bisphosphate (RuBP). Following the carboxylation of RuBP , the resulting 3PG is converted first to ...and then to GAP. The latter sequence is the reverse of two consecutive glycolytic reactions
phosphoribulokinase; 1,3-bisphosphoglycerate (BPG)
88
The second stage of the Calvin cycle begins with the reverse of a familiar glycolytic reaction, the isomerization of GAP to dihydroxyacetone phosphate (DHAP) by ...
triose phosphate isomerase
89
Reactions 6 and 9 are aldolase-catalyzed ... in which DHAP is linked to an aldehyde.
aldol condensations
90
Reactions 7 and 10 are ...reactions that are catalyzed, respectively, by fructose bisphosphatase (FBPase; Section 15-4B) and sedoheptulose bisphosphatase (SBPase
phosphate hydrolysis
91
In Reactions 8 and 11, both catalyzed by transketolase, a C2 keto unit ) is transferred from a ketose to GAP to form …. (Xu5P), leaving the aldoses erythrose-4-phosphate (E4P) in Reaction 8 and ribose-5-phosphate (R5P) in Reaction 11. The E4P produced by Reaction 8 feeds into Reaction 9.
xylulose5-phosphate
92
The Xu5Ps produced by Reactions 8 and 11 are converted to Ru5P by ...in Reaction 12. The R5P from Reaction 11 is also converted to ...by ribose phosphate isomerase in Reaction 13, thereby completing a turn of the Calvin cycle
phosphopentose epimerase ; Ru5P
93
The enzyme that catalyzes CO2 fi xation, ribulose bisphosphate carboxylase (...), is arguably the world’s most important enzyme since nearly all life on earth ultimately depends on its action.
RuBP carboxylase
94
The ...subunit is made of a β sheet domain and an α/β barrel domain that contains the enzyme’s catalytic site (Fig. 19-27c). The function of the S subunit is unknown;
L
95
Abstraction of the C3 proton of RuBP, the reaction’s rate-determining step, generates an enediolate that nucleophilically attacks CO2. The resulting β-keto acid is rapidly attacked at its C3 position by H2O to yield an adduct that splits, by a reaction similar to aldol cleavage, to yield the two product 3PG molecules. The driving force for the overall reaction is provided by the cleavage of the β -keto acid intermediate to yield an additional ...
resonance-stabilized carboxylate group.
96
RuBP carboxylase activity requires Mg2+, which probably stabilizes developing negative charges during catalysis. The Mg2+ is, in part, bound to the enzyme by a catalytically important carbamate group (—NH—COO−) that is generated by the reaction of a nonsubstrate CO2 with the ε-amino group of Lys 201. This essential reaction is catalyzed in vivo by the enzyme....in an ATP-driven process
RuBP carboxylase activase
97
The overall stoichiometry of the Calvin cycle is 3 CO2 + 9 ATP + 6 NADPH →
GAP + 9 ADP + 8 Pi + 6 NADP+
98
… is the precursor of the higher order carbohydrates characteristic of plants.
G1P
99
..., a disaccharide of glucose and fructose (Section 8-2A), is the major transport sugar for delivering carbohydrates to nonphotosynthesizing cells and hence is the major photosynthetic product of green leaves.
Sucrose
100
..., which consists of long chains of β(1 → 4)-linked glucose units and is the major polysaccharide of plants, is also synthesized from UDP–glucose.
Cellulose
101
, plants must have a ... to prevent the Calvin cycle from consuming this catabolically produced ATP and NADPH in a wasteful futile cycle.
light-sensitive control mechanism
102
three best candidates for flux control in the Calvin cycle are the reactions catalyzed by …, …, and...
RuBP carboxylase, FBPase, and SBPase
103
The activity of RuBP carboxylase responds to three light-dependent factors:
1. … On illumination, the pH of the stroma increases from ∼7.0 to ∼8.0 as protons are pumped from the stroma into the thylakoid lumen. RuBP carboxylase has a sharp pH optimum near pH 8.0.
2. … Recall that the light-induced infl ux of protons to the thylakoid lumen is accompanied by the effl ux of Mg2+ to the stroma (Section 19-2D). This Mg2+ stimulates RuBP carboxylase.
3. the transition state analog ….
pH; [Mg2+]; 2-carboxyarabinitol-1-phosphate (CA1P).
104
. Thus, in plants, light stimulates the Calvin cycle while deactivating ..., whereas darkness has the opposite eff ect (that is, the so-called dark reactions do not occur in the dark).
glycolysis
105
illuminated plants consume O2 and evolve CO2 in a pathway distinct from oxidative phosphorylation. In fact, at low CO2 and high O2 levels, this … process can outstrip photosynthetic CO2 fi xation.
photorespiration
106
...competes with CO2 as a substrate for RuBP carboxylase (RuBP carboxylase is therefore also called RuBP carboxylase–oxygenase or RuBisCO).
O2
107
B). The glyoxylate can be converted to glycine in a transamination reaction, as is discussed in Section 21-2A, and exported to the mitochondrion. There, two molecules of glycine are converted to one molecule of serine and one of CO2. This is the ...generated by photorespiration
origin of the CO2
108
The net result of this complex photorespiration cycle is that some of the ATP and NADPH generated by the light reactions is uselessly dissipated and previously fi xed ... is lost.
CO2
109
. Photorespiration may confer a selective advantage by protecting the photosynthetic apparatus from photooxidative damage when... is available to otherwise dissipate its absorbed light energy.
insuffi cient CO2
110
On a hot, bright day, when photosynthesis has depleted the level of CO2 at the chloroplast and raised that of O2, the rate of photorespiration approaches the rate of ...
photosynthesis
111
certain species of plants, such as sugarcane, corn, and most important weeds, have a metabolic cycle that concentrates .., thereby almost totally preventing p hotorespiration. The leaves of plants that have this so-called C4 cycle have a characteristic anatomy. Their fi ne veins are concentrically surrounded by a single layer of so-called ... which in turn are surrounded by a layer of ...cells.
CO2 in their photosynthetic cells; bundle-sheath cells,; mesophyll
112
The C4 pathway thereby concentrates CO2 in the bundle-sheath cells at the expense of ..ATP equivalents. Consequently, photosynthesis in C4 plants consumes a total of ...ATP per CO2 fi xed versus the ... ATP required by the Calvin cycle alone.
2; 5; 3
113
C 4 plants occur largely in tropical regions because they grow faster under hot and sunny conditions than other, so-called ...plants (so named because they initially fi x CO2 in the form of three-carbon acids
C3
