BCMB 2902 L21-24

L21

Light reactions of photosynthesis

Part 1

Introduction to the energetic and molecular basis for photosynthesis

Photosynthetic organisms use light energy to form what?

Photosynthetic organisms use light energy to form ATP and NADPH, which are then used to synthesize carbohydrates from CO2 and H2O.

For light energy to be useful in photosynthesis, the energy in a quantum of light must do what?

For light energy to be useful in photosynthesis, the energy in a quantum of light must match the energy required to excite the chromophore (the absorbing molecule).

The chromophore in its excited state can lose its energy — forming the ground state — in which four ways?

The chromophore in its excited state can lose its energy—forming the ground state—in four ways, producing heat, fluorescence, exciton transfer or charge transfer.

In plant cells, the light-dependent and carbon-assimilation reactions occur in where?

In plant cells, the light-dependent and carbon-assimilation reactions occur in the chloroplasts.

The light reactions occur in where?

The carbon-assimilation reactions occur in where?

The light reactions occur in the thylakoid membranes, whereas the carbon-assimilation reactions occur in the stroma.

Chloroplast thylakoids are topologically what and stack to form what?

Chloroplast thylakoids are topologically continuous and stack to form grana.

Part 2

Pigments and photosystems in photosynthesis

What are the critical pigments in the thylakoid membranes?

Chlorophylls are the critical pigments in the thylakoid membranes.

The tetrapyrrole cage of chlorophyll (Chl) coordinates what?

The tetrapyrrole cage of chlorophyll (Chl) coordinates a Mg atom (cf. Fe in haem).

Chloroplasts of plants contain what?

Chloroplasts of plants contain both Chl a and Chl b, which have different absorbance spectra.

Cyanobacteria and red algae use what as their light-harvesting pigments

Cyanobacteria and red algae use phycobilins as their lightharvesting pigments.

Carotenoids are what kind of pigment. 

Carotenoids are accessory pigments, which absorb light at wavelengths not absorbed efficiently by chlorophylls.

Carotenoids protect what machinery from what?

Carotenoids protect the photosynthetic machinery from sinlet oxygen, a reactive oxygen species (ROS) formed when intense light exceeds the system’s capacity to accept electrons.

Does each photosynthetic pigment have a different action spectrum?

Yes, each photosynthetic pigment has a different action spectrum.

Action spectra help identify what?

Action spectra help identify pigments responsible for a light-induced biological effect.

Absorbed light energy is funnelled to where?

Absorbed light energy is funnelled to reaction centres in photosystems.

Each photosystem (in spinach) contains how many chlorophyll and carotenoid molecules?

Each photosystem (in spinach) contains ~200 chlorophyll and ~50 carotenoid molecules.

A special pair of chlorophyll molecules, (Chl)₂, forming what? The other pigments in the photosystem act as what?

A special pair of chlorophyll molecules, (Chl)2, forming the photochemical reaction centre, converts light energy into chemical energy. The other pigments in the photosystem act as antenna molecules.

Chlorophylls and other pigments are bound specifically to what?

Chlorophylls and other pigments are bound specifically to photosystem proteins.

Some antenna pigments are part of a what around the reaction centre?

Other antenna pigments form what around what?

Some antenna pigments are part of a core complex around the reaction centre.

Other antenna pigments form light-harvesting complexes (LHCs) around the periphery of the core complex.

– The functional unit of LHCII (Light-Harvesting Complex II), for example, is a trimer with 36 chlorophyll and six lutein molecules.

Part 3

Charge separation and electron transport

What can lead to charge separation?

Light absorption leads to charge separation.

One of the chlorophyll molecules in (Chl)₂ receives energy via what, promoting an electron to what?

One of the chlorophyll molecules in (Chl)2 receives energy via exciton transfer, promoting an electron to a higher-energy level.

After being promoted to a higher-energy level, the electron is then donated to what?

After being promoted to a higher-energy level, this electron is then donated to a nearby electron acceptor that is part of the chloroplast electrontransport chain. The electron acceptor acquires a negative charge.

The electron lost by the reaction-centre chlorophyll is replaced by what?

The electron lost by the reactioncentre chlorophyll is replaced by an electron from a neighbouring electron-donor molecule.

The light-induced excitation of (Chl)₂ causes what?

The light-induced excitation of (Chl)₂ causes charge separation and initiates a series of redox reactions.

What allows proton pumping and ATP synthesis?

Photosynthetic electron transport allows proton pumping and ATP synthesis.

Flow of electrons through the electron transport chain causes what?

Flow of electrons through the electron transport chain causes protons to be pumped across the thylakoid membrane, forming a proton gradient used to produce ATP.

The cytochrome bc1 complex is analogous to what?

The cytochrome bc1 complex is analogous to Complex III in the mitochondrial electron transport chain.

Photosynthetic electron transport can be cyclic or non-clyclic, depending on what?

Photosynthetic electron transport can be cyclic or non-cyclic, depending on the organism and biochemical conditions.

Photosynthetic bacteria use relatively simple photosystems and what as the electron donor.

Photosynthetic bacteria use relatively simple photosystems and H₂S as the electron donor.

Charge separation requires what of photosystem components?

Charge separation requires precise orientation of photosystem components.

The precise structure of the photosystems ensures what?

The precise structure of the photosystems ensures that the excited state of the reaction centre does not decay to its ground state by internal conversion, which is extremely rapid (~10 ps).

Exciton transfer from an antenna chlorophyll to the reaction takes how long and with what efficiency.

Exciton transfer from an antenna chlorophyll to the reaction centre takes <100 ps with >90% efficiency.

Within 3 ps of the excitation of P870, pheophytin has received what and become what? <200 ps later, the electron has reached what?

Within 3 ps of the excitation of P870, pheophytin has received an electron and become negatively charged. <200 ps later, the electron has reached the quinone Q_B.

The reactions are extremely rapid and thermodynamically downhill (negative ΔG’^o)—essentially irreversible.

Photosystems are huge complexes of what?

Photosystems are huge complexes of integral membrane proteins with bound pigments.

Each photosystem has a characteristic what?

Each photosystem has a characteristic wavelength of light that it absorbs.

In plants, photosystem I (PSI) absorbs most strongly in where?

PSII absorbs most strongly with what?

In plants, photosystem I (PS1) absorbs most strongly in the far red (700 nm) whereas PSII absorbs most strongly with red light (680 nm).

– The PSI/PSII combination evolved from two bacterial photosystems.

Electrons are transfered from H₂O to NADP⁺ in a what scheme?

Electrons are transfered from H₂O to NADP⁺ in a ‘Z-scheme’

Part 4

Integration and structure of photosystems

One electron is transferred from H₂O to NADP⁺ for every how many photons absorbed?

One electron is transferred from H₂O to NADP⁺ for every two photons absorbed?

One molecule of O₂ is formed for every how many photons absrobed?

One molecule of O₂ is formed for every eight photons absrobed?

Cyclic electron transfer occurs when?

Cyclic electron transfer occurs when electrons move from ferredoxin back to the cytochrome b₆f complex, producing more ATP and less NADPH than in non-cyclic electron transfer.

The mechanism of PSII is similar to that of what?

The mechanism of PSII is similar to that of the photosystem in purple bacteria.

Is PSII monomeric or dimeric?

PSII is dimeric. Each monomer consists of 19 proteins, 16 of which have transmembrane segments.

There are chlorophyll-binding proteins, carotenoids, a non-haem and the inorganic complex Mn₄CaO₅.

The three peripheral proeins stablise th Mn₄CaO₅ complex.

Is PSi asymmetric or symmetric? Dimer or trimer?

PSI is a symmetric trimer with at least 16 proteins.

Have the structures of all the components of the plant photosystems and ATP synthase been solved?

Structures of all the components of the plant photosystems and ATP synthase have been solved.

Lecture questions

Which of the following is not common to the processes associated with the production of ATP in both the chloroplast and mitochondrion?

a. Electron donation from water.

b. Electron transport through a series of electron carriers.

c. Build-up of a proton gradient across an internal membrane.

d. Utilisation of proton motive force to drive ATP synthesis.

e. Catalysis by an F-type ATP synthase.

a. Electron donation from water.

Which of the following statements about carotenoids in thelight reactions of plant photosynthesis is incorrect?

a. Carotenoids are known as accessory pigments.

b. Carotenoids have absorption spectra that complement those of the chlorophylls.

c. Carotenoids protect the photosynthetic machinery from singlet oyxgen.

d. The molecular structures of beta-carotene and lutein are closely related.

e. All antenna molecules in a photosystem are carotenoids.

e. All antenna molecules in a photosystem are carotenoids.

For the Z-scheme describing the light reactions of photosynthesis in plants, which of the following statements is incorrect?

a. The units of the y-axis of the Z-scheme are electron volts.

b. Photosystem I is the oxygen-evolving complex.

c. Electrons are transferred from H₂0 to NADP⁺.

d. Both photosystem I and photosystem II evolved from bacterial photosystems.

e. The cytochrome b₆f complex links photosystem II to photosystem I.

b. Photosystem I is the oxygen-evolving complex.

L22

The Carbon Assimilation Reactions of Photosynthesis

Part 1

Dependence of carbon-fixation on products of the light reactions

The carbon assimilation reactions are dependent on two products of the light reactions, what are they?

ATP and NADPH

What have many enzymes and biochemical pathways absent in chemoheterotrophs (e.g., humans)

Photoautotrophs (e.g., plants) have many enzymes and biochemical pathways absent in chemoheterotrophs (e.g., humans).

Plants reduce atmospheric CO2 to what, which are then used as precursors for synthesis of cellulose, starch, lipids, proteins and other organic molecules.

Plants reduce atmospheric CO2 to trioses, which are then used as precursors for synthesis of cellulose, starch, lipids, proteins and other organic molecules.

Part 2

The Calvin cycle and RuBisCo

Carbon dioxide is assimilated via what?

Carbon dioxide is assimilated via the Calvin Cycle.

What are the three stages in the Calvin cycle.

1. Fixation of CO2

2. Reduction of 3-phosphoglycerate to a triose

3. Regeneration of the carbon acceptor, ribulose 1,5-bisphosphate (RuBP)

The Calvin cycle has a stoichiometry that allows it to regenerate what?

The Calvin cycle has a stoichiometry that allows it to regenerate its own intermediates.

What portion of the triose molecules produced in the Calvin cycle are used to regenerate RbBP.

Five sixths of the triose molecules produced in the Calvin cycle are used to regenerate RuBP.

What portion of the triose phosphate, the product of photosynthesis, is used to make hexoses (fuel and building material), sucrose (energy transport) or starch (energy storage).

One sixth of the triose phosphate, the product of photosynthesis, is used to make hexoses (fuel and building material), sucrose (energy transport) or starch (energy storage).

All of the 13 enzymes in the reductive pentose phosphate pathway (part of the Calvin cycle) are located in where.

All of the 13 enzymes in the reductive pentose phosphate pathway (part of the Calvin cycle) are located in the chloroplast stroma.

The fixation of CO₂ is catalysed by what?

The fixation of CO₂ is catalysed by RuBisCO.

What is the full name of RuBisCO?

Ribulose 1,5-bisphosphate carboxylase/oxygenase

RuBisCO fixes CO₂ to what and cleaves teh unstable 6-C product to what?

RuBisCO fixes CO2 to RuBP and cleaves the unstable 6-C product to two molecules of 3-phosphoglycerate.

Plants with 3-phosphosphoglycerate as the first stable product of photosynthesis are known as what?

Plants with 3-phosphosphoglycerate as the first stable product of photosynthesis are known as C₃ plants.

What are the two distinct forms of RuBisCO?

Form I, found in vascular plants, algae and cyanobacteria.

Form II, found only in certain photosynthetic bacteria.

Plant RuBisCO has how many identical large subunits and how many identical small subunits?

Plant RuBisCO has eight identical large subunits (each 53 kD) encoded by the chloroplast genome, and eight identical small subunits (each 14 kD) encoded in the nuclear genome.

Is RuBisCO abundant?

RubisCO is one of the most abundant enzymes in the biosphere.

Does RuBisCO have low or high K_cat? What does that mean for plants?

RuBisCO is one of the most abundant enzymes in the biosphere. RuBisCO has an extremely low kcat; it fixes only three molecules of CO₂ per second (at 25 °C). Thus, plants need huge amounts of RuBisCO to fix sufficient amounts of carbon.

What in the RuBisCO active site are critical for the enzyme’s mechanism.

A Mg²⁺ ion and a lysine residue that is carbamoylated by RuBisCO activase in the active site are critical for the enzyme’s mechanism.

The second stage of CO₂ assimilation involves synthesis of what?

The second stage of CO₂ assimilation involves synthesis of a carbohydrate.

The third stage of CO₂ assimilation involves interconversions of what?

The third stage of CO₂ assimilation involves interconversions of triose and pentose phosphates.

All the reactions of the Calvin cycle occur in animals apart from what?

What does that mean?

Apart from those catalysed by RuBisCO, sedoheptulose 1,7-bisphosphatase and ribulose 5-phosphate kinase, all the reactions of the Calvin cycle occur in animals but the lack of these three enzymes means animals cannot carry out net conversion of CO₂ to glucose.

How many NADPH and ATP are needed to synthesise each triose phosphate.

6 NADPH and 9 ATP are needed to synthesise each triose phosphate.

NADPH and ATP are produced in the light reactions in about the same ratio (2 : 3) as they are consumed in what?

NADPH and ATP are produced in the light reactions in about the same ratio (2 : 3) as they are consumed in the Calvin cycle.

The production of ATP and NADPH by photophosphorylation, and the fixation of CO₂ into triose phosphate, cease in what?

The production of ATP and NADPH by photophosphorylation, and the fixation of CO₂ into triose phosphate, cease in the dark.

The Pi-triose antiporter of the inner chloroplast membrane exchanges what for what?

The imported Pi is used in what? The exported dihydroxyacetone phosphate can be used for what?

The same antiporter can transport what and act indirectly in what?

The Pi-triose antiporter of the inner chloroplast membrane exchanges a Pi for a triose phosphate.

The imported Pi is used in photophosphorylation. The exported dihydroxyacetone phosphate can be used for sucrose synthesis.

The same antiporter can transport 3-phosphoglycerate and acts indirectly in the export of ATP and reducing equivalents.

Part 3

Oxygenation of ribulose-1,5-bisphosphate and photorespiration.

Photorespiration is the side reaction of what with a what cost?

Photorespiration is a side reaction of photosynthesis with a substantial metabolic cost.

Like mitochondrial respiration, photorespiration consumes what and produces what but is dependent on what?

Like mitochondrial respiration, photorespiration consumes O₂ and produces CO₂ but is dependent on light.

Photorespiration is due to what, forming what?

Photorespiration is due to the oxygenation of RuBP by RuBisCO, forming only one molecule of 3-phosphoglycerate and one molecule of the wasteful 2-phosphoglycolate.

The oxygenation of RuBP occurs partly because of the large ratio of what to what in the atmosphere?

The oxygenation of RuBP occurs partly because of the large ratio of O₂ to CO₂ in the atmosphere.

RuBisCO has a Km for CO₂ of what while the Km for O₂ is what – so the enzyme is quite specific for what?

RuBisCO has a Km for CO₂ of ≈ 9 µM while the Km for O₂ is ≈ 350 µM – so the enzyme is quite specific for CO₂.

Why the oxygenation of RuBP is very substantial when RuBisCO is quite specific for CO₂.

The atmosphere contains ≈ 0.04 % CO₂ and ≈ 20 % O₂, giving concentrations in aqueous solution of ≈ 11 µM CO₂ and 250 µM O₂.

As temperature rises, the ratio of O₂ to CO₂ in solution increases and the affinity of RuBisCO for CO₂ decreases.

As CO₂ is consumed in the carbon assimilation reactions, the O₂ : CO₂ ratio further increases.

Photorespiration occurs via what cycle?

Photorespiration occurs via the oxidative photosynthetic carbon cycle.

The oxidative photosynthetic carbon cycle (also called the glycolate pathway) involves what three organelles?

The oxidative photosynthetic carbon cycle (also called the glycolate pathway) involves three organelles: the chloroplast, peroxisome and mitochondrion.

The oxidative photosynthetic carbon cycle converts what to what and eventually to what?

Some carbon is lost as what and how is the oxygen consumed?

The cycle converts 2-phosphoglycolate to serine and eventually to 3-phosphoglycerate, thus recovering most of the lost carbon.

Some carbon is lost as CO₂, and oxygen is consumed at two steps during the cycle.

Is glycine decarboxylase required in abundance for photorespiration?

Glycine decarboxylase is required in abundance.

In bright sunlight, photorespiration can produce about how many times more CO₂ than the citric-acid cycle.

In bright sunlight, photorespiration can produce about five times more CO₂ than the citric-acid cycle.

The glycine decarboxylase complex, which oxidises glycine to CO₂ and NH₃, makes up how much of the total protein in the mitochondrial matrix (in leaves of pea and spinach).

The glycine decarboxylase complex, which oxidises glycine to CO₂ and NH₃, makes up half the total protein in the mitochondrial matrix (in leaves of pea and spinach).

Photorespiration does not conserve energy and may reduce the accumulation of biomass by as much as how much?

Photorespiration does not conserve energy and may reduce the accumulation of biomass by as much as 50 %.

Part 4

The C₄ pathway of the carbon-fixation reactions of photosynthesis.

C₄ plants concentrate CO₂ around what, thus minimising what?

C₄ plants concentrate CO₂ around RuBisCO, thus minimising photorespiration.

In C₄ photosynthesis, PEP carboxylase fixes what to what, forming what as the first stable intermediate.

In C₄ photosynthesis, PEP carboxylase fixes HCO₃⁻ to phosphoenolpyruvate, forming oxaloacetate as the first stable intermediate.

PEP carboxylase cannot use what as a substrate?

PEP carboxylase cannot use O₂ as a substrate, so O₂ does not compete with HCO₃⁻.

PEP carboxylase and RuBisCO are located in where?

PEP carboxylase and RuBisCO are located in separate, neighbouring cells: mesophyll and bundle-sheath, respectively.