Biochemistry: Chapter 15
This diagram shows the basic pattern of electron transport through the four major protein complexes in the thylakoid membrane of a chloroplast. For each step of photosynthetic electron flow from water to NADP+, drag the appropriate label to indicate whether or not that step requires an input of energy.
1. Water -> P680+ no energy input required 2. P680 -> Pq (plastoquinone) energy input required 3. Pq -> P700+ no energy input required 4. P700 -> Fd (ferredoxin) energy input required 5. Fd -> NADP+ no energy input required
In cyclic photophosphorylation, it is estimated that two electrons must be passed through the cycle to pump enough protons to generate one ATP. Assuming that the ΔG for hydrolysis of ATP under conditions existing in the chloroplast is about -50 kJ/mol, what is the corresponding percent efficiency of cyclic photophosphorylation, using light of 700 nm?
14.6%
Arrange the following sequence of events in the correct order for the Calvin cycle: 1) ATP is consumed in a reaction that yields 1,3-bisphosphoglycerate. 2) CO2 is fixed by reaction with ribulose-1,5-bisphosphate, catalyzed by Rubisco. 3) The unstable intermediate of this reaction is immediately cleaved into two molecules of 3-phosphogycerate. 4) NADPH is employed to prepare glyceraldehyde-3-phosphate. 5) Ribulose-1,5-bisphosphate is regenerated.
2-3-1-4-5
In Engelmann's experiment, he used aerotactic (oxygen-seeking) bacteria to determine which wavelengths of visible light were most effective in driving the reactions of photosynthesis in green algae. A diagram of his apparatus is shown below. Can you deduce the logical link between light of different wavelengths and the distribution of bacteria that Engelmann observed? Drag the labels onto the flowchart to show the relationship between the production of protons by the sun (Engelmann's light source) and the distribution of bacteria that Engelmann observed under his microscope. Not all labels will be used.
2. Prism disperses sunlight into individual wavelengths. 3. Alga's photosynthetic pigments absorb photons at specific wavelengths. 4. Absorbed photons drive photosynthesis in alga. 5. Alga gives off oxygen as it photosynthesizes. 6. Bacteria attracted to regions of highest oxygen concentration.
What is the standard free energy change per mol O2 produced?
273 kJ/mol O2
Calculate the amount of energy required to make one mole of hexose from CO2 and water by photosynthesis if the average photon has an energy of 2.77 x 10^-19 J and 48 photons are required for each 6C sugar. Assume the efficiency of photosynthesis is ~35%.
2802 kJ/mol
Place the following events in the correct order for the two-photosystem light reactions. 1) Electrons are transferred to the b6F complex with concomitant influx of protons into the thylakoid lumen. 2) Electrons are passed through to ferredoxin and then ultimately to NADP+, reducing the latter to NADPH. 3) Electrons are transferred to plastoquinones, which serve as intermediate electron transporters. 4) Light energy is absorbed by photosystem II, placing it in an excited state, causing it to release electrons, which are replaced by the oxidation of water to oxygen. 5) Electrons are transferred to photosystem I via plastocyanin.
4-3-1-5-2
Approximately what wavelength of light is best absorbed by chlorophyll a, the pigment that participates directly in the light reactions?
435 nm
Which wavelength of light is best absorbed by chlorophyll b?
455 nm
Which of these equations best summarizes photosynthesis?
6CO2 + 6H2O -> C6H12O6 + 6O2
The flux of solar energy reaching the Earth's surface is approximately 7 J/s cm^2. Assume that all of this energy is used by a green leaf (10 cm^2 in area), with the maximal efficiency of 35%. How many moles of hexose could the leaf theoretically generate in an hour? Use 600 nm for an average wavelength. Express your answer using two significant figures.
9.2x10^-3 mol
BLANK splits water into 1/2 O2, H+, and e-.
A
Chlorophyll can be found in BLANK.
A and C
J. C. Servaites, in Plant Physiol. (1985) 78:839-843, observed that Rubisco from tobacco leaves collected before dawn had a much lower specific activity than the enzyme collected at noon. This difference persisted despite extensive dialysis, gel filtration, or heat treatment. However, precipitation of the predawn enzyme by 50%% (NH4) 2SO4 restored the specific activity to the level of the noon-collected enzyme. Suggest an explanation. Check all that apply.
- It indicates presence of a strong Rubisco inhibitor. - This substance has strong binding with the enzyme. - It is a low-molecular-weight phosphorylated compound.
The substance dichlorophenyldimethylurea (DCMU) is an herbicide that inhibits photosynthesis by blocking electron transfer between plastoquinones in photosystem II. Normally, DCMU blocks O2 evolution, but addition of ferricyanide to chloroplasts allows O2 evolution in the presence of DCMU. Explain.
Addition of ferricyanide as an electron donor allows a Hill reaction.
Suppose a brief pulse of 14CO2 is taken up by a green plant. Trace the 14C label through the steps leading to fructose-1,6-biphosphate synthesis, showing which carbon atoms in each compound should carry the label during the first cycle. Drag the terms on the left to the appropriate blanks on the right to complete the sentences.
After being attached to C-2 of ribulose-1,5-biphosphate, the 14C will become the carboxylate (C-1) of one of the two molecules of 3-phosphoglycerate. This becomes the carbonyl carbon (C-1) in 3-phosphoglyceraldehyde, or C-1 in dihydroxyacetone phosphate. Upon condensation to form fructose-1,6-bisphosphate, the 14C will chow up in C-3 and/or C-4.
Which of the following processes describe the way excited molecules can return to their ground state? 1) They can return via radiationless dissipation of energy. 2) They can lose this energy as a different wavelength of light in a process known as fluorescence. 3) They can transfer this energy to adjacent molecules in a process known as resonance transfer. 4) They can transfer an excited electron to a nearby molecule with a slightly lower excited state in a process known as electron transfer.
All of the listed statements are true.
Which of the following statements about the two-system light reactions are true?1) The first reaction takes electrons from water, releasing oxygen. 2) The electrons released above are then accepted by NADP+. 3) During the transfer of electrons throughout this series of reactions, protons are pumped into the thylakoid lumen, generating a proton gradient. 4) The proton gradient represents a large difference in free energy, sufficient to drive the synthesis of ATP.
All of the listed statements are true.
In photosynthesis, a redox compound that is produced in the light reactions is required to drive other redox reactions in the Calvin cycle, as shown in this figure along with other components of photosynthesis. Drag the terms to the appropriate blanks to complete the following sentences summarizing the redox reactions of photosynthesis. Terms may be used once, more than once, or not at all.
1. In the light reactions, light energy is used to oxidize H2O to O2. 2. The electrons derived from this oxidation reaction in the light reactions are used to reduce NADP+ to NADPH. 3. The Calvin cycle oxidized the light-reactions product NADPH to NADP+. 4. The electrons derived from this oxidation reaction in the Calvin cycle are used to reduce CO2 to G3P.
BLANK releases energy that is used to pump hydrogen ions from the stroma into the thylakoid compartment.
B
If a photosynthetic organism is illuminated in a closed, sealed environment, it is observed that the CO2 and O2 levels in the surrounding atmosphere reach a constant ratio. Suggest an explanation.
Because both O2 and CO2 are being consumed and produced by the opposing processes of photorespiration and photosynthesis, a steady-state ratio will be attained.
ATP synthesis in chloroplasts is very similar to that in mitochondria: Electron transport is coupled to the formation of a proton (H+) gradient across a membrane. The energy in this proton gradient is then used to power ATP synthesis. Two types of processes that contribute to the formation of the proton gradient are: -processes that release H+ from compounds that contain hydrogen, and -processes that transport H+ across the thylakoid membrane. Drag the labels to the appropriate locations on the diagram of he thylakoid membrane. Use only the blue labels for the blue targets, and only the pink labels for the pink targets. Note: One blue target and one pink target should be left empty.
Bottom left Red: site of H+ release Very left Blue: H+ pumped across membrane Very right Blue: H+ diffuses across membrane Top right Red: site of ATP synthesis
Energized electrons from BLANK enter an electron transport chain and are then used to reduce NADP+.
C
Suppose ribulose-5-phosphate, labeled with 14C in carbon 1, is used as the substrate in dark reactions. In which carbon of 3PG will the label appear?
Carbon 3
Where does the Calvin cycle occur?
E
Which of these phosphorylates ADP to make ATP?
E
How does it compare with the energy in an einstein of 700 nm photons?
E = 171 kJ
A proton gradient is formed across the thylakoid membrane with the stroma becoming more acidic than the lumen.
False
Photons with any energy value are able to cause transitions in the electronic states of organic molecules to drive reactions and thus capture light energy in chemical form.
False
Which of the following intermediates of the Calvin cycle is NOT used directly to regenerate ribulose-1,5-bisphosphate? -Dihydroxyacetone phosphate -Glyceraldehyde 3-phosphate -Fructose 6-phosphate -Fructose 1,6-bisphosphate
Fructose 1,6-bisphosphate
Which of the following statements about photosynthesis is FALSE?
In plants much of the carbohydrate formed is stored as sucrose.
Which of the following statements about the cyclic electron flow system is NOT true?
In this process, electrons excited in P700 are transferred to NADP+.
From the following choices, identify those that are the inputs and outputs of the light reactions. (Recall that inputs to chemical reactions are modified over the course of the reaction as they are converted into products. In other words, if something is required for a reaction to occur, and it does not remain in its original form when the reaction is complete, it is an input.) Drag each item to the appropriate bin. If the item is not an input to or an output from the light reactions, drag it to the "not input or output" bin.
Input: NADP+, ADP, water, light Output: ATP, NADPH, O2 Not input or output: G3P, glucose, CO2
From the following choices, identify those that are the inputs and outputs of the Calvin cycle. Drag each item to the appropriate bin. If the item is not an input to or an output from the Calvin cycle, drag it to the "not input or output" bin.
Input: NADPH, ATP, CO2 Output: ADP, NADP+, G3P Not input or output: light, glucose, O2
How does this compare to the energy required to drive the synthesis of ATP?
It's several times more.
If algae are exposed to 14CO2 for a brief period while illuminated, the labeled carbon is initially found almost entirely in the carboxyl group of 3-phosphoglycerate. However, if illumination is continued after the label pulse, other carbon atoms become labeled. Explain.
Most of the 3PG that is initially labeled is reused in replacing RuBP, which then reenters the cycle.
Which of the following molecules is the primary product of photosystem I?
NADPH
The substance dichlorophenyldimethylurea (DCMU) is an herbicide that inhibits photosynthesis by blocking electron transfer between plastoquinones in photosystem II. Would you expect DCMU to interfere with cyclic photophosphorylation?
No. Plastoquinones are not involved in this process.
Suppose a researcher is carrying out studies in which she adds a nonphysiological electron donor to a suspension of chloroplasts. Illumination of the chloroplasts yields oxidation of the donor. How could she tell whether photosystem I, II, or both are involved?
Observe the oxidation state of plastoquinones and plastocyanin using illumination at both 700 and 680 nm. If only p700 is involved, these will be oxidized. If only p680, they will be reduced. If both are involved, simultaneous illumination at the two wavelengths will give enhanced oxidation of the donor.
Can you tell from these absorption spectra whether red light is effective in driving photosynthesis?
One cannot tell from this graph, but because chlorophyll a does absorb red light, we can predict that it would be effective in driving photosynthesis.
Which process produces oxygen?
Photosynthesis
Which of the following represents the CORRECT order of events in the two-photosystem light reactions of photosynthesis?
Photosystem I receives electrons from plastocyanin at the end of the electron transfer chain from the excited reaction center of photosystem II.
The light reactions require the cooperation of two photosystems to power linear electron flow from water to NADP+. Drag each item into the appropriate bin depending on whether the process is associated with Photosystem II (PS II) only, Photosystem I (PS I) only, or both PS II and PS I. Note that "electron transport chain" here refers to the electron transport chain between the two photosystems, not the one that functions after PS I.
Photosystem II (PS II) only: -oxidation of water -reduction of electron transport chain between the two photosystems Photosystem I (PS I) only: -reduction of NADP+ -oxidation of electron transport chain between the two photosystems both PS II and PS I: -reduction of primary electron acceptor -light absorption
Which of the following reactions ensures that the Calvin cycle can make a continuous supply of glucose?
Regeneration of RuBP
Which of the following is NOT true considering the regulation of photosynthesis?
Rubisco is activated by 2-carboxy-D-arabinitol-1-phosphate, which is synthesized in the dark.
Photosynthesis takes place in all higher plants and algae in chloroplasts. Which of the following statements accurately describe these organelles? 1) In plants, they are located in mesophyll cells. 2) They are semiautonomous in that they carry their own DNA and ribosomes. 3) Unlike mitochondria, they have three membranes. 4) The carbohydrates produced by this reaction are stored as cellulose.
Statements 1, 2, and 3 are true.
Chlorophyll molecules are the most abundant pigments in plants that are responsible for collecting light for photosynthesis. Which of the following statements is not true? 1) The two main forms of chlorophyll are chlorophyll a and b. 2) These molecules resemble the protoporphyrins in hemoglobin and myoglobin, except these contain Fe+2 rather than Mg+2. 3) They require the presence of accessory pigments such as beta-carotene and lutein in order to function. 4) They absorb light in the blue and red region of the visible spectrum, which is why they appear green.
Statements 1, 3, and 4 are true.
Which of following statements about photosynthesis are true? 1) This process describes the catabolism of carbohydrates to CO2. 2) For plants, water is the ultimate reducing agent in this process. 3) The process releases oxygen as a byproduct. 4) The energy required for this transformation comes in the form of light.
Statements 2, 3, and 4 are true
Daniel Arnon and coworkers carried out experiments with intact, salt-washed chloroplasts to study photophosphorylation. When the chloroplasts were illuminated in the presence of ADP+Pi, ATP was produced, but oxygen was not produced or nor consumed. ATP formation was not accompanied by a measurable electron transport involving any external electron donor or acceptor. The overall reaction for this result is: ADP+Pi⟶(hν) ATP When NADP+ was included in addition to the ADP + Pi, illumination of the intact chloroplasts again resulted in the photophosphorylation of ADP to ATP. In addition, the NADP+ was reduced to NADPH + H+ and O2 was produced. Moreover, the light-induced reduction of NADP+ was greatly decreased if ADP + Pi were omitted. The equation for this reaction is: NADP+ + H2O + ADP + Pi ->(hv) NADPH + H+ + ATP + 1/2O2 Briefly describe the mechanism(s) of these two types of photophosphorylation that explain all of these results. Drag the appropriate items to their respective bins.
The first reaction: -cyclic photophosphorylation -only PS I is involved -the electrons are transferred back into the electron transport chain -O2 is not produced -reduced NADPH is not produced -the internal electron transport stimulates ATP synthesis The second reaction: -the electrons is PS I are used to reduce NADP+ -the omission of ADP + Pi decreases the reduction of NADP+ -NADPH is produced -both PS I and PS II are involved -noncyclic photophosphorylation -O2 is produced
If only chlorophyll a were involved in the light reactions, would blue light (wavelength about 490 nm) be effective in driving photosynthesis?
The graph indicates that chlorophyll a absorbs very little blue light, so we can predict that blue light would not be effective.
Which set of reactions uses H2O and produces O2?
The light-dependent reactions
Which of the following statements best describes the relationship between the light-dependent and light-independent reactions of photosynthesis?
The light-dependent reactions produce ATP and NADPH, which are then used by the light-independent reactions.
What is the importance of the light-independent reactions in terms of carbon flow in the biosphere?
The light-independent reactions turn CO2, a gas, into usable carbon in the form of sugars.
Engelmann counted the number of bacteria that were attracted to the algal filament associated with each color of light. As shown in the image below, most of the bacteria were attracted to the regions of the alga illuminated by red or violet-blue light. This distribution of bacteria shows that red and violet-blue wavelengths are most effective in driving photosynthesis. By measuring oxygen production with aerotactic bacteria, Engelmann described an action spectrum for photosynthesis. The action spectrum (indicated by the black line plot in the image above) shows the relative effectiveness of each color of light in driving photosynthesis. What assumptions did Engelmann make in order to conclude that red and violet-blue light were more effective than green light in driving photosynthesis? Select the two that apply.
The number of bacteria clustered at each wavelength (color) was approximately proportional to the amount of oxygen being produced by that portion of the alga. The distribution of chloroplasts within each algal cell was approximately the same.
The rate of O2 production by the light reactions varies with the intensity of light because light is required as the energy source for O2 formation. Thus, lower light levels generally mean a lower rate of O2 production. In addition, lower light levels also affect the rate of CO2 uptake by the Calvin cycle. This is because the Calvin cycle needs the ATP and NADPH produced by the light reactions. In this way, the Calvin cycle depends on the light reactions. But is the inverse true as well? Do the light reactions depends on the Calvin cycle? Suppose that the concentration of CO2 available for the Calvin cycle decreased by 50% (because the stomata closed to conserve water). Which statement correctly describes how O2 production would be affected? (Assume that the light intensity does not change.)
The rate of O2 production would decrease because the rate of ADP and NADP+ production by the Calvin cycle would decrease.
If a photosynthetic organism is illuminated in a closed, sealed environment, it is observed that the CO2 and O2 levels in the surrounding atmosphere reach a constant ratio. What factor would you think primarily determines the value of this ratio?
The relative affinity of rubisco for CO2 and O2.
An action spectrum plots the rate of photosynthesis at various wavelengths of visible light, and it shows that blue light with a wavelength of about 490 nm is effective in driving photosynthesis. Based on this information and the absorption spectra shown at left, what role may chlorophyll b and carotenoids play in photosynthesis?
These pigments are able to absorb more wavelength of light (and thus more energy) than chlorophyll a alone can absorb. As part of light-harvesting complexes in photosystems, they broaden the range of light that can be used in the light reactions.
Which statement about the Hill reactions is NOT true?
These reactions are thermodynamically favorable, even in the absence of light energy.
You obtain the pigments called carotenoids in your diet when you eat carrots. Why do carotenoids appear yellow and orange?
They absorb blue/green light and reflect yellow and red wavelengths of light.
What is the biological significance of the light-independent reactions of photosynthesis?
They convert carbon dioxide to sugar.
Will all molecules of fructose-1,6-biphosphate carry two 14C atoms? Explain.
They may carry 0, 1, or 2 labeled carbons, depending on what combination of labeled and unlabeled trioses has been used in their formation.
Photosynthesis is the major source of oxygen in Earth's atmosphere.
True
The C4 photosynthesis pathway helps to provide CO2 to the carboxylase active site of Rubisco in conditions of high O2 concentration.
True
True or false? The light-dependent reactions of photosynthesis use water and produce oxygen.
True
Water is oxidized to oxygen at the oxygen-evolving complex of photosystem II and released into the lumen of the thylakoid and diffuses out of the chloroplast.
True
Upon which region of the light spectrum does photosynthesis mainly depend?
Visible and infrared
The net reaction of the Calvin cycle is the conversion of CO2 into the three-carbon sugar G3P. Along the way, reactions rearrange carbon atoms among intermediate compounds and use the ATP and NADPH produced by the light reactions. In this exercise, you will track carbon atoms through the Calvin cycle as required for the net production of one molecule of G3P. For each intermediate compound in the Calvin cycle, identify the number of molecules of that intermediate and the total number of carbon atoms contained in those molecules. As an example, the output G3P is labeled for you: 1 molecule with a total of 3 carbon atoms. Labels may be used once, more than once, or not at all.
a. 3 molecules 3 carbons b. 6 molecules 18 carbons c. 6 molecules 18 carbons d. 5 molecules 15 carbons e. 3 molecules 15 carbons f. 3 molecules 15 carbons
The Calvin cycle depends on inputs of chemical energy (ATP) and reductant (NADPH) from the light reactions to power the conversion of CO2 into G3P. In this exercise, consider the net conversion of 3 molecules of CO2 into 1 molecule of G3P. Drag the labels to the appropriate targets to indicate the numbers of molecules of ATP/ADP, NADPH/NADP+, and Pi (inorganic phosphate groups) that are input to or output from the Calvin cycle. Labels can be used once, more than once, or not at all.
a. 6 ATP 6 ADP b. 6 NADPH 6 NADP+ c. 6 Pi d. 2 Pi e. 3 ADP 3 ATP
In eukaryotes, all the reactions of photosynthesis occur in various membranes and compartments of the chloroplast. Identify the membranes or compartments of the chloroplast by dragging the blue labels to the blue targets. Then, identify where the light reactions and Calvin cycle occur by dragging the pink labels to the pink targets. Not that only blue labels should be placed in blue targets, and only pink labels should be placed in pink targets.
a. stroma b. thylakoid membrane c. cytosol d. location of Calvin cycle e. thylakoid space f. location of light reactions g. envelope membranes
Plants are best known for their ability to perform photosynthesis, the process by which light energy is converted to chemical energy in the form of sugars. But plants don't just make sugars; they use them, too. Like animals, plants must break sugars down to fuel cellular work. In this activity, you will complete a concept map showing the interrelatedness of sugar production and sugar breakdown in a plant cell. Drag the labels from the left to their correct locations in the concept map on the right. Not all labels will be used.
a. sunlight b. photosynthesis c. chloroplasts d. sugar e. chlorophyll f. carbon dioxide g. cellular respiration h. mitochondria
Photosynthesis relies predominantly on light in the visible and near-infrared region of the electromagnetic spectrum. This light is capable of __________.
causing transitions in the electronic states of organic molecules that can drive reactions
The following data, presented by G. Bowes and W.L. Ogre in J. Biol. Chem. (1972) 247: 2171-2176, describe the relative rates of incorporation of CO2 by Rubisco under N2 and under pure O2. Decide whether O2 is a competitive or uncompetitive inhibitor.
competitive
Chlorophylls, carotenes and xanthophylls absorb light in the visible part of the spectrum because they contain large BLANK double-bond systems.
conjugated
In C4 and CAM plants carbon dioxide is fixed in the BLANK of mesophyll cells.
cytoplasm
It is believed that the ratio of cyclic photophosphorylation to noncyclic photophosphorylation changes in response to metabolic demands. In each of the following situations, would you expect the ratio to increase, decrease, or remain unchanged? Chloroplasts carrying out both the Calvin cycle and the reduction of nitrite (NO−2) to ammonia (This process does not require ATP)
decrease
It is believed that the ratio of cyclic photophosphorylation to noncyclic photophosphorylation changes in response to metabolic demands. In each of the following situations, would you expect the ratio to increase, decrease, or remain unchanged? Chloroplasts carrying out not only the Calvin cycle but also extensive active transport
increase
It is believed that the ratio of cyclic photophosphorylation to noncyclic photophosphorylation changes in response to metabolic demands. In each of the following situations, would you expect the ratio to increase, decrease, or remain unchanged? Chloroplasts using both the Calvin cycle and the C4 pathway
increase
In C3 plants the conservation of water promotes BLANK.
photorespiration
One of the assumptions that Engelmann made was that the sun (his light source) emits equal numbers of photons at each wavelength in the visible spectrum. In reality, the sun's emission peaks in the yellow region of the spectrum, with relatively fewer photons emitted in the red and violet-blue regions. Recall that the action spectrum from Engelmann's experiment plotted the rate of photosynthesis (as measured by oxygen production) versus wavelength. In each of the following graphs, the black line shows Engelmann's original action spectrum deduced from the distribution of aerotactic bacteria around the alga. Which red line shows the same action spectrum corrected for the unequal number of photons emitted across the visible spectrum?
red line on top lines only touch around the 550 nm mark
The energy of a photon absorbed by an antenna molecule of the photosystem finds it way to a chlorophyll molecule in the reaction center of a light-harvesting complex by a process of:
resonance transfer and electron transfer.
C4 plants differ from C3 and CAM plants in that C4 plants BLANK.
transfer fixed carbon dioxide to cells in which the Calvin cycle occurs
Assume a pH gradient of 4.0 units across a thylakoid membrane, with the lumen more acidic than the stroma. What is the standard free energy change per mol of protons associated with this gradient at 25 degrees C?
uH = 22.8 kJ/molH+
The light reactions of photosynthesis use BLANK and produce BLANK.
water ... NADPH
According to the figure below, upon excitation the P700 reaction center is raised in potential from about +0.4 to -1.3 volts. To what value of ΔG∘′ does this correspond? Express your answer using three significant figures.
ΔG∘' = 164 kJ/mol
What is the longest wavelength of light that could provide enough energy per photon to pump one proton against this gradient, assuming a 20% efficiency in photosynthesis and T = 25 C?
λ =1050 nm
