PART A - Functions of the photosystems
The light reactions require the cooperation of two photosystems to power linear electron flow from water to NADP+.
PHOTOSYSTEM 2 ONLY:
-oxidation of water
-reduction of electron transport chain between the two photosystems
PHOTOSYSTEM 1 ONLY:
-reduction of NADP+
-oxidation of electron transport chain between the two photosystems
BOTH PS2 AND PS1:
-reduction of primary electron acceptor
[The key function of each of the two photosystems is to absorb light and convert the energy of the absorbed light into redox energy, which drives electron transport.
-In PS II (the first photosystem in the sequence), P680 is oxidized (which in turn oxidizes water), and the PS II primary electron acceptor is reduced (which in turn reduces the electron transport chain between the photosystems).
-In PS I, the PS I primary electron acceptor is reduced (which in turn reduces other compounds that ultimately reduce NADP+ to NADPH), and P700 is oxidized (which in turn oxidizes the electron transport chain between the photosystems).]
PART B - Energetics of electron transport
This diagram shows the basic pattern of electron transport through the four major protein complexes in the thylakoid membrane of a chloroplast.
ELECTRON TRANSPORT STEP:
1. Water → P680⁺
2. P680 → Pq (plastoquinone)
3. Pq → P700⁺
4. P700 → Fd (ferredoxin)
5. Fd → NADP⁺
ENERGY INPUT REQUIRED?
1. no energy input required
2. energy input required
3. no energy input required
4. energy input required
5. no energy input required
[In both PS II and PS I, light energy is used to drive a redox reaction that would not otherwise occur. In each photosystem, this redox reaction moves an electron from the special chlorophyll pair (P680 in PS II and P700 in PS I) to that photosystem's primary electron acceptor.
The result in each case is a reductant (the reduced primary electron acceptor) and an oxidant (P680+ in PS II and P700+ in PS I) that are able to power the rest of the electron transfer reactions without further energy input.]
PART C - Proton gradient formation and ATP synthesis
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.
top left pink: empty
bottom left: site of H+ release
top right pink: site of ATP synthesis
left blue: H+ pumped across membrane
middle blue: empty
right blue: H+ diffuses across membrane
[Photosynthetic electron transport contributes to the formation of a proton (H+) gradient across the thylakoid membrane in two places.
-In PS II, the oxidation of water releases protons into the thylakoid space.
-Electron transport between PS II and the cytochrome complex (through Pq) pumps protons from the stroma into the thylakoid space.
The resulting proton gradient is used by the ATP synthase complex to convert ADP to ATP in the stroma.]
PART A - Photosynthesis and respiration in plants
Drag the labels from the left to their correct locations in the concept map on the right. Not all labels will be used.
Photosynthesis and respiration in plants
Drag the labels from the left to their correct locations in the concept map on the right. Not all labels will be used.
f. carbon dioxide
g. cellular respiration
[A mutually dependent relationship exists between chloroplasts and mitochondria in the plant cell. Photosynthesis, which occurs in chloroplasts, generates the sugars and oxygen gas that are used in mitochondria for cellular respiration. Cellular respiration generates carbon dioxide, which in turn is used as a carbon source for the synthesis of sugars during photosynthesis. Cellular respiration also generates ATP and water, which are used in various chemical reactions in the plant cell.]
PART A - Experimental technique: Using bacteria to estimate rates of photosynthesis
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 photons 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
[In this experiment, Engelmann was able to determine which wavelengths (colors) of light are most effective at driving photosynthesis.
-First, Engelmann used a prism to disperse white light from the sun into the colors (wavelengths) of the visible spectrum.
-Then, using a microscope, he illuminated a filament of green algae with the visible spectrum. The photosynthetic pigments in the alga absorbed some of the wavelengths of light, using the absorbed energy to drive the reactions of photosynthesis, including oxygen production.
-Engelmann used his recently discovered aerotactic bacteria to determine which wavelengths of light caused the alga to photosynthesize most. Because the aerotactic bacteria were attracted to areas of highest oxygen concentration, they congregated around the regions of the alga that photosynthesized the most.
-He then counted the bacteria associated with each region of the alga illuminated by the various colors of light.
Engelmann found that some wavelengths of light attracted more bacteria, suggesting that these wavelengths drive more photosynthesis than others.]
PART B - Experimental results: Colors of light that drive photosynthesis
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.
[For Engelmann to be able to draw meaningful conclusions from his experiment, he had to assume that the number of bacteria at any location on the slide was proportional to the amount of oxygen produced by the alga at that location. If this were not the case, the distribution of the bacteria around the alga would be of no use in determining the amount of photosynthesis that occurs at each wavelength.
Similarly, it was necessary for Engelmann to assume that the distribution of chloroplasts among the cells in the algal filament was approximately equal. Fewer chloroplasts in one cell compared to another would mean a lower potential for oxygen production at any color. Engelmann's microscope was sufficiently powerful to see that Cladophora cells contain many small chloroplasts that were nearly uniform in their distribution within and between cells.
In contrast, Engelmann did not assume that the alga absorbed the same number of photons at each wavelength. In fact, most photosynthetic pigments absorb more strongly in the red and blue parts of the spectrum and less strongly in the yellow and green parts.
He also did not assume that all absorbed photons drive photosynthesis, and thus oxygen production, by the alga. In fact, all photosynthetic organisms contain some pigments that absorb photons but do not contribute to photosynthesis or oxygen production.]
PART C - Experimental prediction: The action spectrum
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 higher than black line
-touch around 550 nm
[An action spectrum is typically plotted so that the "action" shown on the y-axis is measured with an equal number of photons at each wavelength of the visible spectrum. But our sun does not emit equal numbers of photons at each wavelength.
Instead, the sun emits the most photons in the yellow part of the spectrum, with relatively fewer photons emitted in the red and violet-blue parts of the spectrum. Thus, the red and violet-blue regions of Engelmann's action spectrum were measured with fewer photons than in the yellow part of the spectrum.
To correct for this, you have to consider how increasing the number of photons in the red and violet-blue parts of the spectrum--to match the emission level in the yellow part--would change the amount of oxygen produced, and thus the number of bacteria that accumulated, in the red and violet-blue spectral regions. The corrected action spectrum would show higher peaks in the red and violet-blue parts of the spectrum, but the plot in the yellow part of the spectrum would be approximately the same as Engelmann's.]
Approximately what wavelength of light is best absorbed by chlorophyll a, the pigment that participates directly in the light reactions?
Which wavelength of light is best absorbed by chlorophyll b?
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.
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.
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.
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 wavelengths 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.
PART A - Inputs and outputs of the light reactions
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.)
NOT INPUT OR OUTPUT:
[In the light reactions, the energy of sunlight is used to oxidize water (the electron donor) to O2 and pass these electrons to NADP+, producing NADPH. Some light energy is used to convert ADP to ATP. The NADPH and ATP produced are subsequently used to power the sugar-producing Calvin cycle.]
PART B - Inputs and outputs of the Calvin cycle
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.
NOT INPUT OR OUTPUT:
[In the Calvin cycle, the energy outputs from the light reactions (ATP and NADPH) are used to power the conversion of CO2 into the sugar G3P. As ATP and NADPH are used, they produce ADP and NADP+, respectively, which are returned to the light reactions so that more ATP and NADPH can be formed.]
PART C - Redox reactions of photosynthesis
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 oxidizes 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.
[In the light reactions, light energy is used to remove electrons from (oxidize) water, producing O2 gas. These electrons are ultimately used to reduce NADP+ to NADPH.
In the Calvin cycle, NADPH is oxidized back to NADP+ (which returns to the light reactions). The electrons released by the oxidation of NADPH are used to reduce three molecules of CO2 to sugar (G3P), which then exits the Calvin cycle.]
PART D - Chloroplast structure and function
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.
the dense fluid surrounded between the inner envelope membrane and the thylakoid membranes
b) thylakoid membrane-
the third membrane system; surrounds the thylakoid; chrorophyll resides in the thylakoid membrane
the fluid portion of cytoplasm surrounding (outside) the chloroplast
d) location of Calvin Cycle-
the calvin cycle occurs in the stroma
e) thylakoid space-
the space within the thylakoid sacs
f)location of light reactions-
the light reactions occur in the thylakoids of the chloroplast
g) envelope membranes-
the inner and outer membranes of the chloroplast
[The chloroplast is enclosed by a pair of envelope membranes (inner and outer) that separate the interior of the chloroplast from the surrounding cytosol of the cell. Inside the chloroplast, the chlorophyll-containing thylakoid membranes are the site of the light reactions.
Between the inner envelope membrane and the thylakoid membranes is the aqueous stroma, which is the location of the reactions of the Calvin cycle. Inside the thylakoid membranes is the thylakoid space, where protons accumulate during ATP synthesis in the light reactions.]
PART A - Following carbon atoms around the Calvin cycle
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
[Counting carbons—keeping track of where the carbon atoms go in each reaction—is a simple way to help understand what is happening in the Calvin cycle.
-To produce 1 molecule of G3P (which contains 3 carbons), the Calvin cycle must take up 3 molecules of CO2 (1 carbon atom each).
-The 3 CO2 molecules are added to 3 RuBP molecules (which contain 15 total carbon atoms), next producing 6 molecules of 3-PGA (18 total carbon atoms).
-In reducing 3-PGA to G3P (Phase 2), there is no addition or removal of carbon atoms.
-At the end of Phase 2, 1 of the 6 G3P molecules is output from the cycle, removing 3 of the 18 carbons.
-The remaining 5 G3P molecules (15 total carbon atoms) enter Phase 3, where they are converted to 3 molecules of R5P.
-Finally, the R5P is converted to RuBP without the addition or loss of carbon atoms.]
PART B - Quantifying the inputs of ATP and NADPH and output of Pi
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+
e) 3 ADP 3 ATP
[The Calvin cycle requires a total of 9 ATP and 6 NADPH molecules per G3P output from the cycle (per 3 CO2 fixed).
-In Phase 2, six of the ATP and all of the NADPH are used in Phase 2 to convert 6 molecules of PGA to 6 molecules of G3P. Six phosphate groups are also released in Phase 2 (derived from the 6 ATP used).
-In the first part of Phase 3, 5 molecules of G3P (1 phosphate group each) are converted to 3 molecules of R5P (also 1 phosphate group each). Thus there is a net release of 2 Pi.
-In the second part of Phase 3, 3 ATP molecules are used to convert the 3 R5P into 3 RuBP.
Note that in the entire cycle, 9 ATP are hydrolyzed to ADP; 8 of the 9 phosphate groups are released as Pi, and the ninth phosphate appears in the G3P output from the cycle.]
PART C - Do the light reactions of photosynthesis depend on the Calvin cycle?
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 depend 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.
[A reaction or process is dependent on another if the output of the second is an input to the first. For example, the light reactions are dependent on the Calvin cycle because the NADP+ and ADP produced by the Calvin cycle are inputs to the light reactions.
Thus, if the Calvin cycle slows (because of a decrease in the amount of available CO2), the light reactions will also slow because the supply of NADP+ and ADP from the Calvin cycle would be reduced.]
PART A - Making a graph with the data
To explore the relationship between the two variables, it is useful to graph the data in a scatter plot, and then draw a regression line. But first, you must determine which variable should go on each axis of the graph.
What variable did the researchers intentionally vary in the experiment, and what are the units for this variable? (This is the independent variable.)
concentration of CO2 in the air, in parts per million
[This is the independent variable, which goes on the x-axis.]
What variable's response to the independent variable was measured by the researchers, and what are the units for this variable?
average dry mass of one plant, in grams
[This is the dependent variable, which goes on the y-axis.]
Now that you have determined which variable goes on each axis, the graph can be constructed. An effective graph marks off the axes with just enough evenly spaced tick marks to accommodate the full set of data.
Assuming that the x-axis tick marks will be separated by 200 (0, 200, 400, and so on), what is the largest value that should appear on the x-axis?
Assuming that the y-axis tick marks will be separated by 10 (0, 10, 20, and so on), what is the largest value that should appear on the y-axis?
To compare the effect of rising atmospheric CO2 on C3 versus C4 plants, data from both types of plants can be placed on the same graph. You should plot the data points for corn and velvetleaf using different symbols for each set of data, and add a key for the two symbols.
Which of the following graphs correctly presents the data from the experiment?
Corn (C4) is on top decreasing between 90-80
Velvet leaf (C3) is on the bottom increasing between 30-50
Next you should draw a "best-fit" regression line for each set of points. A best-fit line does not pass through all or even most points. Instead, it is a straight line that passes as close as possible to all data points from that set.
Which of the following graphs has the best-fit regression lines correctly drawn?
doesn't quite go through all the points
PART G - Interpreting the graph
Select Figure 2 from the drop-down menu above the data table to see the final graph (Figure 1).
What is the relationship between increasing concentration of CO2 and the dry mass of corn?
As CO2 increases, the dry weight of corn decreases.
What is the relationship between increasing concentration of CO2 and the dry mass of velvetleaf?
As CO2 increases, the dry weight of velvetleaf increases.
PART I - Making a prediction
Considering that velvetleaf is a weed invasive to cornfields, how would you predict increased CO2 concentration to affect interactions between the two species
As atmospheric CO2 rises, velvetleaf weeds may grow larger and better compete with corn, reducing grain yields.
PART J - Using data from the graph
What is the estimated dry mass of corn and velvetleaf plants at an atmospheric CO2 concentration of 390 ppm (current levels)?
corn: 91 g; velvetleaf: 38 g
What is the estimated dry mass of corn and velvetleaf plants at an atmospheric CO2 concentration of 800 ppm?
corn: 84 g; velvetleaf: 50 g
If atmospheric CO2 concentration increases from 390 ppm to 800 ppm, what is the estimated percentage change in dry mass for corn? For velvetleaf?
The dry mass of corn would decrease by about 8%, and the dry mass of velvetleaf would increase by about 32%.
PART M - Evaluating a hypothesis
Do these results support the conclusion from other experiments that C3 plants have a better growth response than C4 plants under increased CO2 concentration? Why or why not?
Yes, because C3 velvetleaf had a positive growth response to increased CO2, but C4 corn had a negative growth response to it.
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