Homework 12: Photosynthesis and Carbon Fixation

Select the correct statement about the Calvin cycle.

The Calvin cycle has three phases: carbon fixation, reduction, and regeneration of RuBP.

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.

C4 plants differ from C3 and CAM plants in that C4 plants _____.

transfer fixed carbon dioxide to cells in which the Calvin cycle occurs In C3 and CAM plants carbon dioxide fixation and the Calvin cycle occur in the same cells.

Which process is most directly driven by light energy?

transfer of energy from pigment molecule to pigment molecule

The electrons derived from this oxidation reaction in the Calvin cycle are used to reduce _______ to ________.

CO2 to G3P

Carbon fixation involves the addition of carbon dioxide to _____.

RuBP

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.

In C4 and CAM plants carbon dioxide is fixed in the _____ of mesophyll cells.

cytoplasm

In C3 plants the conservation of water promotes _____.

photorespiration Conserving water simultaneously reduces the amount of carbon dioxide available to the plant.

The electrons derived from the oxidation reaction in the light reactions are used to reduce _______ to ________

NADP+.......NADPH

After 3-PGA is phosphorylated, it is reduced by _____.

NADPH NADPH supplies the electrons that reduce the phosphorylated 3-PGA.

. The Calvin cycle oxidizes the light-reactions product _______to _____.

NADPH to NADP+.

In the Calvin cycle, how many ATP molecules are required to regenerate RuBP from five G3P molecules?

3

How many carbon dioxide molecules must be added to RuBP to make a single molecule of glucose?

6 Six carbon dioxide molecules are required to produce two G3P molecules, which can be combined to make one glucose molecule.

What is the basic role of CO2 in photosynthesis?

CO2 is fixed or incorporated into organic molecules.

In the light reactions, light energy is used to oxidize _____ to _______

H20......O2

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.

How is photosynthesis similar in C4 plants and CAM plants?

In both cases, rubisco is not used to fix carbon initially.

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. Choose between light, glucose, CO2, NADPH, G3P, NADP+, O2, ADP, and ATP

Input: ATP, NADPH, CO2 Output: ADP, NADP+, G3P Not input or output: light, glucose, O2 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 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.) Choices are-light, water, ATP, ADP, NADP+, NADPH, O2, CO2, glucose, and G3P

Input: Light, water, NADP+, and ADP Output: O2 ATP, and NADPH Neither Input or Output: CO2, glucose, and G3P 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.

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. Note that only blue labels should be placed in blue targets, and only pink labels should be placed in pink targets.

a) stroma- 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 c)cytosol- 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.