Chapter 11 Mastering

Inputs of light reactions

light, water, ADP, NADP+

PS II only

-Oxidation of water -reduction of electron transport chain between the two photosystems

PS I only

-oxidation of electron transport chain between the two photosystems -reduction of NADP+

Summarize the redox reactions in photosynthesis in 4 sentences.

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.

Describe the Calvin cycle in terms of how many molecules are present and how many carbons make up that molecule in a series of 6 steps.

1. To produce 1 molecule of G3P (which contains 3 carbons), the Calvin cycle must take up 3 molecules of CO2 (1 carbon atom each). 2. 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). 3. In reducing 3-PGA to G3P (Phase 2), there is no addition or removal of carbon atoms. 4. At the end of Phase 2, 1 of the 6 G3P molecules is output from the cycle, removing 3 of the 18 carbons. 5. The remaining 5 G3P molecules (15 total carbon atoms) enter Phase 3, where they are converted to 3 molecules of R5P. 6. Finally, the R5P is converted to RuBP without the addition or loss of carbon atoms.

Outputs of Calvin Cycle

ADP, NADP+, G3P

A plant uses solar energy to make ATP and NADPH, which then drive the synthesis of carbohydrates in the leaves. At least one carbohydrate, sucrose, is translocated to nonphotosynthetic parts of the plant (stems, roots, flowers, and fruits) for use as a source of energy. Thus, ATP is used to make sucrose, and the sucrose is then used to make ATP. It would seem simpler for the plant just to make ATP and translocate the ATP itself directly to other parts of the plant, thereby completely eliminating the need for a Calvin cycle, a glycolytic pathway, and a TCA cycle. Suggest several reasons that plants do not manage their energy economies in this way.

ATP is highly polar and would be difficult to move across the membrane. Generation of ATP stops as soon as the sun sets, and there is no practical way to store enough of it to meet the ongoing energy needs of the organism at night. ATP would be a very difficult way to move energy about, because it has a molecular weight of about 500 daltons and contains 2 high-energy phosphate bonds. Sucrose, in contrast, has a molecular weight of about 340 daltons and can give 72 molecules of A T P elsewhere in the plant.

Outputs of light reactions

ATP, NADPH, O2

Inputs of Calvin Cycle

CO2, ATP, NADPH

both PS II and PS I

Light absorption and the reduction of primary electron acceptor.

Under laboratory conditions, a photosynthetic organism might convert 31% of the light energy striking it to chemical bond energy of organic molecules. In reality, however, photosynthetic efficiency is far lower, closer to 5% or less. Considering a plant growing in a natural environment, suggest the reasons for this discrepancy. Select the four correct answers.

Many leaves may be shaded by the leaves of the canopy above them. Some of the energy is lost as heat and entropy. Not all light will be of appropriate wavelength. The capacity of a photosystem to absorb energy may be saturated by a level of light far below the intensity of sunlight.

Neither inputs nor outputs of the Calvin cycle

O2, light, glucose

Assume that you have an illuminated suspension of Chlorella cells carrying out photosynthesis in the presence of 0.1%% carbon dioxide and 20%% oxygen. What will be the short-term effects of the following changes in conditions on the levels of 3-phosphoglycerate (PGAPGA) and ribulose-1,5-bisphosphate (RuBPRuBP)? Carbon dioxide concentration is suddenly reduced 1000-fold.

PGA Down, RuBP Up

Assume that you have an illuminated suspension of Chlorella cells carrying out photosynthesis in the presence of 0.1% carbon dioxide and 20% oxygen. What will be the short-term effects of the following changes in conditions on the levels of 3-phosphoglycerate (PGA) and ribulose-1,5-bisphosphate (RuBP)? An inhibitor of photosystem IIII is added.

PGA Up, RuBP Down

Assume that you have an illuminated suspension of Chlorella cells carrying out photosynthesis in the presence of 0.1% carbon dioxide and 20% oxygen. What will be the short-term effects of the following changes in conditions on the levels of 3-phosphoglycerate (PGA) and ribulose-1,5-bisphosphate (RuBP)? Light is restricted to green wavelengths (510-550 nmnm).

PGA Up, RuBP Down

Assume that you have an illuminated suspension of Chlorella cells carrying out photosynthesis in the presence of 0.1% carbon dioxide and 20% oxygen. What will be the short-term effects of the following changes in conditions on the levels of 3-phosphoglycerate (PGA) and ribulose-1,5-bisphosphate (RuBP)? Oxygen concentration is reduced from 20% to 1%.

PGA Up, RuBP Unchanged

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 the thylakoid membrane. Use only the blue labels for the blue targets, and only the pink labels for the pink targets.

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.

Describe the Calvin cycle in terms of the production of ATP and NADH, as well as the movement of inorganic phosphate.

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.)

Describe the membranes and compartments of a 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.

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. 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.

p680 to Pq (plastiquinone)

energy input required

p700->Fd (ferredoxin)

energy input required

Neither input nor output of light reactions

glucose, G3P, CO2

Fd (ferrodoxin) to NADP+

no energy input required

PQ->p700+

no energy input required

water->p680+

no energy input required