CHAPTER SUMMARY
8.1 PHOTOSYNTHESIS IS THE MAJOR PATHWAY BY WHICH ENERGY AND CARBON ARE INCORPORATED INTO CARBOHYDRATES.
- In photosynthesis, water is oxidized to form oxygen, and carbon dioxide is reduced to form carbohydrates.
- Photosynthesis consists of two sets of reactions: (1) the Calvin cycle, in which carbon dioxide is reduced to form carbohydrates, and (2) light-harvesting reactions, in which ATP and NADPH are generated to drive the Calvin cycle.
- The evolution of photosynthesis involved horizontal gene transfer in bacteria and endosymbiosis in eukaryotes.
- In eukaryotes, photosynthesis takes place in chloroplasts: The Calvin cycle occurs in the stroma, and the light-harvesting reactions take place on the thylakoid membrane.
8.2 THE CALVIN CYCLE IS A THREE-STEP PROCESS THAT RESULTS IN THE INCORPORATION OF CARBON DIOXIDE INTO CARBOHYDRATES.
- The three steps of the Calvin cycle are (1) carboxylation; (2) reduction; and (3) regeneration.
- The first step is the addition of CO2 to the 5-carbon sugar RuBP. This step is catalyzed by the enzyme rubisco, considered the most abundant protein on Earth. The resulting 6-carbon compound immediately breaks down into two 3-carbon compounds.
- The second step is phosphorylation of the 3-carbon compounds by ATP followed by reduction by NADPH to produce 3-carbon triose phosphate molecules that are exported from the chloroplast to the cytosol, where they are used to build larger sugars.
- The third step is the regeneration of RuBP from five 3-carbon molecules.
- Starch formation provides chloroplasts with an osmotically inactive way of storing carbohydrates.
8.3 THE LIGHT-HARVESTING REACTIONS USE SUNLIGHT TO PRODUCE ATP AND NADPH REQUIRED BY THE CALVIN CYCLE.
- Visible light is absorbed by chlorophyll. The absorbed light energy can be released as heat, reemitted as light (fluorescence), or transferred to an adjacent chlorophyll molecule. Special chlorophyll molecules in the reaction center transfer both energy and electrons, thus initiating the photosynthetic electron transport chain.
- The electron transport chain consists of multisubunit protein complexes and diffusible compounds. Water is the electron donor and NADP+ is the final electron acceptor.
- Antenna chlorophylls transfer absorbed light energy to the reaction center.
- Reaction centers are located within pigment–protein complexes known as photosystems.
- The linear transport of electrons from water to NADPH requires the energy input of two photosystems.
- Photosystem II pulls electrons from water, resulting in the production of oxygen and protons on the lumen side of the membrane. Photosystem I passes electrons to NADP+, producing NADPH for use in the Calvin cycle.
- The buildup of protons drives ATP synthase to produce ATP on the stroma side of the membrane, where it is used by the Calvin cycle.
8.4 PHOTOSYNTHESIS FACES SEVERAL CHALLENGES TO ITS EFFICIENCY.
- An imbalance between the light-harvesting reactions and the Calvin cycle can lead to the formation of reactive oxygen species.
- Protection from excess light energy includes antioxidant molecules that neutralize reactive oxygen species and xanthophyll pigments that dissipate excess light energy as heat.
- Rubisco can act catalytically on oxygen as well as on carbon dioxide. When it acts as an oxygenase, there is a loss of energy and reduced carbon from the Calvin cycle.
- Rubisco has evolved to favor carbon dioxide over oxygen, but the cost of this selectivity is reduced speed.
- The synthesis of carbohydrates via the Calvin cycle results in significant energy losses, largely due to photorespiration.
- The maximum theoretical efficiency of photosynthesis is approximately 4% of total incident solar energy.
Self-Assessment Question 1
Write the overall photosynthetic reaction and identify which molecules are oxidized and which molecules are reduced.
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Model Answer:
The overall photosynthetic reaction is CO2 + H2O → C6H12O6 + O2, in which the CO2 is reduced to C6H12O6 and the H2O is oxidized to O2.
Self-Assessment Question 2
Describe how photosynthesis evolved in prokaryotes and in eukaryotes.
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Model Answer:
In cyanobacteria two different photosystems evolved to produce one single photosynthetic electron-transport chain. One hypothesis is that a cyanobacterium with one photosystem gained the second photosystem through horizontal gene transfer from another cyanobacterium. Another hypothesis is that the two photosystems evolved together in the same organism. Photosynthesis is hypothesized to have evolved in eukaryotic cells through an endosymbiotic event where a cyanobacterium was incorporated into a plant cell and eventually evolved to be the organelle chloroplast.
Self-Assessment Question 3
Explain the functions of the Calvin cycle and the light harvesting reactions in photosynthesis.
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Model Answer:
The photosynthetic pathway harnesses the energy of the sun to produce energy (in the form of ATP and NADPH) that will then drive the Calvin cycle to convert CO2 to a usable form of energy, a carbohydrate.
Self-Assessment Question 4
Name the major inputs and outputs of the Calvin cycle.
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Model Answer:
The major inputs of the Calvin cycle are CO2 (from the atmosphere) and ATP and NADPH (from the photosynthetic pathway). The major outputs of the Calvin cycle are ADP, NADP+, and carbohydrates (triose phosphates). Larger sugars such as glucose and sucrose are synthesized from triose phosphates in the cytoplasm.
Self-Assessment Question 5
Describe the three major steps in the Calvin cycle and the role of the key enzyme rubisco.
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Model Answer:
See Figure 8.6. Briefly, CO2 enters the Calvin cycle and is added to the 5-carbon compound RuBP catalyzed by the enzyme rubisco, generating 3-phosphoglycerate (3-PGA). After this carboxylation reaction, the 3-PGA is reduced through conversions of ATP to ADP and NADPH to NADP+. The resulting triose phosphate, the carbohydrate, then exits the cycle to be used as energy for the cell. In the final step of the cycle, some of the carbohydrate is used to regenerate RuBP through reactions with ATP. Then the cycle starts again with more CO2.
Self-Assessment Question 6
Explain why photosynthetic electron transport requires two photosystems.
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Model Answer:
Photosynthetic electron transport requires two photosystems because each has a particular function. Photosystem II is able to pull electrons from water (something that Photosystem I is not capable of doing). Photosystem I, however, can harness enough light energy to convert NADP+ to NADPH (something that Photosystem II is incapable of). Since both the ability to split water for electrons and the production of NADPH is important in the photosynthetic pathway, both photosystems are critical for successful carbohydrate production.
Self-Assessment Question 7
Describe the role of cyclic electron transport.
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Model Answer:
The role of cyclic electron transport is to prevent damage to the cell by an excess of high-energy electrons. These electrons are routed into an alternative pathway (the cyclic electron-transport system) that not only protects the cell but also creates ATP by setting up a proton gradient that drives the conversion of ADP to ATP by the enzyme ATP synthase.
Self-Assessment Question 8
List two strategies that plants use to limit the formation and effects of reactive oxygen species.
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Model Answer:
Two strategies that plants use to limit the formation and effects of reactive oxygen species are antioxidants and xanthophylls. Antioxidants like ascorbate and beta-carotene neutralize reactive oxygen species. Xanthophylls are yellow-orange pigments that slow the formation of reactive oxygen species by reducing excess light energy. They do this by accepting absorbed light energy directly from chlorophyll and then converting this energy to heat.
Self-Assessment Question 9
Explain the trade-off that rubisco faces in terms of selectivity and enzymatic speed.
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Model Answer:
The ability of rubisco to favor CO2 over O2 requires a high degree of selectivity (because carbon dioxide and oxygen look very similar to a large enzyme). This selectivity makes the reaction rate of rubisco very slow.
Self-Assessment Question 10
Estimate the overall efficiency of photosynthesis and where in the pathway energy is dissipated.
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Model Answer:
The photosynthetic electron-transport chain captures at most about 24% of the sun’s usable energy. Chlorophyll can only absorb visible light, which has the appropriate energy levels to produce the high-energy electrons needed for photosynthesis. Most of the sun’s output (~60%) is not absorbed by chlorophyll. Some of the visible light energy is reflected off the leaf or passes through it (~8%). Some of the light energy absorbed cannot be transferred to the reaction center and is thus given off as heat (~8%). Another larger energy loss comes with photorespiration (~20%). In all, the maximum energy conversion efficiency of photosynthesis is about 4%.