Chapter 8 Summary

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Core Concepts Summary

8.1 Photosynthesis is the major pathway by which energy and carbon are incorporated into carbohydrates.

In photosynthesis, water is oxidized, releasing oxygen, and carbon dioxide is reduced, forming carbohydrates. page 155

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. page 155

In eukaryotes, photosynthesis takes place in chloroplasts: The Calvin cycle takes place in the stroma, and the light-harvesting reactions take place in the thylakoid membrane. page 156

8.2 The Calvin cycle is a three-step process that uses carbon dioxide to synthesize carbohydrates.

The three steps of the Calvin cycle are (1) addition of CO2 (carboxylation); (2) reduction; and (3) regeneration. page 157

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. page 157

The second step is the donation of a phosphate group to the 3 carbon compounds by ATP followed by reduction by NADPH to produce 3 carbon triose phosphate molecules. Some of these triose phosphates are exported from the chloroplast to the cytosol, where they are used to build larger sugars. page 158

The third step is the regeneration of RuBP from five 3-carbon triose phosphates. page 158

Starch formation provides chloroplasts with a way of storing carbohydrates that will not cause water to enter the cell by osmosis. page 159

8.3 The light-harvesting reactions use sunlight to produce the ATP and NADPH required by the Calvin cycle.

Visible light is absorbed by chlorophyll. page 160

Antenna chlorophyll molecules transfer absorbed light energy to the reaction center. page 162

Reaction centers are located within pigment–protein complexes known as photosystems. Special chlorophyll molecules in the reaction center transfer excited-state electrons to an electron-acceptor molecule, thus initiating the photosynthetic electron transport chain. page 162

The electron transport chain consists of a series of electron transfer or redox reactions that take place within both protein complexes and diffusible compounds. Water is the electron donor and NADP+ is the final electron acceptor. page 163

The linear transport of electrons from water to NADPH requires the energy input of two photosystems. page 163

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. page 163

The buildup of protons in the lumen drives the production of ATP by oxidative phosphorylation. The ATP synthase is oriented such that ATP is produced on the stroma side of the membrane. page 164

Cyclic electron transport involves the redirection of electrons from ferredoxin back into the electron transport chain and increases ATP production. page 166

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. page 166

Protection from excess light energy includes antioxidant molecules that neutralize reactive oxygen species and xanthophyll pigments that dissipate excess light energy as heat. page 168

Rubisco can act catalytically on oxygen as well as on carbon dioxide. When it acts on oxygen, there is a loss of energy and of reduced carbon from the Calvin cycle. page 168

Rubisco has evolved to favor carbon dioxide over oxygen, but the cost of this selectivity is reduced speed. page 169

The synthesis of carbohydrates through the Calvin cycle results in significant energy losses, which are due in part to photorespiration. page 169

The maximum theoretical efficiency of photosynthesis is approximately 4% of total incident solar energy. page 170

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8.5 The evolution of photosynthesis had a profound impact on life on Earth.

The ability to use water as an electron donor in photosynthesis evolved in cyanobacteria. page 170

Cyanobacteria evolved two photosystems either by the transfer of genetic material, or by gene duplication and divergence. page 171

Photosynthesis in eukaryotes likely evolved by endosymbiosis. page 171

All of the oxygen in Earth’s atmosphere results from photosynthesis by organisms containing two photosystems. page 171

Self-Assessment

  1. Write the overall photosynthetic reaction and identify which molecules are oxidized and which molecules are reduced.

    Self-Assessment 1 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.

  2. Compare the overall reactions of photosynthesis and cellular respiration.

    Self-Assessment 2 Answer

    In photosynthesis, energy from sunlight is captured in chemical forms (ATP and NADPH) that are used to synthesize carbohydrates from CO2. In cellular respiration, carbohydrates are oxidized to CO2, releasing energy that is ultimately used to synthesize ATP. In photosynthesis, H2O is the ultimate electron donor and O2 is produced as a by-product. In cellular respiration, O2 is the ultimate electron acceptor and H2O is produced as a by-product.

  3. Name the major inputs and outputs of the Calvin cycle.

    Self-Assessment 3 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.

  4. Describe the three major steps in the Calvin cycle and the role of the key enzyme rubisco.

    Self-Assessment 4 Answer

    See Fig. 8.6. Briefly, in the first step, CO2 enters the Calvin cycle and is added to the 5- carbon compound RuBP in a reaction catalyzed by the enzyme rubisco, generating 3-phosphoglycerate (3-PGA). In the second step, the 3-PGA is reduced through conversion of ATP to ADP and NADPH to NADP+, producing triose phosphate, the carbohydrate. Some of the triose phosphate molecules exit the cycle to be used as an energy source for the cell. In the final step of the cycle, triose phosphate molecules that did not exit the cell are used to regenerate RuBP through reactions with ATP. Then the cycle starts again with more CO2.

  5. Contrast what happens when antenna chlorophylls absorb light energy with what happens when the reaction center absorbs light energy. Why are antenna chlorophylls so important in photosynthesis?

    Self-Assessment 5 Answer

    Antennae chlorophyll transfer absorbed light energy to adjacent chlorophyll molecules; reaction center molecules transfer energy and electrons to an adjacent electron acceptor molecule.

  6. Explain why using water as an electron donor requires a photosynthetic electron transport chain with two photosystems.

    Self-Assessment 6 Answer

    Photosystem II is needed to pull electrons from water, whereas Photosystem I can harness enough light energy to convert NADP+ to NADPH.

  7. Show in a diagram how energy from sunlight is used to produce ATP.

    Self-Assessment 7 Answer

    Diagram should show the two ways in which electron transport results in a high concentration of protons in the lumen: (1) as a result of water splitting and (2) the coupled movement of electrons and protons that involves both plastoquinone and the cytochrome b6f complex and results in protons being moved across the thylakoid.

  8. List the products of linear electron transport and cyclic electron transport, and describe the role of cyclic electron transport.

    Self-Assessment 8 Answer

    Linear electron transport makes both NADPH and ATP; cyclic electron transport produces only ATP. The role of cyclic electron transport is to increase the production of ATP to the ratio required by the Calvin cycle. These electrons are routed into an alternative pathway (the cyclic electron-transport system) that creates ATP by setting up a proton gradient that drives the conversion of ADP to ATP by the enzyme ATP synthase.

  9. Describe two strategies that plants use to limit the formation and effects of reactive oxygen species.

    Self-Assessment 9 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 accept absorbed light energy directly from chlorophyll and convert this energy to heat.

  10. Explain the trade-off that rubisco faces in terms of selectivity and enzymatic speed.

    Self-Assessment 10 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.

  11. Estimate the overall efficiency of photosynthesis and describe where in the pathway energy is dissipated.

    Self-Assessment 11 Answer

    Because chlorophyll can only absorb visible light, 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 is absorbed but not transferred to the reaction center and is consequently given off as heat (~8%). Thus, the photosynthetic electron-transport chain captures at most about 24% of the sun’s usable energy. An additional energy loss comes during the Calvin cycle, in part from photorespiration (~20%). In all, the maximum energy conversion efficiency of photosynthesis is about 4%.

  12. List three major steps that are hypothesized to have occurred in the evolutionary history of photosynthesis.

    Self-Assessment 12 Answer

    (1) The earliest step was the evolution of pigments that can use light energy to drive the movement of electrons. (2) Cyanobacteria incorporated two different photosystems in their photosynthetic electron-transport chain. One hypothesis is that an ancestral cyanobacterium with one photosystem gained the second photosystem through the transfer of genetic material from another bacterium. Another hypothesis is that a single photosystem was duplicated and then the copy diverged in sequence and function. (3) Photosynthesis is hypothesized to have evolved in eukaryotic cells through an endosymbiotic event during which a cyanobacterium was incorporated into a plant cell and eventually evolved to be the chloroplast.