2106 Chapter 8

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 on oxygen, there is a loss of energy and of 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 through the Calvin cycle results in significant energy losses, which are due in part to photorespiration. The maximum theoretical efficiency of photosynthesis is approximately 4% of total incident solar energy.

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?

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

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

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.

Compare the overall reactions of photosynthesis and cellular respiration.

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.

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 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. 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.

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

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

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. Cyanobacteria evolved two photosystems either by the transfer of genetic material, or by gene duplication and divergence. Photosynthesis in eukaryotes likely evolved by endosymbiosis. All of the oxygen in Earth's atmosphere results from photosynthesis by organisms containing two photosystems.

Name the major inputs and outputs of the Calvin cycle.

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.

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

The overall photosynthetic reaction is CO2 + H2O → C6H12O6 + O2, in which the CO2 is reduced to C6H12O6 and the H2O is oxidized to O2.

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. 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 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. The third step is the regeneration of RuBP from five 3-carbon triose phosphates. Starch formation provides chloroplasts with a way of storing carbohydrates that will not cause water to enter the cell by osmosis.

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

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.

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

Visible light is absorbed by chlorophyll. Antenna chlorophyll molecules transfer absorbed light energy to the reaction center. 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. 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. 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 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. Cyclic electron transport involves the redirection of electrons from ferredoxin back into the electron transport chain and increases ATP production.