Photosynthesis

How do plants harvest energy from sunlight?

● Chloroplasts harvest light energy from the Sun using chlorophyll molecules, which absorb photons to excite electrons. The excited electrons are transferred to nearby carrier molecules.

● The photosynthetic electron transport system consists of linked redox reactions that function in much the same way as the mitochondrial electron transport system. Energy available from redox reactions is used to translocate protons across an internal membrane for ATP synthesis and to generate the reductant NADPH.

● ATP and NADPH produced by the photosynthetic electron transport system are used in the Calvin–Benson cycle to fix CO₂ in the form of triose phosphates. The triose phosphates can be used to form hexose sugars, which the plant uses as a source of chemical energy for mitochondrial electron transport.

● Photosynthetic organisms are autotrophs because they use light to supply all of their energy needs during the day and night. In contrast, heterotrophs are nonphotosynthetic organisms that depend on autotrophs for chemical energy.

How is light energy converted to chemical energy by Photosystems I and II?

● Light absorption by plant pigments excites an e^– from the ground state to a higher orbital called the excited state. The e^– can then (1) return to the ground state and transfer the absorbed energy to a nearby chlorophyll molecule (resonance energy transfer); (2) be transferred to a nearby acceptor molecule of higher reduction potential and thereby be reduced (photooxidation); or (3) return to the ground state and the absorbed energy is lost (fluorescence).

● Many light-harvesting protein complexes in a plant leaf absorb light and transfer the energy to nearby chlorophyll molecules, which eventually pass the energy to one of two reaction centers (PSII or PSI), where photooxidation takes place. About 300 energy transfer events occur for every one photooxidation.

● The PSII reaction center absorbs light at 680 nm, whereas the PSI reaction center absorbs light at 700 nm. The PSII and PSI reaction centers are linked by e^– carriers: plastoquinone (PQ), which shuttles an e^– from PSII to cytochrome b₆f; and plastocyanin (PC), which shuttles an e^– from cytochrome b₆f to PSI.

● Redox reactions in photosynthesis constitute the Z scheme, which requires the absorption of 4 photons at PSII and 4 photons at PSI in order to transfer 4 e^– by photooxidation. In the process, water is fully oxidized to generate oxygen in the reaction 2 H₂O  O₂ + 4 H⁺ + 4 e^–.

● A total of 12 H⁺ accumulate inside the thylakoid lumen for every O₂ produced by the oxidation of 2 H₂O. This includes the translocation of protons from the stroma to the thylakoid lumen by the PQ cycle (4 H⁺) and proton translocation by cytochrome b₆f (4 H⁺), as well as the release of 4 H⁺ by the oxidation of 2 H₂O.

How does photophosphorylation generate ATP in chloroplasts?

● The mechanism of ATP synthesis in photosynthetic organisms is called photophosphorylation because light absorption is linked to ATP synthesis through the chemiosmotic gradient produced by the photosynthetic electron transport system.

● The chloroplast ATP synthase complex is structurally similar to the yeast mitochondrial ATP synthase complex; however, 4 H⁺ cross through the chloroplast ATP synthase CF_o complex for every 1 ATP that is generated.

● Cyclic photosynthetic electron transport bypasses electron transfer to ferredoxin-NADP⁺ reductase; instead, ferredoxin transfers the electron to plastoquinone and recirculates through the electron transport system. The net result is higher rates of ATP synthesis and lower rates of NADPH generation. This is a good way to control ATP-to-NADPH ratios for biosynthesis.

How is carbon dioxide converted to carbohydrate by the Calvin-Benson Cycle?

● Although reactions in the Calvin–Benson cycle have been called the dark reactions because they technically can function for a few seconds without light, this is a misnomer because the Calvin–Benson cycle does not actually run during the night under environmental conditions due to limiting amounts of ATP and NADPH.

● The Calvin–Benson cycle consists of three stages. Stage 1: CO₂ fixation by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase, also called RuBisCO. Stage 2: reduction of 3-phosphoglycerate to form glyceraldehyde-

3-phosphate. Stage 3: regeneration of ribulose-1,5- bisphosphate to continue the cycle back at stage 1.

● The activity of Calvin–Benson cycle enzymes is controlled by light in three ways: (1) enzyme inhibitor proteins, which are present at higher levels in the dark than in the light; (2) activation of Calvin–Benson cycle enzymes by elevated pH and increased Mg21 levels in the stroma (this only occurs in the light, when photosynthetic electron transport is fully active); and (3) thioredoxin-mediated activation of Calvin–Benson cycle enzymes through reduction of disulfide bridges.

● RuBisCO can also catalyze an oxygenase reaction, which is called photorespiration and is wasteful because carbon is not easily fixed into carbohydrate. Some plants have evolved to avoid photorespiration using malate as a carbon carrier. These two pathways are called the (1) The C4 pathway, which uses two separate cell types: one for CO₂ uptake and the other for RuBisCO-mediated carboxylation and (2) The CAM pathway, which captures CO₂ at night when transpiration rates are low.

How do plants use the Glyoxylate Cycle to initiate plant growth after germination?

● The glyoxylate cycle is a metabolic pathway in plants that converts acetyl-CoA into succinate, which serves as a carbon source for glucose biosynthesis. Animals do not have glyoxylate cycle enzymes and cannot convert acetyl-CoA into glucose.

● Isocitrate lyase and malate synthase are glyoxylate cycle enzymes that are localized to glyoxysomes in plant cells and are also present in bacteria. The succinate produced by the glyoxylate cycle is converted to malate, which is then exported to the cytosol and oxidized to oxaloacetate, a substrate for glucose synthesis by gluconeogenesis.