Photosynthesis

How do chloroplasts convert light energy into chemical energy (ATP and NADPH)?

• Chloroplasts harvest light energy from the Sun by 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 non-photosynthetic organisms that depend on autotrophs for chemical energy.

• Photosynthesis and CO₂ fixation consume H₂O and generate O₂ and phosphorylated carbohydrates, whereas aerobic respiration consumes carbohydrate and O₂ and generates H₂O and CO₂.

• Chloroplasts contain three membranes: (1) a permeable outer membrane; (2) an impermeable inner membrane, which surrounds the stromal compartment; and (3) a proton-impermeable thylakoid membrane, which contains the ATP synthase complex and proteins in the photosynthetic electron transport system.

How do photosystems II and I absorb light, oxidize water, and reduce NADP⁺?

• Nuclear fusion reactions occurring in the Sun release energy in the form of light, which reaches Earth and is absorbed by protein-embedded chromophores in plants. Different chromophores—for example, chlorophylls, β-carotene, and phycoerythrobilin—possess different light-absorbing properties.

• Light absorption by plant pigments excites an electron from the ground state to a higher orbital called the excited state. The electron 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, in which case 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 electron carriers: plastoquinone (PQ), which shuttles an electron from PSII to cytochrome b₆f; and plastocyanin (PC), which shuttles an electron 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 electrons by photooxidation. In the process, water is fully oxidized to generate oxygen in the reaction 2 H₂O → O₂ + 4 H⁺ + 4 e⁻.

• Electron transfer between PSII and cytochrome b₆f is mediated by plastoquinone through the PQ cycle mechanism, which is analogous to ubiquinol transfer of electrons from complexes I or II to complex III in mitochondria. Similarly, electron transfer between cytochrome b₆f and PSI is mediated by plastocyanin, which is a mobile electron carrier protein analogous to cytochrome c in mitochondria.

• 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.

• The 4 photons absorbed by PSI are used to reduce 2 NADP⁺ to 2 NADPH in the stroma through a series of redox reactions requiring the electron carrier ferredoxin and the enzyme ferredoxin-NADP⁺ reductase.

• The net increase of 12 H⁺ in the thylakoid from light absorption at PSII is used to generate ATP by the chloroplast ATP synthase complex; these 12 H⁺, along with the 2 NADPH generated by light absorption at PSI, are used for CO₂ fixation in the stroma by the Calvin–Benson cycle reactions.

How does photophosphorylation generate ATP using a light-driven H⁺ gradient?

• 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.

• Differential localization of PSII and PSI reaction center complexes within the thylakoid membrane helps to control rates of cyclic photophosphorylation by increasing the frequency of energy transfer from the light-harvesting complexes to PSI rather than PSII. The PSI and ATP synthase complexes are concentrated in unstacked lamellar regions of the thylakoid membranes, whereas PSII complexes are mostly localized to the stacked membranes (grana). Redistribution of PSII and PSI complexes between these two regions is controlled by phosphorylation.

How does the Calvin–Benson cycle use ATP and NADPH to mediate CO₂ fixation?

• Photosynthetic organisms use the Calvin–Benson cycle to assimilate CO₂ into triose phosphates. These triose phosphates can be converted into hexose sugars, which then serve as a source of metabolic energy. The Calvin–Benson cycle reactions occur entirely in the chloroplast stroma.

• Although reactions in the Calvin–Benson cycle have been called the dark reactions, this is a misnomer because the Calvin–Benson cycle does not run during the night due to limiting amounts of ATP and NADPH, which are available only during the daylight as a function of photosynthetic electron transport and photophosphorylation.

• The Calvin–Benson cycle consists of three stages. Stage 1: CO₂ fixation by the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (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.

• To produce one net glyceraldehyde-3-phosphate molecule and regenerate the ribulose-1,5-bisphosphate starting material, the Calvin–Benson cycle requires 3 CO₂, 6 NADPH, 9 ATP, and 6 H₂O.

• RuBisCO catalyzes an energetically favorable reaction (ΔG°′ = −35 kJ/mol), in which 1 mole of ribulose-1,5-bisphosphate is first carboxylated and then cleaved into 2 moles of 3-phosphoglycerate. RuBisCO consists of eight large catalytic subunits and eight small stabilizing subunits arranged in a ring structure. Because 85% of the organisms on Earth are photosynthetic and require RuBisCO for CO₂ fixation by the Calvin–Benson cycle, RuBisCO is the most abundant enzyme on the planet.

• Formation of a carbamylated Lys201 in the RuBisCO active site is required for the carboxylation reaction; this CO₂ group is not the CO₂ that combines with RuBP. The carbamylation reaction of Lys201 is facilitated by the hexameric chaperonin protein RuBisCO activase, which restructures the RuBisCO protein by using an ATP-dependent mechanism to remove the inhibitory sugar phosphate 2-carboxyarabinitol-1-phosphate (CA1P).

• The carbamylated Lys201-CO₂ functional group coordinates binding of Mg²⁺ to the RuBisCO enzyme active site, which is required for carboxylation of the RuBP enediolate intermediate (step 2 in the RuBisCO reaction).

• 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 Mg²⁺ levels in the stroma (this occurs only 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; one of these redox-regulated enzymes is RuBisCO activase.

• In addition to carboxylation of RuBP, RuBisCO also catalyzes an oxygenase reaction, leading to the generation of 2-phosphoglycolate. This molecule must be converted to 3-phosphoglycerate through the glycolate pathway, a complex set of reactions requiring an investment of 1 ATP. This oxygenase reaction is therefore wasteful, and plant cells try to inhibit it by minimizing O₂ levels.

• Because high temperatures differentially increase levels of soluble O₂ relative to CO₂, plants that grow in high-temperature climates have adapted by trapping CO₂ in the form of malate and are called C4 plants.

• C4 plants have evolved two mechanisms to limit photorespiration: (1) the Hatch–Slack pathway, found in tropical plants such as sugarcane, uses two separate cell types; and (2) the CAM pathway, found in desert succulents such as the giant saguaro cactus, captures CO₂ at night when transpiration rates are low.

How does the glyoxylate cycle convert acetyl-CoA into carbohydrate in plants?

• 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.

Everyday example: CO-15: Paraquat: The Marijuana Herbicide