12.2 Photosynthetic reaction centers and electron transport
2 min read•july 24, 2024
Photosynthetic reaction centers are the powerhouses of light-driven energy conversion in plants. These specialized complexes contain chlorophyll molecules that absorb light and initiate electron transfer, along with accessory pigments and electron acceptors that form an efficient electron transport chain.
The process of charge separation in reaction centers is lightning-fast. When light excites electrons in the special pair, they quickly transfer to acceptors, creating a charge-separated state. This initiates a cascade of electron transfers that ultimately powers the production of ATP and NADPH for plant growth.
Photosynthetic Reaction Centers
Components of photosynthetic reaction centers
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- Special pair of chlorophyll molecules absorb light energy and initiate electron transfer as primary electron donor (P680 in PSII, P700 in PSI)
- Accessory pigments enhance light absorption and funnel energy to special pair (carotenoids, phycobilins)
- Primary electron acceptor receives electron from excited special pair (pheophytin in PSII, A0 chlorophyll in PSI)
- Secondary electron acceptors form electron transport chain (quinones, iron-sulfur clusters)
- Protein scaffold holds components in optimal arrangement for efficient electron transfer
- Redox-active cofactors facilitate electron transfer between components (plastoquinone, plastocyanin)
Charge separation in reaction centers
- Light absorption by special pair excites electrons to higher energy state
- Excited electrons transfer from special pair to primary acceptor within picoseconds
- Charge-separated state forms with positive special pair and negative primary acceptor
- Rapid electron transfer through acceptors prevents charge recombination
- Final electron acceptor reduces (QB in PSII, ferredoxin in PSI)
- Oxidized special pair regains electron from donor (water in PSII, plastocyanin in PSI)
Electron Transport and Energy Conversion
Electron transport chain in energy conversion
- Sequential redox reactions transfer electrons from higher to lower redox potential
- Proton pumping across thylakoid membrane creates electrochemical gradient
- ATP synthesis occurs via using proton gradient energy
- NADP+ reduction to NADPH by ferredoxin-NADP+ reductase
- Cyclic electron flow around PSI produces additional ATP without NADPH
Photosystem I vs photosystem II
- PSII initiates electron transport, oxidizes water to produce oxygen, reduces plastoquinone
- PSI receives electrons from PSII via cytochrome b6f complex, reduces ferredoxin
- Light absorption spectra: PSII maximum at 680 nm, PSI maximum at 700 nm
- Electron flow: PSII to PSI (linear transport), PSI only (cyclic transport)
- Final electron acceptors: PSII (plastoquinone), PSI (ferredoxin)
- PSII contains P680 reaction center, PSI contains P700 reaction center
- PSII involved in water oxidation, PSI involved in NADP+ reduction
Key Terms to Review (11)
ATP Synthase: ATP synthase is a vital enzyme complex that synthesizes adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate (Pi) during cellular respiration and photosynthesis. This process occurs in the inner mitochondrial membrane in eukaryotic cells and the thylakoid membrane in chloroplasts, where it harnesses the proton motive force generated by electron transport chains to power the production of ATP, which is essential for cellular energy.
Energy transfer efficiency: Energy transfer efficiency refers to the effectiveness with which energy is transferred from one part of a system to another, particularly in the context of converting light energy into chemical energy during photosynthesis. This concept is crucial for understanding how light-harvesting complexes capture solar energy and subsequently transfer that energy to reaction centers, ultimately leading to electron transport and the formation of chemical energy carriers such as ATP and NADPH.
Excited State: An excited state refers to a higher energy state of an atom or molecule that occurs when it absorbs energy, causing one or more electrons to move to a higher energy level. This state is crucial for understanding processes like absorption and emission of light, as well as various electronic transitions within molecules.
Melvin Calvin: Melvin Calvin was an American biochemist who won the Nobel Prize in Chemistry in 1961 for his research on the carbon fixation process in photosynthesis, commonly known as the Calvin Cycle. His groundbreaking work elucidated how plants convert carbon dioxide into organic compounds, connecting directly to the mechanism of photosynthetic reaction centers and electron transport.
Oxidative phosphorylation: Oxidative phosphorylation is the process by which ATP is produced from ADP and inorganic phosphate through the transfer of electrons in the electron transport chain, coupled with the reduction of oxygen to water. This process takes place in the inner mitochondrial membrane and is vital for cellular respiration, providing the energy needed for various biological functions.
Photophosphorylation: Photophosphorylation is the process of converting light energy into chemical energy in the form of ATP through the addition of a phosphate group to ADP, facilitated by light-driven electron transport. This process occurs in the thylakoid membranes of chloroplasts during photosynthesis, connecting light absorption to energy production.
Photosystem I: Photosystem I (PSI) is a complex of proteins and pigments that plays a critical role in the light-dependent reactions of photosynthesis, primarily involved in the conversion of light energy into chemical energy. It is responsible for the absorption of light and the transfer of electrons, ultimately leading to the reduction of NADP+ to NADPH. PSI works in tandem with Photosystem II and is essential for the overall process of photosynthetic electron transport.
Photosystem II: Photosystem II is a protein-pigment complex found in the thylakoid membranes of chloroplasts that plays a critical role in the light-dependent reactions of photosynthesis. It captures light energy, which excites electrons, leading to their transfer through a series of proteins and ultimately resulting in the production of ATP and NADPH. This process is essential for converting solar energy into chemical energy.
Quantum Yield: Quantum yield is a measure of the efficiency of a photochemical process, defined as the ratio of the number of events (like the formation of a product) to the number of photons absorbed. This concept is crucial in understanding how light interacts with matter, as it helps quantify how effectively light energy is converted into chemical energy or emitted as light, linking absorption and emission phenomena.
Robert Hill: Robert Hill was a British biochemist known for his pivotal work in the field of photosynthesis, particularly for the discovery of the Hill reaction. This reaction describes the process by which light energy is used to drive the transfer of electrons from water to electron carriers in the chloroplasts, which is fundamental to understanding how photosynthetic organisms convert light into chemical energy.
Stroma: Stroma refers to the fluid-filled space surrounding the thylakoids in chloroplasts, where the Calvin cycle of photosynthesis takes place. This matrix contains enzymes, DNA, and ribosomes, playing a crucial role in converting carbon dioxide and energy into glucose. The stroma is essential for facilitating the synthesis of organic molecules, linking the light-dependent reactions with the light-independent reactions.