15.1 Light Reactions and Photosystems
2 min read•august 9, 2024
Photosynthesis powers life on Earth. Light reactions, occurring in thylakoid membranes, kick off this process. Two photosystems work together, capturing light energy and converting it into chemical energy.
Electrons flow through these systems, driving ATP production. Meanwhile, is formed, storing energy for later use. Together, ATP and NADPH fuel the Calvin cycle, turning CO2 into sugar.
Photosystems and Light Harvesting
Structure and Components of Photosystems
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- contains absorbs light at 700 nm wavelength
- houses reaction center captures light at 680 nm wavelength
- serves as primary pigment in photosystems absorbs red and blue light reflects green
- Light-harvesting complex surrounds reaction centers consists of proteins and accessory pigments (, )
- Reaction center acts as the core of photosystems where light energy converts to chemical energy
- illustrates electron flow between photosystems represents overall process of light reactions
Function and Interaction of Photosystems
- Photosystem II initiates light reactions by splitting water molecules
- Electrons from PSII travel through to Photosystem I
- PSI further energizes electrons using additional light energy
- Light-harvesting complex funnels light energy to reaction centers increases efficiency of light capture
- Chlorophyll molecules in reaction centers become excited by absorbed light energy
- Z-scheme demonstrates how electrons move from water to through both photosystems
Electron Transport and Energy Production
Electron Flow and ATP Synthesis
- Electron transport chain consists of protein complexes (, , )
- Electrons flow through ETC from PSII to PSI generating proton gradient
- utilizes proton gradient to produce ATP through rotational catalysis
- drives ATP production by harnessing energy from proton concentration difference
NADPH Production and Energy Storage
- NADP+ receives energized electrons from PSI forming NADPH
- NADPH serves as reducing agent for carbon fixation in Calvin cycle
- catalyzes final step of NADPH production
- ATP and NADPH produced during light reactions power dark reactions of photosynthesis
Thylakoid Membrane and Photolysis
Thylakoid Membrane Structure and Function
- forms flattened sacs within chloroplasts
- Membrane contains integral proteins including photosystems ATP synthase and electron carriers
- consist of stacked thylakoids increase surface area for light absorption
- connect grana stacks allow for efficient energy transfer
Photolysis and Oxygen Evolution
- occurs in of PSII
- Process splits water molecules into protons electrons and oxygen
- in OEC catalyzes water-splitting reaction
- Oxygen released as byproduct of photolysis essential for aerobic life
- Protons from photolysis contribute to proton gradient used in ATP synthesis
- Electrons from water splitting replace those lost by P680 in PSII reaction center
Key Terms to Review (24)
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 enzyme plays a crucial role in energy production, harnessing the proton gradient established across membranes to drive the phosphorylation of ADP, connecting energy conversion processes in both mitochondria and chloroplasts.
Carotenoids: Carotenoids are a class of pigments found in plants, algae, and photosynthetic bacteria that play a crucial role in photosynthesis and provide coloration to many fruits and vegetables. These pigments absorb light energy primarily in the blue and green wavelengths, helping to capture light for photosynthesis and protect plant tissues from damage caused by excess sunlight.
Chemiosmosis: Chemiosmosis is the process through which ATP is synthesized using the energy derived from the proton gradient created across a membrane. This mechanism is vital in cellular respiration and photosynthesis, as it couples the flow of protons back into the mitochondrial or thylakoid lumen with the conversion of ADP and inorganic phosphate into ATP, a key energy currency for cells.
Chlorophyll: Chlorophyll is a green pigment found in the chloroplasts of plants, algae, and some bacteria that plays a crucial role in photosynthesis by absorbing light energy. This pigment allows organisms to capture light energy from the sun, converting it into chemical energy stored in glucose during the light reactions of photosynthesis. Its primary function is to absorb blue and red light, reflecting green, which is why plants appear green.
Cytochrome b6f complex: The cytochrome b6f complex is a crucial protein complex found in the thylakoid membranes of chloroplasts, playing a significant role in the light reactions of photosynthesis. It functions as an electron transport chain component, facilitating the transfer of electrons from plastoquinone to plastocyanin while contributing to the generation of a proton gradient that is essential for ATP synthesis. This complex also connects photosystem II and photosystem I, linking the two systems together in the process of harnessing light energy.
Electron transport chain: The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane that facilitate the transfer of electrons from electron donors, such as NADH and FADH2, to electron acceptors, ultimately generating ATP through oxidative phosphorylation. This process plays a critical role in cellular respiration and is essential for converting energy stored in nutrients into a usable form for the cell.
Ferredoxin-nadp+ reductase: Ferredoxin-NADP\^+ reductase (FNR) is an enzyme that catalyzes the transfer of electrons from ferredoxin to NADP\^+, producing NADPH during the light reactions of photosynthesis. This reaction is crucial for converting light energy into chemical energy, as NADPH serves as a reducing agent in various biosynthetic processes. FNR plays a key role in the electron transport chain of photosystems, linking the light capture process to the synthesis of energy-rich compounds.
Grana: Grana are stacked structures found within chloroplasts, primarily consisting of thylakoid membranes that contain chlorophyll and other pigments. These stacks play a crucial role in the light reactions of photosynthesis by increasing the surface area for light absorption and housing the necessary components for the conversion of light energy into chemical energy.
Manganese cluster: The manganese cluster is a vital metal complex found in the photosystem II of plants, algae, and cyanobacteria, responsible for water splitting during the light reactions of photosynthesis. This cluster consists of four manganese ions and is crucial for the process of converting light energy into chemical energy, facilitating the release of oxygen as a byproduct.
NADP+: NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) is a coenzyme involved in various biochemical reactions, primarily in photosynthesis. It serves as an electron carrier that facilitates the transfer of electrons during the light reactions, playing a crucial role in converting solar energy into chemical energy in the form of glucose and other carbohydrates.
NADPH: NADPH, or nicotinamide adenine dinucleotide phosphate, is a coenzyme that plays a critical role in anabolic reactions by acting as a reducing agent, providing the necessary electrons for biosynthetic processes. It is primarily produced during the light reactions of photosynthesis and is essential for the Calvin cycle, as well as other metabolic pathways like the pentose phosphate pathway and fatty acid synthesis.
Oxygen-evolving complex: The oxygen-evolving complex (OEC) is a crucial component of photosynthesis, specifically in the light reactions that occur in the thylakoid membranes of chloroplasts. This complex is responsible for the splitting of water molecules into oxygen, protons, and electrons, providing the necessary electrons to replenish the reaction center of photosystem II. By releasing oxygen as a byproduct, the OEC plays an essential role in sustaining life on Earth through the oxygenation of the atmosphere.
P680: p680 is a chlorophyll molecule that plays a crucial role in photosynthesis, specifically within photosystem II. This pigment is named for its absorption peak at a wavelength of 680 nanometers, where it captures light energy to initiate the light reactions of photosynthesis, leading to the production of ATP and NADPH.
P700: p700 is a pigment-protein complex found in the photosystem I (PSI) of plants, algae, and cyanobacteria, crucial for the light reactions of photosynthesis. It plays a key role in capturing light energy and transferring it to electrons during the process of converting solar energy into chemical energy.
Photolysis: Photolysis is the process in which light energy, particularly from sunlight, is used to break down molecules, typically involving water in photosynthetic organisms. This process is crucial during the light reactions of photosynthesis, as it generates oxygen and supplies electrons needed for the conversion of light energy into chemical energy.
Photosystem I: Photosystem I is a multi-protein complex located in the thylakoid membranes of chloroplasts that plays a crucial role in the light reactions of photosynthesis. It primarily absorbs light energy at a wavelength of 700 nm and is essential for the production of NADPH, which is a key electron carrier in the photosynthetic pathway. Photosystem I works in conjunction with Photosystem II to ensure efficient energy capture and conversion.
Photosystem II: Photosystem II is a complex of proteins and pigments in the thylakoid membranes of chloroplasts that plays a crucial role in the light-dependent reactions of photosynthesis. It is responsible for capturing light energy and using it to energize electrons, which then participate in a series of reactions that ultimately lead to the production of ATP and NADPH, essential for the synthesis of glucose during the Calvin cycle.
Phycobilins: Phycobilins are water-soluble pigments found in cyanobacteria and red algae, playing a critical role in photosynthesis by capturing light energy. These pigments are essential for light-harvesting because they absorb light in wavelengths that chlorophyll does not, allowing organisms that contain them to efficiently utilize a broader spectrum of light. Phycobilins are organized into structures called phycobilisomes, which are attached to the thylakoid membranes of these organisms and enhance their ability to carry out photosynthesis.
Plastocyanin: Plastocyanin is a copper-containing protein that plays a crucial role in the electron transport chain during photosynthesis, specifically in the light reactions. This protein facilitates electron transfer from the photosystem II complex to the cytochrome b6f complex, making it essential for the synthesis of ATP and NADPH. Plastocyanin contributes to the overall efficiency of photosynthesis by helping to convert light energy into chemical energy.
Plastoquinone: Plastoquinone is a lipid-soluble electron carrier found in the thylakoid membranes of chloroplasts, playing a crucial role in the light reactions of photosynthesis. It transports electrons from photosystem II to the cytochrome b6f complex, facilitating the transfer of energy during the conversion of light energy into chemical energy. This process ultimately contributes to the formation of ATP and NADPH, essential for the subsequent reactions in photosynthesis.
Reaction center: The reaction center is a crucial component in photosynthesis, particularly during the light reactions, where it serves as the site for converting light energy into chemical energy. It consists of specialized pigments and proteins that capture photons and initiate the process of electron transfer, ultimately leading to the generation of ATP and NADPH. This center plays a key role in facilitating the energy conversion necessary for sustaining life through photosynthesis.
Stroma lamellae: Stroma lamellae are membranous structures in chloroplasts that connect thylakoids and provide structural support to the photosynthetic machinery. They play a crucial role in the light reactions by facilitating the organization of thylakoids, which house the chlorophyll and other pigments necessary for capturing light energy.
Thylakoid membrane: The thylakoid membrane is a highly specialized structure found within chloroplasts, primarily involved in the light-dependent reactions of photosynthesis. It is organized into flattened sacs called thylakoids, which are stacked to form structures known as grana. This membrane contains essential proteins, pigments like chlorophyll, and electron transport chains that facilitate the conversion of light energy into chemical energy.
Z-scheme: The z-scheme is a model that describes the flow of electrons during the light-dependent reactions of photosynthesis, illustrating how energy from sunlight is converted into chemical energy. This scheme shows the sequence of electron transfers through two photosystems, PSII and PSI, highlighting the energy changes as electrons are energized by light, leading to the production of ATP and NADPH.