Question
G. reactions of photosynthesis that use energy from light to produce ATP and NADPH
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The reactions of photosynthesis that use energy from light to produce ATP and NADPH are known as the **light-dependent reactions** or the **photochemical phase** of photosynthesis. These occur in the thylakoid membranes of chloroplasts and can be broken down into several key steps:
### Key Steps of Light-Dependent Reactions:
1. **Photon Absorption**: Chlorophyll and other pigments absorb light energy, primarily at the blue and red wavelengths. This energy excites electrons in the chlorophyll molecules.
2. **Water Splitting (Photolysis)**: The absorbed light energy drives the splitting of water molecules (H₂O) into oxygen (O₂), protons (H⁺), and electrons (e⁻). This reaction is catalyzed by the enzyme photosystem II (PSII).
\[
2H₂O \rightarrow 4H⁺ + 4e^- + O₂
\]
3. **Electron Transport Chain (ETC)**: The excited electrons from chlorophyll are transferred to the electron transport chain. As the electrons move through a series of proteins (including plastoquinone, cytochrome b6f, and plastocyanin), they release energy that is used to pump protons into the thylakoid lumen, creating a proton gradient.
4. **ATP Synthesis**: The proton gradient created across the thylakoid membrane drives ATP synthesis through ATP synthase (a process known as **chemiosmosis**). As protons flow back into the stroma through ATP synthase, ATP is produced from ADP and inorganic phosphate (Pi).
\[
ADP + Pi \rightarrow ATP
\]
5. **NADPH Formation**: The electrons that move through the ETC eventually reach photosystem I (PSI). Here, they are re-excited by photons and ultimately used to reduce NADP⁺ (with the help of the enzyme NADP+ reductase) to form NADPH.
\[
NADP^+ + 2e^- + H^+ \rightarrow NADPH
\]
### Summary of Products:
- **ATP**: Provides energy for the subsequent light-independent reactions (Calvin cycle).
- **NADPH**: Serves as a reducing agent, providing the necessary electrons for the reduction of carbon dioxide in the Calvin cycle.
### Overall Equation for Light-Dependent Reactions:
The overall reaction that summarizes the light-dependent processes can be represented as:
\[
2H₂O + light \rightarrow O₂ + ATP + NADPH
\]
These products (ATP and NADPH) are essential for the subsequent light-independent reactions, where carbon fixation occurs to ultimately produce glucose and other carbohydrates.
### Key Steps of Light-Dependent Reactions:
1. **Photon Absorption**: Chlorophyll and other pigments absorb light energy, primarily at the blue and red wavelengths. This energy excites electrons in the chlorophyll molecules.
2. **Water Splitting (Photolysis)**: The absorbed light energy drives the splitting of water molecules (H₂O) into oxygen (O₂), protons (H⁺), and electrons (e⁻). This reaction is catalyzed by the enzyme photosystem II (PSII).
\[
2H₂O \rightarrow 4H⁺ + 4e^- + O₂
\]
3. **Electron Transport Chain (ETC)**: The excited electrons from chlorophyll are transferred to the electron transport chain. As the electrons move through a series of proteins (including plastoquinone, cytochrome b6f, and plastocyanin), they release energy that is used to pump protons into the thylakoid lumen, creating a proton gradient.
4. **ATP Synthesis**: The proton gradient created across the thylakoid membrane drives ATP synthesis through ATP synthase (a process known as **chemiosmosis**). As protons flow back into the stroma through ATP synthase, ATP is produced from ADP and inorganic phosphate (Pi).
\[
ADP + Pi \rightarrow ATP
\]
5. **NADPH Formation**: The electrons that move through the ETC eventually reach photosystem I (PSI). Here, they are re-excited by photons and ultimately used to reduce NADP⁺ (with the help of the enzyme NADP+ reductase) to form NADPH.
\[
NADP^+ + 2e^- + H^+ \rightarrow NADPH
\]
### Summary of Products:
- **ATP**: Provides energy for the subsequent light-independent reactions (Calvin cycle).
- **NADPH**: Serves as a reducing agent, providing the necessary electrons for the reduction of carbon dioxide in the Calvin cycle.
### Overall Equation for Light-Dependent Reactions:
The overall reaction that summarizes the light-dependent processes can be represented as:
\[
2H₂O + light \rightarrow O₂ + ATP + NADPH
\]
These products (ATP and NADPH) are essential for the subsequent light-independent reactions, where carbon fixation occurs to ultimately produce glucose and other carbohydrates.
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