What are the 7 steps of the light dependent reactions ?
The Correct answer and Explanation is:
The light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts and convert light energy into chemical energy (ATP and NADPH), which are then used in the Calvin cycle. The process involves several steps, which can be outlined as follows:
7 Steps of Light-Dependent Reactions:
- Photon absorption by Photosystem II (PSII):
- Light photons strike chlorophyll in PSII, exciting electrons to a higher energy level. This energy is transferred to the reaction center, where the electron is ejected.
- Photolysis of water (Water splitting):
- PSII extracts electrons from water molecules to replace the ones lost. This reaction splits water into oxygen (O₂), protons (H⁺), and electrons (e⁻). Oxygen is released as a byproduct.
- Electron transport from PSII to the cytochrome complex:
- The excited electron is passed through the electron transport chain (ETC), which includes proteins like plastoquinone (Pq) and cytochrome b6f complex. As electrons move through the chain, energy is used to pump protons into the thylakoid lumen, creating a proton gradient.
- Proton gradient formation:
- The electron transport leads to the buildup of H⁺ ions inside the thylakoid lumen, creating an electrochemical gradient, crucial for ATP synthesis.
- Photon absorption by Photosystem I (PSI):
- Electrons lose energy as they move through the ETC and reach Photosystem I. Here, light is again absorbed, re-energizing the electrons.
- Electron transport from PSI to NADP⁺ reductase:
- Re-energized electrons are passed through a second electron transport chain, involving ferredoxin (Fd), and finally reach the enzyme NADP⁺ reductase, which uses the electrons to convert NADP⁺ into NADPH.
- ATP synthesis (Photophosphorylation):
- The proton gradient drives the enzyme ATP synthase, which facilitates the synthesis of ATP from ADP and inorganic phosphate. This process is known as chemiosmosis.
Explanation:
Light-dependent reactions are the initial phase of photosynthesis where sunlight is harnessed to produce energy molecules, ATP and NADPH, essential for the Calvin cycle. These reactions occur in the thylakoid membranes, where photosystems (PSII and PSI) play key roles in absorbing light and exciting electrons. The energy from these electrons is used to generate a proton gradient across the membrane, which then powers ATP synthase to produce ATP. Simultaneously, electrons are transferred to NADP⁺ to form NADPH. Both ATP and NADPH are essential energy carriers that fuel the synthesis of glucose in the later stage of photosynthesis. Importantly, oxygen is produced as a byproduct during the photolysis of water, contributing to the Earth’s oxygen supply.