Short o vs Long o | Quizalize

PHOTOSYNTHESIS LIGHT DEPENDENT REACTION 1. Photosystem II (PSII) – Light Absorption & Water Splitting • Light energy (photons) excites electrons in chlorophyll molecules. • These high-energy electrons leave PSII and are passed into the electron transport chain (ETC). • Meanwhile, water molecules are split (photolysis) into: o O₂ (released as a by-product into the atmosphere) o H⁺ ions (protons, which build up inside the thylakoid) o Electrons (e⁻), which replace the ones lost by PSII. 2. Electron Transport Chain (ETC) • Excited electrons move through protein carriers embedded in the thylakoid membrane. • As they move, their energy pumps H⁺ ions into the thylakoid space, creating a proton gradient (high H⁺ inside, low outside). 3. ATP Production (ATP Synthase) • The buildup of H⁺ ions acts like a “waterfall” of potential energy. • These protons flow back across the membrane through ATP synthase, a protein complex that acts like a turbine. • This flow drives the conversion of ADP + Pi → ATP, which provides energy for the Calvin cycle. 4. Photosystem I (PSI) • Electrons arriving from the ETC enter PSI. • Sunlight excites them again, boosting them to a higher energy level. 5. NADPH Production • The energized electrons are transferred to NADP⁺. • Along with a proton (H⁺), this forms NADPH, another energy carrier. • NADPH is then delivered to the Calvin cycle to help build glucose. End Products of Light-Dependent Reactions: • ATP (energy source for Calvin cycle) • NADPH (reducing power for glucose synthesis) • O₂ (released into the atmosphere as waste) Light-Independent Reactions (Calvin Cycle) • These reactions do not directly require sunlight. • They occur in the stroma of the chloroplast (the fluid-filled space surrounding the thylakoids). • The inputs are ATP and NADPH (from light-dependent reactions) and CO₂ (from the atmosphere). • The outputs are glucose (C₆H₁₂O₆) and other carbohydrates. Think of the Calvin cycle as a factory that uses the energy and “raw materials” made in Stage I (ATP & NADPH) to build sugars. The 3 Main Steps of the Calvin Cycle 1. Carbon Fixation • CO₂ from the atmosphere enters the chloroplast and diffuses into the stroma. • Each CO₂ molecule attaches to a 5-carbon sugar called RuBP (ribulose-1,5-bisphosphate). • This reaction is catalyzed by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase — the most abundant enzyme on Earth!). • The result is a short-lived 6-carbon compound, which immediately splits into two 3-carbon molecules called 3-PGA (3-phosphoglycerate). Summary: CO₂ + RuBP → 2 × 3-PGA 2. Reduction Phase • The 3-PGA molecules are “energized” and converted into G3P (glyceraldehyde-3-phosphate), a more energy-rich 3-carbon sugar. • This transformation requires: o ATP (provides energy) o NADPH (provides high-energy electrons and hydrogen atoms). • Some of the G3P molecules will eventually be combined to form glucose and other sugars. Summary: 3-PGA + ATP + NADPH → G3P 3. Regeneration of RuBP • Not all G3P molecules leave the cycle. Most of them are used to regenerate RuBP so the cycle can continue. • This regeneration also requires ATP. • For every 3 turns of the cycle, 5 G3P molecules are recycled to regenerate 3 molecules of RuBP. Summary: G3P + ATP → RuBP The Full Cycle Balance • To make one G3P molecule that can exit the cycle (and later form glucose), the cycle must run 3 times, fixing 3 molecules of CO₂. • To make one glucose molecule (C₆H₁₂O₆), the cycle must run 6 times (since glucose needs 6 carbon atoms). Inputs (for 1 glucose): • 6 CO₂ • 18 ATP • 12 NADPH Outputs: • 1 glucose (C₆H₁₂O₆) • 18 ADP + 18 Pi • 12 NADP⁺ Day vs Night Clarification • The Calvin Cycle is called light-independent, but that doesn’t mean it only happens at night. • It usually happens during the day because it depends on ATP and NADPH, which are only produced in light-dependent reactions (when sunlight is available). Simplified Analogy • Carbon fixation = The factory brings in CO₂ as raw material. • Reduction = Workers use energy (ATP & NADPH) to shape the raw material into useful products (G3P). • Regeneration = Some products are recycled to keep the factory running (RuBP is re-formed). • Output = After enough cycles, the factory produces glucose, the “food” of the plant.

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