BP 09/30/25 I. Light-Dependent Reactions The light-dependent reactions convert light energy into chemical energy in the form of ATP and NADPH, and produce oxygen as a byproduct. They occur in the thylakoid membranes of chloroplasts. A. Photosystems Definition: Complexes of photosynthetic pigments (e.g., chlorophyll, carotenoids, phycobilins) and special proteins embedded in the thylakoid membrane. Components: Antenna Complex: Light-harvesting pigments (represented as discs) that absorb light energy and transfer it to the reaction center. Reaction Center: Contains special chlorophyll molecules where the actual conversion of light energy to chemical energy begins. Energy Transfer: Light energy is absorbed by pigments in the antenna complex. Energy is transferred from pigment to pigment until it reaches the special chlorophyll molecules in the reaction center. These chlorophyll molecules become excited and release an electron. Electron Excitation: The electron gains significant energy, moving to a higher orbital level. This represents a form of chemical energy. B. Photosystem II (PSII) / P680 Function: Initiates the electron transport chain, absorbing light most efficiently at a wavelength of 680 nm. Water Splitting (Hydrolysis): When PSII loses an electron, it must be replenished. PSII enzymes split water molecules (H₂O) into: Electrons (e⁻): Replenish those lost by chlorophyll. Hydrogen ions (H⁺) / Protons: Released into the thylakoid lumen. Oxygen (O₂): Released as a waste product. Oxygen Production: Oxygen gas (O₂) is a direct product of water splitting in PSII. Isotopic Tracing: Experiments using heavy oxygen isotopes (e.g., O-18 in water) confirmed that the oxygen produced in photosynthesis comes from water, not carbon dioxide. C. Electron Transport Chain (ETC) Definition: A series of protein complexes embedded in the thylakoid membrane that transfer electrons from PSII to PSI. Electron Movement: The high-energy electron from PSII is passed along the ETC proteins. As the electron moves, it gradually loses energy. This energy loss is likened to an "energetic roller coaster" where the electron's energy decreases. Proton Pumping: The energy released by the electron as it moves through the ETC is used to pump hydrogen ions (H⁺) from the stroma (outside the thylakoid) into the thylakoid lumen (inside the thylakoid). This process contributes to building a high concentration of H⁺ inside the thylakoid lumen. D. Photosystem I (PSI) / P700 Function: Re-energizes electrons after they have passed through the ETC, absorbing light most efficiently at a wavelength of 700 nm. Discovery Order: PSI was discovered before PSII, hence the numbering, even though PSII acts first in the electron flow. Electron Re-energization: The electron, having lost energy in the ETC, receives another boost of light energy at PSI, increasing its energy level again. NADPH Production: The re-energized electron from PSI is passed to a final electron acceptor, NADP⁺. NADP⁺, along with the electron and a hydrogen ion (H⁺), is reduced to NADPH. NADPH: A crucial electron carrier molecule that "holds on" to high-energy electrons, which will be used in the subsequent carbon fixation stage. E. ATP Production (Photophosphorylation) Proton Gradient: The splitting of water in PSII releases H⁺ into the thylakoid lumen. The ETC pumps additional H⁺ from the stroma into the thylakoid lumen. This creates a high concentration of H⁺ inside the thylakoid lumen and a lower concentration in the stroma, establishing a proton gradient. This concentration difference across the membrane represents a form of potential energy. ATP Synthase: H⁺ ions naturally tend to move from an area of high concentration to low concentration to achieve equilibrium (homeostasis). They flow out of the thylakoid lumen, through a specialized channel protein complex called ATP synthase. The flow of H⁺ causes ATP synthase to rotate. This rotation provides the energy to catalyze the phosphorylation of ADP (adenosine diphosphate) by adding a phosphate group (Pᵢ) to form ATP (adenosine triphosphate). ATP: The primary energy currency of the cell. F. Products of Light-Dependent Reactions ATP: Chemical energy source. NADPH: Electron carrier for reduction reactions. O₂: Oxygen gas (waste product). II. Light-Independent Reactions (Calvin Cycle) The light-independent reactions (Calvin Cycle) use the ATP and NADPH produced during the light-dependent reactions to convert inorganic carbon dioxide into organic carbohydrates. They occur in the stroma of the chloroplast. A. Overview Other Names: Dark reactions, carbon fixation reactions, Calvin cycle. Location: Stroma of the chloroplast. Purpose: To fix inorganic carbon (CO₂) from the atmosphere into organic carbon compounds (e.g., glucose, other carbohydrates). Dependence on Light Reactions: Although called "light-independent," these reactions indirectly rely on light because they require the ATP and NADPH generated by the light-dependent reactions. Without light, the production of ATP and NADPH ceases, and the Calvin cycle eventually shuts down. Carbon Dioxide Intake: Plants take in CO₂ from the atmosphere through small pores on their leaves called stomata. In environments with higher CO₂ levels, plants may develop fewer stomata. B. Stages of the Calvin Cycle The Calvin cycle can be divided into three main stages: 1. Carbon Fixation Definition: The process of attaching inorganic carbon (CO₂) to an existing organic molecule. Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase). RuBisCO is an extremely important and abundant enzyme, responsible for capturing atmospheric CO₂ for nearly all life on Earth. Process: One molecule of CO₂ is attached to a 5-carbon sugar called Ribulose-1,5-bisphosphate (RuBP). This forms an unstable 6-carbon intermediate molecule. The 6-carbon molecule immediately splits into two molecules of 3-phosphoglycerate (a 3-carbon compound). 2. Reduction Purpose: To convert the 3-phosphoglycerate molecules into higher-energy 3-carbon sugars. Energy Input: This stage consumes ATP and NADPH from the light reactions. ATP donates a phosphate group. NADPH donates electrons. Process: Each 3-phosphoglycerate molecule is phosphorylated by ATP. Then, it is reduced by NADPH. This produces Glyceraldehyde-3-phosphate (G3P or PGAL), a 3-carbon sugar. G3P Significance: G3P is the direct product of the Calvin cycle. It is a versatile precursor molecule. Two molecules of G3P can be combined to form one molecule of glucose (a 6-carbon sugar). G3P can also be used to synthesize other organic molecules needed by the plant, such as amino acids and fatty acids. 3. Regeneration Purpose: To regenerate the starting molecule, RuBP, so the cycle can continue. Energy Input: This stage consumes additional ATP. Process: Most of the G3P molecules produced are used to regenerate RuBP. Through a series of complex rearrangements of carbon bonds, the 3-carbon G3P molecules are converted back into 5-carbon RuBP molecules. This ensures that the cycle can continue to fix more CO₂. C. Glucose Production and Cycle Turns Net Gain: For every 6 turns of the Calvin cycle, one molecule of glucose (C₆H₁₂O₆) is produced. Each turn fixes one molecule of CO₂. Glucose has 6 carbon atoms, so 6 CO₂ molecules must be fixed. Thus, 6 turns are required for a net gain of one glucose molecule. Overall Goal: The ultimate goal of the Calvin cycle is to produce G3P, which can then be used to synthesize glucose and other essential organic compounds for the plant. III. Summary of Photosynthesis Light-Dependent Reactions: Location: Thylakoid membranes. Inputs: Light energy, H₂O, ADP, NADP⁺. Outputs: O₂ (waste), ATP, NADPH. Light-Independent Reactions (Calvin Cycle): Location: Stroma. Inputs: CO₂, ATP, NADPH. Outputs: G3P (used to make glucose and other carbohydrates), ADP, NADP⁺.