C2 - Photosynthesis Stores Energy in Organic Compounds (Updated 2024) PDF

C2 - Photosynthesis Stores Energy in Organic Compounds I can… Explain, in general terms, how energy is absorbed by pigments, transferred through the reduction of NADP to NADPH, and then transferred as chemical potential energy to ATP by chemiosmosis Explain how the products of l...

C2 - Photosynthesis Stores Energy in Organic Compounds I can… Explain, in general terms, how energy is absorbed by pigments, transferred through the reduction of NADP to NADPH, and then transferred as chemical potential energy to ATP by chemiosmosis Explain how the products of light-dependent reactions are used to reduce carbon in light independent reactions to produce glucose Describe where in the chloroplast these processes occur Recall: Photosynthesis 6 CO2 + 6 H2O + light energy → 6 O2 + C6H12O6 Photosynthesis takes place in the chloroplasts of plants Photosynthesis combines carbon dioxide, water, and light energy to generate glucose and oxygen This process, however, consists of two sets of reactions: light-dependent and light-independent Overview of Reactions 1. Light-Dependent Reactions Generate the high-energy molecules (ATP & NADPH) that are required to power the light-independent reactions Also release O2 as a by-product 2. Light-Independent Reactions Serve to transform high-energy molecules into G3P, which is used to create glucose The second set of reactions in photosynthesis is the Calvin Cycle Overview of Reactions Overview of Reactions In the light-dependent reactions, solar energy is trapped by chlorophyll molecules and used to generate two high-energy compounds, ATP and NADPH In the light-independent reactions, the energy of ATP and the reducing power of NADPH are used to reduce carbon dioxide and make glucose Terminology Light-dependent reactions: First set of reactions in photosynthesis ○ Light energy excites electrons in chlorophyll molecules ○ Powers chemiosmotic ATP synthesis ○ Results in reduction of NADP+ to NADPH Carbon fixation: Process of incorporating CO2 into carbohydrate molecules Calvin Cycle: Cyclic set of reactions, occur in stroma of chloroplasts ○ Fixes carbon of CO2 into carbohydrate molecules ○ Also known as… Light-independent reactions: Second set of reactions in photosynthesis ○ Do not require solar energy Light-Dependent Reactions Light-Dependent Reactions The first step of photosynthesis is the light-dependent reactions. A These reactions require energy from the sun to take place. We can break the reactions down into two categories: ○ Those which involve a membrane protein called photosystem II (PSII) ○ And those which involve a membrane protein called photosystem I (PSI). The activities of PSII use a process called chemiosmosis to produce ATP Whereas the activities of PSI are used to generate a compound called NADPH Light-Dependent Reactions Various pigment molecules produce free electrons when light hits them Free electrons are passed along to the reaction centre (specialized chlorophyll in molecule) When electron is in reaction centre, it gets excited by the addition of energy and goes to electron acceptor molecules ○ This reduces the electron-acceptor and puts it at a high-energy level Steps After Energized Electron Reaches Electron Acceptor 1. Electron leaves reaction center of PSII and joins electron acceptor Leaves an “electron hole” in PSII Electron must be replaced before PSII can absorb more light energy Source of electron is a water molecule Enzymes break down water molecule - releases H+ ions, electrons, and oxygen **This step in photosynthesis produces oxygen gas **PSII comes FIRST Steps After Energized Electron Reaches Electron Acceptor 2. Electron acceptor transfers energized electrons to series of electron-carrying molecules (electron transport system) As electrons moves through system, it loses energy Lost energy from electrons are used to push H+ ions across stroma and thylakoid membrane and into thylakoid space (area inside thylakoid) - produces concentration gradient Now, there are more H+ ions in thylakoid space than in stroma Concentration gradient serves as source of potential energy to generate ATP from ADP This process is called chemiosmosis Chemiosmosis Energy from electrons in PSII is used to produce ATP indirectly Energy of electrons is used to push H+ ions against concentration gradient into thylakoid space ○ H+ ions cannot diffuse back across membrane because its impermeable to charged particles Chemiosmosis: Linking the movement of H+ ions to the production of ATP Steps that occur: 1. To return to stroma, H+ ions must move through structure known as ATP synthase which provide the only pathway for H+ ions to move down their concentration gradient 2. ATP synthase uses movement of H+ ions to run a mechanism that bonds together ADP and free phosphates to form ATP Steps After Energized Electron Reaches Electron Acceptor 3. While step 1&2 are taking place, PSI is absorbing light Energy is transferred to reaction centre when electron is excited and is passed to high-energy electron acceptor Electron lost from PSI is replaced by electron arriving through electron transport system from PSII Steps After Energized Electron Reaches Electron Acceptor 4. Excited electron from PSI is used to reduce NADP+ (Nicotinamide Adenine Dinucleotide Phosphate) to form NADPH NADPH’s reducing power is then used later in a light-independent reaction Summary Light reactions use solar power of photons absorbed by both PSI and PSII to provide chemical energy ○ Takes place in thylakoid membranes of chloroplast End result of light-dependent reactions is ATP and NADPH Light-Independent Reactions Light-Independent Reactions The energy from ATP and NADPH can be used to synthesize glucose This process, also known as the Calvin Cycle, may occur in the absence of light because it does not rely on the excitation of electrons to initiate any reactions However, it does usually take place during the day time. Light-Independent Reactions Involves three essential steps: 1. Carbon fixation 2. Reduction 3. Regeneration of RuBP 1. Carbon Fixation 3 CO2 molecules are required to initiate the cycle Each CO2 binds to a molecule of RuBP in the stroma with the help of an enzyme called rubisco This creates an unstable 6-carbon compound, which immediately breaks down into two 3-carbon compounds called 3-PGA End result is 6 molecules of 3-PGA 3 CO2 + 3 RuBP → 3 unstable C6 → 6 3-PGA 2. Reduction Each 3-PGA molecule binds to a molecule of ATP This causes each 3-PGA molecule to become “activated” (gains energy) The 6 activated 3-PGA molecules each bind to a molecule of NADPH, creating a new compound called G3P Two G3P molecules can combine to form one glucose molecule End result is 6 molecules of G3P 6 3-PGA + 6 ATP → 6 activated 3-PGA + 6 NADPH → 6 G3P 3. Regeneration of RuBP Of the 6 molecules of G3P produced in step 2, 5 are required to replace RuBP Replacement of RuBP also requires an additional 3 ATP molecules The cycle needs to repeat to produce 1 full glucose molecule! 6 G3P + 3 ATP → 3 RuBP + ½ Glucose Video https://www.youtube.com/watch?v=0UzMaoaXKaM https://www.youtube.com/watch?v=sQK3Yr4Sc_k