Photosynthesis PDF

# ENERGY TRANSFORMATION ## Introduction # METABOLISM - It encompasses all of the chemical reactions in cells, including those that build new molecules and those that break down existing ones. - Each reaction rearranges atoms into new compounds, and each reaction either absorbs or releases energy...

# ENERGY TRANSFORMATION ## Introduction # METABOLISM - It encompasses all of the chemical reactions in cells, including those that build new molecules and those that break down existing ones. - Each reaction rearranges atoms into new compounds, and each reaction either absorbs or releases energy. - **Catabolism** is the set of metabolic pathways that breaks down molecules into smaller units that are either oxidized to release energy or used in other anabolic reactions. - **Anabolism** is the set of metabolic pathways that construct molecules from smaller units, these reactions require energy, known also as an endergonic process. # OXIDATION-REDUCTION REACTIONS - Most energy transformations in organisms occur in oxidation-reduction (“redox”) reactions, which transfer energized electrons from one molecule to another. # ELECTRON TRANSPORT CHAIN - Groups of proteins that are electron-shuttling “specialists” often align in membranes. - In an electron transport chain, each protein accepts an electron from the molecule before it and passes it to the next. - As a result, each protein in the chain is first reduced and then oxidized. - Small amounts of energy are released at each step, and the cell uses this energy in other reactions. # ATP: THE ENERGY CURRENCY OF CELLS - The covalent bonds of ATP (adenosine triphosphate) temporarily store much of the released energy. - Cells then use the energy in ATP to power reactions that require energy input. - Removing the endmost phosphate group of ATP yields ADP and a free phosphate group. The cell uses the released energy to do work. - Cells use ATP hydrolysis, a reaction that releases energy, to fuel reactions that require energy input. The cell regenerates ATP in other reactions, such as cellular respiration. ## COUPLED-REACTIONS A cell uses ATP as an energy source by transferring its phosphate group to another molecule. # ENZYMES SPEEDS UP REACTIONS - Enzymes speed reactions by lowering the activation energy, the amount of energy required to start a reaction. - The enzyme brings reactants (also called substrates) into contact with one another, so that less energy is required for the reaction to proceed. # ENERGY TRANSFORMATION ## Photosynthesis # PHOTOSYNTHESIS - It is the process by which plants, algae, and some bacteria harness solar energy and convert it into chemical energy. - It is a series of chemical reactions that use light energy to assemble carbon dioxide (CO2) into glucose (C6H12O6) and other carbohydrates. - The plant uses water (H2O) in the process and releases oxygen gas (O2) as a byproduct. ## STAGES OF PHOTOSYNTHESIS ### Stage 1: Light Reaction - Input: Light energy (photons), H2O - Output: NADPH and ATP, O2 - Location: Thylakoid membrane ### Stage 2: Light-independent Reaction (Calvin Cycle/Carbon Reactions) - Input: NADPH, CO2 - Output: Sugar (C6H12O6) - Location: Stroma ### Step 1:Excitation of Photosystems by Light Energy Photosystems are groups of photosynthetic pigments (including chlorophyll) embedded within the thylakoid membrane. - Photosystems are classed according to their maximal absorption wavelengths (PS I = 700 nm ; PS II = 680 nm). - When a photosystem absorbs light energy, delocalized electrons within the pigments become energized or excited. - These excited electrons are transferred to carrier molecules within the thylakoid membrane. ### Step 2:Production of ATP via an Electron Transport Chain - Excited electrons from Photosystem II (P680) are transferred to an electron transport chain within the thylakoid membrane. - The electrons are passed to plastoquinone (PQ). The b6-f complex receives the electrons that provides energy to translocate H+ions into the thylakoid. The electrons are then passed to plastocyanin (PC). - This build up of protons within the thylakoid creates an electrochemical gradient, or proton motive force. - The H+ions return to the stroma (along the proton gradient) via the transmembrane enzyme ATP synthase (chemiosmosis). - ATP synthase uses the passage of H+ions to catalyze the synthesis of ATP (from ADP + Pi). - This process is called photophosphorylation- as light provided the initial energy source for ATP production. - The newly de-energized electrons from Photosystem II are taken up by Photosystem I. ### Step 3: Reduction of NADP+ and the Photolysis of Water - Excited electrons from Photosystem I may be transferred to a carrier molecule and used to reduce NADP+. - This forms NADPH – which is needed (in conjunction with ATP) for the light independent reactions. - The electrons lost from Photosystem I are replaced by de-energized electrons from Photosystem II. - The electrons lost from Photosystem II are replaced by electrons released from water via photolysis - water is split by light energy into H+ions (used in chemiosmosis) and oxygen (released as a by-product). ## Photosynthetic Pigments That Capture Sunlight Sunlight reaching Earth consists of ultraviolet radiation, visible light, and infrared radiation, all of which are just a small part of a continuous spectrum of electromagnetic radiation. Photons with the shortest wavelengths carry the most energy. - **Chlorophyll** – plant cell's pigment molecule that captures light energy - **Chlorophyll a** - a green photosynthetic pigment in plants, algae, and cyanobacteria. - **Chlorophyll b and Carotenoids** – other photosynthetic pigments ## The Calvin cycle has three phases: ### Phase 1: Carbon fixation - The binding of CO2 to a five-carbon sugar named ribulose bisphosphate (RuBP) is catalyzed by RuBP carboxylase-oxygenase, or rubisco. - The six-carbon intermediate molecule is immediately split into two molecules of 3- phosphoglycerate (for each CO2 fixed). ### Phase 2: Reduction - Each molecule of 3-phosphoglycerate is altered through phosphorylation by six ATP and reduction by six NADPH to ultimately produce a G3P sugar. - For every three CO2 molecules that enter the cycle, six molecules of G3P are formed. - Only one of these can be counted as a net gain of carbohydrate. ### Phase 3: Regeneration of the CO2 acceptor (RuBP) - The remaining five molecules of G3P are rearranged in a complex series of reactions yielding three molecules of RuBP. - Three additional molecules of ATP are used to facilitate the regeneration of RuBP. # PHOTOSYNTHESIS ## LIGHT REACTIONS - Are carried out by molecules in the thylakoid membranes - Convert light energy to the chemical energy of ATP and NADPH - Split H2O and release O2 to the atmosphere ## CALVIN CYCLE REACTIONS - Take place in the stroma - Use ATP and NADPH to convert CO2 to the sugar G3P - Return ADP, inorganic phosphate, and NADP+ to the light reactions.