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Respiration Intro. Energy: required by all living systems for: SYNTHESIS, TRANSPORT & MOVEMENT; much of that energy is obtained via Cellular Respiration (CR) - The breakdown of glucose is exergonic, having a free energy change that has a –ΔG value = indicates that the products of the chemical reaction stores less energy than the reactants; (because energy was released to to cell). Remember… The more - ∆G, more work can be done. Chemical Energy: stored in chemical bonds - combustion: breaking of bonds to release energy - controlled combustion: when that energy is harnessed via coupled reactions. coupled reactions: energy from an exergonic reaction can drive an endergonic reaction. Cellular Respiration: process of converting food energy to ATP ATP: a usable form of energy (“energy currency”) ATP provides the cell with the free energy to drive endergonic reactions so exergonic reactions can take place… Exergonic reactions (from CR) provide the energy to regenerate ATP… - The release of energy during the hydrolysis of ATP comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves. Exergonic - The cell taps this energy source by using enzymes (kinases) to transfer phosphate groups from ATP to other compounds, which are said to be phosphorylated… phosphorylation: a is transferred to a molecule (increased E) MAKES IT MORE REACTIVE. - Release of P used to drive SYNTHESIS, TRANSPORT & MOVEMENT Two types of phosphorylation creating ATP: 1. Substrate-level phosphorylation = transfer of a from a substrate molecule (usually an intermediate sugar) to ADP making/recharging ATP. 2. Oxidative phoshorylation = a mode of ATP synthesis powered by redox reactions. In order to understand the process of making energy (ATP), we must review redox reactions: Oxidation-Reduction: REDOX reactions in which energy is transferred between molecules. Electron’s energy associated with stability: with highly electronegative atom (ex: O of O2): - more tightly held - more stable - less energy to be extracted - with less electronegative atom (like C-H bond in glucose): less tightly held less stable energy can be yielded if moved to lower energy state (from a more electronegative atom taking e- from C-H) Organic molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of electrons with high potential energy. They also have the the potential to “drop” the energy when they move closer to oxygen. The change in covalent status of electrons as hydrogen (and associated e-) is tranferred to oxygen is what liberates the energy. OXIDATION: - removal of electrons from a compound/atom - sometimes removing whole H atoms = (dehydrogenation) [e- + proton] - may also happen with addition of oxygen (inorganically = "rusting") releases energy lowers energy in compound which is oxidized - compound doing the oxidizing (the compound taking the removed e- (or removed H) = oxidizing agent; when it takes the e-, it becomes reduced. REDUCTION: - addition of electrons (reduces the amount of + charge of that atom.) - also, addition of whole H atoms - may also happen with the removal of oxygen requires energy raises energy in compound which is reduced - compound doing the reducing (the compound giving up the e-) = reducing agent; when it loses e- it becomes oxidized Oxidation-reduction always occurs together = redox rxns - if reduction requires energy, oxidation must supply it - reactions can be coupled to transfer energy -G = spontaneous = exergonic oxidation +G = not spontaneous (requires E from exergonic rxns) = endergonic = reduction Oxidation & reduction refer not to isolated substances but to changes occurring to a substance during a rxn. o compounds have lower energy after oxidization o so, if we can reduce them again they'll be "recharged" (capable of yielding electrons and energy again) Importance to CR = A redox reaction that relocates ecloser to oxygen releases chemical energy which can be put to work. In CR, e- “fall” from glucose to oxygen in a series of steps in creating ATP. During the combustion of glucose, glucose is oxidized and oxygen is reduced. Meanwhile, e- lose potential energy along the way. - So,… what are these compounds from where these e- “fall from and to” in the transfer of energy stored in glucose in forming ATP? - In other words, what compound carry these e- from glucose to the final e- acceptor (oxygen), in forming the final energy storage molecule = ATP? = Electron Carriers! Think of it as a game of “hot potato” as electrons move from one reaction to the next in each process of CR. Electron carriers in biological systems (the players in the game of hot potato): NAD+ & NADH - NAD+: nicotinamide adenine dinucleotide (oxidized form) - NADH: reduced NAD carries electrons (and Energy ) from glycolysis & citric acid cycle (CAC) to elctron trasport system/chain (ETS) oxidation of NADH will ultimately yield ATP at the end of CR (≈3 ATP / NADH) FAD & FADH2: carry e-'s from (one step) of CAC to ETS (≈2 ATP / FADH2) NADP+ & NADPH: involved in photosynthesis (more to come…) CR in a nut shell: Glycolysis Junction rxn Citric Acid Cycle (Kreb’s) Electron Transport Chain (system) [ETS] Chemiosmosis ATP