Cellular Respiration - Atoms, Electrons Lecture Notes

Cellular Respiration Atoms  Atoms are joined into molecules by covalent bonds. Each covalent bond is formed when electrons are shared.  In a carbon-hydrogen bond, the pair of electrons is more or less equally shared between the two nuclei.  In contrast, the shared electrons in an oxygen-hydrogen bond are not equally shared—the shared electrons are more strongly attracted to the oxygen nucleus.  This is very important to understanding how energy can be released when molecules are rearranged. Electrons          Electrons held more tightly to a nucleus are at a lower energy state. Electrons not held tightly to a nucleus are at a higher energy state. So, in a carbon-hydrogen bond the shared electrons are in a higher energy state than the shared electrons in an oxygen-hydrogen bond. When a carbon-hydrogen bond is broken and the hydrogen is combined with oxygen, the electrons essentially “fall” from a higher-energy state to a lowerenergy state. Since energy is never destroyed, the difference in energy is released. The potential chemical energy in food compounds is in bonds such as the C-H and C-C bonds found in glucose. This chemical energy can be released when these bonds are broken and reformed to produce lower-energy bonds such as O-H bonds. energy is given off as the bonds of the molecule are rearranged from highenergy bonds to lower-energy bonds. Cellular respiration is the metabolic process by which cells harvest the energy of food molecules and store it in molecules of ATP. ATP is the cell’s “fuel” to spend now. In aerobic cellular respiration, electrons move from the chemical bonds of organic molecules (food) to oxygen. The energy of moving electrons is captured and used to make ATP in three main modules:    Glycolysis Citric acid (Krebs) cycle Electron transport chain Biochemical pathways are series of chemical reactions. Each step involves only a small change but the process as a whole produces major change. One of the greatest secrets, how cells use food as a source of energy, has become reasonably well understood only recently. Redox Reaction   removing an electron is oxidation o oxidized adding an electron causes reduction (in charge) o reduced Electron Carrier o 2 states: carrying electrons and not Summary of Aerobic Respiration • Food is broken down into carbon dioxide and water • Much of the energy released is used for the formation of ATP. Glucose + Oxygen ------> Carbon Dioxide + Water + Energy (ATP and Heat) All aerobic organisms do this (e.g., plants, animals, fungi, protists, some bacteria). o o We inhale, we bring in oxygen for this process. We exhale we expel the CO2 that is formed as a by-product. Glycolysis - does not require oxygen outside of mitochondria What Happens? Glucose (six-carbon) is broken into two three-carbon molecules (pyruvate) How? Enzymes catalyze the small sequential steps (10 steps) What results from glycolysis? o o o Pyruvate (three-carbon) Two ATP produced per glucose molecule Electrons are loaded on to electron carriers: two NADH are made per glucose molecule At the end of glycolysis most of the energy originally stored in glucose is still present in the pyruvate molecules. Please notice, only two ATP and only two NADH are made per glucose molecule catabolized. Glycolysis o o Ancient process: evolved before appearance of atmospheric oxygen and before the evolution of cellular organelles. Takes place in virtually all living cells o NAD+ (electron acceptor) required Metabolic Fork “In The Road” o o In the presence of oxygen the products of glycolysis go on to the citric acid (Krebs) cycle and the electron transport chain. If oxygen is not present fermentation takes place. Citric Acid (Krebs) cycle: a biochemical cycle Where? inside the mitochondrion's inner space (matrix) What Happens? in a series of chemical reactions pyruvate from glycolysis is broken down to carbon dioxide How? enzymes catalyze the small sequential steps What results from the Citric Acid (Krebs) cycle? o o o ATP is produced (2) Electrons are loaded on to electron carriers (high yield) CO2 is given off (e.g., in our breath) Electron Transport Chain oxygen is required Where? in the inner membrane of mitochondria What is it? A collection of molecules, mostly proteins; a series of electron carriers What Happens? a) The electron carriers from glycolysis and the Krebs cycle (mostly NADH) donate their electrons to the electron transport chain b) ) The passage of electrons through the electron transport chain is an energy releasing process, and this energy is used to produce a proton gradient. a. Protons build up between the inner and outer membranes of the mitochondria. b. At the end of the electron transport chain oxygen receives the electrons. c. This (reduced) oxygen reacts immediately with hydrogen ions to form water. c) Protons move down their concentration gradient through ATP synthase, a membrane protein, into the inner compartment (matrix) of the mitochondrion. a. Passage of protons through ATP synthase drives the formation of ATP (from ADP plus phosphate). b. High yield of ATP. Summary: Electrons travel the “downhill” route: food --> NADH --> electron transport chain --> Oxygen What does Aerobic Respiration produce? o o With abundant oxygen, thirty something molecules of ATP per glucose molecule catabolized. Fermentation is a form of anaerobic (no oxygen) respiration o Fermentation is glycolysis plus a few reactions that regenerate “emptyhanded” electron carriers (NAD+). And this allows glycolysis to keep going; glycolysis would stop without NAD+ o Fermentation allows glycolysis to continue by providing a continuous supply of NAD+ o Glycolysis is the first process of aerobic cellular respiration, but it is also the first process of fermentation. o Pyruvate produced by glycolysis does not enter the citric acid (Krebs) cycle, but instead undergoes one or two chemical steps to become ethanol or lactic acid. o Only two ATP are produced per glucose molecule consumed. The ATP produced are from glycolysis only. Review: NAD+ receives electrons (is reduced) during glycolysis Fermentation’s Economic Importance Alcohol and CO2 o o o o Yeast (a fungus) consume sugar to make their own ATP and give off carbon dioxide and ethanol as by-products. Wine is fermented fruit juice Beer - barley, rice, corn (chicha) fruits CO2 produced during fermentation makes bread rise (ethanol evaporates while the bread bakes). Lactic Acid Fermentation • In our muscle cells, when insufficient oxygen is present, pyruvate is converted (reduced) to lactic acid. Sauerkraut is made from cabbage. Bacteria do lactic acid fermentation (to make ATP for themselves) if you exclude oxygen. *Note that there is a much greater yield of ATP from aerobic respiration than from fermentation o o Proteins, Carbohydrates, Fats are all broken down to make ATP