SHS 1024 Cell Biology and Genetics Aerobic Respiration and Mitochondria PDF

SHS 1024 Cell biology and Genetics Aerobic respiration and Mitochondria Mitochondria The mitochondria, are called the “powerhouses” of the cell. Without them, cells would be unable to extract enough energy from the nutrients,...

SHS 1024 Cell biology and Genetics Aerobic respiration and Mitochondria Mitochondria The mitochondria, are called the “powerhouses” of the cell. Without them, cells would be unable to extract enough energy from the nutrients, and essentially all cellular functions would cease. Mitochondria are present in all areas of each eukaryotic cell’s cytoplasm, but the total number per cell varies from less than a hundred up to several thousand, depending on the amount of energy required by the cell. Further, the mitochondria are concentrated in those portions of the cell that are responsible for the major share of its energy metabolism. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics  They are also variable in size and shape. Some are only a few hundred nanometers in diameter and globular in shape, whereas others are elongated—as large as 1 micrometer in diameter and 7 micrometers long. The basic structure of the mitochondrion, is composed mainly of two lipid bilayer– protein membranes: an outer membrane and an inner membrane. Many infoldings of the inner membrane form shelves onto which oxidative enzymes are attached. In addition, the inner cavity of the mitochondrion is filled with a matrix that contains large quantities of dissolved enzymes that are necessary for extracting energy from nutrients. These enzymes operate in association with the oxidative enzymes on the shelves to cause oxidation of the nutrients, thereby forming carbon dioxide and water and at the same time releasing energy. The liberated energy is used to synthesize a “high-energy” substance called adenosine triphosphate (ATP). ATP is then transported out of the mitochondrion, and it diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions. Mitochondria are self-replicative, which means that one mitochondrion can form a second one, a third one, and so on, whenever there is a need in the cell for increased amounts of ATP. Indeed, the mitochondria contain DNA similar to that found in the cell nucleus. The DNA of the mitochondrion plays a similar role, controlling replication of the mitochondrion itself. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics Cellular Respiration Aerobic respiration consumes organic molecules and O2 and yields ATP Anaerobic respiration partial degradation of sugars that occurs without O2 Aerobic respiration o The breaking down of sugar to produce energy where oxygen is present. Glucose + Oxygen Carbon dioxide + water + energy o Stages of Harvesting of energy from glucose I. Glycolysis (breaks down glucose into two molecules of pyruvate) II. The citric acid cycle (TCA cycle, Krebs cycle) (completes the breakdown of glucose) III. ETC or Oxidative phosphorylation (accounts for most of the ATP synthesis Arnold Arulnesan MSc, BSc. Cell Biology and Genetics  Glycolysis The Glycolytic pathway describes the oxidation of glucose to pyruvate with the generation of ATP and NADH. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics  Glycolysis is a universal pathway; present in all organisms: from yeast to mammals. In eukaryotes, glycolysis takes place in the cytosol. It does not require oxygen. In the presence of O2, pyruvate is further oxidized to CO2. In the absence of O2, pyruvate can be fermented to lactate or ethanol. Net Reaction: Glucose + 2NAD+ + 2 Pi + 2 ADP 2 pyruvate + 2 ATP + 2 NADH + 2 H2O Stages of Glycolysis Stage 1- Is the investment stage. 2 mols of ATP are consumed for each mol of glucose Glucose is converted to fructose-1,6-bisphosphate. Glucose is trapped inside the cell and at the same time converted to an unstable form that can be readily cleaved into 3-carbon units. Stage 2 Fructose-1,6-bisphosphate is cleaved into glycerladehyde-3 phosphate. Stage 3 is the harvesting stage. 4 mols of ATP and 2 mols of NADH are gained from each initial mol of glucose. This ATP is a result of substrate-level phosphorylation. Glyceraldehyde-3-phosphate is oxidized to pyruvate. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics Transition Reaction Transition reaction connects glycolysis to the Krebs cycle. In this reaction, pyruvate is converted to a two-carbon acetyl group attached to coenzyme A. This redox reaction removes electrons from pyruvate by dehydrogenase using NAD+ as coenzyme. Reaction occurs twice for each original glucose molecule. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics The citric acid cycle (TCA cycle) TCA cycle reactions occur in matrix of mitochondria. Cycle is named for Sir Hans Krebs, who received Nobel Prize for identifying these reactions. Cycle begins by adding C2 acetyl group to C4 molecule, forming citrate; also called the citric acid cycle. The acetyl group is then oxidized to two molecules of CO2. During the oxidation process, most electrons (e-) are accepted by NAD+ and NADH is formed. In one instance, electrons are taken by FAD, forming FADH2. NADH and FADH2 carry these electrons to electron transport system. Some energy released is used to synthesize ATP by substrate-level phosphorylation, as in glycolysis. One high-energy metabolite accepts a phosphate group and passes it on to convert ADP to ATP. Krebs cycle turns twice for each original glucose molecule. Products of the Krebs cycle per glucose molecule include, 4 CO2, 2 ATP, 6 NADH and 2 FADH2 Arnold Arulnesan MSc, BSc. Cell Biology and Genetics The Electron Transport System Electron transport system is located in cristae of mitochondria o Consists of carriers that pass electrons. o Some protein carriers are cytochrome molecules. o Electrons that enter the electron transport system are carried by NADH and FADH2. o NADH gives up its electrons and becomes NAD+; next carrier gains electrons and is reduced. o At each sequential oxidation-reduction reaction, energy is released to form ATP molecules. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics  Oxygen serves as terminal electron acceptor and combines with hydrogen ions to form water. Because O2 must be present for system to work, it is also called oxidative phosphorylation. NADH delivers electrons to system; by the time electrons are received by O2, three ATP are formed. If FADH2 delivers electrons to system, by the time electrons are received by O2, two ATP are formed. Coenzymes and ATP recycle Cell needs a limited supply of coenzymes NAD+ and FAD because they constantly recycle. Once NADH delivers electrons to electron transport system, it is free to pick up more hydrogen. Components of ATP also recycle. Efficiency of recycling NAD+, FAD and ADP eliminates need to synthesize them a new. Electron transport system consists of three protein complexes and two protein mobile carriers that transport electrons between complexes. Energy released from flow of electrons down electron transport chain is used to pump H+ ions, carried by NADH and FADH2, into intermembrane space. Accumulation of H+ ions in this inter-membrane space creates a significant electrochemical gradient. ATP synthase complexes are channel proteins that also serve as enzymes for ATP synthesis. Arnold Arulnesan MSc, BSc. Cell Biology and Genetics  As H+ ions flow from high to low concentration, ATP synthase synthesizes ATP;actual mechanism is still unknown. "Chemi osmosis" term used since ATP production tied to electrochemical (H+) gradient across a membrane. Once formed, ATP molecules diffuse out of the mitochondrial matrix through channel proteins. Energy Yield From Glucose Breakdown Anaerobic respiration The oxidation of molecules in the absence of oxygen to produce energy Occurs, In muscle cells Yeast Arnold Arulnesan MSc, BSc. Cell Biology and Genetics  Two Types Lactic Acid Fermentation Alcoholic Fermentation Advantage and Disadvantage of Fermentation Despite low yield of two ATP molecules, fermentation provides quick burst of ATP energy for muscular activity. Disadvantage is that lactate is toxic to cells. When blood cannot remove all lactate from muscles, lactate change pH and causes muscles to fatigue. Recovery occurs after lactate is sent to liver, converted into pyruvate; then respired or converted into glucose. Prepared by- Thushara Balasuriya B.Sc in MLS (Special) Arnold Arulnesan MSc, BSc. Cell Biology and Genetics