CELLULAR RESPIRATION Harvesting Chemical Energy Producers and Consumers • A tiny fraction of a tiny fraction • Light powers life • Light energy from sun powers the chemical process that makes organic molecules • Called photosynthesis Producers and Consumers cont. • Autotrophs – plants and other self-feeders who can make all their organic matter • Plant nutrients are all inorganic • Heterotrophs – organisms that cannot make organic molecules from inorganic substances Producers and Consumers • Producers – plants and other autotrophs • Consumers – heterotrophs who obtain food by eating plants or eating animals that have eaten plants Chemical Cycling • Substrates for photosynthesis are carbon dioxide and water • Products of cellular respiration are carbon dioxide and water • Substrates of cellular respiration are the products of photosynthesis Cellular Respiration Aerobic Harvest of Food Energy • A cell requires oxygen to break down its fuel • So cellular respiration is an aerobic process • Respiration requires a cell to exchange 2 gases – O2 and CO2- with the surroundings • Breathing requires body to exchange 2 gases with outside Equation – cellular respiration • Glucose – main fuel for respiration • Oxygen required • Products are carbon dioxide, water and energy – ATP Role of Oxygen in cellular respiration • Cellular respiration transfers electrons from sugar to oxygen forming water • Also transfers hydrogen ions from glucose to oxygen forming water Redox Reactions • Oxidation – reduction reactions transfer electrons from 1 substance to another • Redox reactions • Oxidation – loss of electrons • Reduction – acceptance of electrons Redox Reactions cont. • Glucose is oxidized during cellular respiration losing electrons to oxygen • Oxygen is reduced during cellular respiration accepting electrons and hydrogen from glucose Redox Reactions cont. • When hydrogen and its bonding electrons change partners from sugar to oxygen energy is released • Cellular respiration unlocks chemical energy in small steps Cellular Respiration • 3 stages of respiration occur in cells • Glycolysis, Krebs cycle, electron transport chain Glycolysis • Occurs in cytoplasm • 6 carbon glucose split into 2 molecules of 3 carbon pyruvic acid • Must use 2 molecules of ATP per glucose but form 4 molecules of ATP so have net gain of 2 ATP Glycolysis • The energy in the high energy electrons from the broken carbon bonds is used to reduce NAD+ to NADH Glycolysis cont. • Most of energy in glucose still in pyruvic acid and this energy is harvested in the Krebs cycle Krebs cycle • Pyruvic acid must be modified before entering the Krebs cycle • Krebs cycle uses a 2 carbon compound called acetic acid • Enters the cycle as acetylCoA • Occurs in mitochondria Krebs cycle cont. • Pyruvic acid changed to acetic acid by removing 1 carbon atom • More NAD+ reduced to NADH as acetic acid is attached to Coenzyme A to make acetyl-CoA Krebs Cycle cont. • 2 carbon from pyruvic acid now present in acetic acid attached to acetyl-CoA enters Krebs cycle • Both carbons are removed 1 at a time and carbon dioxide is formed Krebs Cycle cont. • Small amount of ATP is formed in Krebs cycle • Much more NAD+ is reduced to NADH • A 2nd electron carrier is formed FADH2 • Starting molecules are reformed Krebs Cycle cont. • At end of Krebs cycle all carbons in glucose are now in carbon dioxide • Energy from breaking of bonds in glucose molecule now in high energy electrons attached to NADH and FADH2 Electron Transport • In glycolysis and the Krebs cycle NAD+ was reduced to NADH • Now the NADH is oxidized back to NAD+ and the electrons are passed to a chain of proteins called the electron transport chain Electron Transport cont. • Energy in high energy electrons is not used directly to synthesize ATP • Energy used to pump hydrogen ions across the inner mitochondrial membrane Electron Transport cont. • Sets up a gradient of hydrogen ions across the inner mitochondrial membrane • This gradient has potential energy due to location behind the membrane Electron Transport cont. • Hydrogen ions cannot pass through the inner mitochondrial membrane directly as it is a lipid bilayer • A protein complex in the membrane called ATP synthase acts as a channel through the membrane Electron Transport cont. • Electrons move down the concentration gradient through the ATP synthase channel and the energy is used to make lots of ATP • Oxygen is the final electron acceptor at the end of the electron transport chain • Water is formed when oxygen accepts electrons and hydrogen ions Cellular Respiration - Summary • 3 stages • Glycolysis in cytoplasm starting with glucose and ending with pyruvic acid little ATP but some NADH Cellular Respiration - Summary • Krebs Cycle – in mitochondria • Pyruvic acid carbons used to form carbon dioxide and the high energy electrons in the bonds are used to reduce NAD+ to NADH Cellular Respiration - Summary • NADH and FADH2 are oxidized in the mitochondria back to NAD+ and FAD and the electrons are transferred to a series of proteins which are reduced and oxidized • The energy in the high energy electron bonds is removed in small steps Cellular Respiration - Summary • The energy is used to form a hydrogen ion gradient • The hydrogen ions can only go down their concentration gradient through ATP synthase • Many ATP are formed as hydrogen ions pass through Cellular Respiration - Versatility • Cellular respiration can use a variety of food molecules • Each type of food molecule enters the 3 stages of cellular respiration at a particular point. Fermentation Human muscle and microorganisms • When muscle cells have insufficient oxygen they still make some ATP by glycolysis • Pyruvic acid is converted to lactic acid so NADH can be oxidized back to NAD+ Fermentation - Microorganisms • Microorganisms produce ATP in the absence of oxygen – process produces carbon dioxide and ethyl alcohol • NADH is reduced then oxidized