Cellular Respiration Chapter 9

Chemical Pathways Food serves as the source of energy for cells. Stores a lot of energy 1 gram of glucose releases 3811 calories of heat energy when burned in the presence of oxygen calorie: amount of energy needed to raise 1 gram of water 1 degree Celsius

Chemical Pathways Cells don’t burn glucose or other food compounds – they gradually release energy Cellular respiration: the process that releases energy by breaking down glucose and other food molecules in the presence of oxygen Equation: (you must memorize) 6O2 + C6H12O6  6CO2 + 6H2O + energy

Chemical Pathways There are three main stages of cellular respiration: 1st – Glycolysis 2nd – Krebs Cycle 3rd – Electron Transport Chain (ETC) When there is no oxygen present, glycolysis is followed by fermentation.

Cellular Respiration

Glycolysis The process in which one molecule of glucose is broken in half, producing 2 molecules of pyruvic acid (a 3-Carbon compound) Takes place within the cytoplasm ATP In glycolysis, the cell has to put in 2 ATP The cell then releases 4 ATP at the end of glycolysis This results in a net gain of 2 ATP Also produces 2 NADH by adding a pair of high-energy electrons to NAD+

Glycolysis

Fermentation Occurs after glycolysis in the absence of oxygen Fermentation releases energy from food molecules by producing ATP Anaerobic – absence of oxygen is required Cells convert NADH back into the electron carrier NAD+, which is needed for glycolysis This allows glycolysis to continue producing a steady supply of ATP

Fermentation 2 types of fermentation Alcoholic Fermentation Yeasts and some other microorganisms Pyruvic acid + NADH  alcohol + CO2 + NAD+ Lactic Acid Fermentation Occurs in muscles during rapid exercise Pyruvic acid + NADH  lactic acid + NAD+

Krebs Cycle When oxygen is available, glycolysis is followed by Krebs and electron transport  this makes up cellular respiration Aerobic – requires oxygen In eukaryotes, Krebs takes place inside the mitochondria Pyruvic acid is broken down into carbon dioxide in a series of energy-extracting reactions

Krebs Cycle Begins when pyruvic acid (from glycolysis) enters the mitochondria One carbon atom from pyruvic acid becomes part of a carbon dioxide molecule, which is eventually released into the air. The other 2 carbon atoms from pyruvic acid are used in a series of reactions. During these reactions, 2 energy carriers accept high-energy electrons  NAD+ is turned into NADH and FAD is turned into FADH2 NADH and FADH2 carry high-energy electrons to ETC

Krebs

Electron Transport Chain (ETC) Uses the high-energy electrons from Krebs to convert ADP into ATP In eukaryotes, it is composed of a series of carrier proteins located in the inner membrane of the mitochondria In this pathway, high-energy electrons move from one carrier protein to the next. Their energy is used to move hydrogen ions across the membrane through a protein called ATP Synthase Each time an ATP Synthase spins, a phosphate group is added to an ADP to make ATP.

ETC

Energy Totals In the absence of oxygen, all the energy that a cell can extract from a single glucose is 2 ATP – the product of glycolysis In the presence of oxygen, the cell can extract much more Krebs and the ETC allow cell to produce 34 more ATP per glucose Cellular respiration = 36 ATP total