Chapter 9 How Cells Harvest Energy Cellular Respiration

Life is work  Living cells require a constant input of free energy from outside sources for growth, reproduction and maintenance of organization.  The energy stored in the bonds of organic molecules (C-H) is the ultimate result of the transfer of free energy from the sun.

Overview  All of the reactions involved in the breakdown and harvesting of free energy from organic molecules are oxidation reactions.  These reactions are not simple transfers of electrons, they are also dehydrogenations.  Enzyme facilitated REDOX

 Intermediate molecules are used throughout the stages of cell respiration to prevent direct transfer of electrons to O2 directly – BOOM!!  This is accomplished via; 1. Soluble carriers 2. Membrane bound carriers 3. Within membrane carriers

Generation of ATP In cells, there are 2 methods that are used to make ATP: 1. Substrate-level phosphorylation 1. Oxidative Phosphorylation

Substrate level phosphorylation  ATP is formed when a phosphate is transferred to an ADP from a phosphate bearing intermediate molecule.

3 Main Stages of Respiration Cellular Respiration occurs in 3 main stages:  Glycolysis: exergonic / occurs in cytoplasm.  Krebs cycle: exergonic / occurs in mitochondria.  Electron Transport: endergonic / occurs in the mitochondria.

Glycolysis  “splitting of sugar”  Approximately 10 steps involved  Occurs in virtually all living cells.  Considered “ancient metabolism”  Occurs in the cytosol  Requires the expenditure of 2 ATP molecules to initiate the reaction.

 Pyruvic acid is toxic to cells and must be detoxed.

Glycolysis Results 1. 4 ATP 2. 2 NADH Total net gain = 2 ATP 2 NADH 2 NADH * Organisms such as yeast and bacteria can satisfy their energy needs with glycolysis alone.

Citric Acid Cycle (TCA)  This step occurs within the mitochondrial matrix only if molecular oxygen is present.  This path is also called the Krebs cycle after Hans Krebs in the 1930’s.  This cycle oxidizes pyruvate releasing much of its stored energy.

Results of Krebs At this point the original glucose molecule has been completely dismanteled providing the following: 1. 4 ATP NADH 3. 2 FADH CO2 * This total accounts for glycolysis and Krebs.

Electron Transport and Chemiosmosis The spatial arrangement of membrane proteins allows the mitochondrion to use a gradient of H+ ions to drive ATP synthesis.

Grand Finale Total 3 ATP x 2 NADH – glycolysis 3 ATP x 2 NADH – pyruvic acid to acetyl Co 3 ATP x 6 NADH – Krebs 2 ATP x 2 FADH2 – Krebs __________________________ 34 ATP 2 ATP – glycolysis 2ATP – Krebs __________________________ Total 36 – 38 ATP

Fermentation/Anaerobic Respiration Some organisms can thrive in an environment without oxygen as long as:  continuous supply of glucose  continuous supply of NAD+

Types of Fermentation  Alcoholic Fermentation: Yeast and bacteria convert glucose to ethanol and CO2.  Lactic Acid Fermentation: Bacteria in dairy industry and muscle cells convert glucose into lactic acid. In humans this is transported to the liver to be converted back into pyruvic acid.

Glucose  2 Pyruvic acid  2 ethanol Glucose  2 pyruvic acid  2 lactic acid

Poisons Affect ETC  Rotenone : Binds to an electron carrier in the 1 st protein complex. Prevents electrons from passing to next carrier.  Cyanide and carbon monoxide: Bind with a carrier in the 3 rd complex blocking flow of electrons to oxygen. No H+ gradient formed.

 Oligomycin: Antibiotic that blocks the passage of H+ ions through the ATP synthase.  Dinitrophenol (uncouplers): Make the membrane leaky to H+ preventing a concentration build up.

Organic Molecules as Food  Polysaccharides are converted to glucose directly.  Proteins are hydrolyzed to amino acids. The needed parts are converted into pyruvic acid, acetyl CoA or an organic of the Krebs cycle.  Fats are digested. The glycerol is converted into G3P for glycolysis and the fatty acids are changed into acetyl CoA.