Cellular Respiration: Harvesting Chemical Energy Chapter 9 Cellular Respiration: Harvesting Chemical Energy

Principles of Energy Conservation Energy Flow Sunlight –> Chemical Bond Energy –> ATP –> Heat Cellular Respiration and Fermentation Cellular Respiration – ATP-producing catabolic process in which the ultimate electron acceptor is an inorganic molecule such as oxygen. Fermentation – anaerobic process where ATP is produced Food + Oxygen –> Carbon Dioxide + Water + Energy

Principles of Energy Conservation (Continued) ATP – nucleotide with high energy phosphate bonds that the cell hydrolyzes for energy to drive endergonic reactions Phosphorylation – the process of compounds receiving a phosphate group from ATP. This compound is said to be phosphorylated ATP –> ADP+ + P

Principles of Energy Conservation (Continued) Redox Reactions – release energy when electrons move closer to electronegative atoms Oxidation-reduction Reactions – partial or complete transfer of electrons from one reactant to another; called redox reactions Oxidation – partial or complete loss of electrons (NAD+) Reduction – partial or gain of electrons (NADH) Oxygen Electron Accepting Cellular respiration is a redox reaction that transfers hydrogen, including electrons with high potential energy, from sugar to oxygen

Principles of Energy Conservation (Continued) Nicotinamide Adenine Dinucleotide – coenzyme that is involved in redox reactions Accepts electrons stripped from glucose before oxygen receives them Assists enzymes in redox reactions Coenzyme – small nonprotein organic molecule that is required for certain enzymes to function Dehydrogenases – removes a pair of hydrogen atoms (2 electrons and 2 protons) from a substrate (glucose) and delivers two electrons and one proton to NAD+

The Process of Cellular Respiration Respiration involves glycolysis, Krebs Cycle, and ETC. Glycolysis – occurs in the cytosol; partially oxidizes glucose (6C) into two pyruvate (3C) molecules Krebs Cycle – occurs in mitochondrial matrix; completes glucose oxidation by breaking down a pyruvate derivative (acetyl CoA) into carbon dioxide ETC – occurs in the inner mitochondrial matrix; oxygen pulls electrons to lower energy state allowing for the 90% production of ATP Oxidative Phosphorylation – ATP production that is couples with exergonic transfer of electrons from food to oxygen Substrate-level Phosphorylation – ATP production by direct enzymatic transfer of phosphate from an intermediate substrate in catabolism to ADP

The Process of Cellular Respiration (Continued) Glycolysis – catabolic pathway where a 6-carbon glucose is split into two 3-carbon molecules, which are then oxidizes into two pyruvate molecules Occurs whether oxygen is present or not Does not produce carbon dioxide Phase I (Energy Investment) 2 ATP invested to split glucose into two 3-carbon molecules. Phase II (Energy-yielding) 4 ATP produced by substrate-level phosphorylation 2 NAD+ are reduced to 2 NADH which can then be used later for oxidative phosphorylation of ATP Results 2 pyruvate, 2 ATP, 2 NADH, and 2 HOH Process is exergonic where most of the energy is conserved and harnessed in ATP and NADH

The Process of Cellular Respiration (Continued) Krebs Cycle Formation of Acetyl CoA – occurs in the mitochondrial matrix; pyruvate is brought into the matrix by a carrier protein. Carbon dioxide is removed from the pyruvate; NAD+ reduces to NADH while transforming the 2-carbon molecule into acetate; coenzyme A is attached to the acetyl group of the molecule forming a highly reactive molecule called Acetyl CoA. Krebs Cycle – 8 step reaction occurring in the mitochondrial matrix For every turn of the Krebs Cycle: (2 turns for every glucose molecule) 2 carbons enter in the acetyl fragment of acetyl CoA Carbon dioxide leaves Three NADH and one FADH2 are produced. 1 ATP molecule is produced from substrate-level phosphorylation.

The Process of Cellular Respiration (Continued) Electron Transport Chain – occurs in the folding of the cristae of the mitochondria. Pathway of Electron Transport FADH2 and NADH are passed down the electron carrier molecules of the transport chain releasing electrons that are then picked up by oxygen ATP synthesis occurs through oxidative phosphorylation Each successive electron carrier molecule has a higher electronegativity than the previous allowing for the downhill movement of electrons to the final acceptor; oxygen Chemiosmosis – the coupling of exergonic electron flow down an electron transport chain to an endergonic ATP production by the creation of a proton gradient across a membrane ATP Synthase – uses the potential energy stored in the proton gradient to make ATP allowing H+ to diffuse down the gradient, across the membrane H+ protons move from the intermembrane space (cristae) to the matrix (proton- motive force)

Process Glycolysis Net 2 ATP 2 NADH 6-8 Oxidation of Pyruvate 6 ATP produced directly by substrate-level phosphorylation Reduced Coenzyme ATP produced by oxidative phosphorylation Glycolysis Net 2 ATP 2 NADH 6-8 Oxidation of Pyruvate 6 Krebs Cycle 2 ATP 6 NADH 2 FADH2 18 4

Anaerobic Respiration Fermentation – enables cells to produce ATP without the use of oxygen Oxidation of food molecules in aerobic environments Anaerobic Respiration Pyruvate is reduced, and NAD+ is regenerated to enable anaerobic glycolysis to continue Alcohol Fermentation – glucose is oxidized and pyruvate is reduced to produce ethanol Pyruvate loses carbon dioxide and is converted to the 2- carbon compound acetaldehyde. NADH is oxidized to NAD+ and acetaldehyde is reduced to ethanol. This process is carried out by many bacteria and yeast.

Anaerobic Respiration (Continued) Lactic Acid Fermentation – glucose is oxidized and pytuvate is reduced to lactate Commercially important in the production of cheeses and yogurt Oxygen starved muscles produce lactase, but is carried to the liver and is allowed to accumulate where it is converted back to pyruvate while waiting for oxygen to become available

Anaerobic Respiration (Continued) Fermentation and Respiration Compared Similarities Both use glycolysis to oxidize glucose to pyruvate producing 2 ATP by substrate-level phosphorylation Both use NAD+ as an oxidizing agent to accept electrons from food to produce the reduced NADH Differences Fermentation reduces pyruvate to ethanol/lactate by oxidizing NADH to NAD+ Respiration uses a step wise transfer of electrons from NADH to oxygen to drive oxidative phosphorylation and regenerate NAD+ The final electron acceptor in fermentation is pyruvate (alcohol fermentation) or acetaldehyde (lactic acid fermentation) The final electron acceptor is respiration is oxygen Cellular respiration yields 18 times more energy Fermentation does not require oxygen

Anaerobic Respiration (Continued) Evolution of Glycolysis First prokaryotes most likely used glycolysis to produce ATP due to the fact that the atmosphere did not contain oxygen Glycolysis is carried out in the cytosol so it does not require membrane bound organelles such as a mitochondrion

Anaerobic Respiration (Continued) Feedback Mechanisms – can switch on or switch off catabolic/anabolic when supplies are small or large Major step of feedback mechanisms take place in the third step of glycolysis ATP to ADP and Amp reflect the energy status of the cell and phosphofructokinase is sensitive in this ratio When the ratio of ATP:ADP/AMP is low, this enzyme will speed up glycolysis thus speeding up the Krebs cycle