Chapter 9 Cellular Respiration

Cellular Respiration Release of potential energy in organic molecules (food) to produce ATP – which is the form of energy used in the cell. Potential energy is primarily in C-H bonds. When molecules containing these are oxidized, the H and accompanying electrons are released and energy is released. With the input of heat (activation) energy, CH4 + 2O2 CO2 + 2H20 + heat Compare this to what happens to glucose C6H12O6 + 6O2 6CO2 + 6H2O + heat (-686 kcal/m) free energy change In living things, the temp is not adequate to overcome the required activation energy to ignite glucose so enzymes lower the energy of activation.

Releasing energy from carbohydrates Oxidation of glucose Electrons from C-H bonds in glucose can be transferred to a lower energy state and the energy in the bonds converted to ATP bonds or heat. Stepwise oxidation releases the energy in small packets that are manageable rather than in one big bang.

Some energy directly phosphorylates ADP and some of the energy is transferred with electrons to NAD+ to release to an ATP generating pathway. Substrate level phosphorylation Enzyme takes a phosphate from a substrate & attaches to ADP ATP Chemiosmotic phosphorylation (from ETC) H + ion concentration gradient across membrane H + flow from high to low through ATP synthase ADP + P ATP

Important Molecules NAD+ - oxidizing agent - Coenzyme FAD - Coenzyme It causes electrons to leave an organic molecule. It transports those electrons to an ATP generating area where it releases them. FAD - Coenzyme Transports electrons Electron Transport Chain Molecules Molecules built into membranes that receive & release electrons stepwise to lower their energy level to the point where they (electrons) are accepted by O2 (final electron acceptor). In some of these steps, ATP is produced.

Glycolysis – in cytoplasm of almost all cells Glucose 1 Glucose 6-phosphate 2 Fructose 6-phosphate 3 Fructose 1, 6 Diphosphate 4 2 Glyceraldehyde Phosphate (PGAL ) 5 2 Diphosphoglycerate (DiPGA) 6 2 Phosphoglycerate (PGA) 7 2 Phosphoenolpyruvate (PEP) 8 2 Pyruvate Steps 1-3 add energy to glucose so it will be more reactive – USES 2 ATP Step 5 is oxidation w/ electrons transferred to NAD – also phosphorylation Step 6 – ATP produced by pulling P off DiPGA Step 7 – Water removed Step 8 – ATP produced Forms – 4 ATP, 2 NADH Nets – 2 ATP, 2 NADH

Oxidation of Pyruvate Happens in the Mitochondria Everything from this point on happens TWICE for 2 pyruvate from 1 Glucose molecule NAD+ NADH CoA + Pyruvate Acetyl –CoA CO2 (decarboxylation) CoA is a coenzyme made from the vitamin pantothenic acid Produces 2 NADH, 2 CO2 Junction between glycolysis & Krebs Cycle

Krebs Cycle – Acetyl CoA goes in Citrate(6 C) Oxaloacetate 5 C Acid 4 C Acid 4 C Acid 4 C Acid NAD+ NADH NADH NAD+ NAD+ CO2 NADH H2O ADP + P FADH2 ATP FAD

Krebs Cycle Totals Total for 2 Acetyl = 6 NADH shuttles electrons 2 FADH2 to ETC 2 ATP 4 CO2

Purposes of steps 1-3 Glycolysis Oxidation of Pyruvate Krebs Cycle (Citric Acid Cycle)

Electron Transport Chain & Chemiosmosis The molecules of ETC are in the cristae (inner membrane) Electrons released from NADH or FADH2 flow through the cytochromes, to lower & lower energy levels until they are accepted by O2 (the final electron acceptor that produces H2O) As they flow, protons are pumped out of the matrix into the innermembrane space.

ETC, cont. As protons build up to higher concentration, they flow back into the matrix through an enzyme, ATP Synthase, and ATP is made - Chemiosmosis Can also be called oxidative phosphorylation Controlled release of energy for ATP synthesis H+ flows back in once gradient occurs through ATP Synthase to produce ATP ATP Synthase acts as a molecular mill that produces ATP from ADP and P

ETC Diagram

Purpose of ETC ETC

Grand Total of ATP

Anaerobic Catabolism of organic molecules Fermentation Only generates ATP by substrate level phosphorylation As long as there is a sufficient supply of NAD+ Without NAD+, glycolysis would shut down for lack of oxidizing agent Two types Alcohol fermentation Step 1 - Release CO2 from pyruvate & convert to acetaldehyde Step 2 - Acetaldehyde reduced by NADH to ethanol – regenerates NAD+ to continue glycolysis Happens in Yeast and Bacteria

Fermentation, cont. Lactic Acid Fermentation Pyruvate reduced directly by NADH to form lactate with no release of CO2 Human muscle cells make ATP by LAF when O2 is scarce. During early strenuous exercise When sugar catabolism for ATP outpaces muscle’s supply of O2 from blood - Cells switch to fermentation Lactate accumulation – causes fatigue and pain* Lactic acid is carried back to liver and converted back to Pyruvate *New studies believe it is Potassium that causes the muscle pain and fatigue and that lactate actually enhances muscle performance.

Glucose Pyruvate Ethanol Acetyl CoA or Lactate Kreb’s Path of Pyruvate Glucose Pyruvate Ethanol Acetyl CoA or Lactate Kreb’s No O2 present - fermentation O2 present – cellular respiration