How Cells Release Stored Energy Cell respiration

When a molecule has hydrogen, it has more energy and removing them releases the energy. The energy produced from the "burning" of glucose is used to make ATP. In chemistry this process is called the oxidation of glucose. The hydrogen carriers NAD and FAD are used to help release the energy in glucose by moving hydrogens and electrons around.

A simple sugar (C 6 H 12 O 6 ) Atoms held together by covalent bonds Glucose In-text figure Page 136

Summary Equation for Aerobic Respiration C 6 H O 2 6CO 2 + 6H 2 0 glucose oxygen carbon water dioxide

Glycolysis -in cytoplasm -does not use oxygen Occurs in Two Stages Energy-requiring steps –ATP energy activates glucose and its six-carbon derivatives –Actually uses 2 ATP’s Energy-releasing steps –The products of the first part are split into 2 three carbon pyruvate molecules –4 ATP and 2NADH form –4 ATP’s form – 2 ATP’s used = 2 net ATP’s made

Glycolysis is often called anaerobic respriation because it does not need oxygen. This process occurs in the cytoplasm. Two net molecules of ATP are made for cell use. It involves glucose being converted to two molecules of pyruvic acid (or pyruvate). This process is not very efficient at converting the energy of glucose into ATP as only 2 ADP are phosphorylated instead of 32 as in Krebs and chemiosmosis.

Energy-Requiring Steps 2 ATP invested Energy-Requiring Steps of Glycolysis glucose PGAL P P ADP P ATP glucose-6-phosphate P fructose-6-phosphate ATP fructose1,6-bisphosphate PP ADP Figure 8.4(2) Page 137

Energy- Releasing Steps ADP ATP pyruvate ADP ATP pyruvate H2OH2O P PEP H2OH2O P P 2-phosphoglycerate P ADP ATP P 3-phosphoglycerate ADP ATP P 3-phosphoglycerate NAD + NADH PiPi 1,3-bisphosphoglycerate PP NAD + NADH PiPi 1,3-bisphosphoglycerate PP PGAL P P Figure 8.4 Page 137 Phosphorylations

Glycolysis: Net Energy Yield Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP, 2 pyruvic acid and 2 NADH

After glycolysis: -If oxygen is present, the pyruvic acid will go into the mitochondria where the Kreb’s cycle (or citric acid cycle) is performed followed by the ETC. This allows the rest of the energy stored in the hydrogen to be extracted. -If no there is no oxygen, then the Kreb's cycle can not be completed (or started in some organisms). A cell can continue doing glycolysis in the absence of oxygen to produce some ATP, BUT it must regenerate NAD to keep glycolysis going. This is called anaerobic respiration (or fermentation). Pyruvate will form either lactic acid (muscles) or ethanol (bacteria, yeast or plants). In either case NAD is regenerated so that glycolysis can continue.

Fermentation Pathways Begin with glycolysis Do not break glucose down completely to carbon dioxide and water Yield only the 2 ATP from glycolysis Steps that follow glycolysis serve only to regenerate NAD +

Lactate Fermentation C 6 H 12 O 6 ATP NADH 2 lactate electrons, hydrogen from NADH 2 NAD ADP 2 pyruvate 2 4 energy output energy input GLYCOLYSIS LACTATE FORMATION 2 ATP net

Alcoholic Fermentation C 6 H 12 O 6 ATP NADH 2 acetaldehyde electrons, hydrogen from NADH 2 NAD ADP 2 pyruvate 2 4 energy output energy input GLYCOLYSIS ETHANOL FORMATION 2 ATP net 2 ethanol 2 H 2 O 2 CO 2

Anaerobic Electron Transport Carried out by certain bacteria Electron transfer chain is in bacterial plasma membrane Final electron acceptor is compound from environment (such as nitrate), not oxygen ATP yield is low

After glysolysis, if oxygen is present aerobic respiration proceeds in the mitochondria.

Preparatory reactions –Pyruvate is decarboxylated and oxidized (releasing carbon dioxide) –NAD + is reduced –Acetyl Co-A is formed –Ex. 1 PreparatorySteps pyruvate NAD + NADH coenzyme A (CoA) OO carbon dioxide acetyl-CoA Ex. 2

The Krebs Cycle Overall Products Coenzyme A 2 CO 2 3 NADH FADH 2 ATP Overall Reactants Acetyl-CoA 3 NAD + FAD ADP and P i

The enzymes for Kreb's is found in the inner compartment of the mitochondria. Summary of Krebs- Occurs in Mitochondria 2X's Pyruvate---> 3 CO2 6 CO2 1 ADP ---> 1 ATP 2 ATP 4 NAD ---> 4 NADH 8 NADH 1 FAD ---> 1 FADH2 2 FADH2 (also, oxaloacetate regenerates so cycle can continue)

Coenzyme Reductions during First Two Stages Glycolysis2 NADH Preparatory reactions 2 NADH Krebs cycle 2 FADH NADH Total 2 FADH NADH

Occurs in the mitochondria Coenzymes deliver electrons to electron transfer chains Electron transfer sets up H + ion gradients Flow of H + down gradients powers ATP formation (chemiosmotic) Electron Transfer Phosphorylation

The purpose of chemiosmosis is to extract the energy found in NADH and FADH 2 to make more ATP. This involves the cristae. There are electron transport chains that are used. The electrons from the NADH and FADH 2 are used to move on the electron transport chain. As the electrons move down the electron transport chain, H + ions are pumped across the membrane. The electrons from one NADH can pump 6 H + across the membrane, but the electrons from FADH 2 can only pump 4 H + across the membrane.

Creating an H + Gradient NADH OUTER COMPARTMENT INNER COMPARTMENT

The outer compartment of the mitochondria becomes positive and the inside becomes negative like a battery. This "battery" can do work. The hydrogen ions can cross an F1 particle and make ATP. It takes 2 H + to cross the F1 particle to provide enough energy to make ATP.

Making ATP: Chemiosmotic Model ATP ADP + P i INNER COMPARTMENT

Summary 8 NADH 2 x 6 H = 48 H+ 2 FADH 2 (Krebs)x 4 H = 8 H+ 2 FADH 2 (glyc.) X 4 H = 8 H+ 64 H+ 64 H+ --> 32 ATP

Importance of Oxygen Electron transport phosphorylation requires the presence of oxygen Oxygen withdraws spent electrons from the electron transfer chain, then combines with H + to form water

Summary of Energy Harvest (per molecule of glucose) Glycolysis –2 ATP formed by substrate-level phosphorylation Krebs cycle and preparatory reactions –2 ATP formed by substrate-level phosphorylation Electron transport phosphorylation –32 ATP formed

Overview of Aerobic Respiration CYTOPLASM Glycolysis Electron Transfer Phosphorylation Krebs Cycle ATP 2 CO 2 4 CO water 2 NADH 8 NADH 2 FADH 2 2 NADH 2 pyruvate e - + H + e - + oxygen (2 ATP net) glucose Typical Energy Yield: 36 ATP e-e- e - + H + ATP H+H+ e - + H + ATP 2 4 Figure 8.3 Page 135

686 kcal of energy are released 7.5 kcal are conserved in each ATP When 36 ATP form, 270 kcal (36 X 7.5) are captured in ATP Efficiency is 270 / 686 X 100 = 39 percent (at best) Most energy is lost as heat Efficiency of Aerobic Respiration

**Fats/Lipids = Glycerol and 3 fatty acids Glycerol is converted to PGAL and respired in glycolysis. The fatty acids are chopped into 2 carbon acetyl groups and used in the Krebs or citric acid cycle. **Proteins = amino acids The amino acids are sent to the liver where the liver removes the amine group. The left over acid is then used at some point in the Krebs cycle.

Alternative Energy Sources FOOD complex carbohydrates simple sugars pyruvate acetyl-CoA glycogenfatsproteins amino acids carbon backbones fatty acids glycerol NH 3 PGAL glucose-6-phosphate GLYCOLYSIS KREBS CYCLE urea Figure 8.11 Page 145 Other organic molecules can be involved in respiration.

When life originated, atmosphere had little (none) free oxygen Earliest organisms used anaerobic pathways Later, noncyclic pathway of photosynthesis increased atmospheric oxygen Cells arose that used oxygen as final acceptor in electron transport and were more efficient Evolution of Metabolic Pathways

Processes Are Linked sunlight energy water + carbon dioxide PHOTOSYNTHESIS AEROBIC RESPIRATION sugar molecules oxygen In-text figure Page 146

Linked Processes Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide Makes carbs Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide Breaks carbs