CHAPTER 7 CELLULAR RESPIRATION

WHERE IS THE ENERGY IN FOOD? Electrons pass from atoms or molecules to one another as part of many energy reactions. Oxidation is when an atom or molecule loses an electron. Reduction is when an atom or molecule gains an electron. These reactions always occur together: Oxidation-reduction (redox) reactions

WHERE IS THE ENERGY IN FOOD? Redox reactions involve transfers of energy because the electrons retain their potential energy. The reduced form of an organic molecule has a higher level of energy than the oxidized form. e–e– AB + –oo +A*B* A+B Loss of electron (oxidation) Low energy High energy Gain of electron (reduction)

WHERE IS THE ENERGY IN FOOD? The energy for living is obtained by breaking down the organic molecules originally produced in plants. The ATP energy and reducing power invested in building the organic molecules are stripped away as the chemical bonds are broken and used to make ATP. The oxidation of food stuffs to obtain energy is called cellular respiration.

WHERE IS THE ENERGY IN FOOD? Cellular respiration is the harvesting of energy from breakdown of organic molecules produced by plants The overall process may be summarized as 6 CO 2 carbon dioxide (heat or ATP) + energy C 6 H 12 O 6 glucose + 6 O 2 oxygen + 6 H 2 O water

CELLULAR RESPIRATION Cellular respiration takes place in two stages: – Glycolysis Occurs in the cytoplasm. Does not require O 2 to generate ATP.

CELLULAR RESPIRATION – Krebs cycle Occurs within the mitochondrion. Harvests energy-rich electrons through a cycle of oxidation reactions. The electrons are passed to an electron transport chain in order to power the production of ATP.

Glucose Glycolysis Pyruvate NADH Electron transport chain H2OH2O CO 2 NAD + and ADF Mitochondrial matrix Cytoplasm e–e– Mitochondrion Intermembrane space FADH 2 ATP Krebs cycle ATP Pyruvate oxidation Acetyl- CoA Inner mitochondrial membrane

USING COUPLED REACTIONS TO MAKE ATP Glycolysis is a sequence of reactions that form a biochemical pathway. In ten enzyme-catalyzed reactions, the six-carbon sugar glucose is broken into two three-carbon pyruvate molecules.

USING COUPLED REACTIONS TO MAKE ATP The breaking of the bonds yields energy that is used to phosphorylate ADP to ATP. This process is called substrate-level phosphorylation. In addition, electrons and hydrogen atoms are donated to NAD + to form NADH.

11 Glucose Glycolysis Phosphorylation of glucose by ATP. 1 2–3 4–5 6 Rearrangement, followed by a second ATP phosphorylation. The six-carbon molecule is split into two three-carbon molecules of G3P. Krebs cycle Electron transport chain Pyruvate oxidation NADH NAD + Glucose ATP ADP Glucose 6-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate Glyceraldehyde 3- phosphate (G3P) Glyceraldehyde 3- phosphate (G3P) ATP ADP 1,3-bisphosphoglycerate (BPG) P P PP PP PPPP ,5 PiPi PiPi NAD + Oxidation followed by phosphorylation produces two NADH molecules and gives two molecules of BPG, each with one high-energy phosphate bond.

7 8–9 10 Removal of high-energy phosphate by two ADP molecules produces two ATP molecules and gives two 3PG molecules. Removal of water gives two PEP molecules, each with a chemically reactive phosphate bond. Removal of high-energy phosphate by two ADP molecules produces two ATP molecules and gives two pyruvate molecules. ADP ATP ADP ATP ADP ATP ADP ATP 3-phosphoglycerate (3PG) 3-phosphoglycerate (3PG) 2-phosphoglycerate (2PG) 2-phosphoglycerate (2PG) Phosphoenolpyruvate (PEP) Phosphoenolpyruvate (PEP) Pyruvate PP PP PP

USING COUPLED REACTIONS TO MAKE ATP Glycolysis yields only a small amount of ATP. Only two ATP are made for each molecule of glucose. This is the only way organisms can derive energy from food in the absence of oxygen. All organisms are capable of carrying out glycolysis. This biochemical process was probably one of the earliest to evolve.

HARVESTING ELECTRONS FROM CHEMICAL BONDS In the presence of oxygen, the first step of oxidative respiration in the mitochondrion is the oxidation of pyruvate. Pyruvate still contains considerable stored energy at the end of glycolysis. Pyruvate is oxidized to form acetyl-CoA.

ACETYL-COA When pyruvate is oxidized, one of its three carbons is cleaved. This carbon leaves as part of a CO 2 molecule. In addition, a hydrogen and electrons are removed from pyruvate and donated to NAD+ to form NADH. The remaining two-carbon fragment of pyruvate is joined to a cofactor called coenzyme A (CoA). The final compound is called acetyl-CoA. Glycolysis Pyruvate CO 2 NAD + CoenzymeA Lipid NADH Protein Fat ATP CoA– Acetyl–CoA

H H H +e–e– + + H H 321 NADH then diffuses away and is available to donate the hydrogen to other molecules. NAD + Substrate Product Enzymes that harvest hydrogen atoms have a binding site for NAD + located near the substrate binding site. In an oxidation-reduction reaction, the hydrogen atom and an electron are transferred to NAD +, forming NADH. e–e– e–e– NADH and NAD + are used by cells to carry hydrogen atoms and energetic electrons. KEY BIOLOGICAL PROCESS: TRANSFER OF H ATOMS

HARVESTING ELECTRONS FROM CHEMICAL BONDS The fate of acetyl-CoA depends on the availability of ATP in the cell. If there is insufficient ATP, then the acetyl-CoA heads to the Krebs cycle. If there is plentiful ATP, then the acetyl-CoA is diverted to fat synthesis for energy storage.

KREBS CYCLE The second step of oxidative respiration is called the Krebs cycle. The Krebs cycle is a series of 9 reactions that can be broken down into three stages: 1. Acetyl-CoA enters the cycle and binds to a four- carbon molecule, forming a 6-C molecule. 2. Two carbons are removed as CO 2 and their electrons donated to NAD +. In addition, an ATP is produced. 3. The four-carbon molecule is recycled and more electrons are extracted, forming NADH and FADH 2.

THE KREBS CYCLE Note: A single glucose molecule produces two turns of the cycle, one for each of the two pyruvate molecules generated by glycolysis. Pyruvate CO 2 NAD + CoA– Oxidation of pyruvate Glucose Glycolysis Pyruvate oxidation Krebs cycle Electron transport chain Coenzyme A Acetyl-CoA NADH

CO 2 Krebs cycle (4 C) Oxaloacetate Citrate (6 C) Isocitrate (6 C)  -Ketoglutarate (5 C) NAD + S CoA-SH (4 C) Succinate CoA-SH (4 C) Malate H2OH2O (4 C) Fumarate NAD + The cycle begins when a C 2 unit reacts with a C 4 molecule to give citrate (C 6 ) Mitochondrial membrane CoA NADH Succinyl-CoA (4 C) ADP ATP FAD FADH 2 The dehydrogenation of malate produces a third NADH, and the cycle returns to its starting point. A molecule of ATP is produced and the oxidation of succinate produces FADH 2. NADH A second oxidative decarboxylation produces a second NADH with the release of a second CO 2. Oxidative decarboxylation produces NADH with the release of CO 2. CoA

HARVESTING ELECTRONS FROM CHEMICAL BONDS In the process of cellular respiration, the glucose is entirely consumed. The energy from its chemical bonds has been transformed into: 4 ATP molecules. 10 NADH electron carriers. 2 FADH 2 electron carriers.

USING THE ELECTRONS TO MAKE ATP NADH and FADH 2 transfer their electrons to a series of membrane-associated molecules called the electron transport chain. Some protein complexes in the electron transport chain act as proton pumps. The last transport protein donates the electrons to hydrogen and oxygen in order to form water. The supply of oxygen able to accept electrons makes oxidative respiration possible.

THE ELECTRON TRANSPORT CHAIN Intermembrane space Mitochondrial matrix NAD + H 2 O + H + 2H + + – O 2 H+H+ H+H+ H+H+ Glucose Glycolysis e–e– e–e– e–e– Inner mitochondrial membrane Protein complex III Protein complex II Protein complex I 1 2 Electron transport chain Krebs cycle Pyruvate oxidation FADH 2 NADH

USING THE ELECTRONS TO MAKE ATP Chemiosmosis is integrated with electron transport. Electrons harvested from reduced carriers (NADH and FADH 2 ) are used to drive proton pumps and concentrate protons in the intermembrane space. The re-entry of the protons into the matrix across ATP synthase drives the synthesis of ATP by chemiosmosis.

H+H+ H + H + H+H+ H+H+ O2O H 2 O CO 2 NADH Acetyl-CoA e–e– Pyruvate from cytoplasm Inner mitochondrial membrane Intermembrane space Electron transport chain Electrons are harvested and carried to the transport chain. Electrons provide energy to pump protons across the membrane. Oxygen joins with protons and electrons to form water. Protons diffuse back in down their concentration gradient, driving the synthesis of ATP. Mitochondrial matrix e–e– e–e– e–e– FADH 2 Krebs cycle ATP 2 H + ATP synthase – O 2 1 2

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CELLS CAN METABOLIZE FOOD WITHOUT OXYGEN In the absence of oxygen, organisms must rely exclusively on glycolysis to produce ATP. In a process called fermentation, the hydrogen atoms from the NADH generated by glycolysis are donated to organic molecules, and NAD + is regenerated. With the recycling of NAD +, glycolysis is allowed to continue.

FERMENTATION Bacteria can perform more than a dozen different kinds of fermentation. Eukaryotic cells are only capable of a few types of fermentation.

FERMENTATION In yeasts (single-celled fungi), pyruvate is converted into acetaldehyde, which then accepts a hydrogen from NADH, producing NAD + and ethanol. In animals, such as ourselves, pyruvate accepts a hydrogen atom from NADH, producing NAD + and lactate.

GLUCOSE IS NOT THE ONLY FOOD MOLECULE Cells also get energy from foods other than sugars. These complex molecules are first digested into simpler subunits, which are then chemically modified into intermediates. These intermediates enter cellular respiration at different steps.

H2OH2O Macro- molecule degradation Cell building blocks Oxidative respiration Ultimate metabolic products CO 2 NH 3 Nucleic acids Proteins Polysaccharides Lipids and fats NucleotidesAmino acidsSugarsFatty acids β-oxidation Krebs cycle Acetyl-CoA Pyruvate Glycolysis Deamination