FIGHTING ENTROPY II: RESPIRATION BIO 2, Lecture 13 FIGHTING ENTROPY II: RESPIRATION

$10,000 bill (Starch; Unusable) Respiration is the process whereby cells break down complex organic molecules (like starch) and convert them into ATP + heat (not 100% efficient) The cell then uses the ATP to do work $10,000 bill (Starch; Unusable) $1 bills (ATP; Usable)

There are two types of respiration: Anaerobic respiration, also called fermentation, occurs without O2 Example: Ethanol fermentation of glucose C6H12O6 + ADP + Pi  2 C2H5OH + 2 CO2 + ATP + heat Aerobic respiration relies on O2 Is more efficient than anaerobic respiration; generates more ATP per organic molecule, loses less energy as heat C6H12O6 + 6O2 + ADP + Pi  6CO2 + 6H2O + ATP + heat

Cells do three types of work: mechanical, transport, and chemical All must be coupled to the hydrolysis of ATP Overall, the coupled reactions are catabolic An example is the creation of the amino acid glutamine from ammonia and glutamic acid Figure14.2 Crossing pea plants

(b) Coupled with ATP hydrolysis, a catabolic (exergonic) reaction Ammonia displaces the phosphate group, forming glutamine. (a) Anabolic (endergonic) reaction (c) Overall free-energy change P Glu NH3 NH2 i ADP + ATP ATP phosphorylates glutamic acid, making the amino acid less stable. Glutamic acid Glutamine Ammonia ∆G = +3.4 kcal/mol 2 1 Figure 8.10 How ATP drives chemical work: Energy coupling using ATP hydrolysis

Adenine Phosphate groups Ribose Figure 8.8 The structure of adenosine triphosphate (ATP) Ribose

Energy is released from ATP when the terminal phosphate bond is broken The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis Energy is released from ATP when the terminal phosphate bond is broken Inorganic phosphate Energy Adenosine triphosphate (ATP) Adenosine diphosphate (ADP) P + H20 i

ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP) The energy to re-phosphorylate ADP comes from catabolic reactions in the cell P i ADP + Energy from catabolism (exergonic, energy-releasing processes) Energy for cellular work (endergonic, energy- consuming processes) ATP H2O

Although carbohydrates, fats, and proteins can all be broken down during respiration to produce ATP, it is helpful to trace cellular respiration with the sugar glucose The step-wise transfer of electrons (from high energy states in complex organic molecules to lower energy states in simple organic molecules) gently releases the energy stored in glucose to regenerate ATP from ADP + P Figure 14.3 When F1 hybrid pea plants are allowed to self-pollinate, which traits appear in the F2 generation?

In oxidation, a substance loses electrons, or is oxidized Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions In oxidation, a substance loses electrons, or is oxidized In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced) Figure 13.6b Three types of sexual life cycles—plants and some algae

becomes oxidized (loses electron) becomes reduced (gains electron)

Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds, thereby changing the potential chemical energy stored in the molecules Reduced organic molecules carry more potential chemical energy than oxidized forms of the same molecules Starch is a highly reduced form of water and carbon dioxide Therefore, breaking starch down into water and carbon dioxide releases energy

Reactants Products becomes oxidized becomes reduced Methane Oxygen Figure 9.3 Methane combustion as an energy-yielding redox reaction Methane Oxygen Carbon dioxide Water

During cellular respiration, the fuel (such as glucose) is oxidized, and another molecule (such as O2) is reduced The chemical potential energy in the reactants is greater than the chemical potential energy in the products; thus energy is released becomes oxidized becomes reduced

Both anaerobic and aerobic respiration begin with glycolysis Breaks down glucose into two molecules of pyruvate) to produce 2 ATP per glucose Takes place in the cytoplasm In anaerobic respiration, the process stops here and only 2 ATP are generated per glucose

Both steps take place in the mitochondria Aerobic respiration has two additional steps that break down the pyruvate to carbon dioxide and water to produce an additional 36 ATP The citric acid cycle (completes the breakdown of glucose) Oxidative phosphorylation (accounts for most of the ATP synthesis) Both steps take place in the mitochondria

Glycolysis (“splitting of sugar”) has two major phases: Energy investment phase Energy payoff phase In the energy investment phase, 2 ATP are consumed to “kick start” the process In the energy payoff phase, four ATP are produced, yielding a net gain of 2 ATP

Energy investment phase Glucose 2 ADP + 2 P 2 ATP used formed 4 ATP Energy payoff phase 4 ADP + 4 2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net 4 ATP formed – 2 ATP used 2 NADH + 2 H+ Figure 9.8 The energy input and output of glycolysis

The energy stored in NADH is likewise wasted In anaerobic respiration, the process stops with the production of 2 ATP Pyruvate is converted to waste products (like ethanol) but no more ATP is gained The energy stored in NADH is likewise wasted It is important to note, however, that NADH carries high energy electrons and can be harvested to produce ATP if a cell has the machinery to do so ...

Potential source of additional ATP; WASTED in aerobic respiration Anaerobic Respiration Substrate-level phosphorylation ATP Cytosol Glucose Pyruvate Glycolysis Electrons carried via NADH Net = 2 Net = 2 NADH Potential source of additional ATP; WASTED in aerobic respiration Figure 9.6 An overview of cellular respiration

The energy stored in NADH is likewise wasted In anaerobic respiration, the process stops with the production of 2 ATP Pyruvate is converted to waste products (like ethanol) but no more ATP is gained The energy stored in NADH is likewise wasted It is important to note, however, that NADH carries high energy electrons and can be harvested to produce ATP if a cell has the machinery to do so ...

Takes place in cells that have mitochondria Aerobic respiration continues the process of glycolysis to breakdown pyruvate and utilize the high energy electrons stored in NADH Takes place in cells that have mitochondria The citric acid cycle (breaks down pyruvate to CO2 and H2O in the mitochondrial matrix to produce additional ATP, NADH, and FADH2) Oxidative phosphorylation (harvests electrons from NADH and FADH2 in the inner mitochondrial membrane and accounts for most of the ATP synthesis)

Electrons carried via NADH Electrons carried via NADH and FADH2 Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation

Electrons carried via NADH Electrons carried via NADH and FADH2 Oxidative phosphorylation: electron transport and chemiosmosis Glycolysis Citric acid cycle Glucose Pyruvate Mitochondrion Cytosol Figure 9.6 An overview of cellular respiration ATP ATP ATP Substrate-level phosphorylation Substrate-level phosphorylation Oxidative phosphorylation

The Citric Acid Cycle: In the presence of O2, pyruvate (generated by glycolysis) enters the mitochondrion from the cytoplasm Prior to the start of the cycle, pyruvate is converted to acetyl CoA, generating one molecule of NADH The cycle then oxidizes acetyl CoA, generating 1 ATP, 3 NADH, and 1 FADH2 per turn

Pyruvate NAD+ NADH + H+ Acetyl CoA CO2 CoA Citric acid cycle FADH2 FAD 3 3 NAD+ + 3 H+ ADP + P i ATP Figure 9.11 An overview of the citric acid cycle

The cycle is “fed” by acetyl-coA In the first step, the acetyl-coA is combined with oxaloacetate to form citrate The citrate is then broken down in a series of steps to produce energy (in the form of ATP, NADH, and FADH2) + CO2 (gas) The end product of the cycle is oxaloacetate, which can then combine with another molecule of acetyl-coA to run the cycle again ...

Succinyl CoA Citric acid cycle Acetyl CoA CoA—SH Citrate H2O Isocitrate NAD+ NADH + H+ CO2 -Keto- glutarate Succinyl CoA P i GTP GDP ADP ATP Succinate FAD FADH2 Fumarate Citric acid cycle Malate Oxaloacetate +H+ 1 2 3 4 5 6 7 8 Figure 9.12 A closer look at the citric acid cycle

Following glycolysis and the citric acid cycle, NADH and FADH2 carry most of the energy extracted from food These two molecules transport the high energy electrons generated by the breakdown of glucose to pyruvate (during glycolysis) and pyruvate to oxaloacetate and CO2 (during the citric acid cycle) and donate them to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

The electron transport chain is located in the inner membrane of the mitochondrion Most of the chain’s components are proteins, which exist in multi-protein complexes The carriers alternate reduced and oxidized states as they accept (become reduced) and donate (become oxidized) electrons down the chain Electrons drop in free energy as they go down the chain and are finally passed to O2 (gas), forming H2O

Free energy (G) relative to O2 (kcal/mol) NADH NAD+ 2 FADH2 FAD Multiprotein complexes Fe•S FMN Q  Cyt b   Cyt c1 Cyt c Cyt a Cyt a3 IV Free energy (G) relative to O2 (kcal/mol) 50 40 30 20 10 (from NADH or FADH2) 2 H+ + 1/2 O2 H2O e– Figure 9.13 Free-energy change during electron transport

As electrons are transferred down the electron transport chain, the energy released at each step is used by the protein complexes to pump H+ from the mitochondrial matrix to the inter-membrane space H+ then moves back across the membrane, with its diffusion gradient, passing through channels in a protein complex called ATP synthase

ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP from ADP and Pi This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work

Why we breathe O2!! Protein complex of electron carriers H+ Cyt c Q    V FADH2 FAD NAD+ NADH (carrying electrons from food) Electron transport chain 2 H+ + 1/2O2 H2O ADP + P i Chemiosmosis Oxidative phosphorylation ATP synthase ATP 2 1 Why we breathe O2!! Figure 9.16 Chemiosmosis couples the electron transport chain to ATP synthesis

During aerobic respiration, most energy flows in this sequence: glucose  NADH  electron transport chain  proton-motive force  ATP About 40% of the energy in a glucose molecule is transferred to ATP during aerobic respiration, generating about 38 ATP The rest is lost as heat

Anaerobic phase Aerobic phase Citric acid cycle MITOCHONDRION Maximum per glucose: About 38 ATP + 2 ATP + about 34 ATP Oxidative phosphorylation: electron transport and chemiosmosis Citric acid cycle 2 Acetyl CoA Glycolysis Glucose Pyruvate 2 NADH 6 NADH 2 FADH2 CYTOSOL Electron shuttles span membrane or MITOCHONDRION Anaerobic phase Aerobic phase Figure 9.17 ATP yield per molecule of glucose at each stage of cellular respiration

Comparing aerobic and anaerobic respiration: Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate However, the processes have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration Cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule

Respiration can use many different fuels (not just glucose!) Proteins Carbohydrates Amino acids Sugars Fats Glycerol Fatty Glycolysis Glucose Glyceraldehyde-3- Pyruvate P NH3 Acetyl CoA Citric acid cycle Oxidative phosphorylation Respiration can use many different fuels (not just glucose!) Figure 9.20 The catabolism of various molecules from food

(a) Uncontrolled reaction H2O H2 + 1/2 O2 Free energy, G (a) Uncontrolled reaction H2O H2 + 1/2 O2 Explosive release of heat and light energy (b) Cellular respiration Controlled energy for synthesis of ATP 2 H+ + 2 e– 2 H + 1/2 O2 (from food) 2 H+ 2 e– Electron transport chain Figure 9.5 An introduction to electron transport chains

Cellular respiration in mitochondria Organic molecules Light energy ECOSYSTEM Photosynthesis in chloroplasts CO2 + H2O Cellular respiration in mitochondria Organic molecules + O2 ATP powers most cellular work Heat ATP Figure 9.2 Energy flow and chemical recycling in ecosystems