1.7: Cellular Respiration

• Cellular respiration harvests chemical energy.

Mitochondria

• The mitochondria has two membranes.
• The outer membrane is smooth.
• The inner membrane is folded into cristae.
• Cristae increases the surface area for enzymes to synthesize ATP.
• The mitochondrial matrix is fluid-filled.
• It contains DNA, ribosomes, and enzymes.
• Aerobic respiration uses oxygen as a reactant to break down organic molecules and release energy. This is a catabolic process.
• This overall process involves organic compounds reacting with oxygen to produce carbon dioxide, water, and energy.
• Glucose as a reactant: C6H12O6 + 6O2 yields 6CO2 + 6H2O + Energy (ATP + heat)
• Fermentation is another type of catabolic process.
• Fermentation is the partial degradation of sugars without oxygen.

ATP

• The bonds between phosphate groups can be broken by hydrolysis, releasing energy.
• ATP -> ADP + P releases 7.3 kcal of energy per mole of ATP.
• ATP phosphorylates transport proteins for transport work.
• ATP phosphorylates motor proteins for mechanical work.
• ATP phosphorylates key reactants for chemical work.

Metabolic Stages

• Glycolysis occurs in the cytosol.
• Pyruvate oxidation occurs in the mitochondrion
• The citric acid cycle occurs in the mitochondrion.
• Oxidative phosphorylation (electron transport and chemiosmosis) occurs in the mitochondrion.
• Some ATP is generated in glycolysis and the Krebs cycle by substrate-level phosphorylation.

Electron Transport Chain

• Electrons move from molecule to molecule until they combine with oxygen and hydrogen ions to form water (H2O).
• Energy from electron transfer is used to synthesize ATP via oxidative phosphorylation.
• Oxidative phosphorylation produces approximately 90% of the ATP generated by respiration.

Glycolysis

• One glucose molecule (a 6-carbon sugar) is split into two 3-carbon sugars.
• These smaller sugars are oxidized and rearranged to form two molecules of pyruvate.
• Each of the 10 steps in glycolysis is catalyzed by a specific enzyme.
• Glycolysis involves two phases including the energy of investment phase and the energy payoff phase.
• The energy investment phase uses two ATP.
• The energy payoff phase produces four ATP.
• Net is 2 ATP.

Energy Investment Phase

• Hexokinase transfers a phosphate group from ATP to glucose, making it more chemically reactive and trapping it inside the cell.
• Phosphoglucoisomerase converts glucose 6-phosphate to fructose 6-phosphate.
• Phosphofructokinase transfers a phosphate group from ATP to the opposite end of the sugar, investing a second molecule of ATP. This is a key step for regulation of glycolysis.
• Aldolase cleaves the sugar molecule into two different three-carbon sugars: glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).
• Isomerase: Conversion between DHAP and G3P; the G3P is used in the next step as fast as it forms.

Energy Payoff Phase

• Two sequential reactions:
  • (1) G3P is oxidized by the transfer of electrons to NAD+, forming NADH.
  • (2) Using energy from this exergonic redox reaction, a phosphate group is attached to the oxidized substrate, making a high-energy product.
• The phosphate group is transferred to ADP (substrate-level phosphorylation) in an exergonic reaction.
• Enolase causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP), a compound with a very high potential energy.
• The phosphate group is transferred from PEP to ADP (substrate-level phosphorylation), forming pyruvate.
• More than 3/4 of the original energy in glucose is still present in the two molecules of pyruvate.
• If oxygen is present, pyruvate enters the mitochondrion to begin the Krebs cycle.
• Glycolysis can occur with or without oxygen.

Pyruvate Oxidation to Acetyl CoA

• Pyruvate (1 Glucose = 2 Pyruvate) is transported into the mitochondrion.

Krebs Cycle / Citric Acid Cycle

• Oxaloacetate is recycled, and acetate is broken down to CO2.
• Per cycle, 1 ATP is produced by substrate-level phosphorylation.
• The cycle also produced 3 NADH and 1 FADH2 (another electron carrier) per acetyl CoA (x2 per glucose molecule).

Conversion of Pyruvate and Krebs Cycle

• Pyruvate conversion & Krebs cycle produces large quantities of electron carriers, NADH and FADH2.

Oxidative Phosphorylation

• Of the 32 ATP produced by respiration of glucose, 4 are derived from substrate-level phosphorylation.
• Most of the ATP comes from the energy in the electrons carried by NADH and FADH2.
• This energy is used in the electron transport system to power ATP synthesis.
• Thousands of copies of the ETC are found in the cristae, the inner membrane of the mitochondrion.

Electron Transport Chain

• Electrons carried by NADH are transferred to the first molecule in the ETC, flavoprotein (FMN).
• The transfer of the electron includes several cytochrome proteins and one lipid carrier.
• Electrons carried by FADH2 have lower free energy and are added at a later point in the chain.
• Electrons from NADH or FADH2 ultimately pass to oxygen (terminal electron acceptor).
• The ETC does not directly generate ATP.
• ETC functions to break the energy drop from food to O2 into steps to release energy in manageable amounts.
• The movement of electrons along the ETC contributes to chemiosmosis and ATP synthesis.
• Unlike heat release that would occur when hydrogen and oxygen combine, cellular respiration breaks the fall of electrons to oxygen into several steps.
• ATP synthase makes ATP from ADP and Pi.
• ATP uses the energy of a proton gradient to power ATP synthesis.
• Chemiosmosis: Energy stored in the form of a H ion gradient used to drive cellular work such as ATP synthesis.

Proton Gradient

• Several chain molecules can use the exergonic flow of e- to pump H+ from the matrix to the intermembrane space.
• This concentration of H⁺ is the proton-motive force.
• Electron shuttles span the membrane of Liver and Heart cells.
• Brain cells have different cycles.
• 1 NADH = 2.5 ATP
• 1 FADH2 = 1.5 ATP

Summary Of Cellular Respiration

• Glycolysis produces 2 ATP and 2 NADH.
• Pyruvate to Acetyl CoA produces 2 NADH.
• The Citric Acid Cycle produces 2 ATP, 6 NADH, and 2 FADH2.
• Total ATP production: 30 or 32 ATP
• The summary cellular respiration is: C6H12O6 + 6O2 -> 6CO2 + 6H2O

Anaerobic Respiration and Fermentation

• Enables ATP production without O2.
• Glycolysis generates 2 ATP whether oxygen is present (aerobic) or (anaerobic).
• Anaerobic respiration uses ETC but oxygen is not the final electron acceptor.
• Anaerobic catabolism of sugars can occur by fermentation.

Alcohol Fermentation

• Pyruvate is converted to ethanol in two steps.
• Used in beer, wine-making, and baking.

Lactic Acid Fermentation

• Pyruvate is reduced directly by NADH to form lactate.
• Used in cheese, yogurt, and muscles.

Facultative Anaerobes

• Includes yeast and many bacteria.
• Can survive using either fermentation or aerobic respiration.
• Pyruvate leads to 2 alternative routes.