Biology 2E Chapter 7 Cellular Respiration

Energy in Living Systems

• Cellular Respiration Overview: Cells extract energy from food to generate ATP through glucose catabolism or cellular respiration. This process involves the transfer of energy and the movement of electrons. The incremental transfer of electrons allows the cell to transfer food energy in small packages that can be captured in the phosphate bonds of ATP.

• Redox Reactions:

  • Oxidation-reduction (redox) reactions involve the transfer of electrons from one molecule to another.
  • Reducing agents donate electrons, while oxidizing agents accept electrons.
  • Molecules that gain electrons are reduced; those that lose electrons are oxidized.
  • Example: AH + X^+ → A^+ + XH

• Electron Carriers:

  • Molecules like NAD (nicotinamide adenine dinucleotide) exist in oxidized (NAD^+) or reduced (NADH) states.
  • NADH carries 2 electrons and 1 proton (H^+) more than NAD+
  • NAD+ accepts electrons, and NADH donates them, shuttling electrons to electron transport chains for ATP production.
  • Other electron carriers include FAD+, FADH2, NADP+, and NADPH.

• ATP in Living Systems:

  • Hydrolysis of ATP to ADP + Pi is exergonic, providing energy for coupled endergonic reactions.
  • Dephosphorylation is the removal of a phosphate group from a molecule.
  • Phosphorylation is adding a phosphate group to a molecule, making it more reactive.
  • ATP is generated by phosphorylating ADP (ADP + Pi → ATP), an endergonic reaction.
  • Energy for ATP generation comes from:
    • Coupled exergonic reactions (substrate-level phosphorylation).
    • Chemiosmosis, requiring ATP synthase.
  • 90% of ATP is produced by chemiosmosis in mitochondria, chloroplasts, and plasma membranes of aerobic prokaryotes.

• Cellular Respiration Equation:

  • C6H{12}O6 + 6 O2 → 6 CO2 + 6 H2O + ~36 ATP

• Metabolic Pathways Involved:

  1. Glycolysis
  2. Oxidation of Pyruvate and Citric Acid Cycle
  3. Oxidative Phosphorylation

Glycolysis

• Glycolysis is the initial metabolic pathway of glucose metabolism, involving 10 enzymatic reactions.

• It occurs in nearly all organisms, in the cytoplasm, and does not require oxygen.

• Reactants: 1 Glucose, 2 NAD^+, 2 ATP, 4 ADP

• Products: 2 Pyruvate, 2 NADH, 4 ATP, 2 ADP

• Two Halves of Glycolysis:

  • First Half: Uses 2 ATP molecules. Glucose is phosphorylated twice and split into two three-carbon molecules called glyceraldehyde-3-phosphate (G3P).
  • Second Half: Involves phosphorylation without ATP investment and produces two NADH and four ATP molecules, generating two pyruvate molecules.
  • Net ATP production of glycolysis is 2 ATP.

• ATP is formed via substrate-level phosphorylation, where the phosphate group comes from a reactant molecule.

• Net Products of Glycolysis: 2 pyruvate molecules, 2 NADH, and 2 ATP.

Oxidation of Pyruvate and the Citric Acid Cycle

• In eukaryotic cells, if oxygen is present, the two pyruvate molecules enter the mitochondria, where each is converted to Acetyl CoA before entering the citric acid cycle (CAC).

  • A molecule of CO_2 is released.
  • Pyruvate is oxidized, transferring electrons to NADH.
  • Coenzyme A is attached.

• Inputs: 2 pyruvate, 2 NAD^+, 2 coenzyme A

• Outputs: 2 CO_2, 2 NADH, 2 acetyl CoA

• Citric Acid Cycle (CAC):

  • Occurs in the mitochondrial matrix.
  • The acetyl group from acetyl CoA is transferred to oxaloacetate to form citrate.
  • Citrate is oxidized, producing 3 NADH and 1 FADH_2.
  • 2 molecules of CO_2 are released.
  • 1 ATP is produced.
  • The final product of the citric acid cycle is oxaloacetate.
  • The cycle runs continuously with sufficient reactants.

• Products per Glucose:

  • From each pyruvate: 3 NADH, 1 FADH_2, 1 ATP
  • From each glucose: 6 NADH, 2 FADH_2, 2 ATP

• Outputs per Glucose to this Point:

  • 4 ATP by substrate-level phosphorylation (2 from glycolysis, 2 from CAC)
  • 6 CO_2 (2 from oxidation of pyruvate, 4 from CAC)
  • 10 NADH (2 from glycolysis, 2 from oxidation of pyruvate, 6 from CAC)
  • 2 FADH_2 (from CAC)
  • Glucose is completely oxidized at the end of CAC.

Oxidative Phosphorylation

• Oxidative phosphorylation uses NADH and FADH_2 from previous steps to form ATP in the mitochondrion.

• Most ATP is produced during oxidative phosphorylation.

• Oxygen is an input.

• It includes the electron transport chain and chemiosmosis, which generates ATP.

• The electron transport chain produces an [H^+] gradient, which provides the energy to power chemiosmosis.

• Electron Transport Chain (ETC):

  • A series of electron transporters are embedded in the inner mitochondrial membrane.
  • They shuttle electrons from NADH and FADH2 to O2.
  • Protons (H^+) are pumped from the mitochondrial matrix to the intermembrane space, and O2 is reduced to form H2O.
  • Each component of the ETC is more electronegative than the previous.
  • Electrons from NADH are transferred to complex I.
  • Electrons from FADH_2 enter the ETC at complex II.
  • Complex III is called cytochrome oxidoreductase.
  • Cytochrome C carries each electron to complex IV.
  • Oxygen is reduced at complex IV (the final electron acceptor).

• Chemiosmosis:

  • Chemiosmosis uses kinetic energy from protons falling down its gradient to form ATP from ADP + Pi.
  • The complex, integral protein ATP synthase, mediates this reaction.

• ATP Yield:

  • The net ATP yield from chemiosmosis is 32-34 per glucose.
  • Varies by species and how efficiently NADH from glycolysis enters mitochondria.
  • This ATP is formed by oxidative phosphorylation.
  • The net ATP yield from glycolysis and CAC is 4.
  • The total ATP yield from the overall process of cellular respiration is 36-38.
  • Cellular respiration in total stores ~34% of the energy from glucose in ATP.

Metabolism Without Oxygen

• Glycolysis occurs in aerobic and anaerobic environments.

• NAD^+ is an input of glycolysis, regenerated during oxidative phosphorylation when O_2 is present.

• Some organisms can do anaerobic respiration in the absence of oxygen.

• The final electron acceptor is an inorganic molecule, rather than oxygen.

• Fermentation:

  • Some organisms can do fermentation in the absence of oxygen; a different process than anaerobic respiration because it only involves glycolysis.
  • When O_2 is lacking, fermentation regenerates NAD^+; otherwise, glycolysis would halt.
  • Two common types of fermentation:
    • Lactic acid fermentation
    • Alcohol fermentation

• Lactic Acid Fermentation:

  • Occurs in muscle cells when O_2 is limited, mammalian red blood cells, & some bacteria, such as those in yogurt.
  • Lactate dehydrogenase catalyzes this reaction: Pyruvate + NADH → lactate + NAD^+

• Alcohol Fermentation:

  • Anaerobic yeast species.
  • This process is used in baking and brewing.
  • Pyruvate + NADH → ethanol + CO_2 + NAD^+

Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways

• Catabolic pathways for carbohydrates, lipids, and proteins eventually connect to glycolysis or the CAC pathways.
• Amino acids are deaminated, and the nitrogen is excreted in the urine.

Regulation of Cellular Respiration

• Cellular respiration is regulated by many mechanisms, including:
  • Hormonal control of glucose entry into the cell
  • Enzyme reversibility (functioning to substrate product equilibrium) or irreversibility (able to exceed equilibrium)
  • Enzyme sensitivity to pH changes due to lactic acid build-up
  • Feedback controls
• In the presence of insulin, Glut4 vesicles fuse with the plasma membrane, allowing glucose to enter the cell.
• Summary of Feedback Controls in Cellular Respiration:
  • Glycolysis:
    • Hexokinase is decreased by Glucose-6-phosphate.
    • Phosphofructokinase is increased by Fructose-6-phosphate but decreased by Citrate and acidic pH.
  • Pyruvate to acetyl coA:
    • Pyruvate dehydrogenase is increased by ADP and pyruvate but decreased by Acetyl coA, ATP, and NADH.
  • CAC
    • Isocitrate dehydrogenase is increased by ADP but decreased by ATP and NADH.
    • Ketoglutarate dehydrogenase is increased by Calcium ions and ADP but decreased by ATP, NADH, and succinyl coA.
  • ETC
    • ADP Increases
    • ATP: Decreases