03b_-_Energy_Part_III_Notes_-_Cellular_Respiration

I. Introduction

• Energy Storage in Food Molecules

  • Food molecules like glucose and triglycerides store substantial chemical potential energy.

  • This energy is not readily accessible without conversion to ATP.

  • ATP serves as a usable energy form for cellular processes.

• Production of ATP

  • Fermentation

    • Primitive ATP production method.

    • Does not require oxygen, resulting in partial glucose breakdown.

    • Generates small amounts of ATP per glucose, with waste products like lactic acid or ethanol.

  • Aerobic Respiration

    • More efficient process requiring oxygen.

    • Involves complete glucose breakdown into CO2 and H2O.

    • Can utilize carbohydrates, lipids, or proteins as substrates, focusing on glucose.

    • Represented by the equation:

      • C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + Heat)

II. Redox (Reduction-Oxidation) Reactions

• Definitions

  • Reduction: Addition of electrons to a substance.

  • Oxidation: Removal of electrons from a substance.

• Examples

  • Reduction agent: Donates electrons (e.g., sodium).

  • Oxidizing agent: Accepts electrons (e.g., chlorine).

• Electron Sharing

  • Not all redox reactions involve complete electron transfer; some involve shared electrons leading to different electronegativities.

III. Cellular Respiration

A. Overview

• Overall Reaction:

  • C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + Heat)

  • Gibbs free energy change (ΔG): -686 kcal/mol.

• Stages of Cellular Respiration:

  1. Glycolysis

  • Occurs in the cytoplasm.

  • Converts glucose to pyruvate (3 carbon molecules).

  2. Kreb's Cycle

  • Pyruvates converted to acetyl-CoA in the mitochondrion.

  • Acetyl-CoA broken down to produce CO2.

  3. Electron Transport Chain (ETC)

  • Utilizes NADH from glycolysis and Kreb's Cycle to produce ATP.

• Forms of ATP Production

  • Oxidative Phosphorylation: ATP production through electron transfer to O2.

  • Substrate-level phosphorylation: Direct ATP production during glycolysis and Kreb's Cycle.

B. Glycolysis

• Breakdown of glucose into two pyruvate molecules.

• Phases of Glycolysis:

  • Energy Investment Phase: 2 ATP used to add phosphates to intermediates.

  • Energy Payoff Phase: Produces 4 ATP (net gain 2 ATP) and reduces 2 NAD+ to 2 NADH.

• Anaerobic Conditions:

  • Pyruvates converted to lactic acid or ethanol, losing energy.

• Aerobic Conditions:

  • Pyruvates continue to cellular respiration.

C. Kreb's Cycle

• Occurs in the mitochondrion matrix.

• Pyruvates convert to acetyl-CoA; decarboxylation releases CO2.

• NADH and FADH2 production through oxidation of intermediates.

• ATP produced via substrate-level phosphorylation.

• Each Kreb's Cycle turn yields:

  • 3 NADH, 1 FADH2, 1 ATP (or GTP).

• For each glucose, 2 turns of the Kreb's Cycle produce:

  • 6 NADH, 2 FADH2, 2 ATP.

D. Electron Transport Chain (ETC)

• Series of electronegative carriers culminating in oxygen as the final electron acceptor.

• Embedded in the inner mitochondrial membrane (cristae).

• Complex I: NADH donates electrons, leading to H+ pumping into intermembrane space.

• Complex II: FADH2 adds electrons without H+ pumping.

• Cytochrome pathway through complexes III & IV ends with oxygen forming water.

E. Chemiosmosis

• Utilizes the established H+ gradient (proton-motive force) to produce ATP via ATP synthase.

• ATP synthase works like a turbine to generate ATP from ADP and Pi.

F. ATP Yield and Energy Accounting

• Each NADH generates ~2.5 ATP, total of 25 ATP from 10 NADH.

• Each FADH2 generates ~1.5 ATP, total of 3 ATP from 2 FADH2.

• Total yield from one glucose: 32 ATP.

• Efficiency: 34%, with 66% of energy lost as heat.

G. Other Catabolic Pathways

• Glucose isn’t the sole ATP source; carbohydrates, fats, and proteins can also be metabolized.

• Other substrates enter at various cellular respiration stages.

H. Energy for Muscle Contraction

• Aerobic respiration is slow and not suited for short bursts of muscle activity.

• ATP production mechanisms include direct phosphorylation by creatine phosphate and anaerobic respiration leading to lactic acid buildup.

I. Temperature Regulation

• Ectothermic: Body temperature regulated by environmental heat.

• Endothermic: Maintain constant temperature via metabolic heat.