Cellular Respiration

• Definition: Cellular respiration is the process by which cells convert glucose and oxygen into carbon dioxide, water, and energy (ATP).

• Chemical Equation: C6H{12}O6 + 6 O2 ightarrow 6 CO2 + 6 H2O + {ATP}

  • Components:

  • Glucose (C6H12O6)

  • Oxygen (O2)

  • Carbon Dioxide (CO2)

  • Water (H2O)

  • Energy (ATP)

ATP Production Methods

• There are two primary ways to generate ATP:

  1. Substrate Level Phosphorylation

  2. Electron Transport Phosphorylation (Chemiosmosis)

• Chemiosmotic Gradient: The ATP production through electron transport is driven by a chemiosmotic gradient, where protons flow through the ATP synthase channel protein, leading to ATP formation.

Summary of ATP Synthesis Process

• ATP Synthase mechanism:

  • H+ ions (protons)

  • ADP with inorganic phosphate (P) combine to form ATP

Stage 1: Glycolysis

• Overview: Glycolysis occurs in the cytoplasm and breaks down glucose into pyruvate while generating ATP and NADH.

• Steps in Glycolysis:

  1. Energy Investment Phase: Consumes 2 ATP.

  • Changing glucose:

    • Hexokinase Reaction:

    • Glucose + ATP → Glucose-6-phosphate + ADP

    • Phosphoglucomutase: Converts glucose-6-phosphate to fructose-6-phosphate.

    • Phosphofructokinase Reaction:

    • Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP

  1. Cleavage Stage: Breakdown of fructose-1,6-bisphosphate into two 3-carbon molecules (DHAP and G3P).

  2. Energy Payoff Phase: Generates 4 ATP and 2 NADH.

  • Triose-phosphate Isomerase:

    • Converts DHAP to G3P.

  • Glyceraldehyde-3-phosphate Dehydrogenase:

    • G3P + NAD+ + P → 1,3-bisphosphoglycerate + NADH + H+

  • Phosphoglycerate Kinase Reaction:

    • 1,3-bisphosphoglycerate turns into 3-phosphoglycerate with ATP generated.

  • Pyruvate Kinase: Converts phosphoenolpyruvate to pyruvate while making ATP.

• Net Yield:

  • 2 ATP (net gain, after investing 2 ATP)

  • 2 NADH

  • 2 Pyruvate

• Note: Glycolysis does not require oxygen and is considered anaerobic.

Stage 2: Kreb's Cycle (Citric Acid Cycle)

• Overview: Occurs in the mitochondria, turning pyruvate into Acetyl Co-A and producing NADH and FADH2.

• Preparatory Step (Before Kreb's Cycle):

  • Pyruvate interacts with Coenzyme A (CoA) to produce Acetyl Co-A.

  • This step releases 2 CO2 and generates 2 NADH.

• Kreb's Cycle Stages:

  1. Formation of Citrate:

  • Acetyl Co-A combines with Oxaloacetate to form Citrate.

  1. Transformation:

  • Citrate undergoes conversion through multiple intermediates, such as Isocitrate and a-Ketoglutarate.

  • Production of

    • 2 CO2 per Acetyl Co-A.

    • 3 NADH, 1 FADH2, and 1 ATP by substrate-level phosphorylation for each Acetyl Co-A.

• Products of Kreb's Cycle (per 2 Acetyl Co-A):

  • 4 CO2, 6 NADH, 2 FADH2, and a total of 2 ATP.

Stage 3: Electron Transport Phosphorylation

• Overview: Takes place on the inner mitochondrial membrane, utilizing high-energy electrons from NADH and FADH2 to produce ATP through a proton gradient (chemiosmosis).

• Process Description:

  1. Electron Carriers: Transport high energy electrons to the mitochondrial enzymes.

  2. H+ Ion Pumping: As electrons are passed along the chain, H+ ions are pumped into the intermembrane space, creating a proton gradient.

  3. ATP Synthase: H+ ions flow back across the membrane through ATP synthase, producing ATP.

  4. Final Electron Acceptor: Oxygen acts as the final electron acceptor, forming water as a product (O2 + 4H+ + 4e- → 2H2O).

• Average Yield: Approximately 32 ATP generated during the entire process of electron transport phosphorylation.

Overall Summary of Cellular Respiration

• Overall Reaction: C6H{12}O6 + 6 O2
  to 6 CO2 + 6 H2O + 36 {ATP}

• Metabolic Pathways Breakdown:

  1. Glycolysis:

  • Input: Glucose;

  • Output: 2 ATP (net), 2 NADH, 2 Pyruvate.

  1. Preparatory Step in Mitochondria:

  • Input: 2 Pyruvate;

  • Output: 2 CO2, 2 NADH, 2 Acetyl-CoA.

  1. Kreb's Cycle:

  • Input: 2 Acetyl-CoA;

  • Output: 4 CO2, 6 NADH, 2 FADH2, 2 ATP (total output when considering both Acetyl-CoA).

  1. Electron Transport Phosphorylation:

  • Summary: Produces approximately 32 ATP, with oxygen being essential for the process.

Connections

• There is a strong interconnection between these pathways:

  • The end products of glycolysis feed into the Kreb's cycle and the electron transport chain, demonstrating how energy is channeled within cellular respiration.

• Energy Efficiency: The total theoretical yield of ATP from complete oxidation of glucose is about 36 ATP molecules, showcasing how cells efficiently convert energy from nutrients to biochemical energy.