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Cellular Respiration

Overview

Cellular respiration is a biochemical process through which cells convert organic compounds into energy in the form of ATP. It involves several key processes:

  • Glycolysis

  • Fermentation

  • The Mitochondrion

  • Oxidation of Pyruvate

  • The Citric Acid Cycle

  • The Electron Transport Chain and Oxidative Phosphorylation

  • ATP Synthesis

  • The Metabolism of Different Foods

References

  • Pages: 448-451, 456-463, 479-485, 488

Glycolysis

General Description

Glycolysis is the metabolic pathway that converts glucose into two molecules of pyruvate, occurring in the cytoplasm. It consists of ten steps divided into two phases:

  • Energy Investment Phase: Initial steps where ATP is consumed.

  • Energy Generation Phase: Subsequent steps where ATP and NADH are produced.

Reaction Steps

  1. Initial Phosphorylation (Step 1)

    • Substrate: Glucose (C6)

    • Enzyme: Hexokinase

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

    • Definition: Phosphorylation – the addition of a phosphate group to a molecule. ATP is consumed in this reaction.

  2. Isomerization (Step 2)

    • Enzyme: Phosphoglucose Isomerase

    • Reaction: Glucose-6-phosphate ↔ Fructose-6-phosphate

    • Definition: Isomerization – conversion of a compound into its structural isomers.

  3. Phosphorylation (Step 3)

    • Enzyme: Phosphofructokinase

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

    • ATP consumed.

  4. Cleavage (Step 4)

    • Enzyme: Aldolase

    • Reaction: Fructose-1,6-bisphosphate → Dihydroxyacetone phosphate + Glyceraldehyde-3-phosphate

  5. Isomerization of DHAP (Step 5)

    • Enzyme: Triose Phosphate Isomerase

    • Conversion: Dihydroxyacetone phosphate ↔ Glyceraldehyde-3-phosphate (G3P)

  6. Oxidation of G3P (Step 6)

    • Reaction: 2 G3P + NAD+ → 2 1,3-Biphosphoglycerate (BPG) + 2 NADH

    • Pi is attached to each C1.

  7. ATP Synthesis by Substrate-Level Phosphorylation (Step 7)

    • Reaction: 2 BPG + 2 ADP → 2 3-Phosphoglycerate + 2 ATP

  8. Isomerization (Step 8)

    • Enzyme: Phosphoglycerate Mutase

    • Reaction: 3-Phosphoglycerate → 2-Phosphoglycerate

  9. Dehydration (Step 9)

    • Enzyme: Enolase

    • Reaction: 2-Phosphoglycerate → Phosphoenolpyruvate (PEP) + H2O

  10. Final ATP Synthesis (Step 10)

    • Reaction: 2 PEP + 2 ADP → 2 Pyruvate + 2 ATP

Yield of Glycolysis

  • ATP Consumption: 2 ATP during investment phase.

  • ATP Production: 4 ATP during generation phase.

  • Net Yield: 2 ATP and 2 NADH.

Fermentation

  • Under anaerobic conditions, pyruvate and NADH are consumed in fermentation.

  • Products: Lactic acid in muscle cells, ethanol, and CO2 in yeast.

  • Reactions: NADH is oxidized to regenerate NAD+; pyruvate is reduced.

The Mitochondrion

Structure and Function

The mitochondrion is a double-membraned organelle crucial for aerobic respiration in eukaryotic cells. It consists of:

  • Outer Membrane: Smooth and permeable to small molecules.

  • Inner Membrane: Contains proteins for the electron transport chain and ATP synthesis.

  • Cristae: Infoldings of the inner membrane, increasing surface area.

  • Matrix: Contains enzymes for the citric acid cycle and the oxidation of pyruvate.

Key Reactions in the Mitochondrion

  • Oxidation of Pyruvate: Pyruvate reacts with coenzyme A (CoA) to form acetyl CoA and CO2.

    • Reaction: Pyruvate + CoA-SH → Acetyl CoA + CO2 + NADH

  • Citric Acid Cycle: Acetyl CoA enters the cycle, leading to the production of NADH, FADH2, and ATP.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle occurs in the matrix of mitochondria and processes acetyl CoA to produce:

  • CO2: By-products of the cycle.

  • Energy Carriers: 3 NADH and 1 FADH2 per cycle turn.

  • ATP: 1 ATP per cycle turn.

Key Steps of the Citric Acid Cycle

  1. Condensation: Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

  2. Isomerization: Citrate is converted to isocitrate.

  3. Oxidation: Isocitrate is oxidized, releasing CO2 and generating NADH.

  4. Decarboxylation: a-ketoglutarate (5C) converted, generating NADH and releasing another CO2.

  5. Conversion: Succinyl CoA (4C) to succinate (4C) generating GTP/ATP.

  6. Oxidations: Succinate is oxidized, generating FADH2.

  7. Final Oxidation: Malate is oxidized to regenerate oxaloacetate and produce NADH.

Electron Transport Chain and Oxidative Phosphorylation

Overview

Located in the inner mitochondrial membrane, the electron transport chain (ETC) consists of enzyme complexes that transfer electrons from NADH and FADH2, facilitating ATP production through oxidative phosphorylation.

Process

  1. Electron donation: Electrons from NADH and FADH2 are donated to the electron transport chain.

  2. Hydrogen Ion Pumping: Electron transport leads to the pumping of H+ ions into the intermembrane space, creating a proton gradient.

  3. ATP Synthesis: H+ flows back into the mitochondrial matrix through ATP synthase, synthesizing ATP from ADP and inorganic phosphate.

Final Electron Acceptor

  • Oxygen: Combines with electrons and H+ to form water:

    • Reaction: rac{1}{2}O2 + 2H^+ + 2e^- → H2O

ATP Yield in Aerobic Respiration

  • Net Yield per glucose molecule (including glycolysis, citric acid cycle, and oxidative phosphorylation):

    • ATP from glycolysis: 2

    • ATP from citric acid cycle: 2

    • ATP from oxidative phosphorylation: 25 (NADH) + 3 (FADH2) → Total = 32 ATP (theoretical)

    • Actual yield may differ due to transport costs of NADH to mitochondria.

Metabolism of Different Foods

Conversion of Biomolecules

  • Amino Acids: Deaminated to generate pyruvate.

  • Fatty Acids: Broken down through beta-oxidation to generate acetyl CoA.

  • Glycerol: Can also enter glycolysis.

General Pathway Stages

  • Stage 1: Glycolysis

  • Stage 2: Breakdown to Acetyl CoA, producing limited ATP.

  • Stage 3: Complete oxidation in mitochondrion, yielding large amounts of NADH and ATP.

Key Takeaways for Memorization

  • Memorize each step in glycolysis, fermentation, and the citric acid cycle including flowcharts.

  • Know names of intermediates, the carbon number, and yield of energy molecules.

  • Understand where reactions occur within cellular compartments, emphasizing the mitochondrion.

  • Understand oxidative phosphorylation processes and how ATP synthesis relates to the electron transport chain output.