DS

Chemotrophic Energy Metabolism: Aerobic Respiration

Chapter 10: Chemotrophic Energy Metabolism: Aerobic Respiration pt. 1

1. Mitochondria

  • Definition: Eukaryotic organelle whose main job is to produce ATP, often referred to as the "powerhouse of the cell".

  • Structure:

    • Composed of an outer membrane and an inner membrane.

    • Inner Membrane:

    • Contains finger-like folds known as "cristae" which increase surface area.

    • The Electron Transport Chain (ETC) is embedded in this membrane, providing more surface area which facilitates more energy reactions.

2. Mitochondrial Matrix

  • Definition: The fluid-filled space within the inner membrane of mitochondria.

  • Function: The site of the Krebs cycle which occurs following glycolysis.

3. Overview of ATP Production Processes

  • Main Inputs: Glucose

  • Processes involved in ATP production:

    • Glycolysis

    • Krebs Cycle

    • Electron Transport Chain

  • Overall Purpose: Take nutrients (mainly glucose) and store energy in ATP for the body to use.

Detailed Pathway:

  • Glycolysis:

    • Input: Glucose

    • Output: 2 ATP and Pyruvate

  • Krebs Cycle (TCA Cycle):

    • Input: Pyruvate (which undergoes conversion to Acetyl-CoA before entering the cycle)

    • Outputs: 2 ATP, CO₂, and various reduced coenzymes (NAD⁺ → NADH).

  • Oxidative Phosphorylation: Produces approximately 32 ATP from each glucose molecule.

4. Steps of Cellular Respiration (6 Steps):

  1. Glycolysis

  2. Krebs Cycle

  3. Electron Transport Chain

  • Overall Purpose: Convert nutrients like glucose into usable energy in the form of ATP.

5. Glycolysis Breakdown

  • Key Enzymes & Intermediates:

    • Hexokinase: Converts Glucose to Glucose-6-phosphate.

    • Phosphofructo-kinase: Transforms Fructose-6-phosphate to Fructose-1,6-bisphosphate.

    • Pyruvate Kinase: Converts Phosphoenolpyruvate to Pyruvate.

  • Pathway: Glucose → Fructose-1,6-bisphosphate → Glyceraldehyde 3-phosphate → Pyruvate.

6. Krebs Cycle (TCA Cycle) Details

  • Step 0: Link Reaction/Pyruvate Oxidation:

    • From: Pyruvate to Acetyl-CoA via enzyme pyruvate dehydrogenase complex.

    • Outputs: 1 CO₂ and 1 NADH per pyruvate (2 pyruvates produced by glycolysis).

Main Steps of the Krebs Cycle:

  1. Acetyl-CoA + Oxaloacetate → Citrate

    • Enzyme: Citrate synthase.

    • Reaction uses: H₂O and releases CoA-SH.

  2. Citrate → Isocitrate

    • Enzyme: Aconitase.

  3. Isocitrate → α-Ketoglutarate

    • Enzyme: Isocitrate dehydrogenase.

    • Outputs: 1 CO₂ and 1 NADH.

  4. α-Ketoglutarate → Succinyl-CoA

    • Enzyme: α-Ketoglutarate dehydrogenase.

    • Outputs: 1 CO₂ and 1 NADH.

  5. Succinyl-CoA → Succinate

    • Enzyme: Succinyl-CoA synthetase.

    • Outputs: 1 GTP (an ATP equivalent).

  6. Succinate → Fumarate

    • Enzyme: Succinate dehydrogenase.

    • Outputs: 1 FADH₂.

  7. Fumarate → Malate

    • Enzyme: Fumarase.

    • Uses: H₂O.

  8. Malate → Oxaloacetate

    • Enzyme: Malate dehydrogenase.

    • Outputs: 1 NADH.

  • Cycle Continuity: Oxaloacetate re-enters by joining with Acetyl-CoA to form Citrate. Throughout, the cycle produces: NADH, GTP/ATP, FADH₂, and CO₂.

7. Fat Catabolism

  • Concept: Stored fats are mainly in the form of triglycerides.

  • Process: In the absence of carbohydrates, hormones can trigger the catabolism of triglycerides into glycerol and fatty acids.

    • Glycerol: Can be converted into G3P and enter glycolysis.

    • Fatty Acids: Undergo beta oxidation, breaking down long chains into 2-carbon units (acetyl-CoA), which can enter the Krebs cycle to generate ATP.

8. Amphibolic Pathway

  • Definition: A metabolic pathway that functions in both catabolism and anabolism.

    • Example: Acetyl-CoA can be broken down in catabolism and its intermediates can be used to build molecules like cholesterol in anabolism.

9. Glyoxylate Cycle

  • Definition: A metabolic pathway utilized in plants, bacteria, fungi, and protists for alternative carbon sources when glucose is not available.

  • Functionality: Works similarly to the Krebs cycle, but includes two key enzyme swaps - isocitrate lyase and malate synthase.

10. Differences in Cycles

Krebs Cycle Steps:

  1. Oxaloacetate

  2. Citrate

  3. Isocitrate

  4. α-Ketoglutarate

  5. Succinyl-CoA

  6. Succinate

  7. Fumarate

  8. Malate

  9. Oxaloacetate

Glyoxylate Cycle Steps:

  1. Oxaloacetate

  2. Citrate

  3. Isocitrate

  4. α-Ketoglutarate

  5. Succinyl-CoA

  6. Succinate + glyoxylate

  7. Fumarate

  8. Malate

  9. Oxaloacetate

  • Enzymes Swapped in Glyoxylate Cycle: Isocitrate lyase and malate synthase facilitate functioning with alternative inputs.

11. Electron Transport System Carriers (5 Types):

  1. NADH and FADH₂: Provides high-energy electrons to initiate the electron transport chain.

  2. Flavoproteins: Proteins with flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) as cofactors, derived from Vitamin B2 (riboflavin).

    • Example: Complex I uses FMN to accept electrons from NADH.

  3. Iron-Sulfur Proteins: These proteins contain clusters of iron and sulfur, capable of transferring electrons one at a time.

  4. Cytochromes: These are proteins containing heme groups - structures that facilitate iron transitions from Fe³⁺ to Fe²⁺, primarily found in Complexes III and IV.

  5. Ubiquinone (Coenzyme Q, CoQ10): Mobile electron carrier that transfers electrons between Complexes I/II and III.