DS

Glycolysis and Cellular Respiration Review

Chapter 9: Chemotrophic Energy Metabolism: Glycolysis & Fermentation

1. Definitions

  • Anabolic pathways:

    • Definition: Reactions which build larger molecules from smaller ones.

    • Energy Use: Use energy.

  • Catabolic pathways:

    • Definition: Reactions which break down larger molecules into smaller ones.

    • Energy Release: These release energy for use in the body.

2. Adenosine Triphosphate (ATP)

  • Description of ATP:

    • ATP is a nucleotide composed of Adenine, ribose, and three phosphates.

    • ATP is considered the "energy currency" of the cell because it stores and provides energy for cellular processes.

3. ATP Hydrolysis

  • Definition of Hydrolysis:

    • Hydrolysis refers to the process of "splitting with water".

  • ATP Hydrolysis Process:

    • ATP stores energy in a bond between the last two phosphate groups (the “terminal bond”).

    • When ATP is hydrolyzed into Adenosine Diphosphate (ADP) + inorganic phosphate (Pi), energy is released.

  • Catalysis:

    • This reaction is catalyzed by the enzyme ATPase.

  • Energy Released:

    • 7.3 kcal/mol of energy is released under standard conditions.

  • Factors Contributing to Energy Release:

    1. Charge Repulsion:

      • The strong charge repulsion between the negatively charged phosphates in ATP puts “strain” on the covalent bonds between the phosphates, making it energetically favorable to break.

    2. Stability Change:

      • After the bond is broken, ATP moves from a state of repulsion to a more stable, lower-energy state (ADP + Pi).

    3. Resonance Stabilization:

      • When ATP breaks into ADP + Pi, the negative charges "spread their electrons over multiple atoms" which reduces the energy of the molecules.

      • Example of Resonance Stability:

        • A "resonance-stabilized" molecule, like Pi, is where electrons are delocalized enough to have multiple resonance structures. This delocalized state lowers the overall energy and increases stability compared to a structure with localized electrons.

4. Types of Biochemical Reactions

  • Group Transfer Reactions:

    • A biochemical reaction where a functional group is transferred from one molecule (the donor) to another (the acceptor).

  • Dehydrogenation:

    • A type of oxidation reaction where a molecule loses hydrogen atoms, resulting in a loss of electrons.

  • Coenzymes:

    • Small, non-protein organic molecules that help an enzyme carry out its function.

    • Usually derived from vitamins.

    • Example:

      • NAD+/NADH, derived from Vitamin B3 (Niacin), helps in electron transfer in redox reactions.

5. Types of Organisms Based on Oxygen Requirement

  • Obligate Aerobes:

    • Organisms that require oxygen to survive and grow; rely on aerobic respiration to generate ATP where oxygen is the final electron receptor.

  • Obligate Anaerobes:

    • Microorganisms that cannot survive in the presence of oxygen; oxygen is toxic to them as they lack enzymes to detoxify reactive oxygen species (ROS); rely on fermentation or anaerobic respiration.

  • Facultative Anaerobes:

    • Microorganisms capable of growing with or without oxygen, favoring aerobic respiration but switching to anaerobic respiration when oxygen is absent, e.g., E. coli.

6. Cellular Respiration Overview

  • Definition:

    • A series of metabolic reactions converting biochemical energy from nutrients into ATP, using oxygen in aerobic respiration, and releasing waste products.

  • Processes Involved:

    1. Glycolysis

    2. Krebs Cycle

    3. Electron Transport Chain

  • Equation for Cellular Respiration:

    • C6H{12}O6 + 6O2
      ightarrow 6CO2 + 6H2O + ext{ energy } ext{ (~36-38 ATP captured) ~ 686 kcal/mol (released)}

7. Glycolysis

  • Overview of Glycolysis:

    • Glycolysis is the first step in Cellular Respiration.

    • Converts glucose into pyruvate, yielding 2 ATP as a net gain.

  • Major Purpose of Glycolysis:

    1. To extract energy from glucose

    2. To produce pyruvate, which feeds into the Krebs cycle.

    3. To generate NADH, which powers the Electron Transport Chain.

  • Steps of Glycolysis:

    1. Glucose to Glucose-6-Phosphate:

      • Enzyme: Hexokinase

      • Energy: 1 ATP used.

      • An “intermediate” is formed temporarily before the final product is made.

    2. Glucose-6-Phosphate to Fructose-6-Phosphate:

      • Enzyme: Phosphoglucoisomerase.

    3. Fructose-6-Phosphate to Fructose-1,6-Bisphosphate:

      • Enzyme: Phosphofructokinase.

      • Energy: 1 ATP is used.

      • Note: Steps 1-3 constitute the Energy Investment Phase of glycolysis where ATP is consumed.

    4. Fructose-1,6-bisphosphate splits into DHAP and G3P:

      • Enzyme: Aldolase.

    5. DHAP to G3P:

      • Enzyme: Triose Phosphate Isomerase.

    6. G3P to 1,3-Bisphosphoglycerate:

      • Enzyme: Glyceraldehyde-3-phosphate dehydrogenase.

      • Energy: 1 NADH is produced.

    7. 1,3-Bisphosphoglycerate to 3-Phosphoglycerate:

      • Enzyme: Phosphoglycerate kinase.

      • Energy: 1 ATP is formed.

    8. 3-Phosphoglycerate to 2-Phosphoglycerate:

      • Enzyme: Phosphoglycerate mutase.

    9. 2-Phosphoglycerate to Phosphoenolpyruvate (PEP):

      • Enzyme: Enolase.

      • Byproduct: 1 H2O released.

    10. PEP to Pyruvate:

      • Enzyme: Pyruvate kinase.

      • Energy: 1 ATP formed per PEP.

  • Summary of Glycolysis:

    • Input: 1 Glucose molecule, 2 ATP, 2 NAD+, 2 Pyruvate molecules.

    • Output: 4 ATP (gross production), 2 ATP (net production), 2 NADH.

8. Fate of Pyruvate Under Different Conditions

  • Aerobic Conditions:

    • Process: Pyruvate is converted to Acetyl-CoA and enters the Krebs Cycle for further cellular respiration.

    • Final Electron Acceptor: Oxygen is the final electron acceptor in the Electron Transport Chain (ETC).

  • Anaerobic Conditions:

    • In Animals:

      • Pyruvate is converted to lactate.

      • Lactate can be converted back to glucose in the liver via the Cori cycle and returned to muscles for energy.

      • No further ATP is generated beyond glycolysis, termed “Lactic Acid Fermentation.”

    • In Yeast/Microbes:

      • Pyruvate is converted to ethanol and CO2 via fermentation.

      • No ATP is made beyond glycolysis, termed “Alcoholic Fermentation.”

9. Gluconeogenesis

  • Definition:

    • Metabolic pathway which produces glucose from non-carbohydrate precursors such as pyruvate, glycerol (from fat breakdown), and amino acids (mainly alanine).

  • Location:

    • Occurs mainly in the liver.

  • Concept:

    • Think of gluconeogenesis as glycolysis in reverse.

10. Regulation of Glycolysis

  1. Allosteric Enzyme Regulation:
    - Many glycolytic enzymes have allosteric sites for inhibition.
    - Example: If [ATP] builds up, it can inhibit glycolysis via phosphofructokinase.

  2. AMP Activation:
    - When ATP is consumed faster than produced, ADP can convert into AMP.
    - High AMP levels signal low energy reserves, activating phosphofructokinase.

  3. Hormonal Control:
    - Hormones such as Glucagon and Epinephrine signal low blood sugar or stress, inhibiting glycolysis in the liver to reserve glucose for critical organs such as the brain and muscles.

11. Galactosemia

  • Definition:

    • A genetic disorder where the body lacks enzymes to break down galactose, leading to its toxic accumulation and resulting in organ damage, especially in the liver, brain, and eyes.

  • Treatment:

    • Strict lifelong avoidance of galactose (no milk, dairy, or lactose-containing foods).

    • In the U.S., screening for galactosemia is performed shortly after birth.