Chemotropic Energy Metabolism: Glycolysis and Fermentation – Study Notes

Chapter 9: Chemotropic Energy Metabolism – Glycolysis and Fermentation

Metabolism

Metabolism encompasses all chemical reactions that occur within living cells, including both the breakdown and synthesis of biomolecules. These reactions are essential for energy production and cellular function.

• Second Law of Thermodynamics: States that the entropy (disorder) of the universe is always increasing. Chemical reactions may decrease entropy locally but increase it overall.

• Catabolism and Anabolism:

  • Catabolism is the breakdown of complex molecules into simpler ones, releasing energy.

  • Anabolism is the synthesis of complex molecules from simpler ones, requiring energy input.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell. Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate.

• Redox Reactions: Involve the transfer of electrons between molecules. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• NAD+ (Nicotinamide Adenine Dinucleotide): An electron carrier that accepts electrons during catabolic reactions, becoming NADH.

Glycolysis

Glycolysis is a metabolic pathway that converts glucose into pyruvate, generating ATP and NADH. It occurs in the cytosol and does not require oxygen.

• Phases of Glycolysis:

  1. Preparatory (investment) phase: Glucose is phosphorylated and split into two 3-carbon sugars. Requires input of 2 ATP.

  2. Payoff phase: 3-carbon sugars are oxidized, generating 4 ATP and 2 NADH per glucose.

• Key Steps and Enzymes:

  1. Glucose is phosphorylated by ATP (hexokinase/glucokinase) to form glucose-6-phosphate (G-6-P).

  2. G-6-P is isomerized to fructose-6-phosphate (F-6-P).

  3. F-6-P is phosphorylated by ATP (phosphofructokinase) to fructose-1,6-bisphosphate (F-1,6-BP).

  4. F-1,6-BP splits into two 3-carbon sugars: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). DHAP is converted to G3P.

• Energy Generation Steps:

  • Oxidation: G3P is oxidized, reducing NAD+ to NADH and allowing for ATP generation.

  • Substrate-level phosphorylation: Direct transfer of phosphate to ADP to form ATP.

• Products of Glycolysis (per glucose):

  • 2 pyruvate

  • 2 NADH

  • 2 ATP (net gain)

Regulation of Glycolysis

Glycolysis is regulated at key enzymatic steps to ensure efficient energy production and prevent waste.

• Hexokinase/Glucokinase: Inhibited by G-6-P (product inhibition).

• Phosphofructokinase: Allosterically regulated by ATP (inhibitor) and AMP (activator).

• Pyruvate kinase: Regulated by ATP (inhibitor) and fructose-1,6-bisphosphate (activator).

Fermentation

Fermentation allows cells to regenerate NAD+ from NADH in the absence of oxygen, enabling glycolysis to continue.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate, regenerating NAD+. Occurs in muscle cells under anaerobic conditions.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+. Occurs in yeast and some bacteria.

Key Equations

• ATP Hydrolysis: $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

• Overall Glycolysis Reaction: $\text{Glucose} + 2\ NAD^+ + 2\ ADP + 2\ P_i \rightarrow 2\ Pyruvate + 2\ NADH + 2\ ATP + 2\ H_2O$

Comparison Table: Catabolism vs. Anabolism

Process	Function	Energy	Example
Catabolism	Breakdown of molecules	Releases energy	Glycolysis
Anabolism	Synthesis of molecules	Requires energy	Protein synthesis

Additional info:

• Glycolysis is a universal pathway found in nearly all organisms.

• NAD+ and NADH are essential for redox reactions in metabolism.

• Fermentation is less efficient than aerobic respiration but is vital under anaerobic conditions.