Lecture on Glycolysis and Glucose Metabolism

Introduction to Glycolysis

• Glycolysis is a key metabolic pathway for energy production.
• It is the beginning step for cellular respiration or fermentation.
• Uses substrate-level phosphorylation, known for its speed but not efficiency.
• Splits 6-carbon glucose into two 3-carbon pyruvates.

Key Stages of Glycolysis

1. **Stage 1: Preparation

   • Investment of ATP is required to initiate the process.
   • No production occurs yet.

2. **Stage 2: Production

   • NAD+ is reduced to NADH in a rate-limiting step.
   • Production of 4 ATP, net gain of 2 ATP after subtracting the 2 ATP invested.
   • Results in two 3-carbon pyruvates.

3. **Stage 3: Option of Fermentation

   • If no oxygen, glycolysis can continue anaerobically.
   • NADH needs to be oxidized back to NAD+ for the process to continue.
   • Pyruvate acts as an electron acceptor.**

Fermentation

• Uses organic electron acceptors.
• Allows glycolysis to continue without oxygen.
• Can produce various byproducts, like lactic acid or alcohol.
• Not the most efficient but fast.
• Tumor cells and many microbes use fermentation despite oxygen availability.

Considerations and Variations

• Oxygen availability is not the only factor influencing glycolysis or fermentation.
• Membrane space and enzyme availability can also affect whether a cell uses glycolysis alone.
• Cells may have different end products based on metabolism. For example:
  • Yeast: Ethanol (alcohol)
  • Streptococcus: Lactic acid
  • Clostridium: Acetate
• Fermentation byproducts can have various effects, such as causing dental caries from lactic acid.

Summary

• Glycolysis is a rapid but inefficient pathway for ATP generation.
• Fermentation allows continuation of glycolysis in the absence of oxygen and under certain cellular conditions.
• Cells prioritize speed over efficiency depending on their metabolic needs and environmental constraints.

Important Enzymes and Terms

• Kinases: Involved in phosphate transfer (e.g., Hexokinase, Phosphofructokinase).
• Isomerases: Change molecule shape without adding or removing components.
• NAD+/NADH: Essential for glycolysis to proceed.

Additional Notes

• Double-membraned bacteria (gram-negative) relate to endosymbiosis theories.
• Detailed glycolysis pathways are more thoroughly covered in upper-level courses.
• Table 3.4 in the textbook provides further details on fermentation byproducts.