Glucose Metabolism and Cellular Respiration

Overview of Glucose Metabolism

  • Glucose is the most commonly utilized sugar molecule for generating ATP (adenosine triphosphate), which is essential for energy in cells.
  • The average person synthesizes their weight in ATP molecules daily.
  • ATP is crucial for metabolic reactions, growth, tissue maintenance, and healing.

Energy Requirements in Cells

  • Single-celled organisms:
    • Need energy for metabolic reactions, motility (movement), and reproduction.
  • This module will focus on how one glucose molecule generates ATP through a series of reactions.

Catabolic Reactions in Glucose Metabolism

  • Glucose must be broken down through catabolic reactions that transform it into intermediates and generate ATP.
  • Major steps of glucose metabolism include:
    1. Glycolysis: Takes place in the cell cytoplasm.
    • In prokaryotic cells, products move to the plasma membrane.
    • In eukaryotic cells, products move to the mitochondria.
    1. Krebs Cycle (also known as TCA Cycle or Citric Acid Cycle).
    2. Electron Transport Chain (ETC): Occurs in mitochondria.

The Importance of Understanding Glucose Metabolism

  • Glucose metabolism can be intimidating, but it follows a step-by-step process.
  • Focus on the overall picture instead of getting bogged down in details.
  • Key points to consider: reactants and products of each reaction group.

Overview of Cellular Metabolism

  • Metabolism: The sum of all chemical reactions in a cell, divided into two groups:
    1. Catabolism: Breakdown of nutrients to release energy.
    • Releases energy stored in chemical bonds, some of which is lost as heat.
    1. Anabolism: Building larger macromolecules (requires energy).
    • Anabolism uses ATP (the cell's currency) to synthesize necessary components for growth, tissue repair, and cell division.

Cellular Respiration and Energy Production

  • Cellular Respiration: Set of oxidation-reduction (redox) reactions that oxidize nutrients to produce ATP.
  • Stages of Cellular Respiration:
    1. Glycolysis:
    • Function: Splits glucose into two molecules of pyruvic acid.
    • Occurs in the cytoplasm and is anaerobic (does not require oxygen).
    1. Intermediate Step: Converts pyruvate to Acetyl CoA (acetyl coenzyme A).
    2. Krebs Cycle (TCA Cycle): Cycles through reactions that produce energy carriers (NADH, FADH2) and minimal ATP.
    • Named after citric acid (reactant) and Hans Krebs (discoverer).
    1. Electron Transport Chain:
    • Requires oxygen as the final electron acceptor, leading to ATP production.

Glycolysis Details

  • Glycolysis consists of two major phases:
    1. Energy Investment Phase:
    • ATP is used to phosphorylate glucose into Glucose-6-Phosphate.
    • Hydrolysis of ATP provides energy for this reaction.
    • Glucose-6-Phosphate cannot leave the cell and initiates glycolysis.
    1. Energy Payoff Phase:
    • Produces: 2 pyruvate, 2 ATP (net gain), and 2 NADH.
    • NADH: Acts as an energy carrier, transferring high-energy electrons to the mitochondria.
  • Net Yield: Gains 2 ATP from glycolysis after input of 2 ATP.
  • Pyruvate production: Two pyruvate molecules are formed per glucose molecule.

Fermentation Pathways

  • Cells without mitochondria or under anaerobic conditions need to regenerate NAD from NADH.
  • Fermentation: Allows for NADH to be oxidized back to NAD.
    • Two Types of Fermentation:
    1. Alcoholic Fermentation:
      • Performed by yeast; ethanol is produced as a waste product.
    2. Lactic Acid Fermentation:
      • Occurs in muscle cells under low oxygen; lactic acid is the waste product.
    • Fermentation does not produce additional ATP; glycolysis remains the primary energy source for anaerobic cells.

The Krebs Cycle

  • After glycolysis, pyruvate undergoes an intermediate step to form Acetyl CoA.
  • The Krebs Cycle (also known as TCA or Citric Acid Cycle) is a cyclic series of reactions:
    1. Produces ATP, NADH, and FADH2.
    2. Each pyruvate (2 per glucose) allows two turns of the cycle.
    3. From one glucose molecule, the profits are:
    • Total Yield: 2 ATP, 6 NADH, and 2 FADH2 from two cycles.

The Electron Transport Chain (ETC)

  • Occurs within the inner mitochondrial membrane.
  • NADH and FADH2 deliver electrons to the ETC, where oxygen is the final electron acceptor.
  • Oxidative Phosphorylation:
    • Creates a proton gradient (chemo-osmosis) across the membrane.
    • ATP Synthase: Catalyzes the phosphorylation of ADP to ATP through the movement of protons.
  • Key Points:
    • The electrons move through the carrier proteins, losing energy sequentially that pumps hydrogen into the intermembrane space.
    • Oxygen combines with electrons and protons to form water.
  • Different organisms may use different electron carriers, adjust the order of carriers based on environment.

Summary of Overall Cellular Respiration

  • Major stages: Glycolysis, Intermediate Step, Krebs Cycle, and Electron Transport Chain.
  • Glucose is metabolized to generate ATP, which is essential for cellular activities, growth, and repairing tissues.
  • ATP Yield:
    • Prokaryotic Cells: Up to 38 ATP per glucose molecule (considering all steps).
    • Eukaryotic Cells: Approximately 36 ATP per glucose molecule due to mitochondrial factors.
  • Comparison with Fermentation:
    • Fermentation yields lower ATP than aerobic respiration but serves as an essential alternative in the absence of oxygen.

Conclusion

  • Glucose metabolism generates energy through a series of catabolic reactions that ultimately support various cellular processes, using ATP as the key energy currency.