Glycolysis and Pyruvate Processing Notes

Overview of Glycolysis and Pyruvate Processing

  • Glycolysis produces energy and pyruvate.

    • Glucose has 6 carbon atoms.

    • Pyruvate has 3 carbon atoms.

  • This results in two pyruvate molecules from one glucose.

  • Glycolysis also produces NADH and ATP.

Problems to Solve Post-Glycolysis

  • Key issues after glycolysis include:

    • Regeneration of NAD+ to continue glycolysis.

    • Removal of accumulated pyruvate since it is not the end product.

    • There is still energy stored in pyruvate.

Pathways Post-Glycolysis

  • After glycolysis, there are two main pathways for pyruvate:

    • Pyruvate Oxidation (if oxygen is available)

    • Fermentation (if oxygen is limiting)

Conditions for Pathways

  • Eukaryotic or prokaryotic organisms' decisions depend on environmental conditions regarding oxygen availability.

  • Under low oxygen (hypoxic conditions), organisms opt for fermentation.

    • Example in humans: Intense exercise leads to anaerobic conditions, triggering fermentation.

  • If oxygen is present, pyruvate enters the oxidation pathway.

Fermentation Processes

  • Fermentation pathways include lactic acid fermentation and alcoholic fermentation.

Lactic Acid Fermentation

  • Occurs in rapidly contracting muscle cells.

  • Key reactions:

    • Pyruvate is converted to lactate.

    • Coupled reaction: NADH is oxidized to regenerate NAD+.

    • Reduction: Gain of electrons (hydrogen atoms added).

    • Structure changes:

    • Pyruvate (C=C=O) converts to lactate (C-C(OH)).

  • Purpose: Replenishes NAD+ so glycolysis can continue, yielding 2 ATP per glycolytic cycle.

Alcoholic Fermentation

  • Primarily takes place in yeast cells.

  • Key reactions:

    • Pyruvate is decarboxylated (removal of CO2) to form acetaldehyde (2 carbons).

    • Acetaldehyde is reduced to ethanol.

    • Coupling: Oxidation of NADH is again processed to regenerate NAD+.

    • Alcoholic fermentation products:

    • Ethanol and carbon dioxide (byproduct).

  • This process does not occur in muscle cells during intense exercise.

  • In lab settings, yeast fermentation produces visible CO2 accumulation as a product.

Transport of Pyruvate for Cellular Respiration

  • Condition for Oxidation: If oxygen is available, the pyruvate undergoes oxidation to carbon dioxide.

  • Location: These processes occur in mitochondria.

Mitochondrial Structure

  • Mitochondria have:

    • Outer mitochondrial membrane (OMM).

    • Inner mitochondrial membrane (IMM).

    • Intermembrane space (IMS) between the two membranes.

    • Matrix: The innermost compartment where pyruvate oxidation occurs.

Transport Mechanism

  • Pyruvate is transported across the two mitochondrial membranes via active transport.

    • Requires energy due to concentration gradients.

    • Type: Secondary active transport, using protons moving down their gradient to pull pyruvate against its gradient.

Bridge Reaction

  • Link between Glycolysis and Krebs Cycle.

  • Input: Pyruvate, NAD+, Coenzyme A.

  • Output: Acetyl-CoA (2 carbons), NADH, CO2.

  • Coenzyme A carries 2-carbon acetyl groups, facilitating further reactions.

  • No ATP is consumed in this step.

Citric Acid Cycle (Krebs Cycle)

  • Acetyl-CoA combines with oxaloacetate (4 carbons).

  • Produces citrate (6 carbons) as a starting molecule of the cycle.

  • Key outcomes of the cycle:

    • Release of two CO2 molecules.

    • Results in production of:

    • 3 NADH

    • 1 FADH2

    • 1 ATP (or GTP)

    • The cycle creates energy carriers that will be further used in the electron transport chain.

    • Overall oxidation of carbon from glucose (6 carbons total from glucose) down to fully oxidized carbon dioxide.

Summary of Key Takeaways

  • Glycolysis leads to the production of pyruvate and ATP.

  • Pyruvate can undergo fermentation or oxidation depending on oxygen availability.

  • Fermentation results in lactate or ethanol, regenerating NAD+.

  • If oxygen is present, pyruvate enters mitochondrial processes leading to complete oxidation of carbon atoms in the Krebs cycle.

  • Key focus areas: inputs and outputs of glycolysis, pyruvate oxidation, and Krebs cycle are crucial for understanding cellular respiration in depth.

Low Oxygen Conditions and Fermentation

  • Overview of Fermentation

    • Occurs when oxygen levels are very low.

    • Pathway involved: Fermentation.

  • Process of Fermentation

    • Pyruvate is reduced to form:

    • Lactate (in animals)

    • Ethanol and Carbon Dioxide (in yeast)

    • Purpose of Fermentation:

    • To regenerate NAD+ from NADH.

    • This allows glycolysis to continue functioning.

Key Reaction: NADH + H+ ⇌ NAD+ + Lactic acid (or Ethanol + CO2)

Aerobic Conditions and Mitochondrial Processes

  • Aerobic Conditions

    • When oxygen is present, pyruvate is transported into the mitochondria.

  • Conversion of Pyruvate

    • Pyruvate (3 carbons) is oxidized to form Acetyl CoA (2 carbons).

    • The missing carbon from pyruvate is released as Carbon Dioxide (CO2).

    • NADH is also produced during this oxidation.

    • This step is referred to as the Bridge Reaction.

Bridge Reaction: Pyruvate + Coenzyme A → Acetyl CoA + CO2 + NADH

The Krebs Cycle (Citric Acid Cycle)

  • Entry of Acetyl CoA

    • Acetyl CoA combines with Oxaloacetate (OAA) (4 carbons) to form Citrate (6 carbons).

  • Process of the Krebs Cycle

    • As the cycle progresses, acetyl CoA is oxidized.

    • CO2 is released at various steps in the cycle.

    • Production of energy carriers:

    • NADH produced at several steps.

    • FADH2 produced at a specific step.

    • One ATP produced in one turn of the cycle.

Stoichiometry:

  • Two pyruvate molecules (from one glucose) yield:

    • 2 Acetyl CoA

    • Therefore, outputs related to glucose should be multiplied by two for accurate results.

Carbon Accounting and Energy Yield

  • Carbon Balancing

    • Starting Material:

    • Glucose: 6 Carbons

    • Pyruvate: 2 x 3 Carbons = 6 Carbons

    • Bridge Reaction: Releases 2 CO2 (2 Carbons lost)

    • Krebs Cycle: Produces 4 CO2 from 2 Acetyl CoA (4 Carbons lost total).

    • Total CO2 Released: 6 Carbons from glucose converted to 6 CO2.

  • Energy Production

    • Total ATP produced from 1 glucose molecule:

    • 4 ATP total (2 from glycolysis + 2 from Krebs cycle).

    • Total NADH produced:

    • Glycolysis: 2

    • Bridge Reaction: 2

    • Krebs Cycle: 6 (3 from each turn for 2 turns)

    • Total NADH: 10

    • Total FADH2 produced:

    • 2 (one per turn in Krebs cycle).

Summary of Key Outputs per Glucose Molecule

  • Waste products:

    • Carbon Dioxide: 6

  • Energy Carriers:

    • ATP: 4

    • NADH: 10

    • FADH2: 2

Energy Utilization via the Electron Transport Chain

  • Future discussions will involve how the NADH and FADH2 created in these processes will be leveraged in the electron transport chain to synthesize more ATP.

Concept Review

  • The process during pyruvate oxidation resulting in the release of carbon dioxide is called Decarboxylation.