CR

Overview of Cellular Respiration

  • Cellular respiration involves several key processes:
    • Glucose oxidation
    • Glycolysis (may also occur through fermentation)
    • Krebs cycle (involves electron transport and generation)
    • Electron Transport Chain (generates H+ gradient)
    • ATP synthesis (involves the generation of ATP)

Oxidation and Reduction in Cellular Respiration

  • Electron Carriers:

    • Oxidized state: NAD+ / FAD
    • Reduced state: NADH / FADH2

  • Key points:

    1. Electron transporters can be reused.
    2. Cells have a limited supply of these transporters.

  • Coupling of Reactions:

    • Oxidation and reduction reactions are coupled.
    • A molecule cannot be oxidized without another molecule to accept the electrons (be reduced).
    • Example: NAD+ must accept electrons to become NADH.

Step 1: Glycolysis

  • Process:

    • Glycolysis breaks down glucose and occurs in the cytoplasm.
    • It generates ATP in the absence of O2 for a limited time.

  • Key Questions:

    • Which molecules are oxidized, and which are reduced?
    • Where are they going next?
    • Where did this come from?

Step 2: Pyruvate Oxidation

  • Location: Mitochondrial matrix.

  • Process:

    • Pyruvate is partially oxidized and converted into Acetyl-CoA.

  • Key Questions:

    • Where did this come from?
    • Where is it going next?

Step 3: Krebs Cycle

  • Location: Mitochondrial matrix.

  • Process:

    • Acetyl-CoA is completely oxidized.

  • Key Questions:

    • Where did these products come from?
    • Where are they going next?

H+ Gradient Formation and ATP Synthesis

  • Hydrogen Ion Gradient Formation:

    • Steps to form the H+ gradient:
    1. Use electron energy to pump H+ ions into the intermembrane space.
    2. Continue this until a strong H+ concentration gradient is established.
    3. Allow H+ to flow back into the matrix using its own concentration gradient energy.
    4. The flow of H+ spins the ATP synthase “wheel” to generate ATP.

  • Key Terms:

    • Oxidative Phosphorylation: The process of ATP production coupled with electron transport.
    • Chemiosmotic Coupling: Linking the H+ gradient to ATP synthesis.
    • Substrate-Level Phosphorylation: ATP generation that does not require a concentration gradient, including during Pyruvate oxidation.

Inhibition Scenarios

  • Krebs Cycle and ETC Inhibition:
    • The Krebs cycle can be directly inhibited or blocked by drugs or toxins.
    • If inhibited, the Krebs cycle cannot function as it is also indirectly affected.
    • Similarly, the Electron Transport Chain (ETC) can be directly inhibited, leading to indirect inhibition of the Krebs cycle.

Glycolysis and its Functionality

  • Glycolysis requires NAD+ to function and relies on mitochondria to return NAD+ molecules.
  • If mitochondria stop, glycolysis runs out of free NAD+ molecules.
  • Fermentation:
    • Can temporarily replenish NAD+ molecules for glycolysis to continue when mitochondria are not functioning.
    • During fermentation, NADH is oxidized to regenerate NAD+ and reduce pyruvate to lactate if oxygen is not present.

Summary of Processes and Outputs

  • Overall ATP Generation Summary:

    • Glycolysis:

    • Inputs: 1 glucose → Outputs: 2 pyruvate, 2 NADH, 2 ATP (via substrate-level phosphorylation)

    • In anaerobic conditions: potential lactate formation.

    • Pyruvate Oxidation:

    • Outputs: 2 Acetyl-CoA, 2 NADH (and 2 CO2 as waste)

    • Krebs Cycle:

    • Total outputs yield: 6 NADH, 2 FADH2, CO2 waste, and limited ATP (mostly through substrate-level phosphorylation).

    • Electron Transport Chain:

    • Key to producing high ATP yield through oxidative phosphorylation and chemiosmosis; typically yields around 34 ATP.

  • Total Theoretical Yield:

    • Typically around 36-38 ATP is produced during complete cellular respiration from one glucose molecule, factoring in losses and the inefficiency of certain steps (2 ATP used for NADH transporting).

  • End Product Summary:

    • If mitochondrial function is inhibited: limited to substrate-level ATP generation and potential shifts to fermentation leading to lactate formation in absence of oxygen.