Lecture 2/12

MCB 150: Introduction to Cellular Respiration

Announcements

• Exam 1 starts tomorrow; best of luck to all.

• Observations on the differences between "Save & Grade" and "Save only" functionalities.

• Student hours available today from 4:00–5:30 in 124 Burrill Hall.

• No class activities scheduled for Friday.

Overview of Cellular Respiration

• Purpose: Convert energy from food into ATP, the energy currency of the cell.

• Key Concept: The breakdown of glucose into CO2 and H2O.

• Phases of Cellular Respiration:

  • Glycolysis: Splitting of sugar.

  • Pyruvate Oxidation and Krebs Cycle: Continued breakdown of products of glycolysis.

  • Oxidative Phosphorylation: Includes electron transport chain and chemiosmosis.

Phase 1: Glycolysis

• Definition: "Glyco" (sugar) + "lysis" (splitting).

• Process:

  • Start: Begins with glucose (6-carbon sugar).

  • End: Produces two pyruvate molecules (3-carbon each).

  • Energy Dynamics: Initially endergonic (requires ATP); uses stored ATP until first pyruvate molecules are produced.

• Location: Occurs in the cytoplasm of all living cells.

Energy Steps in Glycolysis

• Endergonic Steps: 2 steps consume energy.

• Exergonic Steps: 3 steps produce energy.

• Substrate-Level Phosphorylation (SLP): Energy saved through SLP reactions.

Issues Encountered at the End of Glycolysis

• Problems:

  1. Molecules are not at their lowest energy state.

  2. Energy stored in NADH, which needs to be released to generate ATP.

  3. Continuous depletion of NAD+ without replacement.

Questions Raised

1. How to extract more energy from pyruvate?

2. How to transfer energy in NADH to ATP?

3. How to regenerate NAD+?

Answers to Key Questions

• Oxygen's Role: Depends on availability of oxygen or alternative electron acceptors.

  • Aerobic Respiration: If oxygen is present, cells perform aerobic respiration.

  • Anaerobic Respiration: If oxygen is absent but alternative acceptors exist, anaerobic respiration occurs.

  • Fermentation: If no oxygen or acceptors are present, fermentation might occur.

Aerobic Respiration Process

• Conversion of Pyruvate: Each pyruvate is fully converted to carbon dioxide.

  • Steps:

    • First, pyruvate is converted to acetyl-CoA.

    • Acetyl-CoA then enters the Krebs Cycle (Citric Acid Cycle).

  • Energy Transfer: More NAD+ and FAD are reduced to NADH & FADH2.

  • GTP Production: Another SLP reaction occurs in the Krebs cycle producing GTP (similar to ATP).

• Location:

  • Eukaryotes: Occurs in mitochondria.

  • Prokaryotes: Occurs in the cytoplasm and plasma membrane.

Mitochondria Structure and Function

• Organization of Mitochondria:

  • Inner Membrane:

    • Main site for ATP generation.

    • Composed of >70% protein; impermeable to ions/small molecules (except via transporters).

  • Outer Membrane:

    • Typical protein composition; contains porins.

  • Intermembrane Space (IMS):

    • Similar composition to cytoplasm regarding ions and small molecules.

  • Matrix:

    • Contains Krebs cycle enzymes, DNA, and ribosomes.