BSC1010 Lecture 9

Introduction to Cellular Respiration

• Definition: Cellular respiration and fermentation are catabolic, energy-yielding pathways.

• Importance: Understanding mechanisms of energy extraction from organic molecules.

• Key Concepts to Understand:

  1. Cellular respiration processes recycle ATP.

  2. Redox reactions involve energy release.

  3. Electrons transfer from organic molecules to oxygen during cellular respiration, through a stepwise mechanism involving NAD+ and FAD+.

Coupling of Biological Reactions

• Reaction Types:

  • Exergonic reactions (release energy) and endergonic reactions (require energy).

• ATP Process:

  • Breakdown of glucose yields energy (exergonic).

  • ATP synthesis is endergonic, requiring energy.

• Key Products:

  • Cellular Respiration: Converts glucose into CO2, H2O, and heat.

  • Protein Synthesis involves energy input (endergonic).

ATP: The Energy Currency

• ATP Basics:

  • Structure: Composed of adenine, ribose sugar, and three phosphate groups.

  • ATP vs. ADP: ATP is energetically charged; ADP is less energetic and forms ATP when a phosphate is added.

  • Energy Dynamics: ATP behaves like a loaded spring—releasing the end phosphate releases energy.

• Role of ATP in Cells:

  • Fuel for transport work (moving materials across membranes).

  • Facilitates mechanical work (e.g., moving organelles).

  • Supports chemical work (e.g., synthesis of macromolecules).

Oxidation and Reduction Reactions (Redox)

• Redox Basics:

  • Oxidation: Loss of electrons.

  • Reduction: Gain of electrons.

  • Coupled Reactions: Always occur together during cellular processes like respiration and photosynthesis.

• Coenzymes in Redox:

  • Common coenzymes: NAD+ and FAD.

  • Function: Act as electron carriers, facilitating the transfer of energy.

Cellular Respiration Processes

• Stepwise Breakdown:

  • Glucose is gradually oxidized over multiple enzyme-catalyzed steps, transferring electrons to coenzymes before the Electron Transport Chain (ETC).

• ATP Generation Mechanisms:

  • Oxidative phosphorylation: Major contributor to ATP synthesis via the ETC.

  • Substrate-level phosphorylation: Occurs during glycolysis and Krebs cycle, directly generating ATP.

Glycolysis Overview

• Initial Phase:

  • Location: Cytoplasm; no oxygen required.

  • Process: Splits glucose (6C) into two pyruvate molecules (3C).

• Energy Investment:

  • Starts with phosphorylation using 2 ATP.

  • Ends with a net yield of 2 ATP and 2 NADH.

Krebs Cycle (Citric Acid Cycle)

• Purpose:

  • Oxidizes pyruvate to CO2, extracting energy.

• Inputs & Outputs per cycle (from 1 Acetyl CoA):

  • ATP: 1

  • NADH: 3

  • FADH2: 1

  • CO2: 2

• Total Yield from Glucose:

  • For 1 glucose, total yields are 2 ATP, 8 NADH, 2 FADH2, and 6 CO2.

Electron Transport Chain (ETC)

• Structure:

  • Located in the inner mitochondrial membrane.

  • Composed of proteins (cytochromes) that pass electrons.

• Process:

  • Electrons transferred from NADH and FADH2 through a series of protein complexes leading to ATP synthesis via chemiosmosis.

• Final Step:

  • Oxygen serves as the final electron acceptor, forming water.

ATP Yield and Energy Efficiency

• ATP Production Estimates:

  • Each NADH can produce up to 3 ATP, while FADH2 produces about 2 ATP.

  • Total potential yield from substrate-level phosphorylation and oxidative phosphorylation combined is around 38 ATP per glucose molecule, assuming optimal conditions.

• Energy Efficiency:

  • Approx. 40% of energy from respiration is converted to ATP; remaining energy is lost as heat.

Alternative Pathways of Energy Production

• Carbohydrates, Fats, and Proteins:

  • All can enter glycolysis and Krebs cycle pathways.

  • Fatty acids undergo beta-oxidation to enter Krebs cycle.

• Fermentation:

  • Occurs in oxygen-deprived environments, generating ATP via substrate-level phosphorylation.

• Types of Fermentation:

  • Alcohol Fermentation: Converts pyruvate to ethanol.

  • Lactic Acid Fermentation: Converts pyruvate to lactate.

Conclusion

• Cellular respiration is essential for energy production, processing glucose and other molecules in a multi-step manner to efficiently generate ATP, highlighting the interconnected metabolic pathways of the cell.