Chapter6

Biochemical Pathways

Cellular Respiration

Chapter 6

Energy and Organisms

• Energy Classification of Organisms

  • Autotrophs

    • Use sunlight energy to create organic molecules (sugars)

    • Convert organic molecules into ATP

    • Photosynthesis is the primary process

  • Heterotrophs

    • Obtain organic molecules by consuming autotrophs

    • Use the energy from these molecules to generate ATP

• Cellular Respiration

  • All organisms utilize cellular respiration to extract energy from organic molecules for ATP production

Energy Transformation

• Key Components

  • Sunlight and water (H₂O) are essential for plant energy production (ATP)

  • Chloroplasts in plant cells facilitate photosynthesis, converting CO2 and H₂O into organic molecules

  • Mitochondria in both plant and animal cells are crucial for ATP synthesis

Aerobic Respiration: An Overview

• Definition

  • Comprises a series of enzyme-controlled reactions

  • Oxygen is utilized to oxidize glucose, producing carbon dioxide and water

• Processes Involved

  • Breakdown of C-H and O-H bonds in glucose

  • Electrons are transferred to carriers NAD+ and FAD

  • Glycolysis and Krebs Cycle precede an electron transport chain (ETC)

  • Energy from electrons pumps protons, facilitating ATP synthesis

Aerobic Cellular Respiration: Overview

• Process Description

  • Begins with glucose (6-carbon sugar), undergoing glycolysis

  • Key Outputs:

    • Glycolysis: Produces 2 ATP and 2 NADH

    • Krebs Cycle: Produces 2 ATP, 8 NADH, and 2 FADH2

    • Electron Transport System (ETS): Converts the energy from NADH and FADH2 into ATP

  • Total ATP Generated: Up to 36 ATP per glucose molecule

Glycolysis

• Process

  • Breakdown of glucose into pyruvic acid

  • Energy Dynamics:

    • Initially uses 2 ATP, produces 4 ATP, yielding a net gain of 2 ATP

    • Reduces NAD+ to form NADH

  • Occurs in the cytoplasm

Krebs Cycle

• Alternate Name: Citric Acid Cycle or Tricarboxylic Acid (TCA) Cycle

• Process

  • Converts pyruvic acid into carbon dioxide

  • Produces 1 ATP, 4 NADH, and 1 FADH2

  • Takes place in the mitochondrial matrix

Electron-Transport System

• Function

  • NADH and FADH2 donate electrons to the electron transport chain

• Mechanism

  • Proteins of the ETC transfer electrons and use released energy to pump protons, creating a gradient

  • Oxygen serves as the final electron acceptor, forming water

  • Protons flow back through ATP synthase to generate ATP

Total Yields for Aerobic Cellular Respiration

• Total ATP Production:

  • Glycolysis: 2 ATP, 2 NADH

  • Krebs Cycle: 2 ATP, 8 NADH, 2 FADH2

  • Electron Transport Chain: 24 ATP from NADH and 8 ATP from FADH2

  • Grand Total: 36 ATP per glucose molecule

Aerobic Respiration in Prokaryotes

• Process Overview

  • Similar to eukaryotic respiration, but occurs in the cytoplasm due to absence of mitochondria

  • Can yield 2 additional ATP than eukaryotes

Anaerobic Cellular Respiration

• Overview

  • Some organisms lack Kreb’s cycle or ETC enzymes, allowing them to metabolize glucose without oxygen

  • Characteristics:

    • Involves incomplete oxidation of glucose

    • Fermentation uses organic molecules as electron acceptors

Alcoholic Fermentation

• Process

  • Begins with glycolysis to produce pyruvic acid and 2 ATP

  • Pyruvic acid reduces to form ethanol and carbon dioxide

  • Common in yeast (e.g., leavened bread, sparkling wine)

Lactic Acid Fermentation

• Process

  • Glycolysis leads to pyruvic acid and generates 2 ATP

  • Pyruvic acid reduces to lactic acid; no carbon dioxide is produced

  • Muscle cells can conduct this process during temporary oxygen deprivation but brain cells cannot

  • Causes muscle “burn” sensation

Metabolizing Other Molecules

• Energy Use Priority

  • Carbohydrates are primary energy sources, followed by fats and proteins

  • Complex carbohydrates are transformed into simple sugars

Fat Respiration

• Fat Metabolism

  • Fats broken down into glycerol and fatty acids

  • Glycerol converts to glyceraldehyde-3-phosphate to enter glycolysis

  • Fatty acids convert to acetyl-CoA to enter Krebs Cycle

  • Provides more ATP than glucose

Protein Respiration

• Process Overview

  • Proteins are broken down into amino acids

  • Amino group removal generates keto acids, allowing entry into Krebs Cycle

The Bottom Line

• Energy Sources

  • Carbohydrates, fats, and proteins can all provide energy

  • Glycolysis and Krebs Cycle facilitate the interchange of these molecules

  • Excess caloric intake leads to storage, specifically as fat post-saturation of protein and carbohydrate needs