MCB 150 section 1

Announcements from Professor

• Airbags in the professor's vehicle unexpectedly went off; mechanic mentioned high repair costs due to inflation.

• Normal student hours will be held today from 4:00 - 5:30 pm in 124 Burl.

• Specific exam questions cannot be discussed due to accessibility issues. Additional session for exam discussions will occur after Exam Two and Exam Three.

• No class on Friday; professor will be out of town, but will provide recorded lecture content from last semester on metabolism.

Overview of Metabolism

• Discussed how energy is sourced from food, primarily glucose.

• Energy extraction process involves:

  • Phase 1: Glycolysis - Converts glucose into pyruvates without producing CO2 or water; minimal energy is released.

  • Phase 2: Pyruvate Oxidation and Krebs Cycle - Pyruvates converted, CO2 released as a byproduct, more energy extracted.

  • Phase 3: Electron Transport and Chemiosmosis - ATP synthase produces ATP by combining protons, electrons, and oxygen to form water.

Energy Storage and Usage

• Energy is carried by electrons, often transferred as hydrogen atoms during metabolic processes.

• ATP is generated through:

  • Substrate Level Phosphorylation - Initial ATP production during glycolysis and Krebs Cycle.

  • Oxidative Phosphorylation - Majority of ATP produced via electron transport chains creating a proton gradient.

Electron Transport Chain Concept

• Embedded in membranes to compartmentalize protons and create gradients for ATP synthesis.

• ATP synthase operates due to the gradient established by the transport chain.

Review Questions and Discussions

Why are electron transport chains membrane-bound?

• Membrane compartmentalization facilitates proton gradient formation essential for ATP synthesis.

  • Correct Concept: Membranes allow conservation and generation of electrochemical gradients.

Evaluating Statements Regarding ATP and Electron Transport

1. Statement 1: "Zero molecules of ATP are directly synthesized by an electron transport chain." - FALSE (ATP forms only through ATP synthase triggered by proton flow, not directly from the chain).

2. Statement 2: "Zero protons are pumped against a concentration gradient by ATP synthase." - TRUE (ATP synthase allows proton flow along the gradient, using energy from it).

Metabolic Pathways and Energy Sources

• Aerobic respiration is the central metabolic pathway; however, ATP can be generated from:

  • Fats: Broken down into acetyl-CoA (cycle into Krebs directly).

  • Proteins: Hydrolyzed to amino acids, which undergo deamination and can enter various glycolytic and Krebs Cycle pathways.

Consequences of Oxygen Deprivation

Shortage of Terminal Electron Acceptors

• Cells adapted for aerobic respiration face significant ATP generation reduction during low oxygen availability.

• Fermentation: Emergency backup that recycles NAD+ but does not produce additional energy like aerobic respiration.

Understanding Fermentation vs. Anaerobic Respiration

• Fermentation is specific to aerobically respiring organisms that temporarily lack oxygen. It serves to regenerate NAD+ for glycolysis continuation.

• Different organisms may produce various fermentation byproducts (e.g., ethanol in yeast and lactic acid in muscles).

Regulation of Metabolic Pathways

Enzyme Regulation

• Anabolic and catabolic reactions are coordinated to optimize energy expenditure.

• Enzyme modulation can occur via:

  • Creating new enzymes when needed.

  • Breaking down excess enzymes temporarily.

  • Allosteric regulation: Allows quick adjustments to enzyme activity by binding regulatory molecules.

Feedback Inhibition

• A product of a pathway can inhibit enzyme activity if in excess (e.g., ATP inhibits phosphofructokinase in glycolysis).

• First Committed Step: The first irreversible step specific to a pathway acts as a regulatory point (e.g., C to S is committed to producing T).

Summary of Metabolic Adaptations

1. Cells can adapt to lack of oxygen through fermentation, allowing increased glycolysis rates but at higher substrate consumption.

2. Proper regulation prevents resources from being unnecessarily expended and maintains metabolic efficiency.

To better understand the topics and material of metabolism, it's helpful to break it down into key concepts and interconnections:

1. Metabolism Basics: Metabolism is the process by which organisms convert food into energy. It includes all biochemical reactions in the body, focusing on how energy is extracted from nutrients.

2. Phases of Energy Extraction:

   • Glycolysis is the first stage where glucose is broken down into pyruvate in the cytoplasm. It's anaerobic (doesn't require oxygen) and generates a small amount of ATP.

   • Krebs Cycle occurs in the mitochondria and processes pyruvate further. This stage is aerobic and produces carbon dioxide as a byproduct while extracting more energy.

   • Electron Transport Chain is the final stage that occurs in the inner mitochondrial membrane. It uses high-energy electrons to create a proton gradient, leading to ATP production. This stage requires oxygen and produces water.

3. Energy Storage and Transfer: Energy generated during these processes is stored as ATP. The cell uses ATP rapidly as needed, with two primary production methods: substrate-level phosphorylation (direct generation during glycolysis and Krebs) and oxidative phosphorylation (majority of ATP from the electron transport chain).

4. Adaptation to Oxygen Availability: If oxygen levels drop, cells can switch to fermentation, a less efficient way to generate ATP that regenerates NAD+ for glycolysis, thereby allowing some energy production to continue even without oxygen.

5. Regulation Mechanisms: The body uses several regulatory mechanisms, including feedback inhibition, where the end product of a biochemical pathway reduces the activity of an enzyme involved in its synthesis. This prevents overproduction and helps maintain energy balance.

By understanding these interconnected processes, it becomes easier to see how energy production and regulation work in living organisms, as well as the significance of oxygen in aerobic respiration and energy yield.