Metabolic Mechanisms

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Metabolic Mechanisms

Introduction to Metabolism

While exploring the intricate workings of the cell, it’s vital to remember that every action requires energy. This lecture delves into cellular metabolism, answering the question: How does the cell generate the energy needed for its countless reactions?

Key topics in this lecture:

  • Nutrient Catabolism: Breaking down nutrients to access chemical energy.
  • Citrate Cycle: The central metabolic cycle of the mitochondria.
  • Electron Transport System and Oxidative Phosphorylation: The final stages of cellular respiration.
  • Photosynthesis: A complementary energy process in plants.

All energy on Earth originates from the sun. While animals rely on consuming plants or other animals, plants harness solar energy to create the carbon bonds that sustain life.

Defining Metabolism

Metabolism is the sum of all reactions in the body and can be categorized into two complementary processes:

  1. Catabolism: Breaking down molecules to release energy.
  2. Anabolism: Using energy to build complex molecules, such as proteins, lipids, and carbohydrates.

Catabolic Processes

Catabolism releases energy stored in molecules like carbohydrates, fats, and proteins. This energy fuels cellular functions, primarily through the production of ATP (adenosine triphosphate).

  • ATP acts as the energy currency of the cell.
  • Its energy is stored in high-energy phosphate bonds and is used for:
  • Muscle contraction
  • Nerve signaling
  • Active transport of molecules

Anabolic Processes

Anabolism synthesizes key biomolecules:

  • Nucleic acids
  • Carbohydrates
  • Proteins
  • Lipids

Catabolism in Action: Burning Energy

The term burning energy is metaphorical—no flames exist within the body. However, like a fire burning wood, catabolism:

  1. Breaks carbon bonds.
  2. Requires oxygen as an input.
  3. Produces heat, carbon dioxide (CO₂), and ATP.

ATP: The Energy Mediator

  • ATP mediates between catabolic and anabolic reactions.
  • It powers processes such as:
  • Molecule synthesis
  • Ion transport across membranes
  • Muscle contraction

Cellular Respiration: The Three Stages

1. Nutrient Catabolism

The breakdown of glucose (glycolysis) and fats (lipolysis) provides energy precursors:

  • Glycolysis occurs in the cytosol and ends with pyruvate.
  • Pyruvate can either:
  • Stay in the cytosol for anaerobic glycolysis, producing lactate.
  • Enter the mitochondria for aerobic glycolysis, leading to greater energy yield.

Fun Fact: Humans can produce glucose without dietary carbohydrates via gluconeogenesis.

2. The Citrate Cycle (Krebs Cycle)

  • Occurs in the mitochondria.
  • Processes acetyl-CoA (derived from glucose, fats, or proteins).
  • Produces:
  • 1 ATP
  • 3 NADH
  • 1 FADH₂
  • 2 CO₂

The CO₂ produced is exhaled, completing a cycle where plants use CO₂ for photosynthesis and release oxygen for animals.

3. Electron Transport System and Oxidative Phosphorylation

  • Electrons from NADH and FADH₂ are passed through complexes, creating a proton gradient.
  • Oxygen acts as the final electron acceptor, forming water.
  • The proton gradient powers ATP synthase, producing ATP:
  • 1 NADH = ~2.5 ATP
  • 1 FADH₂ = ~1.5 ATP

Fat Metabolism: Lipolysis and Beta-Oxidation

Lipolysis

  • Breaks down triglycerides into free fatty acids and glycerol.
  • Fatty acids are transported into the mitochondria via the carnitine shuttle.

Beta-Oxidation

  • Cuts fatty acids into two-carbon units.
  • Produces acetyl-CoA, feeding into the citrate cycle.
  • A single 18-carbon fatty acid can yield 90 ATP.

Photosynthesis: The Plant Parallel

Plants reverse cellular respiration:

  • Use solar energy to convert CO₂ into carbon chains (glucose).
  • Store energy in a form consumable by animals, creating a symbiotic relationship.

Statins and Cellular Energy

How Statins Work

  • Inhibit HMG-CoA reductase, reducing cholesterol production.
  • Lower LDL cholesterol in the blood.

Consequences of Statins

  • Reduced cholesterol affects:
  • Ubiquinone (CoQ10) production, impairing the electron transport system.
  • Mitochondrial function, leading to muscle pain and potential heart damage.

Cancer and Glucose Metabolism

  • Cancer cells consume glucose at ~200x the rate of normal cells.
  • Restricting carbohydrates can:
  • Starve cancer cells.
  • Enhance the efficacy of chemotherapeutics.

The Glucose-Ketone Index: A tool for managing glucose and ketone levels to suppress cancer growth.

Closing Remarks

Understanding cellular metabolism reveals the intricate balance between catabolism and anabolism, the pivotal role of ATP, and the interconnectedness of life processes. From the energy derived from the sun to the recycling of CO₂, metabolism is a testament to the beauty and complexity of life on Earth.

“You are now modest masters of biology, equipped with a deeper appreciation for the mechanisms of life.”

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