Metabolism

Metabolism Overview

• Metabolism: Highly coordinated cellular activity involving multienzyme systems (metabolic pathways) to:

  • Obtain chemical energy by:

    • Capturing solar energy

    • Degrading energy-rich nutrients

  • Convert nutrient molecules into the cell's characteristic molecules (precursors of macromolecules)

  • Polymerize monomeric precursors into macromolecules (proteins, nucleic acids, polysaccharides)

  • Synthesize and degrade biomolecules (e.g., membrane lipids, intracellular messengers, pigments)

Classification of Organisms

• Living organisms categorized based on carbon acquisition:

  • Autotrophs

    • Use carbon dioxide as the sole carbon source

    • Examples: Photosynthetic bacteria, green algae, vascular plants

    • Some can use atmospheric nitrogen to generate nitrogenous components (e.g., cyanobacteria)

  • Heterotrophs

    • Cannot utilize carbon dioxide, require organic compounds (e.g., glucose)

    • Most multicellular animals and microorganisms are heterotrophic

  • Autotrophic organisms are self-sufficient, while heterotrophs depend on autotrophs for organic nutrients.

Energy Flow and Recycling

• Energy flow in the biosphere is unidirectional; useful energy declines while unusable energy (heat and entropy) increases.

• Continuous recycling of carbon, oxygen, nitrogen in ecosystems:

  • Autotrophs build organic biomolecules from CO2, heterotrophs consume these products.

  • Nitrogen Cycle: Involves fixation of atmospheric nitrogen (N2) by some bacteria and archaea, linking nitrogen with biological systems.

Metabolic Processes

• Metabolic Pathways: Series of enzyme-catalyzed reactions yielding specific small chemical changes, converting precursors to products via intermediates called metabolites.

• Catabolism: Degradative phase converting nutrients (carbs, fats, proteins) into smaller products; releases energy stored in ATP, NADH, NADPH, and FADH2.

• Anabolism: Biosynthesis phase where simple precursors are built into complex molecules (e.g., lipids, polysaccharides, proteins).

Regulation of Metabolism

• Separate but interconnected pathways:

  • Catabolic and anabolic pathways are reciprocally regulated, preventing wasteful simultaneous activity.

  • At least one step in these pathways utilizes different enzymes, allowing for distinct regulation.

  • Processes often occur in separate cellular compartments (e.g., mitochondria vs. cytosol).

Energy Carrying Molecules

• ATP, NADH, NADPH are vital for energetic transactions in metabolism.

• The interconversion of ADP to ATP provides energy for anabolic reactions.

Overview of Glycolysis

• Glycolysis: Breakdown of glucose into pyruvate via a series of reactions; occurs in two phases:

  • Preparatory Phase: Investment of ATP to activate glucose and generate intermediates (fructose 1,6-bisphosphate).

  • Payoff Phase: Energy extraction resulting in ATP and NADH production.

Detailed Reactions and Pathways

• Preparatory Phase:

  • Conversion of glucose to fructose 1,6-bisphosphate through phosphorylations (steps 1-3).

  • Cleavage into three-carbon sugar phosphates (step 4).

• Payoff Phase:

  • Conversion of G3P to pyruvate with energy conservation as ATP and NADH (steps 6-10).

Fates of Pyruvate

• Three pathways depending on conditions:

  1. Aerobic Conditions: Pyruvate oxidized to acetyl-CoA for citric acid cycle.

  2. Anaerobic Conditions: Reduced to lactate (lactic acid fermentation).

  3. Ethanol Production: Fermented to ethanol (alcohol fermentation) by yeast and some plants.

Summary of Reaction Mechanisms

• Glycolysis Reaction Equation:Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O

• Under cellular conditions, glycolysis is largely irreversible, driven by a substantial decrease in free energy.

In-Depth Metabolism Overview

Metabolism

Metabolism is a highly coordinated cellular activity that involves several multi-enzyme systems known as metabolic pathways. The primary goals of metabolism include:

• Obtaining Chemical Energy:

  • Capturing Solar Energy: Plants and certain bacteria harness solar energy through photosynthesis.

  • Degrading Energy-Rich Nutrients: Breaking down more complex organic molecules to release energy.

• Converting Nutrient Molecules: Transforming nutrients into the cell's characteristic molecules, which serve as precursors for macromolecules.

• Polymerization of Precursors: Synthesizing larger molecules such as proteins, nucleic acids, and polysaccharides from smaller building blocks (monomers).

• Synthesis and Degradation of Biomolecules: Creating and breaking down various chemicals essential for cellular functions (e.g., lipids, pigments).

Classification of Organisms

Living organisms can be categorized based on their carbon acquisition methods:

• Autotrophs:

  • Use carbon dioxide (CO2) as their sole carbon source, enabling self-sufficiency in nutrient synthesis.

  • Examples: Photosynthetic bacteria, green algae, vascular plants.

  • Some can also utilize atmospheric nitrogen for synthesizing nitrogenous components (e.g., cyanobacteria).

• Heterotrophs:

  • Require organic compounds (e.g., glucose) as they cannot use CO2 directly for carbon.

  • Examples: Most multicellular animals and many microorganisms are heterotrophic.

  • Depend on autotrophs for organic nutrients, forming a foundational part of their ecological systems.

Energy Flow and Recycling

• The energy flow in the biosphere is unidirectional, with useful energy dissipating while unusable energy increases (heat and entropy).

• Continuous recycling of essential elements like carbon, oxygen, and nitrogen occurs in ecosystems.

  • Autotrophs build organic biomolecules by utilizing CO2, which heterotrophs then consume.

  • Nitrogen Cycle: Atmospheric nitrogen (N2) is fixed by certain bacteria and archaea, linking nitrogen with biological systems, essential for life processes.

Metabolic Processes

• Metabolic Pathways: A series of enzyme-catalyzed reactions that lead to specific chemical changes, with precursors transforming through intermediates called metabolites.

  • Catabolism: The process whereby larger molecules (carbohydrates, fats, proteins) are broken down into smaller units, releasing energy stored in bonds to form ATP, NADH, NADPH, and FADH2.

  • Anabolism: The biosynthetic phase where simpler precursors are constructed into complex biomolecules (e.g., lipids, polysaccharides, proteins).

Regulation of Metabolism

• Metabolism consists of distinct yet interconnected pathways:

  • The catabolic and anabolic pathways are reciprocally regulated to avoid wasteful simultaneous activities, ensuring efficiency in metabolic processes.

  • There is typically a unique enzyme involved in at least one reaction step in these pathways facilitating independent regulation.

  • Various metabolic processes often occur in different cellular locations (e.g., mitochondria vs. cytosol).

Energy Carrying Molecules

• Key molecules for energy transactions during metabolism include ATP, NADH, and NADPH.

• The conversion of ADP to ATP is crucial for supplying energy required for anabolic reactions.

Overview of Glycolysis

• Glycolysis: The metabolic pathway that breaks down glucose into pyruvate, proceeding through two primary phases:

  • Preparatory Phase: Involves ATP investment to activate glucose, leading to the production of intermediates (e.g., fructose 1,6-bisphosphate).

  • Payoff Phase: Focuses on energy extraction, resulting in the production of ATP and NADH.

Detailed Reactions and Pathways

• Preparatory Phase includes:

  • Conversion of glucose to fructose 1,6-bisphosphate via phosphorylation (steps 1-3).

  • Cleavage into three-carbon sugar phosphates (step 4).

• Payoff Phase encompasses:

  • The conversion of G3P to pyruvate, simultaneously conserving energy as ATP and NADH are produced (steps 6-10).

Fates of Pyruvate

The fate of pyruvate depends on environmental conditions:

• Aerobic Conditions: Pyruvate is oxidized into acetyl-CoA for utilization in the citric acid cycle.

• Anaerobic Conditions: Pyruvate is reduced to lactate, a process known as lactic acid fermentation.

• Ethanol Production: Under fermentation conditions, pyruvate may be converted into ethanol, a process known as alcoholic fermentation occurring in yeast and some plants.

Summary of Reaction Mechanisms

• The generalized glycolysis reaction can be summarized as:

  • Equation: Glucose + 2 NAD⁺ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O.

• Under cellular conditions, glycolysis is predominantly irreversible and is driven by a significant decrease in free energy, indicating its efficiency in energy production.