6.1 Energy and Metabolism

Energy and metabolism are the powerhouses of life. They encompass all chemical reactions in our bodies, from breaking down complex molecules to building new ones. These processes are essential for growth, repair, and maintaining our daily functions.

At the heart of cellular energy is ATP, the universal energy currency. It powers everything from muscle contractions to cell division. Enzymes play a crucial role too, speeding up reactions and regulating metabolic pathways to keep our bodies running smoothly.

Energy and Metabolism

Anabolic vs catabolic pathways

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• Metabolism encompasses all chemical reactions in the body
  • Involves breaking down and building up molecules
• Catabolic pathways break down complex molecules into simpler ones
  • Release energy during the process
  • Glycolysis breaks down glucose (converts glucose to pyruvate)
  • Citric acid cycle oxidizes acetyl-CoA to generate ATP and electron carriers (NADH and FADH2)
  • Fatty acid oxidation breaks down fatty acids to generate acetyl-CoA
• Anabolic pathways build complex molecules from simpler ones
  • Require an input of energy
  • Protein synthesis produces proteins from amino acids
  • Gluconeogenesis generates glucose from non-carbohydrate precursors (amino acids, lactate, glycerol)
  • Lipogenesis synthesizes fatty acids from acetyl-CoA

ATP as cellular energy currency

• ATP (adenosine triphosphate) serves as the primary energy currency in cells
  • Composed of adenosine, ribose, and three phosphate groups
• Energy is stored in the high-energy bonds between the phosphate groups
• Hydrolysis of ATP to ADP (adenosine diphosphate) releases energy
  • $ATP + H_2O \rightarrow ADP + P_i + Energy$
• ATP is regenerated by adding a phosphate group to ADP
  • Requires energy input from catabolic pathways
• ATP powers various cellular processes
  • Muscle contraction enables movement
  • Active transport moves molecules against concentration gradients
  • Synthesis of complex molecules (proteins, nucleic acids, lipids)
  • Cell division (mitosis and meiosis)

Enzymes in metabolic reactions

• Enzymes act as biological catalysts
  • Speed up chemical reactions without being consumed
• Lower the activation energy of reactions
  • Activation energy represents the minimum energy required for a reaction to occur
• Enzymes exhibit specificity to their substrates
  • Active site is the region where the substrate binds
  • Induced fit model suggests that the enzyme changes shape to accommodate the substrate
• Several factors affect enzyme activity
  • Temperature
    1. Optimal temperature for most enzymes is 37°C (human body temperature)
    2. High temperatures denature enzymes by disrupting their structure
  • pH
    1. Each enzyme has an optimal pH range for maximum activity
    2. Extreme pH levels can denature enzymes
  • Substrate concentration
    • Increasing substrate concentration increases reaction rate until enzyme saturation is reached
  • Enzyme concentration
    • Increasing enzyme concentration increases reaction rate
• Enzyme regulation modulates enzyme activity
  • Allosteric regulation involves the binding of molecules at sites other than the active site
    • Activators increase enzyme activity (e.g., fructose-2,6-bisphosphate activates phosphofructokinase)
    • Inhibitors decrease enzyme activity (e.g., ATP inhibits phosphofructokinase)
  • Covalent modification involves the addition or removal of chemical groups
    • Phosphorylation/dephosphorylation (e.g., glycogen phosphorylase activation by phosphorylation)
  • Feedback inhibition occurs when the end product of a pathway inhibits an earlier enzyme in the pathway (e.g., ATP inhibits citrate synthase in the citric acid cycle)

Thermodynamics and Energy in Biological Systems

• Thermodynamics governs energy transformations in living systems
• Free energy (G) represents the energy available to do work in a system
  • Changes in free energy determine the spontaneity of reactions
• Entropy (S) measures the degree of disorder in a system
  • Living organisms maintain low entropy states through energy input
• Electron transport chain and oxidative phosphorylation are key processes in cellular energy production
  • Electron transport chain transfers electrons through a series of protein complexes
  • Oxidative phosphorylation uses the energy from electron transfer to generate ATP