Cellular Metabolism and Bioenergetics

Cellular Metabolism and Bioenergetics

• Metabolism is the sum total of all chemical reactions in an organism. It's a complex study of thousands of reactions, including their coordination, regulation, and energy requirements.
• Organisms are categorized into autotrophs and heterotrophs.
  • Autotrophs ("self-feeding"): use CO2 as their sole carbon source, building complex molecules, often using sunlight for energy, like photosynthetic plants and bacteria.
  • Heterotrophs ("feeding on others"): obtain energy by consuming other organisms or complex carbon-containing compounds. They can be further classified by oxygen needs: aerobes need oxygen and anaerobes do not.
• Metabolism in E. coli involves thousands of reactions, and a human cell contains thousands of enzymes catalyzing specific reactions in a sequence.
• Metabolism proceeds via sequences of consecutive enzyme-catalyzed reactions.
• Cells have a gel-like matrix structure impacting the movement of small molecules, which creates organized "metabolons"—functional units of enzymes for metabolic processes.

Intermediary Metabolism

• A human cell possesses at least 3000 different enzymes.
• Metabolic pathways can be grouped into degradative (catabolism) and biosynthetic (anabolism) pathways.
  • Catabolism: breaks down complex organic molecules (fats, carbohydrates, proteins) into simpler molecules like lactate, pyruvate, CO2, H2O, and NH3.
  • Anabolism: constructs complex biomolecules from simpler precursors. The process includes reactions like converting pyruvate to glucose.

Stages of Metabolism

• Catabolism occurs in 3 stages:
  • Stage 1: Breakdown of macromolecules (proteins, fats, and polysaccharides) into their building blocks (amino acids, fatty acids, monosaccharides).
  • Stage 2: Oxidation of building blocks into the common metabolite acetyl CoA. Energy is captured in the form of NADH and ATP.
  • Stage 3: Acetyl CoA enters the citric acid cycle, where it's oxidized to CO2, and electrons are transferred to electron transport chain to produce ATP.
• Anabolism occurs in three stages:
  • Monosaccharides, polysaccharides, synthesis begin with CO2, oxaloacetate, and pyruvate.
  • Amino acids are created from acetyl CoA and other reactions.
  • Triacylglycerols are created using fatty acids from acetyl CoA.

Chemistry of Metabolism

• Metabolism is characterized by oxidation-reduction reactions (transfer of electrons), group-transfer reactions (transfer of functional groups), hydrolysis reactions (breaking bonds with water), nonhydrolytic cleavage reactions (breaking bonds without water), isomerization reactions (changing molecular structure), and bond formation reactions. 
• Many reactions involve transfer of electrons, which can be done with the help of enzymes like Oxidoreductases.
• Carbon atoms in substrates are assessed during oxidation-reduction reactions.

Combining Reaction Types

• Organisms can carry out multiple types of reactions in a single step (e.g. redox reactions and cleavage) and processes like the decarboxylation of isocitrate are included in metabolic pathways.
• Enzyme-catalyzed reactions can proceed via stabilized carbanions (example: Carboxylation of pyruvate in glucose synthesis).