Biochemistry: Redox Reactions and Metabolism

Redox Reactions

• Redox reactions involve electron transfer.
• Oxidation is the loss of electrons (or hydrogen).
• Reduction is the gain of electrons (or hydrogen).
• Reduced molecules have more potential energy.

Cellular Respiration

• Glucose is oxidized to CO₂ during cellular respiration.
• NAD+ is reduced to NADH, acting as an electron carrier.

Anabolism vs. Catabolism

• Anabolism builds larger molecules from smaller ones, requiring energy.
• Catabolism breaks down larger molecules into smaller ones, releasing energy.

Carbohydrate Metabolism

• Cellular respiration equation: CH₂O + 6O₂ → 6CO₂ + 6H₂O + ~32 ATP
• Glycolysis occurs in the cytoplasm.
• It requires 2 ATP initially and produces a net of 2 ATP.
• Pyruvate fate depends on oxygen levels.
  • Oxygen present: Converted to acetyl-CoA and enters the citric acid cycle.
  • Low oxygen: Converted to lactic acid.
• Lactic acid is processed in the liver to regenerate glucose. (Gluconeogenesis)
• Citric acid cycle occurs in the mitochondrial matrix.
• Carbon and oxygen are released as CO₂.
• Hydrogen is transferred to NAD+ and FAD, reducing them to NADH and FADH₂.
• Electron Transport Chain (ETC) utilizes NADH/FADH₂ electrons to create a proton gradient.
• ATP Synthase generates ATP from this gradient.
• ~28 ATP is produced per glucose molecule from ETC.
• Low oxygen halts ETC, reducing ATP production and increasing lactic acid.
• Cellular respiration releases heat, helping to maintain body temperature.
• ATP is used for cellular functions, recycled into ATP for further respiration.
• Glycogen is a glucose storage polysaccharide in the liver and skeletal muscles.

Lipid Metabolism

• Excess calories are converted into triglycerides for storage.
• Triglycerides are broken down into glycerol (enters glycolysis) and fatty acids (β-oxidation to acetyl-CoA, entering the citric acid cycle).

Amino Acid Metabolism

• Transamination transfers an amino group to another molecule, converting amino acids into intermediates for respiration.
• This process happens in the liver.
• Nitrogen is removed, preparing amino acids for energy production (excreted as urea).

Absorptive vs. Postabsorptive State

• Absorptive state (within 4 hours of eating): Glucose is stored as glycogen; triglycerides stored in adipose; amino acids for protein synthesis.
• Postabsorptive state (digestive tract empty): Glycogen is broken down for glucose, triglycerides for fatty acids/glycerol, and proteins for keto acids.
• Ketones are produced from fats during prolonged fasting for energy.
• Keto acids originate from amino acid metabolism and enter the citric acid cycle.