Cellular Metabolism: Respiration, Photosynthesis, and Energy Cycles

Differences Between Respiration and Photosynthesis

Cellular respiration occurs in cells (specifically the mitochondria), and photosynthesis occurs in plant cells (specifically the chloroplasts).

Respiration is a catabolic process, while photosynthesis is anabolic.

Reactants and Products

• Respiration Reactants: Glucose, fatty acids, and proteins.
• Photosynthesis Reactants: CO₂, water, nitrates, and sulfates.
• Respiration Products: CO₂ and water.
• Photosynthesis Products: Glucose, fatty acids, and amino acids.

Energy Sources and Processes

The energy used in respiration is chemical energy, and in photosynthesis, it is light energy. The energy generation process in respiration is phosphorylation, and in photosynthesis, it is photophosphorylation.

Respiration has two main phases: the Krebs Cycle and Oxidative Phosphorylation. Photosynthesis has light-dependent and light-independent (dark) phases.

Cellular Respiration Types

1. Aerobic Respiration (with O₂)

   Pyruvic acid derived from glucose, fatty acids, glycerol, and amino acids is transformed into CO₂ and water.

2. Anaerobic Respiration or Fermentation (without O₂)

   Pyruvic acid compounds become inorganic. This is an incomplete oxidation process, yielding less energy.

   Types of Fermentation:

   • Lactic Acid Fermentation: Glucose is converted into lactic acid (occurs in muscles causing soreness, and used by Lactobacillus bacteria for yogurt and cheese production).
   • Alcoholic Fermentation: Glucose becomes alcohol (used in obtaining wine, beer, and whiskey).
   • Butyric Acid Fermentation: Starch and cellulose are transformed into butyric acid, producing a foul odor.
   • Putrid Fermentation: Meat fermentation that produces a strong odor.

Key Catabolic Processes

Glycolysis

Glycolysis is the transformation of glucose into two molecules of pyruvic acid. This process occurs in the cytoplasm, yielding a total of 2 ATP and 2 NADH. The pyruvic acid then becomes Acetyl-CoA in the mitochondrial matrix.

Beta-Oxidation of Fatty Acids (Lynen's Proposal)

This is the transformation of fatty acids into Acetyl-CoA. For a fatty acid chain of 'N' carbons, the process yields N/2 Acetyl-CoA molecules and requires N/2 minus 1 cycles. Amino acids become Acetyl-CoA and pyruvate, or glycerol becomes pyruvic acid.

The passage of pyruvate to Acetyl-CoA occurs in the mitochondrial matrix.

Krebs Cycle (Citric Acid Cycle)

Acetyl-CoA enters the Krebs Cycle, yielding the following per Acetyl-CoA molecule:

• 3 NADH
• 1 FADH₂
• 1 GTP (equivalent to 1 ATP)

This total energy yield is approximately 12 ATP per Acetyl-CoA, plus CO₂ and metabolites necessary for the synthesis of amino acids.

Respiratory Chain (Electron Transport Chain)

This is the transport of electrons from reduced coenzymes (NADH and FADH₂) to O₂ to form water. Energy yield estimates are 3 ATP per NADH and 2 ATP per FADH₂.

Anabolic Processes

• Gluconeogenesis

  The synthesis of glucose. This is the reverse of glycolysis, though it involves variations in the pathway.

• Glycogenesis

  The step-by-step synthesis of glycogen from glucose.

• Fatty Acid Synthesis

  The inverse process of beta-oxidation.

• Amino Acid Anabolism

  Synthesis occurs via various specific metabolic routes.