Most organisms break down proteins only when glucose and fats are unavailable. Some bacteria that spoil food, pathogenic bacteria, and fungi normally catabolize proteins as energy sources.
Proteins are too large to cross the cell membrane. Microorganisms secrete an enzyme called protease, which splits the protein into amino acids outside the cell. Amino acids are then transported into the cell, where specialized enzymes split off amino groups in reactions called deamination. These molecules then enter the Krebs cycle.
These are examples of how the catabolism or breakdown of proteins, carbohydrates, and lipids can be sources of electrons and proteins during cellular respiration. The pathways of glycolysis and the Krebs cycle are catabolic roadways or tunnels where high-energy electrons from these organic molecules can flow through on their energy-releasing journey.
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