enzymes split off pairs of hydrogenated carbon atoms that make up the fatty acids.
The enzymes then join these pairs to coenzyme A to form acetyl-CoA. This hap-
pens until the entire fatty acid has been converted to molecules of acetyl-CoA.
Acetyl-CoA is utilized in the cycle to generate ATP.PROTEIN CATABOLISM
Most organisms break down proteins only when glucose and fats are unavail-
able. 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, carbo-
hydrates, and lipids can be sources of electrons and proteins during cellular res-
piration. 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.Photosynthesis
Some organisms use anabolic pathways to synthesize organic molecules from
inorganic carbon dioxide. Most of these phototrophic organisms are autotrophic
and are capable of surviving and growing on carbon dioxide as their only carbon
source. The energy from sunlight is used to reduce CO 2 to carbohydrates. This pro-
cess is called photosynthesis. The ability of an organism to “photosynthesize”
depends on the presence of light-sensitive pigments, called chlorophyll, or related
compounds. These pigments are found in plants, algae, and certain bacteria.
The growth of these photosynthetic organisms can be explained by two sep-
arate types of reactions. In light reactions, light energy is converted into chemi-
cal energy and dark reactionsin which the chemical energy from the light
reactions is used to reduce carbon dioxide (CO 2 ) to carbohydrates. For the
growth and survival of autotrophic organisms, energy is supplied in the form of
an ATP molecule. Electrons for the reduction of CO 2 come from NADPH.(^98) CHAPTER 5 The Chemical Metabolism