Questions: What must occur before a fatty acid can be metabolized by cellular respiration?

The answer is the third one: The fatty acid must be broken down into acetic acid, a two-carbon molecule.

Explanation for each option:

1. The fatty acid chain must be broken down into three-carbon molecules, pyruvic acid.
   This is incorrect. Fatty acids are not directly converted into pyruvic acid. Instead, they undergo beta-oxidation to form acetyl-CoA, a two-carbon molecule, which then enters the Krebs cycle.

2. The fatty acid must be broken down into amino acids before they can enter the Krebs cycle.
   This is incorrect. Fatty acids are not converted into amino acids. Amino acids are a different type of biomolecule and have their own metabolic pathways.

3. The fatty acid must be broken down into acetic acid, a two-carbon molecule.
   This is correct. Fatty acids undergo beta-oxidation, which breaks them down into acetyl-CoA, a two-carbon molecule. Acetyl-CoA is then used in the Krebs cycle for further energy production.

4. The fatty acids must be linked to glycerol and form a neutral fat.
   This is incorrect. While fatty acids can be stored as triglycerides (neutral fats) by linking to glycerol, this is a storage form and not a step in the process of metabolizing fatty acids for energy.

In summary, before a fatty acid can be metabolized by cellular respiration, it must be broken down into acetyl-CoA, a two-carbon molecule, through the process of beta-oxidation.