Recall that fatty acids (like palmitic acid) can yield substantially more ATP compared to glucose. Why doesn't aerobic respiration rely solely on fatty acids as energy sources, rather than glucose?
Fatty acids cannot be used by some vital tissues in the human body, such as the brain.
Fatty acids are rare in the environment and cannot be readily acquired from food sources.
Beta-oxidation—where fatty acids are broken down—requires a great deal of ATP and is not an energy-efficient process for cells.
Aerobic respiration usually does rely on fatty acids in other mammals; however, humans are unique due to their relatively recent appearance on Earth.
Fatty acids cannot be used to generate acetyl-CoA, and so cannot tie into the later stages of aerobic respiration

Fatty acids cannot be used by some vital tissues in the human body, such as the brain.

Beta-oxidation—where fatty acids are broken down—requires a great deal of ATP and is not an energy-efficient process for cells.

Explanation:

Fatty acids are the products of fats in the body. They are known to provide about six times the amount of energy generated by carbohydrates. But some vital organs of the body such as the brain doesn't make use of fatty acids for their energy production. The primary function of fatty acids is not catabolism but protein synthesis. The brain doesn't have energy stores and requires constant supply of energy, this makes it rely mostly on glucose which can provide quick energy for it use. This is also because a major product of fatty acid degradation, superoxide is  injurious to brain cells and many other cells of the body. The beta-oxidation of fatty acids also require great deal of ATP and such process does not conserve energy. Since the brain requires constant supply of energy and with little work, it solely depends on glucose for the production of energy. Only in prolonged starvation does it initiate the use of fatty acids and even proteins.

the answer is d p: cell, q: organ, r: organ system, s: tissue

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