Question

The intermediates of the citric acid cycle are constantly being depleted because they are used to produce many of the amino acids needed to make proteins. The enzyme pyruvate carboxylase converts pyruvate to oxaloacetate to replenish these intermediates. Bacteria, but not animal cells, have additional enzymes that can carry out the reaction acetyl CoA + isocitrate í oxaloacetate + succinate. Which of the following compounds will not support the growth of animal cells when used as the major source of carbon in food, but will support the growth of nonphotosynthetic bacteria?

(a) pyruvate
(b) glucose
(c) fatty acids
(d) fructose

Answer 1

Fatty acids cannot be used by animal cells to produce glucose due to the lack of necessary enzymes; however, nonphotosynthetic bacteria can use fatty acids to grow because they possess the glyoxylate cycle.

Step-by-step explanation:

The question aims to identify which compounds can support the growth of nonphotosynthetic bacteria but not animal cells when used as a major carbon source in food.

One crucial enzyme that bacteria have, but animal cells lack, is capable of converting acetyl CoA directly to oxaloacetate. Fatty acids cannot be converted into glucose through gluconeogenesis in animal cells due to the absence of the glyoxylate cycle and the enzymes needed for acetyl CoA to be a substrate in the process of forming oxaloacetate.

Since animal cells rely on glucose or other substrates that lead directly into the citric acid cycle, they are unable to use fatty acids exclusively for glucose production. In contrast, nonphotosynthetic bacteria possess enzymes for the glyoxylate cycle, which enables them to utilize fatty acids to sustain growth when they are the major carbon source.

Thus, fatty acids meet the criteria of the question, being a compound that can only support the growth of nonphotosynthetic bacteria and not animal cells when used as the major carbon source in food.