Why are metabolic pathways described as porous rather than as closed systems?
ATheir intermediates enter and leave other pathways
BTheir reactions can only ever run one way
CTheir products are all discarded as waste
DTheir enzymes leak out through the membrane
Answer & Solution
Correct answer: A. Their intermediates enter and leave other pathways
1. Substances enter a pathway from other pathways and intermediates leave it for other pathways.
2. These pathways are therefore not closed systems.
3. Many of the substrates, intermediates and products in one pathway are reactants in others.
4. This traffic in both directions is exactly what the word porous is describing.
_Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 7 "Cellular Respiration", section 7.6 Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways_
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