Certain heavy metals, e.g. zinc and chromium, are considered toxic to plants. Research suggests that some of these metals can alter the outcome of the events that occur within the electron transport chain inside plant cells.
Fruit trees grown in soils containing high levels of heavy metals have been found to produce fruits that are much smaller than those produced by trees grown in soil with low concentrations of the metal.
Which of the following statements provides the most important and reasonable explanation for this observation?
Loading of organic compounds in the apoplast pathway cannot occur due to a lack of proton pump activity.
Loading of organic compounds in the apoplast pathway cannot occur due to non-functional membrane transport proteins.
Loading of organic compounds through the symplast pathway cannot occur due to a lack of proton pump activity.
Unloading of organic compounds cannot occur at the source due to a lack of membrane transport proteins.