Researchers applied radioactive phosphorus and radioactive calcium to a freshly cut tree stump. Eight days later, they came back to measure the distribution of the radioisotopes. They found that nineteen different species of shrubs and trees within a seven-meter radius had taken up the radioactive elements. No radioactivity was detected in the air. Radioactively labeled starch was later added to the stump; in subsequent observations, no labeled starch molecules were ever detected in neighboring plants. Which of the following is the most logical explanation for the observation?
a. During transport, the radioactive atoms are removed from the starch molecules.
b. Starch molecules are too large to cross cell membranes.
c. The radioactivity caused the starch molecules to break down.
d. Water in the soil dissolved the starch molecules.
e. The radioactive carbon atoms decayed into other atoms that are undetectable.
Ans: b. Starch molecules are too large to cross cell membranes.
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