Table I.

Exchange rate of ¹⁵N/¹⁴N over the plasma membrane determined by measuring the dilution of ¹⁵N with ¹⁴N (n = 6)

Interval	Exchange Rate of 15N/14N
min	μmol g−1 fresh wt h−1
0–5	29.42  ± 3.95
5–10	19.87  ± 4.35
10–15	6.22  ± 1.79
15–20	3.86  ± 0.68

Exchange rates were calculated on the basis of the ¹⁵N-enrichment measurements illustrated in Figure 2.