If algae are exposed to 14CO2 for a brief period while illuminated, the labeled carbon is initially found almost entirely in the carboxyl group of 3-phosphoglycerate. However, if illumination is continued after the label pulse, other carbon atoms become labeled.

(A) During photosynthesis carbon atoms from 14CO2 will be fixed in the form of hexoses. As a part of metabolism other carbon atoms become labeled.

(B) Most of the 3PG that is initially labeled is condencated by fructose bisphosphate aldolase, and then enters the cycle as fructose-1,6-bisphosphate.

(C) Photosynthesis-photorespiration shunt regulates the energy transfer of the plant cell by moving intermediates (such as 3PG) that is illustrated movement labeled carbon.

(D)Most of the 3PG that is initially labeled is reused in replacing RuBP, which then reenters the cycle.